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The hydrography of the Mozambique Channel from six years of continuous temperature, salinity, and velocity observations
... Annually, the upper ocean circulation of the MCT routes large amounts (about 17 Sv (1 Sv = 10 6 m³ s − ¹)) of IO water southwards to the Agulhas Current 15 (Fig. 1a). The MCT is dominated by the regular southwards propagation of about 300-350-km-wide deep-reaching anticyclonic eddies, which affect the near-bottom currents 15 . ...
... Annually, the upper ocean circulation of the MCT routes large amounts (about 17 Sv (1 Sv = 10 6 m³ s − ¹)) of IO water southwards to the Agulhas Current 15 (Fig. 1a). The MCT is dominated by the regular southwards propagation of about 300-350-km-wide deep-reaching anticyclonic eddies, which affect the near-bottom currents 15 . An enhanced MCT is coupled to the variability in the year-round combined westwards flow of the South Equatorial Current (SEC) and its extension, the Northeastern Madagascar Current (NEMC), north of Madagascar 16 , which bifurcates upon approaching the African coast into the Eastern African Coastal Current (EACC) and the southwards MCT (Fig. 1a). ...
... cm s −1 ) southerly flow, which is directly below the influence of the energetic eddy-induced currents strongly affecting the upper 1,500 m (Fig. 1c, Extended Data Fig. 5). The region is further sheltered from the northward-flowing Mozambique Undercurrent (MUC; about 1-3 Sv; 3.5-4.5 cm s −1 ), which is confined to the African continental slope between about 1,500 m and 2,500 m water depth 15 , and it is also disconnected from down-slope sediment transport off the continental margins (Fig. 1b, Extended Data. Fig. 5c). ...
Today, the eastern African hydroclimate is tightly linked to fluctuations in the zonal atmospheric Walker circulation1,2. A growing body of evidence indicates that this circulation shaped hydroclimatic conditions in the Indian Ocean region also on much longer, glacial–interglacial timescales3–5, following the development of Pacific Walker circulation around 2.2–2.0 million years ago (Ma)6,7. However, continuous long-term records to determine the timing and mechanisms of Pacific-influenced climate transitions in the Indian Ocean have been unavailable. Here we present a seven-million-year-long record of wind-driven circulation of the tropical Indian Ocean, as recorded in Mozambique Channel Throughflow (MCT) flow-speed variations. We show that the MCT flow speed was relatively weak and steady until 2.1 ± 0.1 Ma, when it began to increase, coincident with the intensification of the Pacific Walker circulation6,7. Strong increases during glacial periods, which reached maxima after the Mid-Pleistocene Transition (0.9–0.64 Ma; ref. 8), were punctuated by weak flow speeds during interglacial periods. We provide a mechanism explaining that increasing MCT flow speeds reflect synchronous development of the Indo-Pacific Walker cells that promote aridification in Africa. Our results suggest that after about 2.1 Ma, the increasing aridification is punctuated by pronounced humid interglacial periods. This record will facilitate testing of hypotheses of climate–environmental drivers for hominin evolution and dispersal. A record of flow-speed variations from an ocean core shows that a key component of Indian Ocean circulation rapidly accelerated 2.1 million years ago, coincident with intensification of Pacific Walker circulation.
... The circulation of deep currents is widely described due to the Davie Ridge (Coffin and Rabinowitz, 1987) that separates the channel into two distinct basins (i.e. the Comoros Basin in the north and the Mozambique Basin in the south; Fig. 1). According to some authors, the Davie Ridge represents a topographic obstacle to the circulation of these deep currents flowing from the Atlantic Ocean to the south and from the Indian Ocean to the north (Toole and Warren, 1993;Mantyla and Reid, 1995; Kolla et al., 1980;Fine, 1993;Toole and Warren, 1993;DiMarco et al., 2002;Lutjeharms, 2006;Ullgren et al., 2012). AABW: Antarctic Bottom Water; AAIW: Antarctic Intermediate Water; AC: Agulhas Current; AUC: Agulhas Undercurrent; MC: Mozambique Current; ME: Mozambique Eddies; MUC: Mozambique Undercurrent; NADW: North Atlantic Deep Water; NIDW: North Indian Deep Water; RSW: Red Sea Water; SW: Surface Water including TSW: Tropical Surface Water, STSW: Sub-Tropical Surface Water and SICW: South Indian Central Water. ...
... The MUC and the Agulhas Undercurrent (AUC) carry Antarctic Intermediate Water (AAIW) between 800 and 1500 mbsl (Ullgren et al., 2012;Fig. 1B, C). ...
... The Nd isotope signatures from the surface layer of the Fe-Mn crusts highlight the large variability (between ε Nd = − 7.1 and − 10.1) of water mass compositions (Table 2, Fig. 2). In several hydrodynamic and geochemical studies (Toole and Warren, 1993;Jeandel et al., 1995;McCave et al., 2005;Ullgren et al., 2012;Collins et al., 2016), an average water depth of 1500 mbsl is considered to separate intermediate layers (AAIW between 800 and 1500 mbsl, RSW between 900 and 1200 mbsl) from deep layers (NADW and NIDW at more than 1500-2000 mbsl). Therefore, Nd isotope composition of samples located above and below 1500 mbsl will be presented separately (Fig. 3). ...
The Mozambique Channel plays a key role in the exchange of water masses between the Indian and Atlantic Oceans, which include the North Atlantic Deep Water (NADW) inflow from the south and the North Indian Deep Water (NIDW), an aged form of the NADW spreading poleward from the northern and equatorial Indian Ocean basin. Several authors assume that the Davie Ridge acts as a topographic barrier to the northward advection of NADW, which would therefore be absent in the Comoros Basin. Other studies suggest that the NADW flows from the south of the Mozambique Channel to the Comoros Basin, indicating that the Davie Ridge may not currently constitute a blocking topographic barrier to deep water mass circulation. To address this question, we studied ferromanganese (Fe, Mn) crusts collected over 2000 km in the Mozambique Channel, from the Agulhas Plateau to the Glorieuses Islands. Neodymium (Nd) isotope compositions (ε Nd) of surface scrapings range between ε Nd = − 10.1 above the Agulhas Plateau, which might reflect the NADW inflow, and more radiogenic values between ε Nd = − 8.0 and − 8.2 in the Glorieuses area, highlighting the NIDW influence. However, value of ε Nd = − 9.4 measured north of the Davie Ridge cannot be explained by the sole influence of the NIDW and therefore highlights the advection of the NADW northeast of the Comoros Basin. We estimate that the contribution of the NADW through the channel is up to 68% in the Agulhas Plateau and 60% north of the Davie Ridge. These findings are consistent with previous hydrographic studies and suggest that the Davie Ridge does not currently act as topographic barrier to deep currents.
... One way of resolving seasonality in the LCD flux and the LDI is to analyze time series samples from sediment traps that continuously collect sinking particles in successive time intervals over periods of a year or more. Such studies have been carried out for the U K 37 as well as for the TEX 86 and associated lipids (e.g., Müller and Fischer, 2001;Wuchter et al., 2006;Huguet et al., 2007;Fallet et al., 2011;Yamamoto et al., 2012;Rosell-Melé and Prahl, 2013;Turich et al., 2013). However, very few studies have been undertaken for LCDs. ...
... In addition to the sediment trap sampling, the primary productivity of the surface waters was measured every month using 14 C incubations . For this study, we investigated two periods, i.e., May 1999-May 2000 and July 2002-July 2003 for traps A and B. These years include upwelling and non-upwelling periods, as well as a disastrous flooding event in December 1999(Turich et al., 2013. Turich et al. (2013) identified the upwelling periods, linked to the migration of the ITCZ, as indicated by decreasing SST in the CTD (temperature at −1 m water depth) and satellite-based measurements (indicated by grey boxes in Figs. 8 and 10), and shoaling of the average depths of primary production and increased primary production. ...
... For this study, we investigated two periods, i.e., May 1999-May 2000 and July 2002-July 2003 for traps A and B. These years include upwelling and non-upwelling periods, as well as a disastrous flooding event in December 1999(Turich et al., 2013. Turich et al. (2013) identified the upwelling periods, linked to the migration of the ITCZ, as indicated by decreasing SST in the CTD (temperature at −1 m water depth) and satellite-based measurements (indicated by grey boxes in Figs. 8 and 10), and shoaling of the average depths of primary production and increased primary production. Moreover, Turich et al. (2013) evaluated the U K 37 and TEX 86 proxies for the same two time series for which we analyzed the LCD proxies. ...
In this study we analyzed sediment trap time series from five tropical sites to assess seasonal variations in concentrations and fluxes of long-chain diols (LCDs) and associated proxies with emphasis on the long-chain diol index (LDI) temperature proxy. For the tropical Atlantic, we observe that generally less than 2 % of LCDs settling from the water column are preserved in the sediment. The Atlantic and Mozambique Channel traps reveal minimal seasonal variations in the LDI, similar to the two other lipid-based temperature proxies TEX86 and U37K′. In addition, annual mean LDI-derived temperatures are in good agreement with the annual mean satellite-derived sea surface temperatures (SSTs). In contrast, the LDI in the Cariaco Basin shows larger seasonal variation, as do the TEX86 and U37K′. Here, the LDI underestimates SST during the warmest months, which is possibly due to summer stratification and the habitat depth of the diol producers deepening to around 20–30 m. Surface sediment LDI temperatures in the Atlantic and Mozambique Channel compare well with the average LDI-derived temperatures from the overlying sediment traps, as well as with decadal annual mean SST. Lastly, we observed large seasonal variations in the diol index, as an indicator of upwelling conditions, at three sites: in the eastern Atlantic, potentially linked to Guinea Dome upwelling; in the Cariaco Basin, likely caused by seasonal upwelling; and in the Mozambique Channel, where diol index variations may be driven by upwelling from favorable winds and/or eddy migration.
... The permanent thermocline is mainly composed of South Indian Central Water (SICW) (Fig. 1C, D). Intermediate waters in the Mozambique Channel are composed of Red Sea Water (RSW), that enters into the Mozambique Channel from the north, and Antarctic Intermediate Water (AAIW), that enters from the south as part of the Mozambique Undercurrent (MUC) ( Fig. 1; Ullgren et al., 2012). The deep waters in the Mozambique Channel contain North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) (Fig. 1C, D;van Aken et al., 2004;Ullgren et al., 2012). ...
... Intermediate waters in the Mozambique Channel are composed of Red Sea Water (RSW), that enters into the Mozambique Channel from the north, and Antarctic Intermediate Water (AAIW), that enters from the south as part of the Mozambique Undercurrent (MUC) ( Fig. 1; Ullgren et al., 2012). The deep waters in the Mozambique Channel contain North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) (Fig. 1C, D;van Aken et al., 2004;Ullgren et al., 2012). The NADW is found between about 2000 and 3500 m water depth (wd) and the AABW at > 3500 m wd (Fig. 1C, D). ...
... The NADW is characterised by a relative maximum in salinity and oxygen at about 2500 m wd ( Fig. 1D; Mantyla and Reid, 1995). The upper portions of the NADW and the AAIW flow, as the Mozambique Undercurrent (MUC), northwards along the western margin of the basin through the sill of the Mozambique Channel (Fig. 1A;van Aken et al., 2004;Ullgren et al., 2012). The deep portion of the NADW and the AABW (characterised by an absolute minimum of temperature and a relative minimum of salinity; Fig. 1D) are constrained by the bathymetry, flowing northwards along the Mozambican margin and back southwards along the eastern part (along Madagascar) ( Fig. 1A; van Aken et al., 2004). ...
Mixed turbidite-contourite systems can be found in oceans where bottom currents and turbidity currents interact. The Zambezi turbidite system, located in the Mozambique Channel (SW Indian Ocean), is one of the largest sedimentary systems in the world in length and area of the related catchments. The oceanic circulation in the Mozambique Channel is intense and complex, dominated by eddies flowing southwards and deep currents flowing northwards along the Mozambican margin. Current measurements obtained from moorings at 3400-4050 m water depth in the Zambezi and Tsiribihina valleys show periods of intense currents at the seafloor with peaks of 40-50 cm s(-1) that last up to one month and are not related to turbidity currents. These strong bottom-current events are correlated with a change in current direction and an increase in temperature. The periods of current intensification may be related to eddies, since they present similar frequencies (around 7 per year). Moreover, modelling results show that during periods of intense deep circulation an anticyclonic eddy is present between the Mozambican slope and the centre of the Mozambique Channel, which may block the northward transport of the deep water mass and thus enhance the southward transport along the western slope of Madagascar. According to our hydrodynamic modelling of the circulation near the seafloor, intense currents are often present along the Zambezi Valley, especially along the valley flanks. Multi-channel seismic reflection data show that the Zambezi turbidite system does not show the typical characteristics of turbidite systems, being dominated by erosional processes, which mainly affect the valley flanks. Levees associated with the valley are absent in the main axis of the system. The effect of bottom currents on sedimentation in the basin is evidenced by the low sedimentation rates that witness winnowing in the basin, the presence of contouritic sand in the Zambezi Valley flanks and the abundance of current-related bedforms observed in multibeam bathymetry and seismic data. The intense oceanic processes observed in the Mozambique Channel may transport a large part of the fine sediment out of the basin and erode the seafloor even at great depths. Therefore, the Zambezi turbidite system could at present be considered as a mixed turbidite-contourite system, with important implications for source-to sink studies.
... Because of the scarcity of observational data on currents in the vertical plane on the Sofala Bank, model realism was investigated by taking into consideration knowledge gained from the Long-term Ocean Climate Observation (LOCO) program (Ullgren et al., 2012) and the "parent" model. The vertical distribution of long-term mean alongshore velocity from our 7-year ROMS-BSM simulations (Fig. 4A) was consistent with the vertical current patterns known from the 6-year LOCO observations (Fig. 4B). ...
... The vertical section of modelled mean alongshore currents and hydrography corroborated the surface current patterns, in particular, the mean "Mozambique Current" depicted by the strong southward current (Ullgren et al., 2012) and isoclines of temperature and salinity uplifted in the upper 500 m. However, LOCO, which is the longest in situ dataset in the region, lacks hydrographic sampling in the upper 500 m (Ullgren et al., 2012), making it difficult to evaluate the model hydrography in this layer. ...
... The vertical section of modelled mean alongshore currents and hydrography corroborated the surface current patterns, in particular, the mean "Mozambique Current" depicted by the strong southward current (Ullgren et al., 2012) and isoclines of temperature and salinity uplifted in the upper 500 m. However, LOCO, which is the longest in situ dataset in the region, lacks hydrographic sampling in the upper 500 m (Ullgren et al., 2012), making it difficult to evaluate the model hydrography in this layer. This highlights the relevance of ocean models in a region where observations are scarce. ...
A high-resolution, two-way nested Regional Ocean Modeling System, forced with monthly climatologies, has been set up for the Sofala Bank and adjacent deeper ocean of the Mozambique Channel to investigate the role of offshore mesoscale eddies on the shelf circulation, hydrographic structures and river plumes. The model is shown in comparison with available observations and published studies. Most known oceanographic features are reproduced by our model. We applied Self-Organizing Maps and showed that offshore passing eddies, depending on their strength and proximity to the shelf, modulate the shelf circulation and river plume direction and spread. The presence of a strong cyclonic eddy close to the shelf induces northward surface shelf currents. In contrast, the presence of a strong anticyclonic eddy close to the shelf induces a strong southward current over most of the shelf, except off Beira. Our analyses confirm that the plume of the Zambezi River is bi-directional. The southward-directed plume patterns, opposite to the dominant northwards, occur in response to nearby offshore anticyclonic eddies (26% of occurrence). This behavior could have an influence on water dispersal, shelf ecosystems and important fisheries. Therefore, offshore mesoscale eddies should be taken into account when studying the ocean dynamics of the Sofala Bank.
... Several water masses converge within the Mozambique Channel (Figure F3), transporting Tropical Surface Water (TSW), Subtropical Surface Water (STSW), Equatorial Indian Ocean Water (EIOW), Arabian Sea High Salinity Water (ASHSW), and Red Sea Intermediate Water (RSIW) into the Agulhas Current (de Ruijter et al., , 2005Schott et al., 2009;Collins et al., 2016). At depth, the Mozambique Undercurrent carries North Atlantic Deep Water (NADW), South Indian Central Water (SICW), and Antarctic Intermediate Water (AAIW) toward the Equator along the continental slope Ullgren et al., 2012). In general, the circulation is dominated by mesoscale activity (e.g., Biastoch and Krauss, 1999;de Ruijter et al., 2002;Rid-derinkhof and de Ruijter, 2003;Schouten et al., 2003;Lutjeharms, 2006) and is related to the inflow of the Indonesian Throughflow, the Tasman Strait Throughflow, and the strength of the tropical and subtropical surface gyres in the Indian Ocean (Penven et al., 2006;Palastanga et al., 2006;Ridderinkhof et al., 2010;Schott et al., 2009;Backeberg and Reason, 2010). ...
... Satellite observations (e.g., Schouten et al., 2003;Quartly and Srokosz, 2004) and ocean models (Biastoch and Krauss, 1999) have shown that mesoscale eddies generally form as the channel narrows, close to Davie Ridge (~16°S), between the northern and central basin. These anticyclonic eddies typically maintain rotational velocity that is often >1.5 m/s (Schouten et al., 2003;Ullgren et al., 2012), as ~4-7 eddies migrate southward through the channel per year. Ridderinkhof and de Ruijter (2003) have shown that the hydrographic influence of these eddies extends to 2000 mbsl, with vertical migration of isopycnals occasionally exceeding 40 m per day in the upper layer (σ θ = 25.0 kg/m) during the passage of an eddy (Ullgren et al., 2012). ...
... These anticyclonic eddies typically maintain rotational velocity that is often >1.5 m/s (Schouten et al., 2003;Ullgren et al., 2012), as ~4-7 eddies migrate southward through the channel per year. Ridderinkhof and de Ruijter (2003) have shown that the hydrographic influence of these eddies extends to 2000 mbsl, with vertical migration of isopycnals occasionally exceeding 40 m per day in the upper layer (σ θ = 25.0 kg/m) during the passage of an eddy (Ullgren et al., 2012). The development of eddies in the Mozambique Channel and their southward propagation within the greater Agulhas Current system seed the generation of solitary meanders that progress downstream in the Agulhas Current, the so-called Natal Pulses Tsugawa and Hasumi, 2010). ...
... The system comprises of a series of east-west (E-W) trending submarine canyons that extend ~70 km offshore (Fig. 1a), with low modern terrestrial sediment supply 29 , and is located in a region of complex and highly energetic ocean currents ( Fig. 1b) 12,[30][31][32][33][34][35][36][37] . Using these unprecedented synchronous and spatially-distributed measurements, we quantify the dynamic interactions across a large mixed depositional system and address the following questions. ...
... 1e)and asymmetric canyon and channel cross-sectional morphologies15 . The dominant northward flow, especially in deeper (>1,500 m) water, is consistent with previous observations of the Mozambique Undercurrent35 (MUC; Fig. 1b). ...
Deep-sea transport of sediment and associated matter, such as organic carbon, nutrients and pollutants, is controlled by near-bed currents. On the continental slope, these currents include episodic down-slope gravity-driven turbidity currents and more sustained thermohaline-driven along-slope contour currents. Recent advancements in deep-sea monitoring have catalysed a step change in our understanding of turbidity currents and contour currents individually. However, these processes rarely operate in isolation and the near-bed current regime is still to be quantified in a mixed system. Such measurements are crucial for understanding deep-sea particulate transport, calibrating numerical models and reconstructing palaeoflow. Here we use 4 years of observations from 34 instrument moorings in a mixed system offshore of Mozambique to show that near-bed currents are highly dynamic. We observe spatial variability in velocity over tidal and seasonal timescales, including reversals in current direction, and a strong steering and funnelling influence by local seabed morphology. The observed near-bed currents are capable of mobilizing and distributing sediments across the seabed, therefore complicating deep-sea particulate transport and reconstruction of palaeoceanographic conditions.
... The characteristics of anticyclonic eddies in the MC have been extensively documented Ridderinkhof and De Ruijter, 2003;Schouten et al., 2003). Through moored current measurements, it has been estimated that anticyclonic eddies occur approximately 4-5 times per year in the narrowest part of the MC Schouten et al., 2003;Ridderinkhof et al., 2010;Ullgren et al., 2012). These eddies typically have a diameter of around 300 km and can reach the channel's bottom Ridderinkhof and De Ruijter, 2003). ...
... Therefore, it becomes necessary to combine eddies from various bins at the same latitude in order to accurately determine the occurrence frequency of eddies. The obtained results ( Figure 5) demonstrate a strong concurrence with moored current measurements conducted by Ridderinkhof and De Ruijter, 2003;Schouten et al. (2003); Ridderinkhof et al. (2010) and Ullgren et al. (2012). Figure 5 show good agreement with power spectra analysis of SSHA (Schouten et al., 2003) in the northern and southern MC A B The frequency distribution of eddy radius (A, B) for anticyclonic and cyclonic eddies extracted from dipole eddies; (C) the frequency distribution of eddy lifespans for dipole eddies. ...
The discovery of cyclonic and dipole eddies in the Mozambique Channel (MC) indicates that the understanding of the mesoscale eddy characteristics in the MC is incomplete. The distributions of anticyclonic, cyclonic, and dipole eddies along the MC were elucidated in this study using satellite observations. It was observed that these eddies exhibit a preference for emergence and movement in the western MC. The occurrence frequencies of anticyclonic and cyclonic eddies are four and three times per year, respectively, in the narrowest section of the MC. In contrast, the frequency of mesoscale eddies reaches its peak at nine times per year in the central region of the MC. The occurrence of dipole eddies also reaches its peak twice per year in the middle MC. Dipole eddies are more prevalent in the MC and exhibit larger dimensions and shorter lifespans compared to anticyclonic and cyclonic eddies. Mesoscale eddies, which traverse the narrowest section of the MC and propagate southward, are predominantly generated within the western Comoros Basin due to barotropic instability. The southward branch of the Northeast Madagascar Current (NEMC) plays a crucial role in transporting these eddies to the middle MC. The eastern middle MC is also a generation site for mesoscale eddies in addition to the Comoros Basin, where cyclonic eddies are generated twice per year. These cyclonic eddies are also generated due to barotropic instability.
... Therefore, the emplacement and migration of depocentres is highly sensitive to sea-level fluctuations between glacial lowstands and interglacial highstands (Schulz et al., 2011;van der Lubbe et al., 2014). The margin is also exposed to vigorous surface to bottom oceanic circulation (de Ruijter et al., 2002;Penven et al., 2006;Ullgren et al., 2016Ullgren et al., , 2012 (Figs. 1 and 2) that drives slope sediment transport and plasters the seabed (Breitzke et al., 2017;Fierens et al., 2019;Flemming and Kudrass, 2018;Kolla et al., 1980;Miramontes et al., 2019bMiramontes et al., , 2021Thiéblemont et al., 2019;Wiles et al., 2017). The margin is also affected by giant sliding (Deville et al., 2020). ...
... However, sediment wave formation by lee waves is a complex process that involves both across and along crest currents that can lead to sediment wave crests oblique to the bottom current (Blumsack, 1993;Blumsack and Weatherly, 1989). Furthermore, in the Mozambique channel, bottom circulation is an unsteady phenomenon with contribution from surface circulation in the form of eddies that can affect the whole water column, reach the seabed with high velocities and even revert the northward transport related to the MUC (de Ruijter et al., 2002;Miramontes et al., 2019b;Schouten et al., 2003;Ullgren et al., 2012). Sediment wave crests parallel to the slope may thus be compatible with the complex fluctuating bottom current pattern in the area. ...
The pattern of sediment dispersal and the location of sediment depocentres on continental margins can be very complex in both space and time. We aim at investigating how significantly external and internal factors, such as river runoff, bottom current and sliding can deviate the sediment dispersal from a simplistic fully sea-level controlled spreading. In this study we examined the sedimentation at two transects across the Mozambique-Zambezi slope between 17°20S-20°S, where multibeam bathymetry, sub-bottom profiler data and sediment cores were acquired. The period investigated spans the last 40 cal ka BP with a focus on the contrast between the last glacial (lowstand) and the Holocene (highstand) periods. Results show contrasting patterns of sediment dispersal, deposition and preservation. Sea level fluctuation remains the main forcing and most of the sediment from the Zambezi River settled on the inner shelf since the last sea level rise. However, we found that two major depocentres have developed on the upper slope during the Holocene consequent to the interaction of bottom currents with seabed morphologies at the shelf edge and at the upper slope. Early Holocene sliding in the north-east and in the south-west upper slope is a secondary but yet major factor of sediment transfer to the deep domain. Identified preconditioning factors for sliding on the slope are related to lowstand sediment loading and fluid circulation on the upper slope, and erosion at the base of slope of a plastered drift. Triggering must be related to margin wide mechanisms such as changes in hydrostatic pressure and reorganisation of sediment dispersal, subsequent to the post-glacial sea level rise, or maybe a period of regional seismicity. Climatic conditions in the Zambezi River watershed during the Bolling-Allerod and Younger Dryas periods are recorded and imprinted on the upper slope in the form of a detrital rich layer and a prominent slope-wide high-amplitude reflector. All over the continental slope, plastering of sediment deposits by bottom currents is pervasive and shows a morphological continuum from erosional scours at the base of slope to sediment waves on the upper slope and a possible interaction between along and across slope transport processes. We conclude that, in addition to sea level, the interplay of external and internal factors such as oceanic circulation and sliding, together with margin morphology, lead to the development of unexpected depocentres on the continental slopes. Thus, the study of modern marine analogues is crucial to avoid misleading interpretation of fossil deposits in terms of paleo sea level and more generally of paleo-environmental conditions.
... These currents mainly flow southwards as large anticyclonic eddies with diameters of ≥300 km that can affect the entire water column and reach the seafloor (de Ruijter et al., 2002;Halo et al., 2014). Intermediate (Antarctic Intermediate Water, AAIW) and deep-water masses (North Atlantic Deep Water, NADW, and Antarctic Bottom Water, AABW) enter the Mozambique Channel from the south and flow northwards along the eastern African margin as the Mozambique Undercurrent (MUC) ( Fig. 1; Ullgren et al., 2012). AAIW and the upper part of the NADW continue to flow northwards along the northern Mozambican margin, but the lower part of the NADW and the AABW are constrained by the northward shallowing bathymetry, especially below 2500 m water depth, and flow back southwards along the western margin of Madagascar ( Fig. 1; Ullgren et al., 2012;Miramontes et al., 2019a). ...
... Intermediate (Antarctic Intermediate Water, AAIW) and deep-water masses (North Atlantic Deep Water, NADW, and Antarctic Bottom Water, AABW) enter the Mozambique Channel from the south and flow northwards along the eastern African margin as the Mozambique Undercurrent (MUC) ( Fig. 1; Ullgren et al., 2012). AAIW and the upper part of the NADW continue to flow northwards along the northern Mozambican margin, but the lower part of the NADW and the AABW are constrained by the northward shallowing bathymetry, especially below 2500 m water depth, and flow back southwards along the western margin of Madagascar ( Fig. 1; Ullgren et al., 2012;Miramontes et al., 2019a). This intense circulation controls sedimentation and seafloor geomorphology in the Mozambique Channel at all depths, from the shelf (Flemming and Kudrass, 2018) and top of seamounts (Miramontes et al., 2019c), to the deep part of the basin (Kolla et al., 1980;Breitzke et al., 2017;Fierens et al., 2019;Miramontes et al., 2019a;Thiéblemont et al., 2019a). ...
Oceanic currents can profoundly reshape the seafloor and even modify the characteristics of turbidite systems. Multiple erosional and depositional features directly formed by bottom currents (i.e. contourites), as well as by the interaction between bottom currents and turbidity currents or turbidite systems (i.e. mixed turbidite-contourite systems) have been identified in the Mozambique Channel (SW Indian Ocean) in multibeam bathymetry, seismic reflection data, sub-bottom profiler images and sediment cores. In this study, we characterise the morphology, stacking pattern and sedimentary characteristics of these sedimentary systems, and analysed the properties of bottom currents at these systems using a hydrodynamic numerical model. Modelled bottom currents are the highest at abraded surfaces and moats, but they also display a relatively high variability, suggesting that the observed erosion is not the result of a constant or persistent current but rather of episodes of intense circulation. Modelled bottom currents at contourite terraces are not significantly different from currents at related plastered drifts, where accumulation is enhanced. The formation of contourite terraces can thus not solely be explained by the mean oceanic circulation and eddies, implying that other processes such as internal waves may play a relevant role in their formation. Three different types of mixed turbidite-contourite systems were observed: one characterised by asymmetric channel-levee systems formed by the synchronous interaction of bottom currents and turbidity currents, one characterised by a phased interaction that resulted in the erosion of the channel flanks by bottom currents, and another one in which both synchronous and phased interaction played a relevant role in the evolution of the system. Finally, we propose a simplified classification of contourites that can be applied to any contourite system worldwide, and that comprises erosional and depositional features, including muddy and sandy contourite deposits.
... Southward-flowing Mozambique currents are anticyclonic eddy-dominated western boundary currents (DiMarco et al., 2002;Schouten et al., 2003;Preu et al., 2011). They carry upper layers of Tropical Surface Water (TSW) and Subtropical Surface Water (STSW) flowing at < 200 m (Ullgren et al., 2012), a middle layer of South Indian Central Water (SICW) occupying a permanent thermocline at 200-600 m (Fig. 2c), and a lower layer of the Red Sea Water (RSW) between 900 and 1200 m (Quadfasel, 2001). ...
... The upper volume of the NADW continues its northward flow across the southern Mozambican continental slope while the remaining NADW forms a return current and flows along the MozR (van Aken et al., 2004;Wiles et al., 2014;Fischer and Uenzelmann-Neben, 2018), which may create relatively complex pathways locally (Fig. 1b) and is of special interest to this study. A similar situation also occurs in the Mozambique Channel, where parts of the NADW enter into the Somali Basin and the rest acts as a southerly return current (van Aken et al., 2004;Ullgren et al., 2012;Fig. 1a). ...
... At water depths of 1500-2400 m the Mozambique undercurrent flows northward as a deep western boundary current. The Mozambique undercurrent comprises two water masses; Antarctic Intermediate Water (AAIW) in the upper levels, and North Atlantic Deep Water (NADW) in the lower levels (de Ruijter et al., 2002;DiMarco et al., 2002;Ullgren et al., 2012;Wiles et al., 2014). Although the AAIW and upper NADW cross the Davie Ridge Transform Fault Zone, the lower component of NADW retroflects against shoaling bathymetry, turning eastwards across the Mozambique Channel, and then southwards (de Ruijter et al., 2002;Donohue and Toole, 2003). ...
... These features are characterised by wavelengths 0.5-10 km and height up to 50 m, although some examples attain wavelengths of > 10 km, amplitudes of > 150 m, and are aligned oblique to the slope up which they migrate (Flood et al., 1993;Flood, 1994;Howe, 1996;Manley and Caress, 1994;Cunningham and Barker, 1996;Stow et al., 2013). Within the study area, much of the Mozambique Channel experiences some level of current/seafloor interaction as AAIW and NADW migrate through the channel (de Ruijter et al., 2002;DiMarco et al., 2002;Ullgren et al., 2012;Wiles et al., 2014). The NADW and AABW are credited with significant seafloor interactions elsewhere in the southwest Indian Ocean creating contourites, sediment waves and driving sediment accumulation in basins to the south on the study area (Dingle et al., 1987;Uenzelmann-Neben, 2007, 2008;Wiles et al., 2013Wiles et al., , 2014. ...
In this contribution, high resolution multibeam swath bathymetry and PARASOUND sediment echosounder data are used to describe a region within the distal part of the central Mozambique Channel. The study area marks a transition from abyssal plain to abyssal hill type morphology within the sediment-rich Mozambique Fan and associated with a zone of extension in response to East African Rift System kinematics. Hosted within the abyssal hill lies an east-west orientated, elongate (80 km × 11 km) depression (relief of ca.175 m). Multibeam bathymetry and PARASOUND data show that the region surrounding the depression is variable in geomorphology including rugged irregular seafloor and sediment waves. Low gradient, smooth sea floor dominates the abyssal plain which returns several, distinct, sub-parallel sub bottom echoes. The flanks of the abyssal hill are marked by seafloor undulations likely evidence of bottom-current controlled geomorphology, and mass wasting deposits. The floor of the depression is characterised by hyperbolic echoes commonly associated with very rugged seafloor and basement outcrop with little sediment cover. The present-day geomorphology of the study area is the product of deep-seated ocean circulation and soft sediment deformation superimposed upon the antecedent geological framework, influenced by present-day kinematics of the East African Rift System. Faulting associated with these kinematics is manifest at the seafloor as the elongate steep-flanked depression; the result of an extensional regime expressed across the Mozambique channel from south-southwest to north-northeast. This contribution highlights the local, marine, ramification of a continental-scale largely terrestrial tectonic regime.
... Both sharks dived to cold temperatures of between 5.1 and 10°C while in oceanic waters. We have no contemporaneous data on vertical temperature profiles in this region, but an array of oceanographic sensors across the Mozambique Channel from 2003 to 2009 found that on average, the temperature was close to 10°C at 500 m, 6°C at 1000 m and 4°C at 1500 m (Ullgren et al. 2012). However, the passage of eddies southwards through the array resulted in up to 8°C changes in temperature at 110 m depth, so the relationship between depth and temperature is not strictly linear within this dynamic system (Ullgren et al. 2012). ...
... We have no contemporaneous data on vertical temperature profiles in this region, but an array of oceanographic sensors across the Mozambique Channel from 2003 to 2009 found that on average, the temperature was close to 10°C at 500 m, 6°C at 1000 m and 4°C at 1500 m (Ullgren et al. 2012). However, the passage of eddies southwards through the array resulted in up to 8°C changes in temperature at 110 m depth, so the relationship between depth and temperature is not strictly linear within this dynamic system (Ullgren et al. 2012). A whale shark tagged with a pop-up archival tag by Brunnschweiler et al. (2009) in the southern Mozambique Channel recorded temperatures of 9.2°C at 1092 m, 5.5°C at 1087 m and 4.2°C at 1264 m. ...
... Both sharks dived to cold temperatures of between 5.1 and 10°C while in oceanic waters. We have no contemporaneous data on vertical temperature profiles in this region, but an array of oceanographic sensors across the Mozambique Channel from 2003 to 2009 found that on average, the temperature was close to 10°C at 500 m, 6°C at 1000 m and 4°C at 1500 m (Ullgren et al. 2012). However, the passage of eddies southwards through the array resulted in up to 8°C changes in temperature at 110 m depth, so the relationship between depth and temperature is not strictly linear within this dynamic system (Ullgren et al. 2012). ...
... We have no contemporaneous data on vertical temperature profiles in this region, but an array of oceanographic sensors across the Mozambique Channel from 2003 to 2009 found that on average, the temperature was close to 10°C at 500 m, 6°C at 1000 m and 4°C at 1500 m (Ullgren et al. 2012). However, the passage of eddies southwards through the array resulted in up to 8°C changes in temperature at 110 m depth, so the relationship between depth and temperature is not strictly linear within this dynamic system (Ullgren et al. 2012). A whale shark tagged with a pop-up archival tag by Brunnschweiler et al. (2009) in the southern Mozambique Channel recorded temperatures of 9.2°C at 1092 m, 5.5°C at 1087 m and 4.2°C at 1264 m. ...
Whale sharks Rhincodon typus, the world's largest fish, are routinely sighted offthe northwest coast of Madagascar, particularly offthe island of Nosy Be. Dedicated whale shark tourism has been developing in the area since 2011. During our first dedicated survey, from September to December 2016, we photo-identified 85 individual whale sharks ranging from 3.5 to 8 m in total length (all juveniles). None had been previously identified from surrounding countries. We tagged 8 sharks with tethered SPOT5 tags in October 2016, with tracking durations of 9 to 199 d. Kernel density plots showed that the main activity hotspot for tagged sharks was around the Nosy Be area. Three individuals were resighted back at Nosy Be in late 2017 after having lost their tags. A secondary hotspot was identified offPointe d'Analalava, 180 km southeast of Nosy Be. Five sharks swam offthe shelf into the northeastern Mozambique Channel, between Madagascar and Mayotte, and one of these continued to near the Comoros islands. Two sharks swam to southern Madagascar, with minimum track distances of 3414 and 4275 km. The species is presently unprotected in Madagascar, although a small proportion of the high-use area we identified in this study is encompassed within 2 marine protected areas adjacent to Nosy Be. Whale sharks are globally endangered and valuable to the local economy, so there is a clear rationale to identify and mitigate impacts on the sharks within the 2 hotspots identified here.
... Located in the southwest Indian Ocean, between Madagascar and the African continent, the Mozambique Channel experiences an average southward oceanic transport of approximately 14 Sv (1 Sv = 10 6 m 3 s 1 ) (Ganachaud & Wunsch, 2000), with estimates ranging from 5 to 18 Sv (Harlander et al., 2009) and with a large standard deviation of ∼15 Sv (Ullgren et al., 2012). This region is indeed among the most turbulent in the world's oceans, with surface eddy kinetic energy exceeding 1,500 cm 2 s 2 . ...
During the RESILIENCE cruise aboard the R/V Marion Dufresne II (April 19-24 May 2022), a high-resolution in situ observation campaign investigated a mesoscale dipole in the Mozambique Channel, composed of a large anticyclonic ring and a cyclonic eddy. Using an innovative adaptive sampling strategy to track its movement, we employed continuous observing systems, including a Moving Vessel Profiler and Acoustic Doppler Current Profilers, to capture high-resolution vertical sections. The results revealed a distinct dipolar structure: The 250 km-wide anticyclonic ring featured low chlorophyll and homogeneous waters, while the smaller cyclonic eddy exhibited higher chlorophyll concentrations and pronounced salinity variations. These include patches, vertically stacked layers, and filaments, reflecting a mix of contrasted water masses from the southern Mozambique Channel and the Sofala Bank. A central jet between the eddies exhibited horizontal velocities up to 130 cm s-1 , facilitating significant offshore transport exceeding 10 Sverdrups in the upper 250 m and emphasizing the dipole's role in eastward water movement. Vertical velocities, derived from the Quasi-Geostrophic Omega equation, highlighted the influence of smaller-scale structures in driving vertical motions, reaching 40 m day-1 at depth. Lagrangian particle trajectories revealed the dipole's spiraling structure and its connectivity to coastal waters. These findings show that Mozambique Eddy-Ring Dipoles efficiently transport properties from the continental shelf to the open ocean, enhancing regional ecosystem connectivity. This work provides new insights into their biogeochemical, biological and ecological significance, challenging traditional cyclonic/anticyclonic eddy paradigms, and setting the foundation for future studies on mesoscale dipoles in the region.
... The volume transport through the Mozambique Channel (16.7±15.8 Sv; Ullgren et al. 2012) and by the southeast Madagascar Current (37±10 Sv; Nauw et al. 2008) are the main sources of Agulhas transport. We show large values of volume transport in BRAN, ORAS5, and SODA and they also characterize large variability in seasonal cycle (Fig. 12a, b, e). ...
... The warm and surface water masses (>1200 m) coming from the northern Indian Ocean and the Red Sea, constitute the Mozambique Current (MC) and flow preferentially southward along the African coast (Fig. 1). Conversely, the intermediate and deep-water masses from the Atlantic (North Atlantic Deep Water "NADW" 2200-3500 m) and Antarctic (Antarctic Intermediate Water "AAIW" 800-1500 m and Antarctic Bottom Water "AABW" >4000 m) flow northwards (de Ruijter et al., 2002;Ullgren et al., 2012;Miramontes et al., 2019). Bottom currents strongly interact with the seafloor and their circulation is impacted by the submarine reliefs. ...
Contourites encompass a wide variety of sedimentary facies. Some of them show common facies with others deep-water deposits, such as turbidites and hemipelagites. Sedimentological characterization at macro- and microscales is valuable to discriminate those close facies but the distinction is not always clear. Contourites are increasingly used in paleoceanographic and paleoclimatic reconstructions. Improving their characterization is therefore essential to their interpretations.
This study aims at characterizing a foraminiferal sandy contourite facies from a sediment core collected on top of a contourite drift under the influence of the Antarctic Intermediate Water/North Atlantic Deep-Water interface located in the Limpopo Corridor (Mozambique margin, Indian Ocean) at 2000 m depth. This work is based on a detailed analysis of the sedimentary record using physical properties (gamma density and magnetic susceptibility) as well as laser grain size and X-Ray Fluorescence core scanning data.
Our results show that: (1) the foraminiferal sand is vertically continuous and homogenous over 633 kyr with an average sedimentation rate of 0.26 cm.kyr−1; (2) this contourite facies results from sedimentation with low terrigenous inputs under stable hydrodynamic conditions over time, with relatively strong bottom currents for this water depth (∼16 cm.s−1); (3) during glacial stages, sedimentation rates are lower and bottom current speed is higher than during interglacial stages. We also propose the concept of “Contourite Graphical Chart” which summaries the theoretical distribution of contourite endmembers from cross-plot of “sorting versus grain-size median D50” and their related sedimentary processes.
This study highlights the relevance of condensed foraminiferal sandy contourites as long time-scale paleoceanographic archives and provides new insights for studies related to paleoclimatology and paleoecology.
... The AAIW and the upper part of the NADW probably continue to flow northward along the northern Mozambique margin. The lower part of the NADW and the AABW are constrained by the northward decrease in the bathymetry and flow back southward along the western margin of Madagascar ( Fig. 1A; van Aken et al., 2004;Ullgren et al., 2012;Charles et al., 2020). ...
Bottom currents are key processes that contribute to the shaping of submarine slopes, with the redistribution of sediments in contourite systems. Despite numerous recent studies on contourite systems, the complexity and diversity of these sedimentary systems are still not fully understood and often underestimated. Their understanding requires comprehensive works integrating all scales from seismic architecture to microfacies. This paper focuses on a contouritic ridge located between 2000 m and 2500 m water depth on the Mozambique margin. Bathymetry, seismic data and piston cores collected during the PAMELA‐MOZ3 cruise allow a multi‐scale study from large depositional geometries to sedimentary facies. At the seismic scale, the contouritic ridge show three stages of evolution with: (i) initiation and development of the drift/moat system; (ii) an intermediate stage with successive incisions and aggradations; and (iii) moat infill and drift erosion during the Plio‐Quaternary. Plio‐Quaternary deposits are composed of hemipelagic, turbiditic and contouritic facies filling the moat. Coarse‐grained contouritic facies, dominated by planktonic foraminifera, are identified on the western flank of the ridge between the moat infill and the erosional area at the top of the ridge. They consist of condensed deposits, with sedimentation rate about 0.3 cm/ka, indicating a strong and stable bottom current that winnows away the fine‐grained component. This facies could be present more generally in intermediate position between erosion and depositional areas in contourite systems. At present, the contourite system is located at the transition depth between North Atlantic Deep Water and Antarctic Intermediate Water. Trajectories of bottom currents are complex and interact with sporadic turbidity currents and anti‐clockwise eddies that participate in reshaping the sea floor morphology. Although Plio‐Quaternary depositional geometries indicate the end of drift/moat development, the moat filling and drift erosion are also related to bottom currents and constitute atypical contouritic sedimentation.
... February 2022 | Volume 9 | Article 752066 Modern oceanographic setting and current system offshore East Africa. (König and Jokat, 2006;Lutjeharms, 2006;Ullgren et al., 2012;Franke et al., 2015;Castelino et al., 2016;Reeves et al., 2016;Uenzelmann-Neben, 2018b, 2018a;Reeves, 2018;Thiéblemont et al., 2019Thiéblemont et al., , 2020. ...
Submarine lobes form at the distal end of sediment gravity flow systems and are globally important sinks for sediment, anthropogenic pollutants and organic carbon, as well as forming hydrocarbon and CO2 reservoirs. Deep-marine, near bed or bottom currents can modify gravity flow pathways and sediment distribution by directly interacting with the flow or by modifying seafloor morphology. Deciphering the nature of gravity- and bottom currents interaction, particularly in ancient systems, remains a challenge due to the lack of integrated datasets and the necessary oceanographic framework. Here we analyse high-resolution 3D seismic reflection and core data from the Upper Cretaceous interval offshore Tanzania to reveal the interaction of turbidite lobes with fine-grained sediment waves and contourite drift deposits. Contourite drift morphology governs the large-scale confinement style and shape of lobes that range from frontally confined and crescent shaped, to laterally confined and elongated, to semi-confined lobes. Core data reveals massive to cross-laminated high density turbidites in the lobe axis position that show no direct interaction between gravity flows and contour currents. Lobe off-axis and fringe deposits consist of parallel- and ripple-laminated, low density turbidites, which are inter-bedded with bioturbated, muddy siltstones that represent the toes of contourite drifts. Starved ripples, and streaks of up to fine-grained sandstone above individual turbidite beds indicate reworking by bottom currents. This facies distribution reflects the temporal interaction of quasi-steady bottom currents and turbidity currents that interact with the topography and build lobes over short periods of time. Frontally confined turbidity currents form lobes in a fill-and-spill fashion, in which the confinement of turbidity currents causes rapid deposition and obscures any bottom current signal. Lateral confinement causes increased turbidity current runout length, and promotes the development of lobe fringes with a high proportion of bottom current reworked sands. During times when sediment gravity flows are subordinate, contourites accumulate on top of the lobe, confining the next flow and thus modifying the overall stacking pattern of the lobe complex. Although sediment volumes of these bottom current modified lobe complexes are comparable to other deep-marine systems, bottom currents considerably influence facies distribution and deposit architecture.
... The Mozambique Channel is located between the countries of Mozambique in mainland and the Island of Madagascar. The main water masses that characterize the upper layers are the Tropical Surface Waters (TSW, < 25 kg m −3 ) and Subtropical Surface Water (STSW, 25.8 kg m −3 ) [1]. To the north, warm and less saline surface waters of the TSW are brought into the area by the South Equatorial Current [2]. ...
... In contrast, the transport through the Indonesian Archipelago, representing the exchange between the Pacific and Indian Ocean, is at the upper end of observational estimates with around 15.2 Sv (13 Sv, Gordon et al., 2010). Similarly for the 24 Sv transport through the Mozambique Channel (16.5 Sv, Ullgren et al., 2012). Both transports represent a large portion of the upper limb of the global overturning circulation, even though they are also determined by the regional and basin-scale wind-driven 25 circulations. ...
Abstract. A new Earth system model, the Flexible Ocean and Climate Infrastructure (FOCI), is introduced. A first version of FOCI consists of a global high-top atmosphere (ECHAM6.3) and an ocean model (NEMO3.6) as well as sea ice (LIM2) and land surface model components (JSBACH), which are coupled through the OASIS3-MCT software package. FOCI includes a number of optional modules which can be activated depending on the scientific question of interest. In the atmosphere, interactive stratospheric chemistry can be used (ECHAM6-HAMMOZ) to study, for example, the effects of the ozone hole on the climate system. In the ocean, a biogeochemistry model (MOPS) is available to study the global carbon cycle. A unique feature of FOCI is the ability to explicitly resolve mesoscale ocean eddies in specific regions. This is realized in the ocean through nesting; first examples for the Agulhas Current and the Gulf Stream systems are described here. FOCI therefore bridges the gap between coarse-resolution climate models and global high-resolution weather prediction and ocean-only models. It allows to study the evolution of the climate system on regional and seasonal to (multi-) decadal scales.
The development of FOCI resulted from a combination of the long-standing expertise in ocean and climate modeling in several research units and divisions at GEOMAR. FOCI will thus be used to complement and interpret long-term observations in the Atlantic, enhance the process understanding of the role of mesoscale oceanic eddies for large-scale oceanic and atmospheric circulation patterns, study feedback mechanisms with stratospheric processes, estimate future ocean acidification, improve the simulation of the Atlantic Meridional Overturning Circulation changes and their influence on climate, ocean chemistry and biology.
In this paper we present both the scientific vision for the development of FOCI as well as some technical details. This includes a first validation of the different model components using several configurations of FOCI. Results show that the model in its basic configuration runs stably under pre-industrial control as well as under historical forcing, and produces a mean climate and variability which compares well with observations, reanalysis products and other climate models. The nested configurations reduce some long-standing biases in climate models and are an important step forward to include the atmospheric response in multi-decadal eddy-rich configurations.
... g Corg m -2 yr -1 at depths less than 1 km and 2.15-1.94 g Corg m -2 yr -1 at depths exceeding 3 km (Lutz et al., 2007) Water below 1200 m, and Antarctic Bottom Water beyond 3500 m (Ridderinkhof et al., 2010;Ullgren et al., 2012;Van Aken et al., 2004). Furthermore, the MC contributes substantially to the seasonality of the climatologically important Agulhas current (Biastoch et al., 1999), while simultaneously facilitating inter-ocean connectivity due to its interaction with the Atlantic and Indian Oceans (Ternon et al., 2014). ...
... Northward flowing bottom currents transporting North Atlantic Deep Water and Antarctic Intermediate Water masses are found at depths below 2000 m (Collins et al., 2016). Annual mean velocities of the southward flow decrease progressively in the upper 1500 m of the water column within the Mozambique Channel (de Ruijter et al., 2002;Ullgren et al., 2012). ...
Provenance determination of late Eocene to Quaternary sediments deposited at the DSDP 25 Site 242 may help to evaluate the mechanisms that have controlled the sediment deposition in the Mozambique Channel. To determine the provenance of the clay fraction, we measure major and trace element concentrations as well as strontium (⁸⁷Sr/⁸⁶Sr) and neodymium isotopic compositions (expressed as ƐNd(0)) of thirty-six sedimentary samples from DSDP 25 Site 242 on the eastern flank of the Davie Fracture Zone. Light Rare Earth Element (LREE) enrichment and Heavy REE (HREE) depletions against Post Archean Australian Shales (PAAS) associated with Ce anomalies higher than or equal to 0.90 for all the analyzed samples suggest very little influence of authigenic smectite incorporation. The absence of correlation of Al/Si and CIA with REE content, Eu/Eu*, Cr/Th, Th/Sc, and ⁸⁷Sr/⁸⁶Sr and ƐNd(0) suggests that chemical weathering has not modified these provenance proxies. Slight increase of Eu/Eu*, Cr/Th ratios, ƐNd(0) values associated with slight decrease in ⁸⁷Sr/⁸⁶Sr and Th/Sc ratios from the late Eocene to the Quaternary point to input of less differentiated and younger detritus to the DSDP 25 Site 242 with age. Based on variations in ƐNd(0) and Nd concentrations, we estimate an overall decrease of the Southeast African rivers (Congo/Zambezi) contribution from ~65.7% (Standard Deviation (S.D.) ~13.2) in the late Eocene to ~28.5% (S.D. ~10.0) in the early Pliocene. The causes for the low ƐNd(0) values and high Southeast African contribution between the late Eocene and the early Oligocene remain to be determined. It could record the onset of modern sedimentation in the Zambezi delta or the discharge of the Paleo Congo river in the Indian Ocean. The decrease of Zambezi contribution between the late Oligocene and early Pliocene is attributed to the cumulative effect of increasing tectonic activity of the Davie Fracture Zone and intensification of the Mozambique Current. The late Pliocene to Quaternary glacial-interglacial cycles promoted higher Zambezi contribution during glacial sea-level lowstands and in turn lower ƐNd(0) detrital values, whereas low Zambezi contributions are favored during interglacial highstand causing higher ƐNd(0) values. Hence, the modern dispersal pattern of fine-grained sediments within the Mozambique Channel is no younger than late Pliocene.
... The eddies inside the MC also play an important role in SST anomalies inside the MC. Previous studies show that eddy activities are strong inside the MC [33,34]. We applied the eddy detection method introduced in Section 2 to the AVISO geostrophic current data from 2000 to 2014, and obtained an eddy dataset including positions, radii, lifetimes, and intensities of the eddies inside the MC. ...
Based on both satellite remote sensing sea surface temperature (SST) data and numerical model results, SST warming differences in the Mozambique Channel (MC) west of the Madagascar Island (MI) were found with respect to the SST east of the MI along the same latitude. The mean SST west of the MI is up to about 3.0 °C warmer than that east of the MI. The SST differences exist all year round and the maximum value appears in October. The area of the highest SST is located in the northern part of the MC. Potential factors causing the SST anomalies could be sea surface wind, heat flux and oceanic flow advection. The presence of the MI results in weakening wind in the MC and in turn causes weakening of the mixing in the upper oceans, thus the surface mixed layer depth becomes shallower. There is more precipitation on the east of the MI than that inside the MC because of the orographic effects. Different precipitation patterns and types of clouds result in different solar radiant heat fluxes across both sides of the MI. Warm water advected from the equatorial area also contribute to the SST warm anomalies.
... Depth (m) 3000 A complex system of energetic and turbulent mesoscale eddies, approximately 100-300 km across dominate flow in the Mozambique Channel ( Ternon et al., 2014a) (Fig. 4.3). At-sea observations as well as a long-term current survey at 16°S (Ullgren, Van Haken, Ridderinkof, & De Ruijter, 2012), allowed characterization of their structure and dynamics. Their ro- tation extends down thousands of meters, eventually to the bottom (Swart, Lutjeharms, Ridderinkhof, & de Ruijter, 2010). ...
The Northern Mozambique Channel is a treasure of unique oceanography, rich coral reefs, migrating tuna, and whales, bounded by the Comoros, France, Madagascar, Mozambique, Seychelles, and Tanzania. Its living resources are relatively intact and of great importance for food and livelihood security and the developing economies of its surrounding countries. It holds newly discovered natural gas fields of global importance that could fuel unprecedented development pressure, and its 10 million coastal population could grow an order of magnitude by the turn of the century. With these immense pressures against the backdrop of rapid climate change, effective governance across different sectors and among countries will be essential to maintain the region's natural capital and deliver on the promise of sustainable development and “blue economy” subscribed by the surrounding countries. This chapter outlines some of the novel ocean management and governance approaches that may provide the best chance to deliver on a prosperous future.
... 2000 m), large-scale (diameter>300 km), southward propagating, anticyclonic eddies (de Ruijter et al., 2002). Flowing beneath the MCE train, at ca. 1500 -2400 m depth, lies the northward flowing Mozambique current (de Ruijter et al., 2002;DiMarco et al., 2002;Ullgren et al., 2012) the lower portion (below ca. 2500 m) is forced, by bathymetric confinement, to retroflect, while, the upper portion continues across the Davie Ridge (Donohue and Toole, 2003). ...
The continental slope of Madagascar is underrepresented in the literature in comparison to other continental slopes worldwide. In particular, the submarine geomorphology of this zone has not been discussed in detail. During research cruise SO230 approximately 1900 km² of high resolution multibeam bathymetric data were collected along the northwest continental slope of Madagascar. These data show four, previously unrecognised, submarine canyons extending to the toe of the continental slope in the eastern Mozambique Channel. Measured canyon thalweg lengths vary from 40 to 51 km and exhibit straight to sinuous paths. The thalweg profiles include concave, linear and slightly convex characteristics. Canyon relief decreases with depth downslope, while canyon width increases. The interfluves of the upper canyon reaches are dominated by large, square to rectangular, block-like features of 100–200 m vertical relief. These features decrease in prominence down slope. Canyons mark the coast-perpendicular flank of the blocks, while the coast-parallel block flanks are delineated by elongate valleys. The geomorphology of the canyons is best explained by varied stages (youthful to mature) of canyon evolution. The discovery of these canyons highlights the complexity of the Madagascan Continental slope, and the future potential for multidisciplinary research in this region.
... In the south-west Indian Ocean, the South Equatorial Current separates in two branches against the east coast of Madagascar [de Ruijter et al., 2004]. The northward branch, the North Madagascar Current, bifurcates again along the African continent and forms a poleward flow in the Mozambique channel dominated by large anticyclonic rings [Ullgren et al., 2012], with an estimated mean southward transport of 14 Sv [de Ruijter et al., 2002]. ...
Cruise datasets, satellite remote sensing observations and model data analyses are combined to highlight the existence of a coastal surface poleward flow in the south-west of Madagascar: the South-west MAdagascar Coastal Current (SMACC). The SMACC is a relatively shallow (<300 m) and narrow (<100 km wide) warm and salty coastal surface current, which flows along the south western coast of Madagascar toward the south, opposite to the dominant winds. The warm water surface signature of the SMACC extends from 22∘S (upstream) to 26.4∘S (downstream). The SMACC exhibits a seasonal variability: more intense in summer and reduced in winter. The average volume transport of its core is about 1.3 Sv with a mean summer maximum of 2.1 Sv. It is forced by a strong cyclonic wind stress curl associated with the bending of the trade winds along the southern tip of Madagascar. The SMACC directly influences the coastal upwelling regions south of Madagascar. Its existence is likely to influence local fisheries and larval transport patterns, as well as the connectivity with the Agulhas Current, affecting the returning branch of the global overturning circulation.
... Satellite observations (e.g., Schouten et al., 2003;Quartly and Srokosz, 2004) and ocean models (Biastoch and Krauss, 1999) have shown that mesoscale anticyclonic eddies generally form as the channel narrows close to Davie Ridge (at about 16°S), between the northern and central basin of the Mozambique Channel. These eddies typically maintain a high rotational velocity, often >1.5 m/s Ullgren et al., 2012) as they migrate southward through the channel at a mean rate of ~4-7 eddies per year. Accordingly, processes in the Mozambique Channel have downstream implications for the Agulhas Current including leakage into the South Atlantic Ocean (e.g., Bryden et al., 2005;Biastoch et al., 2008Biastoch et al., , 2009b. ...
... Additionally, Chapman et al. (2003) reported that Lagrangian floats at 800-900 m depth occasionally took a shortcut directly toward the South African coast, making the southern contribution even more efficient. In this regard, Siedler et al. (2009) estimated that about 50% of the SEMC transport continues toward the Agulhas Current, and thus joins the southbound volume transport through the Mozambique Channel of about 17 Sverdrup (Sv) near 30 • S (see Harlander et al. 2009;Ridderinkhof et al. 2010;Ullgren et al. 2012Ullgren et al. , 2016Halo et al. 2014). The circulation in the Mozambique Channel is dominated by large anticyclonic eddies moving southward (Lutjeharms et al. 2000;Ridderinkhof and de Ruijter 2003;van der Werf et al. 2010;Halo et al. 2014). ...
The South Equatorial Current (SEC) in the Indian Ocean bifurcates when it reaches Madagascar leading, respectively, to the North East Madagascar Current (NEMC), which contours the island flowing northwestward, and to the South East Madagascar Current (SEMC), which flows southwestward. Both branches eventually contribute to the greater Agulhas Current system and thus play a part in the global ocean circulation. In addition, these currents have important effects on the local conditions for marine life. In this study, the NEMC and the SEMC are investigated based on a comprehensive, multidisciplinary survey of the east coast of Madagascar in 2008. Results from conductivity-temperaturedepth stations, underway measurements with thermosalinograph and fluorometer, and ship-mounted acoustic Doppler current profiler are discussed along with concomitant remotely sensed data. Maximum core velocities of >150 cm s⁻¹ were observed in both the NEMC and the SEMC. The SEMC appeared as a nearshore southward jet, which extended at its widest nearly 200 km offshore. Near the southern tip of Madagascar, the SEMC was flanked by reverse velocities: a northeastward current about 30 km wide along the coast and a northeastward offshore current greater than 140 km wide suggesting the presence of the South Indian Ocean Countercurrent. The NEMC had the structure of a narrow nearshore jet, with weak currents beyond 100 km offshore. Volume transport for the upper 1,100 m was estimated to be 22 Sverdrup (Sv) for the SEMC and 48 Sv for the NEMC. The high NEMC transport compared with earlier estimates might be because of anomalously high influx of the SEC during the measurement period. Off the south coast, an undercurrent below 900 m carried an equatorward transport of 3.1 Sv. No undercurrent was observed in the north, but a geostrophic velocity minimum and a similar vertical velocity shear between surface and subsurface currents were found at the level where such a countercurrent has been previously observed from direct measurements. © 2017 Aksel Voldsund, Borja Aguiar-González, Tor Gammelsrød, Jens-Otto Krakstad, and Jenny Ullgren.
Mesoscale eddies are widespread in the global ocean, significantly influencing the physical, chemical, and biological structures of water column. Based on the CTD data and suspended particulate matter (SPM) data collected at 36 hydrographic stations during a field cruise in southern Mozambique Channel, combined with satellite altimeter observations, we identified a series of mesoscale eddies traversing the Mozambique Channel. Our hydrographic measurements, coupled with in situ chlorophyll fluorescence data, reveal that these eddies significantly influence thermohaline structure and chlorophyll distribution, which in turn affects primary productivity and SPM concentrations in the upper ocean. The cyclonic eddies facilitate the upwelling of cold subsurface water, leading to a shallowing of the pycnocline and the creation of a low-temperature anomaly with variable salinity anomalies at different depths. Conversely, anticyclonic eddies submerge warm surface water, deepening the pycnocline, and resulting in a high-temperature anomaly accompanied by distinct salinity patterns. Significantly, a coastal anticyclonic eddy was observed to intercept terrestrial material from the Delagoa Bight, redirecting it west of 36°E. This study presents unique and quasi-synchronous CTD datasets capturing mesoscale eddy impacts, and provided valuable insights into SPM variability within the often-neglected southern Mozambique Channel.
The Southern Ocean is a high‐nutrient, low‐chlorophyll (HNLC) region characterized by incomplete nitrate (NO3⁻) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the ¹⁴N‐ versus the ¹⁵N‐bearing form of NO3⁻, quantified as the NO3⁻ assimilation isotope effect (¹⁵ε). Previous summertime estimates of the ¹⁵ε from HNLC regions range from 4 to 11‰. While culture work has shown that the ¹⁵ε varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the ¹⁵ε varies in the field. Here we estimate the ¹⁵ε from water‐column profile and surface‐water samples collected in the Indian sector of the Southern Ocean—the first leg of the Antarctic Circumnavigation Expedition (December 2016–January 2017) and the Crossroads transect (April 2016). Consistent with prior work in the mid‐to‐late summer Southern Ocean, we estimate a higher ¹⁵ε (8.9 ± 0.6‰) for the northern Subantarctic Zone and a lower ¹⁵ε (5.4 ± 0.9‰) at and south of the Subantarctic Front. We interpret our data in the context of coincident measurements of phytoplankton community composition and estimates of iron and light stress. Similar to prior work, we find a significant, negative relationship between the ¹⁵ε and the average mixed‐layer photosynthetically active radiation flux of 30–100 μmol m⁻² s⁻¹, while above 100 μmol m⁻² s⁻¹, ¹⁵ε increases again. In addition, while we observe no robust relationship of the ¹⁵ε to iron availability or phytoplankton community, mixed‐layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude.
The ability of coastal nations to manage their coastal and marine environments is vital in the development and maintenance of national blue economies following the 2030 Agenda. Thus, capacity development (CD) is an important priority area to strengthen education and training for various stakeholders to help create an appropriately trained workforce able to develop, implement, and expand blue economies, especially in developing countries. The chapter is separated into three parts. Part one provides a synthesis of existing global and regional initiatives that build the foundation of CD for ocean sciences. Some multilateral initiatives that are discussed are the Intergovernmental Oceanographic Commission (IOC) of UNESCO, the Scientific Committee on Oceanic Research (SCOR), and the Partnership for Observation of the Global Ocean (POGO). The Western Indian Ocean Marine Science Association (WIOMSA) is presented as a prime example for a regional CD organization. Part two showcases lessons learned from case studies and success stories from training programs such as summer schools that bring together students from various backgrounds for intensive theoretical and practical training on cross-cutting topics. Part three provides recommendations for scientists, policymakers, and the private industry to accelerate global CD efforts and responses to achieve SDG 14 in the current decade.KeywordsCapacity developmentOcean scienceOcean observations
The ocean’s contributions to humanity exceeds the products available from it, by absorbing more than 90% of the heat resulting from anthropogenic greenhouse gas emissions. The ocean plays a major role in the global cycles of oxygen, carbon dioxide, nitrous oxide and other gases and rebalances the heat differential between poles and the equator, governing the climate to maintain life on our planet. The need to sustainably observe all areas of the ocean—as well as its unlimited potential for renewable ocean energy—are providing inspiration for new technological innovations. However, it is becoming more evident from recent scientific findings that ocean health is more at risk than previously thought, because different pressures add up and contribute to rapid and unpredictable changes in ocean ecosystems. With renewed, revitalized, and changing global scenarios and the United Nations declaring this decade as the UN Decade of Ocean Sciences for Sustainable Development, countries are moving up the ocean in their national policy agendas. Coastal countries, especially small island developing states, are advocating for socially equitable and environmentally sustainable growth. This will require systematic in-situ ocean data collection to understand today’s ocean and for forecasts, disaster risk reduction and early warning systems for coastal society and infrastructure and for the assessment and management of ocean resources. This chapter discusses in detail the need for and importance of ocean observations linked to the blue economy, using case studies to understand how under-resourced countries are facing the complex challenges of ocean observing.
A regional assessment of three global ocean reanalysis products is presented for southern Africa’s major oceanographic features. The reanalyses include Mercator Ocean’s Global Reanalysis (GLORYS), the Commonwealth Scientific and Industrial Research Organisation’s (CSIRO) Bluelink Reanalysis (BRAN) and the Fleet Numerical Meteorology and Oceanography Center’s (FNMOC) global Hybrid Coordinate Ocean Model (HYCOM) reanalysis. The aim is to provide modelers with sufficient information for selecting the appropriate product for use as boundary conditions to force their regional ocean models, as well as to provide marine industries, relevant government agencies and academics with insight into the optimal reanalysis product for their purposes. The reanalyses are compared to both assimilated and independent observational datasets spanning various regions within the southern African marine environment. While all reanalysis products reproduce the eastern and western boundary current systems surrounding southern Africa, limitations exist. BRAN outperforms the other reanalyses in its representation of the Mixed Layer Depth, contributing to its good representation of coastal SSTs in the Benguela upwelling system, whereas GLORYS and HYCOM’s misrepresented MLD result in significant warm biases in this region. The Angola-Benguela Frontal Zone and it’s variability is best reproduced by BRAN and HYCOM. The Agulhas Current system’s major components are well reproduced by both GLORYS and BRAN. HYCOM, however, simulates considerably more early retroflections than are observed which have resulted in its mean eastward location. While all the reanalyses overestimate the occurrence of Agulhas meanders, GLORYS and BRAN resolve the associated variability best. Agulhas Current transport is best resolved by GLORYS, unlike BRAN and HYCOM which largely overestimate the magnitude of its south-westward flow, linked to their misrepresentation of the Current’s vertical structure. The bay-scale and nearshore evaluations highlighted issues pertaining to the resolution of the reanalyses and their use at such a small scale. The reanalyses are limited by their resolution, as well as by their misrepresentation of submesoscale processes or lack thereof, prompting the need for the development of regional downscaled models in and around the southern African oceans based on the global ocean reanalysis products.
Skipjack tuna play a significant role in global marine fisheries and are of particular interest for socio-economy in the tropical waters of the Mozambique Channel. However , human-induced climate change has been leading to a reduction and reallocation of biomass, along with other ecological changes, thereby creating a feedback loop with negative socioeconomic consequences for fisheries-reliant coastal communities. The objective of this study was to predict the potential skipjack tuna fishing grounds by 2050 and 2100. To that end, skipjack tuna catch data were collected from Spanish purse seine fleets, and subsequently, generalized additive models were used to model these data against a combination of environmental variables and future pathway projections from BIO-ORACLE models under optimistic (RCP2.6) and pessimistic (RCP8.5) scenarios. Both optimistic and pessimistic scenarios by 2050 predicted that the potential fishing grounds will relocate southward from tropical to more temperate waters, with moderate shifts in the potential fishing grounds of purse seines to the latitude >16 S, whereas the pessimistic scenario predicted higher displacement catches of purse seines in the southernmost part (>24 S) and moderate to high catches in northern (>20 S) of the Mozambique Channel by the end of the century. Despite the degree of uncertainty surrounding the climate change impacts on skipjack tuna, we argue that fisheries stakeholders, administrators and regional tuna fisheries management organizations should work towards building resilience and ensuring sustainability while reducing or mitigating vulnerability and climate change impacts on local and regional communities and their livelihoods.
The purpose of this study is to differentiate and characterise contouritic sands in two different locations with variable sediment compositions (siliciclastic and bioclastic) based on a multiproxy approach that includes the analysis of sedimentary texture, semi-quantitative geochemistry, microfacies and ichnological information, as well as a Principal Component Analysis (PCA) applied to geochemical and sedimentary data on sediment cores. The integration of sedimentological analyses and the PCA permits also to the differentiation between fine-grained deep-water deposits such as hemipelagites, muddy contourites and hyperpycnites. A depositional model is proposed here, based on geochemical and sedimentological characteristics of the Holocene-highstand Mozambique Channel upper slope sands, and glacial-lowstand Corsica Contourite Depositional System middle slope sands. The upper continental slope of the Mozambican margin is characterised by siliciclastic sandy contourites, muddy contourites and muddy hyperpycnites. Mozambique siliciclastic sandy contourites constitute large accumulations of well-sorted very fine to coarse sand with evidences of strong winnowing and reworking under high-energy conditions. The sedimentary facies represents highstand contourite sands and shows a reversely-graded trend. The contourite drift on the Pianosa ridge (eastern flank of the Corsica Trough) consists of bioclastic sandy and muddy contourites and hemipelagites. Bioclastic sandy contourites are made up of shallow-marine winnowed bioclasts with a reversely- and normally-graded trend and represent lowstand contourite sands. The PCA in the two environments —showing a distinctive geochemical signal— allows for differentiation of the contourite deposits. In siliciclastic sands, reworking is marked by an accumulation of Si, Zr, and Sr in fine- to medium-grained sands. In bioclastic sands, reworking is less evident but it is characterised by accumulations of Ca and Sr. The reworking and winnowing bottom current effects are also observed at the microfacies scale. Both types of contourite deposits show evidences of intermittent depositional conditions depending on the ichnodiversity, distribution and abundance of trace fossils. This work is useful to discriminate similar fine-grained deposits in different continental margins which would contribute to a better understanding of sedimentary deposits and processes in deep-marine environments.
The Zambezi turbidite system (Mozambique Channel, Western Indian Ocean) is one of the largest turbidite systems in the world and yet still remains poorly understood. Newly acquired high-resolution multibeam bathymetry, seismic reflection and sedimentological data allowed to investigate the architecture evolution and depositional patterns since the Oligocene in order to understand the main forcing factors that control the deep sea sedimentation in the Mozambique Channel. It was found that the Zambezi turbidite system is composed of two adjacent depositional systems: the channelized Zambezi Fan and a semiconfined fan in the lntermediate Basin. Moreover, results and interpretations indicate: (1) important tectonic control since the Miocene that caused deep incision of the Zambezi Valley and limited overflow of turbidite currents; (2) an important influence of bottom-currents that induces scarcity of fine-grained turbidites, valley flanks erosion and widespread occurrence of sediment waves; (3) low turbidite activity for the last 700 kyr that shows no relationship with sea-level changes as turbidite activity occurred irrespective of glacial or interglacial periods; (4) peaks in terrigenous flux with maxima in local summer insolation, reflecting that monsoon controls the sediment inputs towards the deep marine depositional system; (5) an on-off evolution of the Zambezi Fan that demonstrates a depocenter shift from the distal Zambezi Fan to the proximal Intermediate Basin. All our findings underline the high complexity in depositional environments of the Zambezi turbidite system.
A new Earth system model, the Flexible Ocean and Climate Infrastructure (FOCI), is introduced. A first version of FOCI consists of a global high-top atmosphere (European Centre Hamburg general circulation model; ECHAM6.3) and an ocean model (Nucleus for European Modelling of the Ocean v3.6; NEMO3.6) as well as sea-ice (Louvain-la-Neuve sea Ice Model version 2; LIM2) and land surface model components (Jena Scheme for Biosphere Atmosphere Coupling in Hamburg; JSBACH), which are coupled through the OASIS3-MCT software package. FOCI includes a number of optional modules which can be activated depending on the scientific question of interest. In the atmosphere, interactive stratospheric chemistry can be used (ECHAM6-HAMMOZ) to study, for example, the effects of the ozone hole on the climate system. In the ocean, a biogeochemistry model (Model of Oceanic Pelagic Stoichiometry; MOPS) is available to study the global carbon cycle. A unique feature of FOCI is the ability to explicitly resolve mesoscale ocean eddies in specific regions. This is realized in the ocean through nesting; first examples for the Agulhas Current and the Gulf Stream systems are described here. FOCI therefore bridges the gap between coarse-resolution climate models and global high-resolution weather prediction and ocean-only models. It allows to study the evolution of the climate system on regional and seasonal to (multi)decadal scales.
The development of FOCI resulted from a combination of the long-standing expertise in ocean and climate modeling in several research units and divisions at the Helmholtz Centre for Ocean Research Kiel (GEOMAR). FOCI will thus be used to complement and interpret long-term observations in the Atlantic, enhance the process understanding of the role of mesoscale oceanic eddies for large-scale oceanic and atmospheric circulation patterns, study feedback mechanisms with stratospheric processes, estimate future ocean acidification, and improve the simulation of the Atlantic Meridional Overturning Circulation changes and their influence on climate, ocean chemistry and biology.
In this paper, we present both the scientific vision for the development of FOCI as well as some technical details. This includes a first validation of the different model components using several configurations of FOCI. Results show that the model in its basic configuration runs stably under pre-industrial control as well as under historical forcing and produces a mean climate and variability which compares well with observations, reanalysis products and other climate models. The nested configurations reduce some long-standing biases in climate models and are an important step forward to include the atmospheric response in multidecadal eddy-rich configurations.
This study describes previously unrecognized contourite depositional systems (CDSs) in the Mozambique Channel which constrain palaeoceanographic models for this area. The stratigraphic stacking patterns record nine seismic units (SU1 to SU9) separated by eight major discontinuities (a to h, oldest to youngest). Key seismic markers in CDS evolutionary history occur during Aptian-Albian (~122 Ma), late Cenomanian (94 Ma), early (38.2–36.2 Ma) and late (25–23 Ma) Oligocene, and early-middle Miocene (~17–15 Ma) epochs. These record onset (~122 to 94 Ma), growth (94 to 25–23 Ma), maintenance (25–23 to 17–15 Ma), and burial (17–15 Ma to the actual time) stages for CDSs. CDSs first develop during the onset stage which coincides with the opening and deepening of the African-Southern Ocean gateway (at 122 and 100 Ma, respectively). The growth stage, beginning in the late Cenomanian (94 Ma), correlates with the opening and deepening of the Equatorial Atlantic gateway. During the growth stage, two major shifts in sedimentary stacking pattern occur which coincide with palaeoceanographic changes during the early (38.2–36.2 Ma) and late (25–23 Ma) Oligocene. These in turn coincide with the onset and local enhancement of Antarctic water masses. CDS growth continued until the early-middle Miocene during the maintenance stage (~17–15 Ma). Most CDS growth ceased at the end of the maintenance stage. Circulation of the North Atlantic water mass into the Southern Hemisphere led to a deepening of Antarctic water masses in the area.
Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high‐resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex‐shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. On the other hand, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small‐scale sedimentary features generated by internal waves are superimposed on large‐scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record.
In contrast to the vigorous deep ocean circulation system of the north- and southwestern Atlantic Ocean, no systematically sampled datasets of dissolved radiogenic neodymium (Nd) isotope signatures exist to trace water mass mixing and provenance for the more restricted and less well ventilated Angola Basin and the Cape Basin in the southeastern Atlantic Ocean, where important parts of the return flow of the Atlantic Meridional Overturning Circulation are generated. Here, to improve our understanding of water mass mixing and provenance, we present the first full water column Nd isotope (expressed as εNd values) and concentration data for a section across the western Angola Basin from 3° to 30° S along the Zero Meridian and along an E-W section across the northern Cape Basin at 30° S sampled during GEOTRACES cruise GA08. Compared with the southwestern Atlantic basin we find overall less radiogenic εNd signatures reaching −17.6 in the uppermost 200 m of the Angola and Cape basins. In the western Angola Basin these signatures are the consequence of the admixture of a coastal plume originating near 13° S and carrying an unradiogenic Nd signal that likely resulted from the dissolution of Fe-Mn coatings of particles formed in river estuaries or near the West African coast. The highly unradiogenic Nd isotope signatures in the upper water column of the northern Cape Basin, in contrast, originate from old Archean terrains of southern Africa and are introduced into the Mozambique Channel via rivers like the Limpopo and Zambezi. These signatures allow tracing the advection of shallow waters via the Agulhas and Benguela currents into the southeastern Atlantic Ocean. The Nd isotope compositions of the deep water masses in both basins primarily reflect conservative water mass mixing with the only exception being the central Angola Basin, where the signatures are significantly overprinted by terrestrial inputs. Bottom waters of the Cape Basin show excess Nd concentrations of up to 6 pmol/kg (20%), originating from resuspended bottom sediments and/or dissolution of dust, but without significantly changing the isotopic composition of the waters due to similar εNd values of particles and bottom waters ranging between −9.6 and −10.5. Given that bottom waters within the Cape Basin today are enriched in Nd, non-conservative Nd isotopic effects may have been resolvable under past glacial boundary conditions when bottom waters were more radiogenic.
The Agulhas Current, the western boundary current of the South Indian Ocean, has been shown to play an important role in the connectivity between the Indian and Atlantic oceans. The greater Agulhas Current system is highly dominated by mesoscale dynamics. To investigate their influence on the regional and global circulations, a family of high-resolution ocean general circulation model configurations based on the NEMO code has been developed. Horizontal resolution refinement is achieved by embedding “nests” covering the South Atlantic and the western Indian oceans at 1/10∘ (INALT10) and 1/20∘ (INALT20) within global hosts with coarser resolutions. Nests and hosts are connected through two-way interaction, allowing the nests not only to receive boundary conditions from their respective host but also to feed back the impact of regional dynamics onto the global ocean. A double-nested configuration at 1/60∘ resolution (INALT60) has been developed to gain insights into submesoscale processes within the Agulhas Current system. Large-scale measures such as the Drake Passage transport and the strength of the Atlantic meridional overturning circulation are rather robust among the different configurations, indicating the important role of the hosts in providing a consistent embedment of the regionally refined grids into the global circulation. The dynamics of the Agulhas Current system strongly depend on the representation of mesoscale processes. Both the southward-flowing Agulhas Current and the northward-flowing Agulhas Undercurrent increase in strength with increasing resolution towards more realistic values, which suggests the importance of improving mesoscale dynamics as well as bathymetric slopes along this narrow western boundary current regime. The exploration of numerical choices such as lateral boundary conditions and details of the implementation of surface wind stress forcing demonstrates the range of solutions within any given configuration.
We present a combined study of the geomorphology, sedimentology, and physical oceanography of the Mozambique Channel to evaluate the role of bottom currents in shaping the Mozambican continental margin and adjacent Durban basin. Analysis of 2D multichannel seismic reflection profiles and bathymetric features revealed major contourite deposits with erosive (abraded surfaces, contourite channels, moats, furrows and scours), depositional (plastered and elongated-mounded drifts, sedimentary waves), and mixed (terraces) features, which were then used to construct a morpho-sedimentary map of the study area. Hydrographic data and hydrodynamic modelling provide new insights into the distribution of water masses, bottom current circulation and associated processes (e.g., eddies, internal waves, etc.) occurring along the Mozambican slope, base-of-slope and basin floor. Results from this work represent a novel deep-sea sedimentation model for the Mozambican continental margin and adjacent Durban basin. This model shows 1) how bottom circulation of water masses and associated sedimentary processes shape the continental margin, 2) how interface positions of water-masses with contrasting densities (i.e., internal waves) sculpt terraces along the slope at a regional scale, and 3) how morphologic obstacles (seamounts, Mozambique Ridge, etc.) play an essential role in local water mass behaviours and dynamics. Further analysis of similar areas can expand understanding of the global role of bottom currents in deep-sea sedimentation.
In this study we have analyzed sediment trap time series from five tropical sites to assess seasonal variations in concentrations and fluxes of long-chain diols (LCDs) and associated proxies with emphasis on the Long chain Diol Index (LDI). For the tropical Atlantic, we observe that generally less than 2% of LCDs settling from the water column are preserved in the sediment. The Atlantic and Mozambique Channel traps reveal minimal seasonal variations in the LDI, similar to the TEX86 and UK´37. However, annual mean LDI-derived temperatures are in good agreement with the annual mean satellite-derived sea surface temperatures (SSTs). In the Cariaco Basin the LDI shows larger seasonal variation, as do the TEX86 and UK´37. Here, the LDI underestimates SST during the warmest months, which is likely due to summer stratification and the habitat depth of the diol producers deepening to around 20 to 30m. Surface sediment LDI temperatures in the Atlantic and Mozambique Channel compare well with the average LDI-derived temperatures from the overlying sediment traps, as well as with decadal annual mean SST. Lastly, we observed large seasonal variations in the Diol Index, as indicator of upwelling conditions, at three sites, potentially linked to Guinea Dome upwelling (Eastern Atlantic), seasonal upwelling (Cariaco Basin) and seasonal upwelling and/or eddy migration (Mozambique Channel).
Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep‐marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south‐west Indian Ocean). The acquired data include multibeam bathymetry, multi‐channel high‐resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull‐mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0.9 to 9.0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec⁻¹ and maximum speeds of 57 cm sec⁻¹, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high‐frequency variability) and to anticyclonic eddies near the seamounts (low‐frequency variability). This study contributes to a better understanding of the formation of dunes in deep‐marine settings and provides valuable information about carbonate preservation after drowning and the impact of bottom currents on sediment distribution and sea floor morphology.
Deep ocean pressure measurements in two regions of the South-West Indian Ocean (West and East of Madagascar), covering one to two years of data, are analysed for tidal motions. The pressure data are taken both from Bottom Pressure Recorders as well as from mid-water column instruments. Coherent tides are characterised by fixed amplitudes and phases. Those inferred from bottom measurements compare well to tides obtained from satellite altimetry, and cover up to 99 of the pressure variance in the frequency band having periods shorter than 29 h. Long-period tides, in the low-frequency band, are regularly overshadowed by (unwanted) eddy-induced mooring motion (‘blow-down’), which events have therefore been eliminated. In the Mozambique Channel, semidiurnal surface tides are stronger than East of Madagascar, and all appear to be near resonance with a basin mode. Away from the bottom, coherent internal tides were determined. Evidence of the presence of incoherent internal tides has been obtained by applying Harmonic Analyses over a moving time window of 1 year duration. East of Madagascar internal tides appear to be very strong, although its source remains unclear.
Many questions in ocean and climate modelling require the combined use of high resolution, global coverage and multi-decadal integration length. For this combination, even modern resources limit the use of traditional structured-mesh grids. Here we compare two approaches: A high-resolution grid nested into a global model at coarser resolution (NEMO with AGRIF) and an unstructured-mesh grid (FESOM) which allows to variably enhance resolution where desired. The Agulhas system around South Africa is used as a testcase, providing an energetic interplay of a strong western boundary current and mesoscale dynamics. Its open setting into the horizontal and global overturning circulations also requires global coverage.
Both model configurations simulate a reasonable large-scale circulation. Distribution and temporal variability of the wind-driven circulation are quite comparable due to the same atmospheric forcing.
However, the overturning circulation differs, owing each model's ability to represent formation and spreading of deep water masses. In terms of regional, high-resolution dynamics, all elements of the Agulhas
system are well represented. Owing to the strong nonlinearity in the system, Agulhas Current transports of both configurations and in comparison with observations differ in strength and temporal variability.
Similar decadal trends in Agulhas Current transport and Agulhas leakage are linked to the trends in wind forcing.
Although the number of 3D wet grid points used in FESOM is similar to that in the nested NEMO, FESOM uses about two times the number of CPUs to obtain the same model throughput (in terms of simulated model years per day). This is feasible due to the high scalability of the FESOM code.
x3' 1985-1988 forcing NCEP winds MSU precip 1958-1997 forcing NCEP winds-10 333 i 5,_ 5 I ml M-1997 forcing NSCAT winds Xie Arkin precip Started from rest
Analysis of satellite altimetry observations, transports estimates from a mooring array, as well as
output from 2 different numerical ocean circulation models (ROMS and HYCOM), have been used
to investigate the mesoscale eddy properties and transport variability in the Mozambique Channel.
The power spectral density of model transports at 17oS indicates the models ability to represent the
transport variability at mesoscale frequencies (range between 3 y-1 and 10 y-1). The models have
shown an exaggerated representation of the lower frequencies (~ <3 y-1), while underestimating the
higher frequency signals (~ >10 y-1). The overestimation of the seasonal cycle appears in our case
not to be related to a misrepresentation of the mesoscale variability. The eddies were identified
using an automatic eddy tracking scheme. Both anticyclonic and cyclonic eddies appeared to have a
preferred site of formation within the channel. The density distribution showed that the anticyclones
exhibited a bi-modal distribution: the first mode was associated with the typical scale for the
oceanic mesoscale turbulence, while the second mode was related to the passage of large rings at a
frequency of about 4-7 per year. On the other hand, cyclonic eddies had a single mode distribution
that follows the first baroclinic Rossby radius of deformation, which is a typical scale for the
oceanic mesoscale surface eddy variability, suggesting that their formation is associated with
baroclinic instability. Eddy mean amplitudes per class of radius (<100 km), increase linearly with
increasing radius, while no linear relationship exists for the rings. Different from the rings, the
increase on the amplitude of the eddies was consistent with the increase of their life expectancy and
travelling distances.
The concept of a spatially continuous western boundary current in the Mozambique Channel
has historically been based on erroneous interpretations of ships’ drift. Recent observations
have demonstrated that the circulation in the Channel is instead dominated by anti-cyclonic
eddies drifting poleward. It has therefore been suggested that no coherent Mozambique
Current exists at any time. However, satellite and other observations indicate that a continuous
current – not necessarily an inherent part of Mozambique Eddies – may at times be found
along the full Mozambican shelf break. Using a high-resolution, numerical model we have
demonstrated how such a feature may come about. In the model, a continuous current is a
highly irregularly occurring event, occurring about once per year, with an average duration of
only 9 days and with a vertical extent of about 800 m. Surface speeds may vary from 0.5 m/s to
1.5 m/s and the volume flux involved is about 10 Sv. The continuous current may occasionally
be important for the transport of biota along the continental shelf and slope.
The origins of density-compensating anomalies of temperature and salinity (spice) are investigated using a model forced with the most realistic surface products available over the 40 years 1958-97. In this hindcast,the largest interannual spiciness anomalies are found in the Pacific Ocean near the isopycnal σ0 = 25.5, where deviations as great as 1.2°C and 0.6 psu are generated equatorward of winter outcropping in the eastern subtropics in both hemispheres. These source regions are characterized by very unstable salinity gradients and low mean density stratification in winter. Two related signatures of winter mixing in the southeast Pacific (SEP) are density that is well mixed deeper than either temperature or salinity and subsurface density ratios that approach 1. All ocean basins in the model are shown to have regions with these characteristics and signatures; however, the resultant spiciness signals are focused on different isopycnals ranging from σ0 = 25.0 in the northeast Pacific to σ0 = 26.5 in the south Indian Ocean. A detailed examination of the SEP finds that large positive anomalies are generated by diapycnal mixing across subducted isopycnals (e.g., σ0 = 25.5), whereas negative anomalies are the result of a steady isopycnal advection, moderated by vertical advection and heave. There is considerable interannual variability in the strength of anomalies and in the density on which they occur. Historical observations are consistent with the model results but are insufficient to verify all aspects of the hindcast, including the processes of anomaly generation in the SEP. It was not possible to relate isopycnal anomaly genesis to local surface forcing of any kind. A complex scenario involving basinwide circulation of both the ocean and atmosphere, especially of surface water through the subtropical evaporation zones, is put forward to explain the decadal time scale evident in SEP salinity anomalies on σ0 = 25.5. Pacific anomalies generated on σ0 = 25.5 can be traced along mean geostrophic streamlines to the western boundary, where decadal salinity variations at ≈7°S are about 2 times as large (order ±0.1 psu) as at ≈ 12°N, although there may be more variance on shallower isopycnals in the north. At least portions of the σ0 = 25.5 signals appear to continue along the boundary to a convergence at the equator, suggesting that the most robust sources of Pacific spiciness variance coincide with equatorial exchange pathways.
Changes in atmospheric forcing can affect the subsurface water column of the ocean by three different mechanisms. First, warmed mixed-layer water that is subducted into the ocean interior will cause subsurfce warming; second, the subducted surface water can be freshened through changes in evaporation and precipitation; and third, the properties at a given depth may be changed by the vertical displacement of isotherms and isohalines without changes of water masses. Linear relations are derived for the relative strength of each process in terms of the observed changes of potential temperature and salinity in two different coordinate frames: 1) constant density surfaces, and 2) isobaric surfaces. -from Authors
This study focuses on the improved estimation of mesoscale surface ocean circulation obtained by merging TOPEX/Poseidon (T/P) and ERS-1 and -2 altimeter measurements between October 1992 and May 1998. Once carefully intercalibrated and homogenized, these data are merged through an advanced global objective analysis method that allows us to correct for residual long wavelength errors and uses realistic correlation scales of ocean dynamics. The high-resolution (0.25°×0.25°) merged T/P+ERS-1 and -2 sea level anomaly maps provide more homogeneous and reduced mapping errors than either individual data set and more realistic sea level and geostrophic velocity statistics than T/P data alone. Furthermore, the merged T/P+ERS-1 and -2 maps yield eddy kinetic energy (EKE) levels 30% higher than maps of T/P alone. They also permit realistic global estimates of east and north components of EKE and their seasonal variations, to study EKE sources better. A comparison of velocity statistics with World Ocean Circulation Experiment surface drifters in the North Atlantic shows very good agreement. Comparison with contemporary current meter data in various oceanic regimes also produces comparable levels of energy and similar ratios of northward and eastward energy, showing that the maps are suitable to studying anisotropy. The T/P+ERS zonal and meridional components of the mapped currents usually present comparable rms variability, even though the variability in the Atlantic is more isotropic than that in the Pacific, which exhibits strong zonal changes. The EKE map presents a very detailed description, presumably never before achieved at a global scale. Pronounced seasonal changes of the EKE are found in many regions, notably the northeastern Pacific, the northeastern and northwestern Atlantic, the tropical oceans, and the zonally extended bands centered near 20°S in the Indian and western Pacific Occans and at 20°N in the northwestern Pacific.
The collision of anticyclonic, lens-like eddies with a meridional western boundary is investigated as a function of two independent, nondimensional numbers: = {R/fo and s: w/fo, where f0 and/30 are the Coriolis parameler and its rate of change with latitude, respectively, both evaluated at the reference latitude, R is the eddy's radius, and w is its angular frequency. The numerical experiments show that in all cases there is a southward expulsion of mass pr9portional to both / and z, which is estimated during the eddy-boundary interaction. The eddies are invariably deformed with the initial collision, but afi. erward, they reacquire a new circular shape. There is a meridional translation of the eddy along the boundary which depends exclusively on the initial ratio r = s/fl. If r > 1, the eddy goes southward, but if r < 1, the eddy goes northward first and then sothward. As the eddy loses mass and reacquires a new circular shape, there is a readjustment of/3 and s such thatdecreases because its radius becomes smaller andincreases by energy conservation. This implies that the eddies ultimately migrate southward. A formula derived for the meridional speed of the center of mass of the eddy is consistent with the numerical results. A physical interpretation shows that after collision a zonal force is exerted on the eddy by the wall which is balanced by a meridional migration. Nonlinearities induce a southward motion, while high values could produce northward motion, depending on the mass distribution along the wall.
Recent hydrographic data from the South Atlantic Ventilation Experiment cruises and others are combined with historical data and used to map the isopycnal properties corresponding to the Antarctic Intermediate Water (AAIW) in the Atlantic Ocean. The low salinity of the AAIW extends eastward across the South Atlantic just south of the equator (3-4°S). Evidence of a weak eastward flow just north of the equator (1-2°N) is also shown. Lateral and vertical homogenization of properties in the AAIW is found at the equator between 2°S and 2°N there is no clear zonal gradient in salinity just along the equator. These observations suggest enhanced mixing within the equatorial baroclinic deformation radius. The South Atlantic tropical gyre is shown to consist of the following three cells: one cyclonic cell centered at about 7°S, another centered at about 19°S in the west and 23°S in the east, and one anticyclonic cell centered at about 13°S. These cells are associated with a westward extension at 10°S of high salinity and low oxygen which originates in the eastern tropical South Atlantic and a front in these properties at about 15°S in the est and about 20°S in the east.
Moored current measurements of 11-month duration were carried out in the boundary currents east of Madagascar, near 12°S at Cape Amber where the mean current flows northwestward and near 23°S where the mean current flows approximately southward. Transports derived from the moored current measurements in the depth range 150-1100 m compare resonably well with those derived from ship sections by Swallow et al. (this issue). At 12°S, very energetic boundary current transport variations occur in the 40- to 55-day-period band, contributing about 40% to the total transport variance, while at 23°S to the 40- to 55-day-period band fluctuations contribute only 15% to the total transport variance. The fluctuations near 12°S do not seem to be caused by local wind forcing, which does not show an energy peak in this period band. A significant annual cycle cannot be detected in the moored current and transport time series despite significant variation of wind forcing over the subtropical Indian Ocean. A comparison of the observations is carried out with two different numerical Indian Ocean models, both gives mean boundary current transports which compare well with the observations and also shows a small seasonal cycle. The multilayer Geophysical Fluid Dynamics Laboratory model also shows a small seasonal cycle. The observational evidence from the western subtropical Indian Ocean appears to be similar to that from the subtropical North Atlantic east of the Bahamas-Antilles arc where also no significant seasonal boundary current response was detected, despite large annual variation of wind forcing over the ocean. The two observational situations and numerical model results for both oceans are compared.
Geostrophic currents and transports in the boundary currents near Madagascar are described for a section at 23°S off the east coast and at 12°S to the northeast of Cape Amber. The results are based on conductivity-temperature-depth and expendable bathythermograph sections made during cruises of the Marion Dufresne in 1984, and 1985, and 1986, supplemented by those historical data that were sufficiently deep. The reference levels used are 1100 dbar at 12°S and 1170 dbar at 23°S, based on the results from 11 months of current meter records obtained in 1984-1985. Comparisons are made between the geostrophic current profiles and direct observations, both from moored current meters and from acoustic Doppler profiling in the upper 200 m. The mean geostrophic transports above the chosen reference levels are 29.6 Sv northwestard off Cape Amber out to 115 km offshore and 20.6 Sv southward at 23°S out to 110 km offshore. The directly recorded currents show no detectable seasonal signal below 200 m; above that level other historical data suggest a seasonal amplitude in transport of approximately +/-2 Sv at 12°S and +/-0.3 Sv at 23°S.
Maps of the Antarctic Intermediate Water (AAIW) in the Atlantic, and on a global isopycnal which intersects the AAIW in the south, show the location and properties of the salinity and oxygen extrema associated with the AAIW, and the likely sources of AAIW. These are primarily the surface waters in the southeastern Pacific, which produce the South Pacific AAIW, and surface waters in northern Drake Passage and the Falkland Current loop, which produce the South Atlantic AAIW. This latter source is the primary one for AAIW of the Indian Ocean as well. Winter surface properties and annual-averaged Ekman pumping and S verdrup transport for the southern hemisphere suggest that the formation density of the AAIW is the highest density which can be subducted in the South Pacific. The higher density of AAIW in the South Atlantic may result from more complex processes. The connection between the subtropical gyres of the Atlantic and Indian and between the Indian and Pacific Oceans contributes to modification of AAIW as it spreads tortuously northward around the subtropical gyres. Potential vorticity and AAIW salinity and oxygen illustrate the near barrier between the subtropical and tropical regimes, at about 20°to 25°north and south of the equator. Communication between the regimes is primarily through the western boundary currents.
The southwest Indian Ocean is distinguished by discontinuities in the wind-driven Sverdrup circulation. These connect the northern and southern tips of Madagascar with Africa and the southern tip of Africa with South America. In an analytical barotropic model with a flat bottom, the discontinuities produce intense westward jets. Those off the northern tip of Madagascar and the southern tip of Africa are always present, while the strength of that off southern Madagascar depends on the position of the zero curl line in the Indian Ocean (the jet is strong if the line intersects Madagascar but weak if the line is north of the island). All three jets are barotropically unstable by the Rayleigh-Kuo criterion. The authors studied the development of the instability using a primitive equation model, with a flat bottom and realistic coastlines. The model produced westward jets at the three sites and these became unstable after several weeks, generating 200-300-km scale eddies. The eddies generated west of Madagascar are in accord with observations and with previous numerical studies. The model's Agulhas eddies are similar in size to the observed eddies, both the anticyclonic rings and the cyclones that form to the west of the tip of South Africa. However, the model's Agulhas does not retroflect, most likely because of its lack of stratification and topography, and so cannot capture pinching-off events. It is noteworthy nevertheless that a retroflection is not required to produce eddies here.
As a result of its remarkably high salinity and despite its small volume input, remnants of Red Sea Water (RSW) have been identified in the Agulhas Current, over 6000 km distant of their source. This provided thenotivation to investigate the long-terln mean spreading of RSW throughout the Indian Ocean, using a comprehensive set of observations, taken from the National Oceanographic Data Center archives and from the World Ocean Circulation Experilnent Hydro- graphic Program for the Indian Ocean. After emerging from the Gulf of Aden into the Arabian Sea, RSW spreads predominantly southwestward along the Aft'lean continental slope, as indicated by strongly inclined isohalines across the Arabian Sea. There is some monsoovariability, so that during the winter monsoon there is more RSW present in the Gulf of Aden and an intensification of southward spreading along the western boundary, between 12 ø and 5øN. Elsewhere the in- termedlate depth salinity field of the Indian Ocean appears relatively stationary. Between 5 ø and 10øS, in the region of the South Equatorial Current, isohalines of the RSW layer become quasi-zonal across the width of the Indian Ocean, only dipping southward toward the western boundary west of 50øE. South of here, there is a strong tongue of RSW spreading southward through the Mozambique Channel and into the Agulhas Current. These conclusions concur with previous localized investigations of intermediate water properties. Using a simple mixing model, the percentage of RSW throughout the Indian Ocean was quantified. It was found that the flux of salt into the Gulf of Aden fi'om the Red Sea is similar to that estimated to cross 32øS in the Agulhas Current. This result implies that all the RSW which is mixed into the interior of the Indian Ocean may eventually be exported at the western boundary. Furthermore, it implies that RSW is the dominant component of the salt budget for the intermediate layer and that input from the Indonesian Seas and via diapycnic processes are small.
[1] Combining high resolution model output and geostrophic currents derived from satellite altimeter data, it is shown that the formation of mesoscale eddies in the Mozambique Channel (MZC) is connected to variability in the transport of the South Equatorial Current (SEC). Lagged cross-correlations of the currents north of Madagascar and vorticities in the MZC, combined with a composite analysis of the model output, show that eddies form in the narrows of the channel approximately 20 weeks following a westward transport pulse in the SEC. A relationship between MZC eddies and the large-scale variability of the South Indian Ocean may have downstream impacts on the Agulhas leakage, the Atlantic Meridional overturning circulation, and thus climate.
1] Internal waves in the Mozambique Channel were studied, in the narrowest passage between Mozambique and Madagascar. Seven current meter moorings were deployed for a year and a half. The observed baroclinic flow in the semidiurnal frequency band exhibited strong intermittency. Internal tides could be detected at nearly all times from differences between current meter records in amplitude and phase, varying in time. To study the long-term average of the internal tidal field, the overall energy in the semidiurnal bands was computed for each location. Internal tidal currents were everywhere strongest near the surface (around 4 cm/s at 250 m depth up to 12 cm/s near the pycnocline in the generation area), decreased to less than 3 cm/s at 600 m depth, and increased a little near the bottom. The results were compared with numerical results from a two-dimensional internal-tide generation model allowing a description of beam scattering at the pycnocline and repeated reflection. Model results and observations were in qualitative agreement.
To monitor changes in heat content and geostrophic volume transport of abyssal water accurately, 50 moored conductivity–temperature–depth (CTD) recorders used for density measurements were calibrated in situ by simultaneous observations with accurate shipboard CTDs. Comparisons of the data from the moored and shipboard CTDs showed pressure sensitivities of 0–3 mK at 6000 dbar for the temperature sensors of the moored CTDs. From the in situ calibrations, the uncertainties of the moored CTD data for the deep ocean (3000 dbar) were estimated to be 0.6 dbar, 0.6 mK, and 0.0026 for pressure, temperature, and salinity, respectively, relative to the shipboard CTD reference. Time drifts of the moored CTD data, estimated from the in situ calibrations before and after 17-or 14-month mooring deployments in the deep ocean, were considerably smaller than typical stabilities as specified by the manufacturer. However, time drifts of the pressure sensors tended to be negative and the result suggests that pressure data from most present Argo floats, which use the same type of pressure sensor, may have a systematic negative bias. Time series salinity data calculated from the in situ–calibrated CTDs were slightly biased (mean of 0.0014) with respect to the shipboard CTD salinity data, based on potential temperature–salinity relationships, possibly due to a disequilibrium of the moored CTD conductivity sensors during the in situ calibrations.
Between 1995 and 2000, on average 4 eddies per year are observed from satellite altimetry to propagate southward through the Mozambique Channel, into the upstream Agulhas region. Further south, these eddies have been found to control the timing and frequency of Agulhas ring shedding.Within the Mozambique Channel, anomalous SSH amplitudes rise to , in agreement with in situ measured velocities. Comparison of an observed velocity section with GCM model results shows that the Mozambique Channel eddies in these models are too surface intensified. Also, the number of eddies formed in the models is in disagreement with our observational analysis.Moored current meter measurements observing the passage of three eddies in 2000 are extended to a 5-year time series by referencing the anomalous surface currents estimated from altimeter data to a synoptic LADCP velocity measurement. The results show intermittent eddy passage at the mooring location.A statistical analysis of SSH observations in different parts of the Mozambique Channel shows a southward decrease of the dominant frequency of the variability, going from 7 per year in the extension of the South Equatorial Current north of Madagascar to 4 per year south of Madagascar. The observations suggest that frequency reduction is related to the Rossby waves coming in from the east.
We summarize previous estimates of volume transport and property distributions through the Mozambique Channel and offer additional estimates and measurements based on recently acquired hydrographic and float data. Previously published property distributions are consistent with southward spreading through the Channel. Waters of the Mozambique Channel are characterized by shallow and intermediate oxygen minima separated by a relative maximum. Based on hydrographic sections, the intermediate maximum in dissolved oxygen is seen to decrease in value as it spreads southward. The highest values are found in the westward flow of the South Equatorial Current just north of Madagascar and within the western 200 km of the Channel. Similarly, oxygen concentrations at the intermediate oxygen minimum, which derives from the Arabian Sea, increase southwards, while its depth increases from 900 to 1100 m, supporting previous studies and indicating southward spreading and mixing along the Mozambique Channel. Historical transports based on hydrographic data in the Channel vary from 5 Sv northward to 26 Sv southward depending on reference level and time of the year. Balancing transport below 2500 m (the sill depth in the Channel), we estimate the net southward transports above this depth to be 29.1 and 5.9 Sv for the northern and southern sections, respectively—the difference is presumably related to seasonality and eddy variability superimposed on the mean flow. Individual deep float trajectories show the presence of many eddies, but the overall flow in the channel is southward, and broadly consistent with hydrography. Model outputs also show mean southward transport with considerable seasonal variability. Satellite data show high variability in sea surface height anomalies and high eddy kinetic energy associated with eddy activity. Although the geostrophic transport values are consistent with the historical limits, the lowered ADCP measurements suggest a substantial barotropic component to the flow. Direct long-term measurements of the current are needed to quantify its magnitude and variability.
During the preparation of the Atlas on the physical oceanography of the Indian Ocean (WYRTKI, 1971) a comprehensive analysis of its oceanographic conditions had to be undertaken to construct meaningful maps, sections and diagrams, and to interpret the structure and circulation of this ocean, which is in so many ways different from the other oceans. On a large scale 3 distinct circulation systems can be delineated. These are:
I.
the seasonally changing monsoon gyre
II.
the south hemispheric subtropical anticyclonic gyre
III.
the Antarctic waters with the Circumpolar Current
The circulation of deep water in the south-western Indian Ocean has been studied from hydrographic observations and current measurements, obtained during the Dutch–South African Agulhas Current Sources Experiment programme, and from similar public data from the World Ocean Circulation Experiment. The three major water masses involved are the saline North Atlantic deep water (NADW), its derivative in the Antarctic circumpolar current, lower circumpolar deep water (LCDW), and the aged variety of deep water, North Indian deep water (NIDW). Although bound by the shallow topography near Madagascar, about 2×106 m3/s from the upper half of the NADW core appears to flow across the sill in the Mozambique Channel into the Somali Basin, while the remaining NADW flows east at about 45°S and is transformed to LCDW by lateral and diapycnal mixing. East of Madagascar the deep circulation is dominated by the southward flow of NIDW. Northward inflow of LCDW into the Indian Ocean therefore can take place only in the eastern half of the Indian Ocean, along the Southeast Indian Ridge and the Ninetyeast Ridge.
A practical step-by-step guide to wavelet analysis is given, with examples taken from time series of the El NiñoSouthem Oscillation (ENSO). The guide includes a comparison to the windowed Fourier transform, the choice of an appropriate wavelet basis function, edge effects due to finite-length time series, and the relationship between wavelet scale and Fourier frequency. New statistical significance tests for wavelet power spectra are developed by deriving theoretical wavelet spectra for white and red noise processes and using these to establish significance levels and confidence intervals. It is shown that smoothing in time or scale can be used to increase the confidence of the wavelet spectrum. Empirical formulas are given for the effect of smoothing on significance levels and confidence intervals. Extensions to wavelet analysis such as filtering, the power Hovmöller, cross-wavelet spectra, and coherence are described. The statistical significance tests are used to give a quantitative measure of changes in ENSO variance on interdecadal timescales. Using new datasets that extend back to 1871, the Niño3 sea surface temperature and the Southern Oscillation index show significantly higher power during 1880-1920 and 1960-90, and lower power during 1920-60, as well as a possible 15-yr modulation of variance. The power Hovmöller of sea level pressure shows significant variations in 2-8-yr wavelet power in both longitude and time.
A 550-day record of Agulhas Undercurrent transport between March 2003 and August 2004 is constructed from five deep moorings placed on the continental shelf off South Africa at nominally 32°S. The vertical and lateral scales of the undercurrent are estimated to be 2000 m and 40 km, respectively, using the average of seven direct velocity sections, predominantly taken in austral autumn over a 10-yr period from 1995 to 2005. Peak speeds in the undercurrent are some of the greatest ever seen at depth: over 90 cm s−1 at 1400 m. The undercurrent has a transport of 4.2 ± 5.2 Sv (1 Sv = 1 × 106 m3 s−1), in close agreement with a previous estimate from a single current meter record during 1995 of 4.2 ± 2.9 Sv. Records below 1800 m, within the North Atlantic Deep Water (NADW) layer of the undercurrent, show a higher level of coherence and less variance than those at intermediate depths. On average, 2.3 ± 3.0 Sv of NADW is carried northeastward within the undercurrent, an amount similar to that estimated previously by analyses of deep water mass characteristics around South Africa.
Short-term variability in the undercurrent peaks at the semidiurnal period, at a local shear-adjusted inertial period (21.6 h), and at 4.5, 6.5, and 9.5 days. The latter may be associated with topographic Rossby waves, although no evidence for enhanced onshore velocities was found at these periods. The variability of the undercurrent is highly topographically controlled, strongly aligned in the along-stream direction, with significant variance in cross-stream velocity only at 2-day periods or less and isotropic variance only at the (effective) inertial period. For the mesoscale, the deeper current meters within the NADW layer all exhibit broad peaks at 50–60 days, which matches the periodicity of solitary meanders of the Agulhas Current (so-called Natal pulses) presented previously in the literature. The results of this study demonstrate that these meanders are highly barotropic in nature.
Red Sea Intermediate Water (RSIW) has been shown to move down the Agulhas Current as distinct lenses. It has been assumed that this intermittency is the result of variable input. To clarify and quantify the nature of RSIW contributions from the source regions of the Agulhas Current observations at 15 hydrographic sections were examined using a multi-parameter analysis. In the northern Mozambique Channel RSIW is found to be layer-like, but with patches of distinctly different contributions. In the southern part of the channel the layer-like distribution disappears with RSIW mostly confined within anticyclonic and cyclonic eddies exhibiting varying maximum contributions ranging from 15–20% to 25–30% purity. Net transports across the channel ranged from −0.45 to −0.7 Sv. At the southern tip of Madagascar RSIW contributions exhibited similar purity variability ranging from 10–15% to 15–20%. The net southward transport of RSIW in the East Madagascar Current displayed an even greater variability due to changes in the flux of the undercurrent ranging from negligible to −0.3 Sv. Indications therefore were that the transport of RSIW to the Agulhas Current occurs in both cyclones and anti-cyclones through the Mozambique Channel whilst from the East Madagascar Current it is mostly confined to anti-cyclones. This variability in the inflow was also reflected in the northern part of the Agulhas Current proper. The maximum contributions of RSIW range here from 10–15% to 20–25% purity and net transports from −0.75 to −1.39 Sv off Durban. As it was east of Madagascar RSIW was mostly confined to the slope.
The full depth velocity field of the Agulhas Current, the Western Boundary Current of the southwest Indian Ocean, has been directly measured using a lowered acoustic Doppler current profiler (LADCP). Fifteen combined conductivity-temperature-depth (CTD) and LADCP stations were occupied across the current at 32°S off the east coast of South Africa in February-March 1995, as part of the World Ocean Circulation Experiment Indian Ocean program. The deep velocity structure of the Agulhas Current was found to be very different than previously described using geostrophic estimates. In particular, LADCP results reveal a V-shaped pattern for the level of no motion across the Agulhas Current and an Agulhas Undercurrent is observed flowing equatorward below 800 m depth, directly beneath the surface core of the poleward flowing Agulhas Current. Comparisons of direct and geostrophic velocities suggest that the velocity structure of the Agulhas Current is essentially geostrophic below about 200 m depth, where differences are generally less than estimated errors. Above these depths the LADCP-measured shears and geostrophic shears exhibit differences and velocities can diverge. Shipboard ADCP data support LADCP results near the surface, indicating that both instruments are measuring real signals. The geostrophic and LADCP-measured shears are well matched away from the surface, and a depth-averaged fit (below 200 m) between them exhibits small standard deviations. The geostrophic volume transport of the Agulhas Current, as referenced to LADCP, is 73 Sv, and is just 3% different from the direct LADCP transport estimate. The most recent full depth estimate of Agulhas Current transport in the literature is given by Toole and Warren [1993], who estimate a geostrophic volume transport of 85 Sv. The baroclinic velocity structures from their section and from this work are similar, indicating that the difference in the transport estimates is due to the choice of geostrophic reference level and ultimately to the presence of the previously unobserved Agulhas Undercurrent. Combined CTD/LADCP data are very useful for potential vorticity analysis, since velocity and density measurements are simultaneous and coincident. The vorticity structure of the Agulhas Current suggests the presence of a ``mixing boundary'' at middepths, similar to features observed in the Gulf Stream. The feature appears to be associated with a sharp upturn in isopycnals, coincident with the foot of the continental slope. The boundary inhibits cross-stream mixing of intermediate water masses and may help explain the appearance of a discrete filament of Red Sea Water inshore of Antarctic Intermediate Waters in the same density layer, despite intense cross-stream shears.
The characteristics of the deep and bottom waters of the Indian Ocean, when illustrated on potential-density anomaly surfaces, indicate that the waters enter from both the Atlantic and Pacific Oceans. The paths of spreading are constrained by the complex topography, and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments, especially in the northern Indian Ocean. The Weddell Sea contributes to the densest waters found in the western basins and the Ross Sea and Adelie coast to the densest waters found in the eastern basins. Both dense water varieties are altered by and incorporated in the less dense water above; initially, water carried by the circumpolar current, then water from the north Atlantic, and finally by deep water whose characteristics are derived in the northern Indian Ocean. Contact with the sediments increases the silica content of the bottom water in the Southern Ocean. In the northern Indian Ocean the sediments alter the silica of the water at the bottom and, together with enhanced salinity from diffusion of saline overflows from the marginal seas above, imprint unique markers to the deep water that flows back to the south.At middepths the series of ridges between Madagascar and Australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern Indian Ocean characteristics southward. Within the circumpolar current, low-oxygen deep water from the Pacific is carried across the Atlantic and into the Indian Ocean south of Africa. Part flows around the cyclonic Weddell Sea Gyre, and part extends across the Southern Ocean. Water from another Pacific source can be seen near 2000 m extending westward from the Tasman Sea, south of Australia and across the Indian Ocean, and perhaps to the Aghulas Current region southeast of Africa.
Two seasonal hydrographic data sets are used in a mixing model to determine the spreading and mixing of the thermocline waters. The mixing model comprises a system of four major source water masses. They are Indian Central Water (ICW), North Indian Central Water (NICW) interpreted as aged ICW, Australasian Mediterranean Water (AAMW), and Red Sea Water (RSW)/Persian Gulf Water (PGW). The mixing ratios of these water masses are quantified and mapped on four isopycnal surfaces. The mixing ratios and pathways of the thermocline water masses show large seasonal variations, particularly in the upper 400-500 m of the thermocline.
The results obtrained during a cruise in the south-western Indian Ocean, from South Africa to Mauritius in September 1987, are presented. Temperature, salinity, oxygen and nutrients were measured at 70 CTD station, 20 nautical miles apart. The data perspective is enhanced by NOAA satellite infrared imagery and Geosat altimetry. The major features encountered on the cruise were two intense cyclonic eddies in the vicinity of the Mozambique Ridge and the Mozambique Basin and which seemed to form part of the broader western boundary region of the southern Indian Ocean. Evidence was found of a significant equatoward flux at the Madagascar Ridge, which provided input of water from higher latitudes. The passage of North Atlantic Deep Water was constrained by the bottom topography south of the Madagascar Ridge, and this led to a reduction of its salinity and oxygen concentration. The positive correlation between the CTD results and the altimetry enabled the identification, tracking and description of a number of eddies in the region. The transition zone of these eddies from east of Madagascar to the African continent was identified, and so was the apparent closure of the southern entrance of the Mozambique Channel to migrating eddies.
In this paper, we review observations, theory and model results on the monsoon circulation of the Indian Ocean. We begin with a general overview, discussing wind-stress forcing fields and their anomalies, climatological distributions of stratification, mixed-layer depths, altimetric sea-level distributions, and seasonal circulation patterns (Section 2). The three main monsoon circulation sections deal with the equatorial regime (Section 3), the Somali Current and western Arabian Sea (Section 4), and the Bay of Bengal, seasonally reversing monsoon currents south of India and Sri Lanka, and the eastern and central Arabian Sea (Section 5). For the equatorial regime, we discuss equatorial jets and undercurrents, their interactions with the eastern and western boundaries, and intraseasonal and vertically propagating signals. In the Somali Current section, we describe the ocean's responses to the summer and winter monsoon winds, and outline the modelling efforts that have been carried out to understand them. In the Bay of Bengal section, we present observational and modeling evidence showing the importance of remote forcing from the east, which to a large extent originates along the equator. In the following three sections, we review the southern-hemisphere subtropical regime and its associated boundary currents (Section 6), the Indonesian Throughflow (Section 7), the Red Sea and Persian Gulf circulations (Section 8), and discuss aspects of their interactions with other Indian-Ocean circulations. Next, we describe the Indian Ocean's deep and shallow meridional overturning cells (Section 9). Model results show large seasonal variability of the meridional overturning streamfunction and heat flux, and we discuss possible physical mechanisms behind this variability. While the monsoon-driven variability of the deep cell is mostly a sloshing motion affecting heat storage, interesting water-mass transformations and monsoonal reversals occur in the shallow cross-equatorial cell. In the mean, the shallow cell connects the subduction areas in the southern subtropics and parts of the Indonesian Throughflow waters with the upwelling areas of the northern hemisphere via the cross-equatorial Somali Current. Its near-surface branch includes a shallow equatorial roll that is seasonally reversing. We close by looking at coupled ocean-climate anomalies, in particular the large events that were observed in the tropical and subtropical Indian Ocean in 1993/94 and 1997/98. These events have been interpreted as an independent Indian-Ocean climate mode by some investigators and as an ENSO-forced anomaly by others.
The characteristics of the Butterworth low-filter are well known in electrical engineering. Here we disuss its use for oceanographic records and compare its characteristics with other low-pass filters now in use: the cosine-Lanczos filter, the Gaussian filter, and the ideal filter. The Butterworth filte is recursive, i.e., past values of the output are used as input, so a phase shift is introduced unless the data are filtered forward and backward through the same filter. When this is done, the filtered signal differs only slightly from that of other low-pass filters. Because the Butterworth filter uses fewer multiplicative constants for the same effect, there is a reduction in computer time over other low-pass filters; the difference becomes more pronounced as more data points are used.
The combined effect of the three processes is a rather complicated process and the results are counterintuitive. For instance, imagine a lenslike anticyclonic eddy situated on a sloping bottom (analogous to β). This highly nonlinear eddy migrates with shallow water on its right (“westward”) and encounters a meridional wall. Intuitively, it is expected that, once the “westward” migration is arrested by the wall, gravity will pull the eddy downhill (southward) so that the eddy will migrate toward deep water (i.e., toward the equator). Surprisingly, however, the authors’ numerical computations show that the eddy migrates uphill. This bizarre behavior results from the leakage along the wall that, in terms of the eddy energy, compensates for the uphill drift. Namely, the leakage plays a crucial role in the eddy-wall interaction process because it allows the uphill migration. Eventually, it causes a destruction of the lens by completely draining its fluid.
A process study is conducted on the evolution of boundary currents in a two-layer quasigeostrophic model on the f plane. These currents are composed of two strips of uniform potential vorticity (PV), one in each layer, and both hugging the coast. Coastal water separation (“detrainment”) through baroclinic instability and topographic perturbation is examined. It is shown that the key characteristics of the flow finite-amplitude destabilization can be explained with the help of a linear quantity—the critical amplitude Ac—that refers to the location of the line (often called critical layer) where the phase speed of the growing perturbation is equal to the unperturbed flow velocity. Notably, prediction on PV front breaking location is made possible. Different detrainment regimes (i.e., the way fragments of the boundary current are isolated and detached from the initially rectilinear core—e.g., filament formation, eddy shedding) are also identified, related to various Ac value ranges, and compared with observed oceanic events.
During 1994–1996, 215 Autonomous Lagrangian Circulation Explorers (ALACE floats) were released at a nominal 900 m depth in the Indian Ocean as part of the World Ocean Circulation Experiment. Of these, 66 entered the region around Madagascar (2–30°S, 35–55°E), generally at a depth of 800–900 m. Floats approaching the island from the east were deflected either northward or southward depending on latitude, with the bifurcation point being near 20°S. Mean southward velocities in the western boundary current were 8.5 cm s-1 during each 25-day observation period, with mean northward velocities 7.7 cm s-1. Speeds past Cape Amber were about 11 cm s-1. These figures are comparable to those obtained from in situ current meter measurements. Floats rounding the island to the north frequently drifted north of the Comores for many months with no discernable pattern, before exiting the region either to the north in the East African Coastal Current and the equatorial current system or to the south via the Mozambique Channel. Flow rates in this region were highly skewed towards low (less than 5 cm s-1) velocities.
Floats passing south of Madagascar showed little northward movement into the Mozambique Channel, but tended to move steadily westwards towards the African coast, becoming entrained in the Agulhas Current and its recirculation gyres near 28°S. Similarly, floats released within the southern portion of the Channel all tended to move to the south and west despite occasional entanglement with eddies. Mean flow in the Agulhas region was about 18 cm s-1, with maximum velocities over 25 days of up to about 35 cm s.
All flows were extremely variable because of the ubiquity of eddies. In essence, the flow observed near 800 m resembled closely that seen at the surface from TOPEX/POSEIDON altimetry and at 845 m in the OCCAM global model. Temperature data collected by the floats were used to determine when a given float was being acted on by an anticyclonic eddy. Deviations, which were typically 0.5°C or more from the background temperature field, were found only in eddies south of 12°S and occurred in about 9% of float records within the Mozambique Channel. The results support the idea of a net southward flow of water through the Channel from the tropics to the Agulhas Current, with the eddies playing a major role in the transport in the southern part of the region. In the northern and eastern parts of the region, however, eddies were far less prevalent, and topographic steering by the Mascarene Ridge became important.
This study focused on the southwest region of the Indian Ocean, where the poleward-directed Agulhas Current is born, and where dense waters filter through fractures in the Southwest Indian Ridge to form an equatorward-directed deep boundary current east of Madagascar. Both represent major circulation features of the Indian Ocean: the Agulhas in its role as a western-boundary current closes the wind-driven subtropical gyre; the deep western-boundary current renews the bottom waters of the Madagascar, Mascarene, and Somali basins to the north.
A primitive equation model to study the dynamics of the Agulhas system has been developed. The model domain covers the South Atlantic and the south Indian Ocean with a resolution of ⅓ in the Agulhas region while coarser outside. It is driven by a climatology of the European Centre for Medium-Range Weather Forecasts. It is shown that the model simulates the Agulhas Current, its retroflection, and the ring shedding successfully. The model results show baroclinic anticyclonic eddies in the Mozambique Channel and east of Madagascar, which travel toward the northern Agulhas Current. After the eddies reach the current they are advected southward with the mean flow. Due to the limited numerical resolution only a few eddies reach the retroflection region without much modification. These eddies are responsible for drastic enhancement of the heat transfer from the Indian Ocean to the South Atlantic and lead to periodicities in the interoceanic heat transport of about 50 days superimposed on the seasonal variability. Combined satellite data from TOPEX/Poseidon and ERS-1 show that the observed vortices in the Mozambique Channel are comparable to those seen in the model. In contrast to this the simulated eddies east of Madagascar seem not to be well reproduced. Analyses of the energy conversion terms between the mean flow and the eddies suggest that barotropic instability plays an important role in the generation of Mozambique Channel eddies. For the generation of Agulhas rings and other eddy structures in the model the barotropic instability mechanism seems to be minor, and baroclinic instability mechanisms are more likely.
The South Equatorial Current (SEC) is the major westward current in the South Indian Ocean. It crosses the Mascarene Plateau, an extensive range of banks and islands, near 60°E, but how this occurs has until now been unclear. Here, we present the results of a recent survey during June–July 2002 using a suite of modern instrumentation, and provide a detailed examination of this process, and the water masses involved. Upstream from the Plateau the SEC carries 50–55 Sv (1 Sv=106 m3/s) westwards between 10 and 16°S. As it approaches the Plateau, 25 Sv of this is constricted to pass over a narrow sill (about which we provide new information) between the Saya De Malha and Nazareth Banks at 12–13° S. This then forms a northern core to the SEC between 10 and 14°S downstream from the Plateau (25 Sv). The remainder of the inflow passes either around the northern edge of the Saya De Malha Bank (8–9°S) or between Mauritius and the Cargados Carajos Bank (18–20°S). The former may retroflect to flow eastwards near 8°S, joining the South Equatorial Counter Current (SECC), whereas the latter, strengthened near Mauritius by further flows from the south, forms a southern core to the SEC downstream from the Plateau (20–25 Sv between 17 and 20°S). The overall effect of the Plateau is to split the SEC into two cores. On reaching Madagascar, these cores may then form the Northeast and Southeast Madagascar Currents. The SEC also forms a sharp boundary between upper and intermediate level water masses. Subtropical Surface Water (STSW), Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are present on the southern side of the SEC, whereas Arabian Sea High Salinity Water (ASHSW) and Red Sea Water (RSW) are found on its northern side. As they approach the Plateau, the STSW and SAMW are partially drawn northwards, and Tropical Surface Water (TSW) is drawn southwards, in order to flow across the sill near 12–13°S. At deeper levels, North Indian Deep Water (NIDW) passes southwards below the SEC on the western side of the Plateau, and while there is no indication of North Atlantic Deep Water, Antarctic Bottom Water (AABW) is present west of the Plateau. Finally, there is evidence of significant mixing in the upper and intermediate waters (the TSW, STSW, SAMW and RSW) as they pass across the sill at 12–13°S, and also in the deeper waters on the eastern side of the Plateau above the rough bottom topography of the Central Indian Ridge.
Based on hydrographic data from 1977 to 1980 off the coast of Mozambique and historical data from the Mozambique Channel, the general circulation pattern of the area is described. The circulation pattern is characterized by the influence of three anticyclonic gyres covering the northern, the central, and the southern parts of the channel. Additionally, smaller cyclonic eddies are observed, of which some probably are topographically induced. The results strongly indicate that in the upper 1000 m the role of the Mozambique Current as one of the tributaries to the Agulhas Current is of minor significance and draw into question the concept of the Mozambique Current as a continuous one.
The inter-ocean exchange of water masses at the Agulhas Current termination comes about through the shedding of rings, and this process plays an important role in the global thermohaline circulation. Using several hydrographic sections collected during the ARC (Agulhas Retroflection Cruise), MARE (Mixing of Agulhas Rings Experiment) and WOCE (World Ocean Circulation Experiment), this investigation aims to establish the degree to which Red Sea Intermediate Water (RSIW) is involved in this exchange and at what level of purity. To this end a wide range of hydrographic parameters were used. Upstream from the Agulhas Current retroflection water with clear RSIW origin is shown to move downstream on both the landward and seaward sides of the Agulhas Current with the highest water sample purity or water-mass content exceeding 15%. The least mixed water was found close to the continental shelf. At the retroflection the RSIW purity shows considerable variability that ranges between 5% and 20%. This suggests that RSIW moves down the current in patches of considerably varying degrees of previous mixing. This pattern was also observed in a ring sampled during the ARC experiment. The MARE sections in turn indicate that at times RSIW may be entirely absent in the Agulhas Current. RSIW is therefore shown to travel down the current as discontinuous filaments, and this intermittency is reflected in its presence in Agulhas Rings. From the sections investigated it is therefore clear that any calculation of RSIW fluxes involved in inter-ocean exchange can only be done on the basis of event scales. RSIW not trapped in Agulhas Rings flows east with the Agulhas Return Current.
The realization of North Atlantic Deep Water (NADW) replacement in the deep northern Indian Ocean is crucial to the “conveyor belt” scheme. This was investigated with the updated 1994 Levitus climatological atlas. The study was performed on four selected neutral surfaces, encompassing the Indian deep water from 2000 to 3500 m. The Indian deep water comprises three major water masses: NADW, Circumpolar Deep Water (CDW) and North Indian Deep Water (NIDW). Since NADW flowing into the southwest Indian Ocean is largely blocked by the ridges (the Madagascar Ridge in the east and Davie Ridge in the north in the Mozambique Channel) and NIDW is the only source in the northern Indian Ocean that cannot provide a large amount of volume transport, CDW has to be a major source for the Indian deep circulation and ventilation in the north. Thus the question of NADW replacement becomes that of how the advective flows of CDW from the south are changed to be upwelled flows in the north—a water-mass transformation scenario. This study considered various processes causing motion across neutral surfaces. It is found that dianeutral mixing is vital to achieve CDW transformation. Basin-wide uniform dianeutral upwelling is detected in the entire Indian deep water north of 32°S, somewhat concentrated in the eastern Indian Ocean on the lowest surface. However, the integrated dianeutral transport is quite low, about a net of 0.2 Sv (1 Sv=106 m3 s-1) across the lowermost neutral surface upward and 0.4 Sv across the uppermost surface upward north of 32°S with an error band of about 10–20% when an uncertainty of half-order change in diffusivities is assumed. Given about 10–15% of rough ridge area where dianeutral diffusivity could be about one order of magnitude higher (10-4 m2 s-1) due to internal-wave breaking, the additional amount of increased net dianeutral transport across the lowest neutral surface is still within that error band. The averaged net upward transport in the north is matched with a net downward transport of 0.3 Sv integrated in the Southern Ocean south of 45°S across the lowermost surface. With the previous works of You (1996. Deep Sea Research 43, 291–320) in the thermocline and You (Journal of Geophysical Research) in the intermediate water combined, a schematic dianeutral circulation of the Indian Ocean emerges. The integrated net dianeutral upwelling transport shows a steady increase from the deep water to the upper thermocline (from 0.2 to 4.6) north of 32°S. The dianeutral upwelling transport is accumulated upward as the northward advective transport provided from the Southern Ocean increases. As a result, the dianeutral upwelling transport north of 32°S can provide at least 4.6 Sv to south of 32°S from the upper main thermocline, most likely to the Agulhas Current system. This amount of dianeutral upwelling transport does not include the top 150–200 m, which may contribute much more volume transport to the south.
Observations from an array of current meter moorings across the narrowest section of the Mozambique Channel are used to study the general characteristics of the current field and to estimate the volume transport through the channel. The observations cover a period of about 19 months in 2000 and 2001. Due to problems in data return, the mean transport time series is constructed using several assumptions. The resulting mean volume transport through the channel is estimated to be 14 Sv southward. The time variability of the volume transport is large and oscillates remarkably regular with minimum and maximum values varying roughly between 20 Sv northwards and 60 Sv southwards. No seasonal variability has been found. The spatial and temporal variability of the currents can be characterised by two different regimes: one with strong currents across the entire section, the other one with weak currents. The former lasts much longer than the latter. During the strong-current regime, an anti-cyclonic eddy is formed, which subsequently migrates southward. The formation of these eddies occurs regularly and is related to the strength of the volume transport through the channel. At intermediate and deep levels against the African continental slope (1500 and 2500 m), a relatively strong and northward flowing Mozambique Undercurrent is observed. The mean northward speed is 4.6 cm s−1 (1500 m) and 4.5 cm s−1 (2500 m).
Features of the water-property and circulation fields at the southern limit of the continentally bounded Indian Ocean are described on the basis of a transoceanic hydrographic section occupied along roughly Lat. 32°S by the R.R.S. Charles Darwin in November-December 1987. Primary observations consisted of 106 full-depth CTD/O2 stations with discrete measurements of the concentrations of dissolved silica, phosphate and nitrate. The section lies in the southern part of the South Indian subtropical gyre; water-property features in the upper kilometer indicate that the northward interior flow is predominantly in the eastern half of the ocean there, consistent with the forcing pattern of wind-stress curl. The southward return flow is the Agulhas Current, whose transport at Lats 31–32°S is estimated as 85 × 10⁶ m³ s⁻¹. Circumpolar Deep Water flows northward to fill the greater deep Indian Ocean by means of western-boundary currents in the Crozet Basin, Central Indian Basin and Perth Basin. North Atlantic Deep Water entering directly from the mid-latitude South Atlantic is almost entirely confined to the south-western Indian Ocean (Mozambique Basin, Natal Valley) by the topography of the Madagascar Ridge and Mozambique Channel.
As a component of the meridional overturning variability experiment in the tropical North Atlantic, a four-year-long time series of meridional transport of North Atlantic deep water has been obtained from moored end point measurements of density and bottom pressure. This study presents a quality assessment of the measurement elements. Rigorous pre- and post- deployment in situ calibration of the density sensors and subsequent data processing establish an accuracy of O(1.5 Sv) in internal transport in the 1200–5000 dbar range at subinertial time scales. A similar accuracy is reached in the bottom pressure-derived external transport fluctuations. However, for pressure, variability with periods longer than a deployment's duration (presently about one year) is not measurable. This effect is demonstrated using numerical simulations and a possible solution for detecting long-term external transport changes is presented.
Antarctic Intermediate Water and Subantarctic Mode Water are studied by examining their modal density and modal salinity characteristics around Antarctic. The study focuses on 15 regions between 40 and 45°S. Circumpolar distributions for the cores of Antarctic Intermediate and Subantarctic Mode Water and potential temperature-salinity diagrams provide a detailed description of the water masses. The modal salinity and density change gradually across each of the three oceans, presumably due to the alteration of a single water mass. Large changes occur in the transition regions in the Drake Passage, south of Africa, and south of New Zealand. These large changes are due to the presence of distinctly different water masses.Thermohaline finestructure is associated with the water mass transitions. The finestructure intensity between the 27.10 and 27.40 σθ isopycnals is quantified. Although continuous temperature-depth data are not available for the entire region the study indicates that the most intense finestructure occurs in regions of large water mass contrast and decreases by an order of magnitude away from them. Increased vertical temperature gradient variances were observed near a 100-km diameter eddy south of New Zealand.
The Agulhas Current system contains one of the world's strongest western boundary currents, and plays an important part in the warm water path of the global thermohaline circulation. However, there have been few surveys of the source regions of the Agulhas Current, and thus little in situ measurement of their variability. Utilizing the more than 5-year record of SeaWiFS data, we examine the eddy activity present in the southern portion of the Mozambique Channel. The two sources of Agulhas input from the central Indian Ocean (southward flow through the Mozambique Channel and westward flow around the southern limit of Madagascar) both show great temporal variability, with no clear seasonal signal.A number of large (∼200 km diameter) anticyclonic rings intermittently propagate poleward along the western edge of the channel, sweeping coastal waters into mid-channel. Their passage past Maputo appears to affect the circulation of the lee eddy in the Delagoa Bight. The eastern side of the channel is mainly characterized by cyclonic eddies. These are made manifest in the lee of the southern tip of Madagascar, although it is not clear whether many form there or just develop a visible presence due to entrainment of high-chlorophyll coastal waters. Several of these cyclonic eddies then appear to move in west-southwesterly direction. The chlorophyll data do reveal the apparent East Madagascar Retroflection on occasions, but do not show clear examples of the pinching off of anticyclonic eddies. However, surface waters from the East Madagascar Current may reach the African mainland on occasions when no retroflection is present.
The MARE project studies the effects of inter-ocean exchange between the Indian and Atlantic Ocean, via Agulhas rings, on the Atlantic meridional overturning circulation. The field programme of MARE concentrates on the study of the decay and modification of a single Agulhas ring named Astrid, formed in January 2000. The ring was clearly visible in the analysis of satellite altimetry data, and surface drifters confirmed the anti-cyclonic rotation. During a detailed survey of this 2-month-old ring in March 2000, it appeared that the water mass properties of this ring only differed from the surrounding water above the 12°C isotherm. The observed fine-structure near its boundary suggested that exchange of water with its surroundings already had started. Observations with a lowered acoustic Doppler current profiler showed that the ring had a significant barotropic component, additional to the baroclinic flow around its warm centre. Meteorological observations indicated that during the summer survey the ring was losing heat to the atmosphere. This heat loss maintained convective mixing in the surface mixed layer. Compared to other reported rings, Astrid had a very large kinetic energy, a property probably characteristic for very young Agulhas rings. In other aspects Astrid did not differ strongly from the other rings, although Astrid was slightly larger than their typical ‘average’ size.
The circulation of deep water in the south-western Indian Ocean has been studied from hydrographic observations and current measurements, obtained during the Dutch–South African Agulhas Current Sources Experiment programme, and from similar public data from the World Ocean Circulation Experiment. The three major water masses involved are the saline North Atlantic deep water (NADW), its derivative in the Antarctic circumpolar current, lower circumpolar deep water (LCDW), and the aged variety of deep water, North Indian deep water (NIDW). Although bound by the shallow topography near Madagascar, about 2×106 m3/s from the upper half of the NADW core appears to flow across the sill in the Mozambique Channel into the Somali Basin, while the remaining NADW flows east at about 45°S and is transformed to LCDW by lateral and diapycnal mixing. East of Madagascar the deep circulation is dominated by the southward flow of NIDW. Northward inflow of LCDW into the Indian Ocean therefore can take place only in the eastern half of the Indian Ocean, along the Southeast Indian Ridge and the Ninetyeast Ridge.