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Regional Oceanography: An Introduction
Abstract
This is an advanced undergraduate / introductory graduate textbook. It can be downloaded from http://www.mt-oceanography.info/
... The Indian Central Water (ICW) fills most of the subsurface and thermocline waters of the Indian Ocean, and it can eventually reach the surface [1]. It represents a subtropical water mass subducted at the Subtropical Convergence Zone (SCZ) [2]. The Australasian Mediterranean Water (AAMW) originates from the Pacific Ocean Central Water mixing with the Australasian Mediterranean Sea [2]. ...
... It represents a subtropical water mass subducted at the Subtropical Convergence Zone (SCZ) [2]. The Australasian Mediterranean Water (AAMW) originates from the Pacific Ocean Central Water mixing with the Australasian Mediterranean Sea [2]. Its outflow into the Indian Ocean presents low salinity values of up to 600 meters, with temperatures between 7-8 °C. ...
... Above 600 meters, the AAMW is represented by a band of low salinity along 10°S, from east to west [3]. For a detailed historical description of the water masses in the Indian Ocean, see [4], [5], [2]. ...
Water masses in the surface layer of the ocean are harder to define in terms of hydrographic properties because they undergo large seasonal and interannual variations. Therefore, single TS-combinations cannot be used to define surface water masses. The solution to the definition of surface water masses is the temperature/salinity time diagram (TS-time diagram) and/or the temperature/chlorophyll time diagram (TC-time diagram). This work establishes a systematic definition of regional water masses in the surface layer of the tropical Indian Ocean based on TC-time and TS-time diagrams of mean and standard deviation for 5° squares. This classification is based on an analysis of the ratios between semi-annual and annual amplitudes of temperature and a multiple regression analysis between temperature, salinity, and chlorophyll, with Ekman pumping, components of pseudo-stress wind, wind magnitude and net-down-freshwater-flux. Four water masses are identified for the surface of the Indian Ocean: The Arabian Sea Surface Water (ASSW), the Bay of Bengal Surface Water (BBSW), the Equatorial Indian Surface Water (EISW) and the Southern Indian Surface Water (SISW). The geographical representation of the water masses agrees well with the theoretical frame of reference.
... Subsequently, in reference to the variations of subsurface cold water, with the Kuroshio intrusion onto the shelf, the subsurface coldwater uplifts markedly from 18:00 on July 27 to 18:00 on July 28 (6b-6d). The dynamic linkage between onshore-ward Kuroshio intrusion and uplift of subsurface cold water can be explained by the Tomczak and Godfrey (1994), the uplift (dark blue arrows) is a result of the thermocline slope across a current. Temperature field reacts to the presence of a strong current by adjusting to geostrophic equilibrium. ...
... geostrophic adjustment of the temperature field reacting to the presence of a strong current (see Fig. 7 and Tomczak and Godfrey (1994). Because isotherm depths on the offshore-ward side of the current cannot change, thus, this process would lead to a steep rise of the thermocline from the ocean toward the coast (Tomczak and Godfrey, 1994). ...
... geostrophic adjustment of the temperature field reacting to the presence of a strong current (see Fig. 7 and Tomczak and Godfrey (1994). Because isotherm depths on the offshore-ward side of the current cannot change, thus, this process would lead to a steep rise of the thermocline from the ocean toward the coast (Tomczak and Godfrey, 1994). Moreover, the uplifted subsurface cold water feeds in the cooling feature revealed in the SST (see Fig. 5c-5d). ...
... Two main regions can be distinguished in the Indian Ocean, the northern and the southern Indian Ocean (McPhaden, 1982;Tomczak and Godfrey, 1994). The northern Indian Ocean comprises three regions: the equatorial Region (10° N to 10° S), Bay of Bengal and Arabian Sea. ...
... Around July, the Somali Current becomes stronger and the counter current at the equator is replaced by the SW Monsoon Current. Below the equator, the South Equatorial Current, coming from the east, turns north along the African coast and becomes the Somali Current (Tomczak and Godfrey, 1994). This current has an important effect on the local baroclinic adjustment. ...
... One of the main characteristics of the equatorial Indian Ocean is its lack of upwelling. As a result of the dominance of the meridional orientation of the wind, Ekman transport does not result in divergence along the equatorial region and, therefore, no upwelling occurs (Tomczak and Godfrey, 1994). However, the overall pattern with a secondary low during August remains, even where no upwelling is expected. ...
A long-term analysis of temperature can be used to describe the main mechanisms that operate at the surface of the ocean. The average sea surface temperature (SST) contour plots for the Indian Ocean are produced based on the World Ocean Atlas Data Set (1994). SST, together with the independent variables wind (Wx, zonal component of pseudo-stress wind and Wy, meridional component of pseudo-stress wind), net-down-fresh-water-flow (NDFF), and Ekman pumping, are included in a multiple regression analysis to define the relative importance of each one of these variables in the physical processes at the surface of the Indian Ocean. The NDFF data set is based on COADS (Comprehensive Ocean-Atmosphere Data Set). The wind data is obtained from the Florida State University (FSU). The harmonic terms of the variables are calculated, which is considered to be stationary and expressed by a Fourier series as a cosine function. The harmonic terms are multiplied by the maximum amplitude of the variables and then added to their mean annual values. The isotherms are mainly meridional along the western boundary, but zonal in the southern Indian Ocean. The annual component is seen to have a maximum in July, Summer Monsoon (SW Monsoon) and a minimum in January, during the Winter Monsoon (NE Monsoon). The amplitude of the semiannual component is smaller, with two maxima in May and October and two minima in February and August. The small magnitude of these residuals errors is an indication that the temperature variability during this period and for this area can be explained reasonably well by the two harmonic terms. In the Arabian Sea, the final regression equations for SST variability show that it is mainly affected by the Wx, Ekman pumping and NDFF. For most of the areas of the Bay of Bengal, as well as for most of the locations in the southern tropical Indian Ocean, the entered independent variables can explain SST. Two components fit to observation can be used to predict SST together with the regression equations. Although harmonic analysis can be used to study SST variability, a multiple regression analysis is required to identify and quantify the variables related to areas of large annual and semiannual variability. Different techniques are therefore used together to provide more reliable results in SST configuration in the Indian Ocean.
... The dynamic linkage between the strong alongshore flow and the uplift of subsurface cold water can be explained by the geostrophic adjustment of the temperature field reacting to the presence of a strong current (see Figure 8 and Tomczak and Godfrey [38]). According to Tomczak and Godfrey [38], this upwelling is not related to the wind. ...
... The dynamic linkage between the strong alongshore flow and the uplift of subsurface cold water can be explained by the geostrophic adjustment of the temperature field reacting to the presence of a strong current (see Figure 8 and Tomczak and Godfrey [38]). According to Tomczak and Godfrey [38], this upwelling is not related to the wind. Instead, it is a result of the thermocline slope across a current. ...
... The schematic plot of the dynamic linkage between stronger alongshore (northwar and uplift of subsurface cold water surrounding Longdong. According to Tomczak and G [38], this upwelling is a result of the thermocline slope across a current. The oceanic temp field reacts to the presence of a strong current by adjusting to geostrophic equilibrium. ...
In this study, the Regional Ocean Modelling System (ROMS) with a spatial resolution of 2 km is used to understand the cooling responses in the sea northeast of Taiwan associated with multiple typhoons. Sea level measurements derived from in situ tidal gauges and continuous temperature measured by a moored buoy deployed at Longdong were used to validate the model’s performance. Six far-field typhoons with similar tracks, namely, Utor (2001), Dujuan (2003), Sanvu (2005), Nanmadol (2011), Usagi (2013), and Meranti (2016), were systematically investigated to demonstrate the influences of tidal effect on upper ocean responses to typhoon passages surrounding shore regions. After integrating tidal forcing, model-simulated cold wakes behind typhoon passages were essentially enhanced. Compared with observations, the cold wakes were reproduced more realistically. Tides could promote a cooling response through the following potential mechanisms: (1) tidal mixing destratifies the water column, (2) a stronger northward current leads to a sharper bottom thermocline, (3) tidal residual currents (southward) drive the offshore-ward (downslope) bottom Ekman flow and lead to the overturning of cold bottom water and warm subsurface water, (4) the increase in bottom stress due to the interaction of tidal currents and bottom topography results in the destratification of the bottom water column, and (5) the wind–tide coupled effect. Nevertheless, the exact mechanism dominating the process of a typhoon–tide-induced stronger upper ocean response depends on different typhoon activities, tidal regimes, stratification, and bathymetry and needs further investigations. In this study, we suggest that including tidal effects is essential for the modeling of upper ocean responses to typhoon passages near the shore regions.
... During the NE Monsoon, a thermohaline circulation with northward transport against the wind direction occurs along the west coast of India. This northward current is generated by differences in pressure gradient from north to south caused by differences in temperature and salinity during the winter [14]. A remarkable semi-annual event which occurs in the study area is the Equatorial Jet, first described by [16]. ...
... It represents a semi-annual reversal of flow triggered mainly by the zonal wind, although it is not the only mechanism responsible for the event as it is also influenced by equatorial Kelvin waves at greater depths. The situation is reversed when wind decreases speed and a westward flow, together with the South Equatorial Current, replaces the water off Africa [14,5]. ...
... EAC = East Arabian Coast; SJC = South Java Current; ZC = Zanzibar Current; EMC = East Madagascar Current; SC = Somali Currents; STF = Subtropical Front; SAF = Sub-Antarctic Front; PF = Antarctic Polar Front; WGB = Weddell Gyre Boundary. Adapted from[14] ...
Long-term analysis of salinity can be useful to describe the main mechanisms that operate at the surface of the ocean. Average sea surface salinity (SSS) contour plots for the Indian Ocean are produced based on the NODC_WOA94 data provided by the NOAA/OAR/ESRL PSL. Salinity, together with the independent variables wind, ndff (net-down-freshwater flow) and Ekman pumping are included in a multiple regression analysis to define the relative importance of each one of these variables in the physical processes at the surface of the Indian Ocean. The ndff data set is based on COADS (Comprehensive Ocean-Atmosphere Data Set). The wind data is obtained from the Florida State University (FSU). The harmonic terms are considered to be stationary and expressed by Fourier series as a cosine function in which the first and second harmonic terms are multiplied by the maximum amplitude of the variable and added to the mean annual parameter. The salinity contours tend to be zonally orientated away from the coast, while a meridional influence is observed close to the boundaries. A typical zonal pattern of salinity distribution is observed only south of 10°S. Maximum annual amplitude values are observed in the north of the Arabian Sea and the Bay of Bengal. The variability of the annual components is consistent with the distribution of the net-down-freshwater-flow (ndff) contours and wind direction. During the SW Monsoon, the ndff becomes gradually positive towards the east, in the direction of the west coast of India, which results in a peak of maximum salinity in August and decreasing afterwards. During the NE Monsoon, the ndff is negative elsewhere in the Arabian Sea. The annual term plays a dominant role in determining the maximum and minimum salinity observed during August and January, while the semi-annual component provides minor adjustment. The annual component shows the influence of the monsoons through the year, with a high salinity during the NE Monsoon and a secondary peak during the SW Monsoon. Although harmonic analysis can be applied to the study of salinity variability, to identify and quantify the variables related to these areas of large annual and semiannual variability, a multiple regression analysis needs to be applied.
... All rights reserved. southwesterlies during the summer monsoon (June, July, August) and weaker northeasterlies during the winter monsoon (December, January, February) over the BOB (Tomczak & Godfrey, 2001). The BOB is also marked by a complete seasonal reversal of surface ocean circulation (Schott et al., 2009), with saltier, denser water masses from the Arabian Sea entering the BOB via the Southwest Monsoon Current during summer and less saline water masses from the BOB flowing into the Arabian Sea via the Northeast Monsoon Current during winter (Jensen, 2001 and 2003) (Figure 1a-b). ...
... Kuhnt et al., 2004;Hall, 2009;Molnar & Cronin, 2015;Sosdian & Lear, 2020) could have influenced regional SSTs during the late Miocene. In the modern, warm, fresh waters flowing from the Pacific into the Indian Ocean via the ITF contribute significantly to intermediate water masses that occupy the Indian Ocean thermocline (You & Tomczak, 1993;Gordon & Fine, 1996;Tomczak & Godfrey, 2001;Gordon, 2005). Our late Miocene simulations do not account for any potential paleogeographic or ITF changes coeval with the LMGC, because the model resolution is insufficient to allow us to reconstruct the complex geography of the Indonesian Gateway area. ...
During the late Miocene, global cooling occurred alongside the establishment of near‐modern terrestrial and marine ecosystems. Significant (3°C–5°C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at midlatitudes to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital‐resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9–5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. Our SST record reveals a 3.2°C decrease from 7.4 to 5.8 Ma, significantly increasing previous estimates of late Miocene tropical cooling. Analysis of orbital‐scale variability shows that before the onset of cooling, SST variations were dominated by precession‐band (19–23 kyr) variability, whereas tropical temperature became highly sensitive to obliquity (41 kyr) after 7.5 Ma, suggesting an increase in high‐latitude forcing. We compare a revised global SST database with new paleoclimate model simulations and show that a pCO2 decrease from 560 to 300 ppm, in the range suggested by pCO2 proxy records, could explain most of the late Miocene sea surface cooling observed at Site U1443. Using our new Site U1443 record as representative of tropical SST evolution, estimated meridional SST gradients suggest a much more modest increase over the late Miocene than previously suggested, in agreement with modeled meridional SST gradients.
... The strong convective mixing is also supported by a rapid decrease in specific humidity of air from 19 g/kg in October to 10 g/kg in December (S.P. Kumar & Prasad, 1996). The crucial parameters to set convective mixing are the differences in sea-air temperature, specific humidity of air and wind speed (Tomczak and Godfrey, 1994). ...
... P. Kumar and Prasad, 1996;Madhupratap et al., 1996). Based on the climatological data, northern AS contributes to ∼60% of the net evaporation-precipitation (E-P) during winter to net annual evaporation of 1,000 mm/y (Tomczak and Godfrey, 1994). Weller et al. (1998) reported that mean relative humidity over northern AS was ∼70% during winter favored high evaporation leading to an increase in surface density and deepening of MLD (Wiggert et al., 2000). ...
The recent increase in atmospheric pollutants (aerosols) over the northern Indian Ocean due to an increase in industrial activity in South and Southeast Asia decreased air temperature (AT) leading to the strengthening of winter convective mixing and promotion of phytoplankton blooms in the northern Arabian Sea (AS). To test this hypothesis, the variability in meteorological and surface ocean properties was analyzed for a period of 4 decades (1979–2018) in the northern AS. The total and anthropogenic aerosol optical depth (AOD) increased at a rate of 0.05 and 0.07 per decade respectively in the recent decades associated with a decrease in AT over the northern AS during winter. The estimated decrease in AT due to increasing AOD accounts for 0.18°C over the AS and it is close to that of the mean decrease in AT from 1994 to 2018 of 0.25°C. This increased the difference in temperature between the sea surface and atmosphere and enhanced specific humidity in the atmosphere resulting in the strengthening of convective mixing in the northern AS as evidenced by the deepening of the depth of 20°C isotherm in the recent decades during winter. Enhanced nutrient concentration and phytoplankton biomass were observed in the photic zone during winter further confirming that convective mixing strengthened in the recent decades in the northern AS than hitherto hypothesized as weakening due to the warming of the Eurasian landmass. The enhanced nutrients supported diatoms blooms, which are prey for Noctiluca scintillans to initiate the bloom in the AS in the recent decades.
... In this section, the different water masses were characterized based on Talley et al. (2011) and Tomczak and Godfrey (2003). Properties of each end-member are presented in Table 1. ...
... In the western boundary, the SEC splits and joins the East Australian Current that runs along the northwest coast of Australia before flowing eastward to feed the southern branch of the gyre. It then joins the Humboldt Current that flows northward along the continental coast of South America (Tomczak & Godfrey, 2003). Gyre waters then return to the equator through the SEC (Ganachaud et al., 2014). ...
In the Western Tropical South Pacific, a hotspot of dinitrogen-fixing organisms has been identified. The survival of these species depends on the availability of dissolved iron (DFe); however, the source of this DFe is still unclear. DFe was measured along a transect from 175°E to 166°W near 19-21°S. The distribution of DFe showed high spatial variability: low concentrations (∼0.2 nmol kg-1) in the South Pacific gyre and high concentrations (up to 50 nmol kg-1) west of the Tonga arc, indicating that this arc is a clear boundary between iron-poor and iron-rich waters. An optimal multiparameter analysis was used to distinguish the relative importance of physical transport relative to non-conservative processes on the observed distribution. This analysis demonstrated that the shallow hydrothermal sources present along the Tonga-Kermadec arc are responsible for the high concentrations observed in the photic layer. Nevertheless, in contrast to what has been observed for deep hydrothermal plumes, our results highlighted the rapid decrease in DFe concentrations near shallow hydrothermal sources. This is likely due to a shorter residence time of surface water masses combined with several biogeochemical processes at play (precipitation, scavenging, biological uptake, photoreduction). This study clearly highlights the role of shallow hydrothermal sources on the DFe cycle within the Tonga-Kermadec arc where a strong link to biological activity in surface waters can be assessed, despite the small but significant fraction of DFe ultimately stabilized. It also emphasizes the need to consider the impact of these sources for a better understanding of the global iron cycle.
... (Tsuchiya et al., 1989;Qu and Lindstrom, 2002). Originating from the subtropical high evaporation area in the southeastern Pacific, this water mass is characterized by high salinity (~35.7) and low oxygen content (~141 μmol/kg; Tomczak and Hao 1989;Tsuchiya et al., 1989;Tomczak and Godfrey 2003;Kessler and Cravatte 2013b). (Tsuchiya, 1981;Qu et al., 2008;Grenier et al., 2014) and a very loose stratification at about 300 m depth (~σ θ 26.0 kg/m 3 , Tomczak and Hao, 1989;Tomczak and Godfrey, 2003). ...
... Originating from the subtropical high evaporation area in the southeastern Pacific, this water mass is characterized by high salinity (~35.7) and low oxygen content (~141 μmol/kg; Tomczak and Hao 1989;Tsuchiya et al., 1989;Tomczak and Godfrey 2003;Kessler and Cravatte 2013b). (Tsuchiya, 1981;Qu et al., 2008;Grenier et al., 2014) and a very loose stratification at about 300 m depth (~σ θ 26.0 kg/m 3 , Tomczak and Hao, 1989;Tomczak and Godfrey, 2003). ...
Significant progress has been made in the last decade on the understanding of the role of the Coral and Solomon Seas as major suppliers of waters and chemical elements to the equatorial Pacific. Yet, the location, depth, and processes of chemical enrichment of these waters remain poorly constrained. Neodymium (Nd) isotopic compositions ( ε N d ) and rare earth element concentrations (REE) are powerful tracers of land-ocean chemical exchanges. Combined, they can greatly refine the characterization of these exchanges. Here we report profiles of ε N d at 21 stations located in the Coral and Solomon Seas as part of the GEOTRACES GP-12 cruise that complement the rare earth element concentration (REE) profiles of Pham (Chemical Geology, 2019, 524 (May), 11–36). Waters exiting the Solomon Sea are generally slightly more radiogenic than the incoming ones, suggesting inputs of radiogenic material along their pathways across the Solomon Sea. This radiogenic material is brought to the surface waters via natural processes (rivers, volcanic dusts) and likely local mining activities. Noticeable ε N d increases are also observed in subsurface and intermediate layers. All these processes indicate the occurrence of local Boundary Exchange (BE) processes, which are estimated to occur within a few days. Coupling hydrological and chemical tracers allows highlighting the land-ocean interactions affecting some water layers and quantifying the exchanged fluxes of Nd. Modifications of the Nd concentration and isotopic composition in the lower thermocline layer require an external flux of 7.9 ± 2.0 t(Nd)/yr only partly balanced by a scavenging flux of 1.8 ± 2.3 t(Nd)/yr, leading to a net influx of 6.1 ± 1.7 t(Nd)/yr. Regarding the Upper Circumpolar Deep Water, a total net flux of 105 ± 50 t(Nd)/yr is estimated, the external flux is relatively high (86 ± 31 t(Nd)/yr while the scavenging flux remains. These results refine the role of the Solomon Sea as a supplier of continental chemical elements to the Pacific equatorial waters.
... The topography ranges from 0 to 152 m above sea level (average=91 m above sea level (asl) Eisenberg and Lockhart, 1972) and annual temperatures average 27 • C, with two dry periods annually: a short period between January and February, and a longer period from May to September (Mueller-Dombois and Sirisena, 1968;Mueller-Dombois, 1968). Most of the region's rainfall occurs during the first, and second inter-monsoonal periods, between March-April, and September-November (Tomczak and Godfrey, 2003), and averages 1000 mm annually (Eisenberg and Lockhart, 1972). North-Eastern monsoonal rains also occur from December to January (Tomczak and Godfrey, 2003). ...
... Most of the region's rainfall occurs during the first, and second inter-monsoonal periods, between March-April, and September-November (Tomczak and Godfrey, 2003), and averages 1000 mm annually (Eisenberg and Lockhart, 1972). North-Eastern monsoonal rains also occur from December to January (Tomczak and Godfrey, 2003). ...
The decline and extirpation of large carnivore populations can lead to cascading effects in natural ecosystems. An understanding of large carnivore population densities, distribution and dynamics is therefore critical for developing effective conservation strategies across landscapes. This is particularly important in island environments where species face increased extinction risk due to genetic isolation coupled with local losses of finite habitat. The Sri Lankan leopard Panthera pardus kotiya is one of two remaining island-living leopards on Earth and the only apex predator in Sri Lanka. Despite its iconic status in Sri Lanka, robust research on the species has been limited to only a handful of scientific studies, limiting meaningful scientific recommendations for the species’ conservation and management. In this study, we conducted a single season camera trap survey in Sri Lanka’s largest protected area, Wilpattu National Park (1,317 km²), located in the country’s northwest. Our objective was to estimate key ecological state variables of interest (density, abundance, sex-specific movement and spatial distribution) of this leopard subspecies. Our results indicate that Wilpattu National Park supports a density of 18 individuals/100 km² (posterior SD=1.5; 95% HPD interval=16–21) with a mean abundance of 144 (posterior SD=15) individual leopards and a healthy sex ratio (f:m=2.03:1). The estimated activity range for male leopards >2 years old was 49.53 km² (Posterior SD=3.43; HPD interval=43.09–56.41) and for female leopards >2 years old was 22.04 km² (Posterior SD=1.82; HPD interval=18.34–25.65). This density falls at the higher end of published estimates for the species anywhere in its global range, based on similar methods. Given Sri Lanka’s limited size, this national park system should be considered as a critical stronghold that maintains a source population of leopards, contributing to the long-term population viability of leopards in the larger landscape.
... The bottom water at eastern equatorial Indian Ocean Site U1443 (2925 m water depth), with a δ 13 C of ~0.25‰ and oxygen content of ~150 μmol/kg (Fig. 2), is composed of Indian Deep Water (IDW), which occupies the depth range from 3800 m to ~1500 m in the equatorial and northern Indian Ocean. IDW is characterized by high salinities reaching maxima of 34.8 practical salinity units (psu) in the southwestern part of the Indian Ocean and 34.75 psu in the southeastern part, where its upper limit rises to a depth of 500 m (Tomczak and Godfrey, 2003). Its temperature, salinity, and oxygen properties in the high-salinity core are virtually identical with those of NADW in the Atlantic sector of the Southern Ocean, indicating that IDW is mainly of NADW origin and is not formed in the Southern Ocean, in contrast to AABW, which fills the Indian Ocean below 3800 m (Tomczak and Godfrey, 2003). ...
... IDW is characterized by high salinities reaching maxima of 34.8 practical salinity units (psu) in the southwestern part of the Indian Ocean and 34.75 psu in the southeastern part, where its upper limit rises to a depth of 500 m (Tomczak and Godfrey, 2003). Its temperature, salinity, and oxygen properties in the high-salinity core are virtually identical with those of NADW in the Atlantic sector of the Southern Ocean, indicating that IDW is mainly of NADW origin and is not formed in the Southern Ocean, in contrast to AABW, which fills the Indian Ocean below 3800 m (Tomczak and Godfrey, 2003). The northward flow of IDW, concentrated along the western boundary of the Ninetyeast Ridge, penetrates into the Northern Hemisphere, where it is modified by mixing with thermocline water from above and upwelling of AABW from below. ...
The Monterey Formation is a Miocene marine unit that occurs extensively in the Coast Ranges and in the continental margins of California, and analogous biosiliceous deposits are found around the Pacific Rim and elsewhere in the world. Classic studies on the diatomaceous deposits that characterize the hemipelagic/pelagic facies of the Monterey Formation have been key to understanding the oceanographic and tectonic conditions that lead to the preservation of large volumes of organic-rich hemipelagic biosiliceous sediments, and the properties of these sedimentary deposits once they convert into rocks. This volume presents a collection of recent studies on the Monterey and other similar biosiliceous deposits that offer modern and updated interpretations of this classic unit and its analogues. The volume is dedicated to the memory of Professor Bob Garrison.
... Deposition in North-East Greenland took place under relatively deep marine conditions, below storm wave base (see p. 16), but not even close to the oceanic depths of the present North Atlantic Ocean. However, stratification of the water column with oxygenated bottom currents is known to be able to pass submarine barriers in the form of sills at relatively low depths without mixing with surface layers, e.g., the outflow of dense, oxygenated bottom water from the Mediterranean Sea through the Strait of Gibraltar at only 320 m depth (Tomczak & Godfrey 2003). ...
... ITF enters the Indian Ocean through three routes which are: the Lombok straits, Timor passage, and Omboi Straits (Figure 1; Ayers et al., 2014;Valsala & Ikeda, 2007). The ITF, largely affected, seasonally and interannually, by remote forcing from planetary waves and monsoon winds (Ayers et al., 2014), transports large amounts of warm water through the Indonesian Seas into the Indian Ocean (Muhling et al., 2017;Tomczak & Godfrey, 2003). ITF is part of the upper branch of the global heat conveyor belt, and thus important for balancing heat, salt, and nutrient budgets in the oceans. ...
The Argo Abyssal Plain (AAP), south of Java and northwest of Australia in the tropical eastern Indian Ocean, is an oligotrophic region (low in chlorophyll and nutrients), downstream of the Indonesian throughflow. The processes that supply nitrogen to the AAP and support ecosystem production are unknown. Here we quantified lateral advection of particulate organic matter (POM) and the role of this advected POM in supplying new nitrogen (i.e., allochthonous nitrogen supply) to the mixed layer of the AAP using 14 yr of remotely sensed data, combined with sampling undertaken on a research cruise in February 2022. Our results indicate that the average net primary productivity of phytoplankton in this offshore oligotrophic region is 98.5 Gg C d⁻¹ and that lateral advection transports 1.21 Gg C d⁻¹ of particulate organic carbon (POC) into the region. We find that lateral advection of POM within the mixed layer supplies an annual average of 12% (95% C.I. = 5.2%–49%) of allochthonous “new” nitrogen supporting phytoplankton productivity, if regenerated to nutrients through microbial processes. Accounting for lateral transport in the deep euphotic zone, lateral transport supplies an average of 32% (10%–>100%) of new nitrogen, although this calculation is less certain due to inability to constrain subsurface POM fields. Our data suggest that lateral advection is a quantitatively important but not dominant source of nitrogen supporting new production. Overall, approximately half of the lateral transport is driven by transient eddies (mostly during winter) highlighting the importance of mesoscale circulation.
... These water masses subduct beneath warmer subtropical surface waters north of the Sub-Antarctic Front around 36°S-54°S and are transported eastward as SACW across the South Atlantic into the Cape basin [64][65][66] . Here, SACW converges with the Agulhas water from the Indian Ocean to form ESACW that enters the SBUS from the south [67][68][69] . The majority of the SACW circumvents the SBUS along the Benguela Current and enters the BUS from the north via the Angola-Benguela Frontal Zone (ABFZ) as a poleward undercurrent, which, unlike ESACW, is nutrient-enriched and oxygendepleted 70 . ...
Eastern Boundary Upwelling Systems (EBUS) are highly productive ecosystems. However, being poorly sampled and represented in global models, their role as atmospheric CO2 sources and sinks remains elusive. In this work, we present a compilation of shipboard measurements over the past two decades from the Benguela Upwelling System (BUS) in the southeast Atlantic Ocean. Here, the warming effect of upwelled waters increases CO2 partial pressure (pCO2) and outgassing in the entire system, but is exceeded in the south through biologically-mediated CO2 uptake through biologically unused, so-called preformed nutrients supplied from the Southern Ocean. Vice versa, inefficient nutrient utilization leads to preformed nutrient formation, increasing pCO2 and counteracting human-induced CO2 invasion in the Southern Ocean. However, preformed nutrient utilization in the BUS compensates with ~22–75 Tg C year−1 for 20–68% of estimated natural CO2 outgassing in the Southern Ocean’s Atlantic sector (~ 110 Tg C year−1), implying the need to better resolve global change impacts on the BUS to understand the ocean’s role as future sink for anthropogenic CO2.
... The salinity of a marginal basin is closely related to the overall water balance (i.e., evaporation versus precipitation and runoff) and the basin's degree of connection with the open ocean (Matthias and Godfrey, 1994). In general, reduced water exchange with open ocean and/or enhanced freshwater influx would result in low salinity in a (semi-) restricted basin. ...
... Currently, the thermocline in the southern WPWP (sector south of the equator) is mostly comprised of warm, high-salinity water masses subducted within the South Pacific gyre (Grenier et al., 2013;Tomczak and Godfrey, 2013). Low-latitude western boundary currents (WBCs) are the primary conduit for these water masses to the equator (Davis et al., 2012) and therefore play an important role in regulating the WPWP subsurface temperature and salinity (Hu et al., 2015). ...
... Below the AAIW, two deep-water masses enter the Tasman Sea from the south, the shallower Circumpolar Deep Water (CPDW) and the deeper Antarctic Bottom Water (AABW). The AABW has lower temperatures and higher dissolved oxygen concentrations than the CPDW (Bostock et al., 2004;Tomczak & Godfrey, 1994). ...
The Late Miocene‐Early Pliocene Biogenic Bloom (∼9–3.5 Ma) was a paleoceanographic phenomenon defined by anomalously high accumulations of biological components at multiple open ocean sites, especially in certain regions of the Indian, and Pacific oceans. Its temporal and spatial extent with available information leaves fundamental questions about driving forces and responses unanswered. In this work, we focus on the middle part of the Biogenic Bloom (7.4–4.5 Ma) at International Ocean Discovery Program Site U1506 in the Tasman Sea, where we provide an integrated age model based on orbital tuning of the Natural Gamma Radiation, benthic foraminiferal oxygen isotopes, and calcareous nannofossil biostratigraphy. Benthic foraminiferal assemblages suggest changes in deep water oxygen concentration and seafloor nutrient supply during generally high export productivity conditions. From 7.4 to 6.7 Ma, seafloor conditions were characterized by episodic nutrient supply, perhaps related to seasonal phytoplankton blooms. From 6.7 to 4.5 Ma, the regime shifted to a more stable interval characterized by eutrophic and dysoxic conditions. Combined with seismic data, a regional change in paleoceanography is inferred at around 6.7 Ma, from stronger and well‐oxygenated bottom currents to weaker, oxygen‐depleted bottom currents. Our results support the hypothesis that the Biogenic Bloom was a complex, multiphase phenomenon driven by changes in ocean currents, rather than a single uniform period of sustained sea surface water productivity. Highly resolved studies are thus fundamental to its understanding and the disentanglement of local, regional, and global imprints.
... The modern New Zealand subcontinent lies at the interface between the southwest Pacific Ocean and the Southern Ocean (Nelson and Cooke, 2001;Tomczak and Godfrey, 2013). Tectonic reconstructions show significant movement of the New Zealand land mass to the north for the last 40 million years (King, 2000;Nelson and Cooke, 2001). ...
... To show coastal upwelling favorable conditions the cumulative Ekman transport (CET) was used. The CET is described as the cumulative transport effect over time and it corresponds to the wind-generated mass transport per unit of width over the depth of the Ekman layer, which is expressed in kg m − 1 s − 1 (a transport of 1000 kg m − 1 s − 1 is typically for a wind stress of 0.1 N m − 2 at 45 • ; Tomczak and Godfrey, 2003). The CET was integrated monthly between the years 2015 and 2021 and annually between the years 2015 and 2100 forecast. ...
... The contour in fine black lines indicates the extent of the exposed shelves and dark grey shading the extent of ice during the last glacial maximum (Alloway et al., 2007;Davies et al., 2020). Prevailing currents are modified from Tomczak and Godfrey (1994) and Chiswell et al. (2015): South Pacific equatorial current (SPEC), Tasman current (TC), sub tropical front (STF), Antarctic circumpolar current (ACC) and Humboldt current (HC). Reconstructions of the most likely colonization routes followed by G. chilensis are shown as coloured arrows (route of colonization from New Zealand to Chatham Island in red and to Chile in blue). ...
Gracilaria chilensis is the main cultivated seaweed in Chile. The low genetic diversity observed in the Chilean population has been associated with the over-exploitation of natural beds and/or the founder effect that occurred during the post-glacial colonization from New Zealand. How these processes have affected its evolutionary trajectory before farming and incipient domestication is poorly understood. In this study, we used 2,232 SNPs to assess how the species evolutionary history in New Zealand (its region of origin), the founder effect linked to transoceanic dispersion and colonization of South America, and the recent over-exploitation of natural populations have influenced the genetic architecture of G. chilensis in Chile. The contrasting patterns of genetic diversity and structure observed between the two main islands in New Zealand attest to the important effects of Quaternary glacial cycles on G. chilensis. ABC analyses indicated that Chatham Island and South America were colonized independently near the end of the Last Glacial Maximum and emphasized the importance of coastal and oceanic currents during that period. Furthermore, ABC analyses inferred the existence of a recent and strong genetic bottleneck in Chile, matching the period of over-exploitation of the natural beds during the 1970s, followed by rapid demographic expansion linked to active clonal propagation used in farming. Recurrent genetic bottlenecks strongly eroded the genetic diversity of G. chilensis prior to its cultivation, raising important challenges for the management of genetic resources in this incipiently domesticated species.
... Open ocean currents reached the archipelago interior via inter-atoll channels and resulted in the deposition of ten mega-drift sequences in the IS 6). Monsoon induced surface and bottom currents have been the most important factor for periplatform depositional system at the MA resulting in contourite drift bodies surrounding the atolls in the IS (Betzler currents can reach velocities up to 2 m/s within the modern atolls' passages and the currents reach water depths of more than 200 m(Tomczak and Godfrey, 2003). Further research in the area has discovered details of the current activity by forming smaller carbonate contourite drift bodies at the eastern entrance of the KC (Lopez ...
The interaction between bottom currents, platform-derived particles, and the bathymetric framework of isolated carbonate platforms can result in complex current-controlled depositional patterns, which are not entirely understood. The continuous supply and usually local deposition of carbonate particles around platforms, combined with the natural variations in the hydrodynamic regime at various depths, contribute significantly to the heterogeneity of carbonate contourite drifts. This thesis sheds light on the interplay between bottom currents and isolated carbonate platforms in two representative study areas. Moreover, this study presents a classification of carbonate contourite (sediment) drifts related to the architecture and sediment distribution of carbonate platforms based on seismic datasets, as well as the identification of the main driving mechanisms and depositional architecture. Two realms of isolated carbonate platforms were studied, which are located in the Bahamian and Maldivian archipelagos. Both are influenced by major ocean current systems, which were investigated to understand their effects on the depositional patterns. The study areas were targeted by geophysical methods during two scientific cruises, which produced dense grids of high-resolution seismic datasets, allowing new insights to be gained into the sedimentary dynamics of both archipelagos. To achieve the main objectives seismic and hydroacoustic datasets around isolated carbonate platforms were compiled and analyzed, combined with lithostratigraphic and chronological borehole data from multiple sites of the IODP expedition 359 and the ODP Site 1006. (Re-)Interpretation of multichannel seismic cross sections, subordinated echosounder sub-bottom profiling and bathymetric data as well as former inventory data revealed the history of oceanographic processes resulting in various geomorphologies of contourite drifts. The seismic data emphasize the diversity of the sedimentation pattern developed under the influence of long-term bottom current activity.
... The location of Site 758 is currently influenced by the IDW [36] (Fig. 1b), which is formed by vertical mixing of water masses sourced from the Southern Ocean and the IIW, namely, the lower limb of the ITF [35]. The southern-sourced bottom waters (>2000 m) in the northern Indian Ocean are strongly influenced by the AABW, with minor contributions from the North Atlantic Deep Water (NADW) [70]. The AABW is one of the most ancient water masses; it originates from dense shelf water around the Antarctic, supplies the lower limb of the global overturning circulation [71] and might have existed since approximately 40-32 Ma with the progressive opening of the Drake Passage and Tasmanian gateway [72,73]. ...
Enhanced silicate weathering induced by the uplift of the Himalayan-Tibetan Plateau (HTP) has been considered as the major cause of pCO2 decline and Cenozoic cooling. However, this hypothesis remains to be validated, largely due to the lack of a reliable reconstruction of the HTP weathering flux. Here, we present a 37-million-year record of the difference in the seawater radiogenic neodymium isotopic composition (DeNd) of Ocean Drilling Program (ODP) sites and Fe-Mn crusts between the northern and central Indian Ocean, which indicates the contribution of regional weathering input from the South Asian continent to the Indian Ocean. The results show a long-term increase in DeNd and thus provide the first critical evidence of enhanced South Asian weathering input since the late Eocene. The evolution coincided well with major pulses of surface uplift in the HTP and global climatic transitions. Our foraminiferal eNd record suggests that tectonic uplift and silicate weathering in South Asia, especially in the Himalayas, might have played a significant role in the late Cenozoic cooling.
... It is dominantly driven by the South Asian monsoon. The northeast monsoon occurs between November and the subsequent March and the southwest monsoon prevails from May to September, with the latter being significantly stronger than the former [1][2][3][4]. Driven by a strong southwest monsoon, a unique area with a relatively low sea surface temperature (SST) is formed in the southern BOB during summer (June-August), called the southern cold pool (SCP) [5][6][7]. The SCP plays an important role in modulating the climate over the study region and beyond. ...
The southern Bay of Bengal (BOB) cold pool (SCP) plays an important role in the regional climate fluctuation of the BOB. However, the interannual variability in the SCP is still unknown. Multisource satellite remote sensing data and assimilation have been applied to explore the interannual variability in the SCP and its relationship with El Niño–Southern Oscillation (ENSO) events for the period 1982–2020. The anomalous SST of the SCP in the summer following the peak phase (i.e., winter) of the ENSO was closely related to the ENSO events. El Niño (La Niña)-induced the warm (cold) anomaly of the SCP starting from May and persisted throughout August with a peak value appearing in June during the El Niño (La Niña) decaying years. In the El Niño decaying years, the southwest monsoon current (SMC) was weakened, forced locally by the weakening southwesterly wind and remotely by the easterly wind anomaly at the equator associated with El Niño. The El Niño-related weakening SMC and the associated less cold advection led to the warm anomaly of the SCP. In addition, El Niño-related atmospheric heating also made a comparable contribution to the evolution of the SCP’s SST. In the early stage (15 May to 10 June), its contribution to the warming of the SCP was much larger than that of the SMC, whereas from mid-June to August, it reversed to have a cooling effect and partially offset the advection heating induced by the SMC on the SCP. In the La Niña decaying years, similar oceanic and atmospheric processes operated but in an opposite way.
... It presents a well-known intensification in its western margin, the Gulf Stream, with maximum velocities up to 2 m s −1 (Halkin, Rossby, and Rossby, 1985). The currents observed in this western margin of the gyre occupy a small horizontal extension compared to that of the currents in the eastern side, resulting in an asymmetric gyre ( (Stramma, 1984;Tomczak and Godfrey, 2003)). The low intensity of the currents at the eastern boundary made them very little studied until the 1970s, when CINECA (Cooperative Investigations of the Northern Part of the Eastern Central Atlantic) program focused on the productive African upwelling system (Ekman, 1923;Hughes and Barton, 1974;Tomczak, 1979;Hempel, 1982). ...
The main goal of this research work, contextualized within Physical Oceanography
but also with some multidisciplinary nature, is to complete the ideas that have been established throughout history around the Cape Verde Frontal Zone, CVFZ. Nestled within the highly productive coastal upwelling off Northwest (NW) Africa, the CVFZ is located off the coast of Cape Blanc, in the convergence zone of the Subtropical and Tropical gyres of the North Atlantic where the central waters transported by both gyres, NACW and SACW, form a thermohaline front practically compensated in density and with abundant associated mesoscale structure. Based on in situ measurements carried out during
the two cruises of the COCA project and two other cruises of the FLUXES project, the characterization of the CVFZ deepens in order to achieve a better understanding of the physical components that govern its dynamics, from the basin scale, in which the circulation patterns are described north and south of Cape Blanc influenced by the unresolved mesoscale, up to the mesoscale and submesoscale, in which the main thermohaline structures detected above and below 100 m and the associated frontal dynamics are detailed, including the secondary ageostrophic circulation with its respective horizontal and vertical components. In addition, this research work describes the distribution and behavior of the different water masses detected at different levels, as well as, other associated biochemical variables, such as inorganic nutrients, dissolved oxygen and dissolved organic carbon, addressing the uncertainties in the balances of the large-scale biochemical fluxes in the CVFZ.
... Notably, the importance of the b-effect has been evidenced in another unique characteristic of mesoscale eddies, namely, their westward propagation (e.g., Tomczak and Godfrey, 2003;Chelton et al., 2007;Chelton et al., 2011;Early et al., 2011). Nevertheless, an earlier investigation by McGillicuddy et al. (1995) of the b-effect on vertical motion of an isolated eddy overlooked the "eddy b-spiral" effect because they focused on vertical motions near the surface, whereas the "b-spiral"associated w is nearly zero near the surface and increases greatly with depth. ...
Oceanic eddies accompanied by a significant vertical velocity ( w ) are known to be of great importance for the vertical transport of various climatically, biologically or biogeochemically relevant properties. Using quasi-geostrophic w -thinking to extend the classic “ β -spiral” w -theory for gyre circulations to isolated and nearly symmetric oceanic mesoscale eddies, we propose that their w motion will be dominated by a strong east-west dipole pattern with deep ocean penetrations. Contrasting numerical simulations of idealized isolated eddies together with w -equation diagnostics confirm that the w -dipole is indeed dominated by the “eddy β -spiral” mechanism in the β -plane simulation, whereas this w -dipole expectedly disappears in the f -plane simulation. Analyses of relatively isolated warm and cold eddy examples show good agreement with the proposed mechanism. Our studies further clarify eddy vertical motions, have implications for ocean mixing and vertical transport, and inspire further studies.
... A high mountain ridge (ca. 1800 m) in the island's interior creates an obstruction to the dominant northeast trade winds, leading to warmer and sheltered waters in the south of Madeira (Tomczak and Godfrey, 2013;Caldeira et al., 2002;Caldeira and Sangrà, 2012). ...
... Comparatively, the deeper water mass of the ITF has minimal salinity referring to the AAIW which gets contribution from the South Pacific through the NGCUC spreading into the Banda Sea through the Lifamatola and Makassar passages (Zenk et al., 2005). The deep water in the Timor Sea is derived from the Indian Deep Water, ranging from 3800 m to 1500e2000 m water depth (Tomczak and Godfrey, 2001). Hence, the ITF-IW ranging from 1000 to 1500 m in the Timor Sea comprises the deep part of the IIW and the AAIW from the South Pacific through the northwestward-flowing NGCUC that has experienced intense vertical mixing in the Banda Sea. ...
The Indonesian Throughflow (ITF) is the unique low-latitude connector between the Pacific and Indian oceans. During the Plio–Pleistocene, the ITF may have played a significant role in the global climate associated with the tectonic constriction of the Indonesian gateway, which was possibly one of the factors spurring the Northern Hemisphere glaciation. Persistent constriction of the gateway would have restricted the ITF intermediate water (ITF-IW) with a high priority over the ITF upper waters. Due to limited availability of sediment archives, however, no investigations have been carried out on the Plio–Pleistocene evolution of the ITF-IW. In this study, we present our work on a total of 1368 sediment samples spanning the past ∼3.17–1.07 Ma from Site U1482, which was drilled during the International Ocean Discovery Program Expedition 363 in the Timor Passage, the main exit of the ITF to the Indian Ocean. Site U1482 is bathed by the ITF-IW, the evolution of which could thus be monitored by benthic foraminiferal shells preserved in the sediments of the site. Based on establishment of the first high-resolution (∼1.53 kyr) Plio–Pleistocene benthic δ¹⁸O stratigraphy within the ITF region, we recognized that, superimposed over the prominent 41-kyr glacial–interglacial cycles, the long-term change of Site U1482 benthic δ¹⁸O was punctuated at ∼2.51–2.43 Ma (Marine oxygen isotope stage (MIS)100/99–96/95) and at ∼1.6 Ma (MIS 56/55), followed by an interval of decreased glacial–interglacial amplitudes in the duration of ∼1.6–1.2 Ma. We propose that Site U1482 benthic δ¹⁸O was mainly driven by the global ice-sheet growth prior to ∼1.6 Ma, in addition to possibly influenced by the tectonic constriction of the Indonesian gateway. In contrast, the decreased glacial–interglacial amplitudes in the benthic δ¹⁸O during the interval of ∼1.6–1.2 Ma more likely indicate the regional signal carried by the ITF-IW, which is attributed to a freshening of the ITF surface waters that was subsequently transmitted to the intermediate depths via vertical mixing in the Banda Sea, caused by enhanced regional precipitation related to monsoonal and western Pacific warm pool conditions.
... The influence of wind on the ocean surface layer contributes to the vertical mixing of the water column, favoring the input of nutrients to the photic layer, and therefore, increasing the primary production (Tomczak and Godfrey, 1994). Also, the alongshore wind stress forces the upwelling/downwelling of coastal waters (Bakun, 1973). ...
Harmful Algal Blooms (HAB) pose a severe socio-economic problem worldwide. The dinoflagellate species Alexandrium catenella produces potent neurotoxins called saxitoxins (STXs) and its blooms are associated with the human intoxication named Paralytic Shellfish Poisoning (PSP). Knowing where and how these blooms originate is crucial to predict blooms. Most studies in the Chilean Patagonia, were focused on coastal areas, considering that blooms from the adjacent oceanic region are almost non-existent. Using a combination of field studies and modelling approaches, we first evaluated the role of the continental shelf off northern Chilean Patagonia as a source of A. catenella resting cysts, which may act as inoculum for their toxic coastal blooms. This area is characterized by a seasonal upwelling system with positive Ekman pumping during spring-summer, and by the presence of six major submarine canyons. We found out that these submarine canyons increase the vertical advection of bottom waters, and thus, significantly enhance the process of coastal upwelling. This is a previously unreported factor, among those involved in bloom initiation. This finding put this offshore area at high risk of resuspension of resting cysts of A. catenella. Here, we discuss in detail the physical processes promoting this resuspension.
... However, different conclusions were obtained, e.g., with 8 kyr phase lag in Arabian Sea sediments Clemens and Prell, 2003), and without phase lag in Bay of Bengal (Weber et al., 2018;Lauterbach et al., 2020). The equatorial Indian Ocean region is also affected by the Indian monsoon (Figure 1) (e.g., Tomczak and Godfrey, 2003;Betzler et al., 2013;Lüdmann et al., 2013;Bunzel et al., 2017;Alonso-Garcia et al., 2019), but there is a lack of discussion on the phase difference between ISM and NHSI. Therefore, in order to demonstrate whether this phase difference is unique to the sedimentary records of the Indian Ocean, it is necessary to find evidence in wider Indian monsoon control regions, especially in the equatorial Indian Ocean (e.g., Ninetyeast ridge and Maldives). ...
The Indian monsoon is an important part of the global monsoon system, allowing important transfers of moisture at a large geographical scale and deeply affecting human populations and economic prosperity of regions. The tropical summer monsoon in the Northern Hemisphere is generally considered to be driven by low latitude solar radiation. Therefore, the summer monsoon strength is near zero-phase to the maximum of Northern Hemisphere Summer Insolation (NHSI). However, records from the Arabian Sea and some other parts of the Indian Ocean (e.g., Andaman Sea) show that a ∼8 kyr phase difference exists between the Indian summer monsoon (ISM) strength and the northern Hemisphere Summer Insolation maxima, which is obviously different from the records of stalagmites in the East Asia and other marine sediments (e.g., Bay of Bengal). This leads to the “sea-land precession phase paradox” in indian summer monsoon research. This paper systematically summarizes the Indian monsoon variability on orbital scale indicated by various records from the Indian monsoon regions (including oceans and continents) since the late Quaternary. The orbital forcing of Indian monsoon, the potential phase difference between indian summer monsoon and northern Hemisphere Summer Insolation and its possible forcing mechanism(s) are further discussed. The observed phase lag between indian summer monsoon and northern Hemisphere Summer Insolation may be controlled by the Atlantic Meridional Overturning Circulation (AMOC), latent heat transfer between the southern Indian Ocean and the Asian continent, or caused by the lack of tightly coupling between the Arabian Sea summer monsoon proxies and the monsoon intensity. In addition, it is still unclear whether previous monsoon proxies can provide a strong constraint on the intensity of summer monsoon. Environmental magnetism has been widely used in high-resolution dating and the analysis of paleoclimate variabilities in marine and terrestrial sediments, due to its high sensitivity on the rainfall and temperature. Therefore, in order to solve these issues, it is necessary to combine magnetic parameters with geochemical and paleontological parameters for more systematic work in the future.
... The contour in fine black lines indicates the extent of the exposed shelves and dark grey shading the extent of ice during the last glacial maximum (Alloway et al., 2007;Davies et al., 2020). Prevailing currents are modified from Tomczak and Godfrey (1994) and Chiswell et al. (2015): South Pacific equatorial current (SPEC), Tasman current (TC), sub tropical front (STF), Antarctic circumpolar current (ACC) and Humboldt current (HC). Reconstructions of the most likely colonization routes followed by G. chilensis are shown as coloured arrows (route of colonization from New Zealand to Chatham Island in red and to Chile in blue). ...
Gracilaria chilensis is the main cultivated seaweed in Chile. The low genetic diversity observed in the Chilean population has been associated with the over‐exploitation of natural beds and/or the founder effect that occurred during the post‐glacial colonization from New Zealand. How these processes have affected its evolutionary trajectory before farming and incipient domestication is poorly understood. In this study, we used 2,232 SNPs to assess how the species evolutionary history in New Zealand (its region of origin), the founder effect linked to transoceanic dispersion and colonization of South America, and the recent over‐exploitation of natural populations have influenced the genetic architecture of G. chilensis in Chile. The contrasting patterns of genetic diversity and structure observed between the two main islands in New Zealand attest to the important effects of Quaternary glacial cycles on G. chilensis. Approximate Bayesian Computation analyses indicated that Chatham Island and South America were colonized independently near the end of the Last Glacial Maximum and emphasized the importance of coastal and oceanic currents during that period. Furthermore, ABC analyses inferred the existence of a recent and strong genetic bottleneck in Chile, matching the period of over‐exploitation of the natural beds during the 1970s, followed by rapid demographic expansion linked to active clonal propagation used in farming. Recurrent genetic bottlenecks strongly eroded the genetic diversity of G. chilensis prior to its cultivation, raising important challenges for the management of genetic resources in this incipiently domesticated species.
... The climatological-mean surface currents for the winter monsoon show the westward-flowing Northeast Monsoon Current (NMC; Tomczak and Godfrey 2003) south of India and Sri Lanka, which carries water from the Bay of Bengal towards the AS. Along its way, some fraction of the NMC splits off north-eastward toward the Indian Coast, flowing anticyclonically around the Laccadive High with its centre around 10° N, 70° E (Bruce et al. 1994). ...
High resolution mean surface velocity field of the Arabian Sea was derived by combining sea level anomalies based on satellite altimetry data with the surface drifter data. The mean current field exhibits the western boundary Somali Current, weak westward North Equatorial Current, weak West Indian Coastal Current. Besides strong currents, the Arabian Sea exhibits strong mesoscale eddy activity in the western side. The seasonal surface circulation changes of the Arabian Sea and variability associated with the Indian Ocean Dipole (IOD) events were determined. The Somali Current is northward during summer, with a speed above 1.8 m/s in July and it reverses towards south in winter. Significant changes were found in the flow pattern during IOD events. An anticyclonic gyre near the equator was a conspicuous feature during the positive IOD. Further, the Great Whirl shows an earlier formation and dissipation during the positive IOD. Notably, a delay of southward reversal of the Somali Current is found during the positive IOD. The southward flow of West Indian Coastal Current during summer monsoon is much stronger during the positive IOD and the northward flow in fall is prominent during negative IOD. The eastward Equatorial Jet is absent in fall and instead an opposite (westward) flow prevails during the positive IOD. Enhanced Chl-a concentration occurs during the negative IOD, compared to the positive IOD, especially in the western Arabian Sea. The wind-driven upwelling and mesoscale eddies are significantly modifying the Chl-a distribution along the Somali Coast.
... Wind and related current flow in the different hemispheres cause the cloct(wise circulation of the SASG, which represents roughly half of the South Atlantic, and the anti-clockwise circulation of the NASG, which represents just over a third of the North Atlantic ( Fig. 1.1a; Boltovskoy, 1999). Surface equatorial currents include four adjacent and counter-flowing currents Tchernia, 1980;Tomczak and Godfrey, 1994). ...
p>Routine measurements of the phytoplankton community from the first 10 cruises (1995 - 2000) of the Atlantic Meridional Transect (AMIT) programme are examined in the context of spatial and temporal variations in community composition within the subtropical (10 - 40°N/S) and tropical equatorial (10°N - 10°S) Atlantic Ocean. Measurements include size-fractionated chlorophyll a concentration, High-Performance-Liquid-Chromatography (HPLC) determined pigment signature concentration and large (>5 mm) phytoplankton species abundance and identification. Ancillary measurements (CTD, nutrient concentrations and light levels) are used to examine spatial and temporal variations in the hydrographic environment and provide a regional framework for analysis of variations in the phytoplankton community composition. Multivariate statistical analyses (Bray-Curtis similarity) of the phytoplankton community composition from (HPLC) pigment measurements and species identification show both latitudinal and vertical differences in composition; an upper and deep flora in subtropical and equatorial Atlantic waters with a reduction in depth differences in upwelling waters off NW Africa. Such spatial differences are associated with regional changes in the mechanisms important for the formation of the ubiquitous Chlorophyll a Maximum. Analysis of the time-series measurements collected in South Atlantic Subtropical Waters (0 - 30°S) show marked interannual (post-winter) differences in the depth of the mixed layer. Examination of the phytoplankton community in South Atlantic Subtropical Waters indicates interannual variations in the phytoplankton community composition. It is hypothesised that a decrease in winter mixing associated with climatic variability in the South Atlantic Ocean reduce seasonal new nutrient inputs and favour oligotrophic components of the phytoplankton community.</p
... The monsoon triggers seasonal reversing ocean currents, which are driven by southwestern winds from April to October (SW Summer Monsoon Current) and strong northeastern winds from December to March (NE Winter Monsoon Current). Under the influence of the SW monsoon, ocean currents flow predominantly to the east, while during the NE monsoon they flow predominantly to the west (Hydrographic Office, 2007;Schott and McCreary Jr., 2001;Shankar et al., 2002;Tomczak and Godfrey, 2003). The number of inter-atoll channels correlates with the intensity of the monsoon and decreases in southern direction (Gischler et al., 2013). ...
Wind-driven surface currents and deeper water-mass circulation shape carbonate sediment drifts in shallow- to deep-water depositional settings of the Maldives. Drift sediments are located in the platform interior, on top of drowned banks, and around the atolls. They are composed of an admixture of pelagic and shallow-water components. Drift bodies vary in dimension, architecture, and genesis. Small patch drifts and sheeted drifts are present in the platform interior and on top of drowned atolls; large patch drifts preferably form in the current-shadow of large atolls and banks. Small inter-atoll channels and large gateways are dominated by channel-related drifts. The Inner Sea, a large perched basin surrounded by atolls comprises a complex contourite depositional system of giant elongated mounded and sheeted drifts, mixed drifts, as well as delta drifts. This succession of different drift types is organized in sequences, delimited by unconformities caused by changes in the current regime. These may be triggered by a re-configuration of the inter-atoll gateways or by allo-cyclic processes such as climate variability. Effects based on to changes in the global water-mass circulation could not be proven. With exception of the delta drift, carbonate drift types in the Maldives are analogous to their siliciclastic counterparts. By contrast, however, these carbonate drifts are emplaced at a short distance to the shallow-water carbonate factory, which acts as main sediment source. Variability and abundance of drifts in the Maldives emphasize the significant role of current-controlled sediment transport and redistribution in carbonate platform development.
... The changes in the spatial location of the suitable habitat of D. gigas and T. murphyi might be related to the oceanic current conditions. The South Pacific Ocean is mainly driven by cyclonic circulation, and the most direct cause of this cyclone is the Peru-Humboldt current system (Tomczak and Godfrey, 2003;Martinez et al., 2009). In the waters of central and southern Chile, T. murphyi inhabits the waters with subtropical circulation in the South Pacific. ...
Understanding concurrent responses of habitat pattern of pelagic fish species to climate variability is favorable for sustainable exploitation and fisheries management. In this study, the key environmental factors affecting Dosidicus gigas (sea surface salinity (SSS), sea surface height anomaly (SSHA) and water temperature at 400 m (Temp_400m), and Trachurus murphyi (sea surface temperature (SST), mixed layer depth (MLD) and Temp_400m), were used in combination with the Pacific Decadal Oscillation (PDO) index to examine synchronous habitat variations off Chile based on habitat suitability index model (HSI). All environmental factors were significantly related to the PDO. A significantly negative relationship was found between the HSI of D.gigas and the PDO index, while a significantly positive correlation was observed in the HSI of T.murphyi. In the warm PDO regime, MLD was shallower, SST increased, and SSHA decreased from the northeast to the southwest off Chile. SSS and Temp_400m in northern waters off Chile were higher than those in southern waters. The suitable habitats of D.gigas contracted and shifted southwestward. While the area and distribution of T.murphyi changed little, but its habitat quality enhanced. In the cold PDO regime, SST and SSHA decreased, and MLD deepened. Variations in SSS and Temp_400m were consistent with those in the warm PDO regime. The suitable habitats of D.gigas enlarged and moved northeastward. Whereas the suitable habitats of T.murphyi slightly reduced with small change occurred with its spatial location. Our findings suggested that the PDO played important roles in the long-term concurrent habitat variations of Chilean T.murphyi and D.gigas.
... Shetye et al. (1994) and Banse and English (2000) pointed out the importance of the offshore transport mechanism that brings upwelled cold waters along the Arabian coast from 300 to 700 meters deep. During the Southwest Monsoon, the Somali, and East Arabian currents dominate the flow in the Arabian Sea with velocities between 0.5 to 0.8 m s -1 (Tomczak and Godfrey 1994). These currents flow northward along the Arabian coast and generate an offshore transport, which results in water convection from below the photic zone. ...
Background: Several authors have pointed out the relationship between seasonal changes of atmospheric winds, phytoplankton distribution and productivity in the Arabian Sea. The best example of such interaction is the western Arabian Sea region, which presents a large seasonal variation in temperature and chlorophyll because of the monsoons throughout the year. The main feature in the area is the strong western boundary current along the Somali and Arabian Peninsula coasts, which, with favorable winds, result in two important upwellings during the SW Monsoon. This work presents a characterization of the surface of the mixed layer along the Somali and Arabian coasts, based on temperature/salinity time diagram (TS-time diagram), temperature/ chlorophyll time diagram (TC-time diagrams) and harmonic analysis techniques. ABSTRACT Background: Several authors have pointed out the relationship between seasonal changes of atmospheric winds, phytoplankton distribution and productivity in the Arabian Sea. The best example of such interaction is the western Arabian Sea region, which presents a large seasonal variation in temperature and chlorophyll because of the monsoons throughout the year. The main feature in the area is the strong western boundary current along the Somali and Arabian Peninsula coasts, which, with favorable winds, result in two important upwellings during the SW Monsoon. This work presents a characterization of the surface of the mixed layer along the Somali and Arabian coasts, based on temperature/salinity time diagram (TS-time diagram), temperature/ chlorophyll time diagram (TC-time diagrams) and harmonic analysis techniques. Materials and Methods: Geographic distribution of amplitudes and phases of harmonic waves are generated for the chlorophyll, temperature, and salinity data, which are computed for an average year over a 5-degree grid along the Somali and Arabian coasts. They are based on the World Ocean Atlas Data Set and the AVHRR MCSST and CZCS phytoplankton pigment concentration data set. This classification includes the analysis of the annual and semi-annual amplitudes, plus multiple regression analysis between temperature, salinity, chlorophyll, Ekman pumping, components of the pseudo-stress wind and net-down-freshwater flow (ndff). To clarify the processes currently involved with the surface water properties, sea surface temperature (SST), sea surface salinity (SSS) and sea surface chlorophyll (SSC) are treated as pseudo-harmonic terms. Results: TC-time diagrams show the upwelling along the Arabian and Somali coasts. Two peaks of low temperature are observed during the Northeast and Southwest Monsoons. The highest chlorophyll concentration and low temperature are related to the upwelling that occurs in these areas during the SW Monsoon. The chlorophyll bloom starts in May and declines in October. The maximum chlorophyll values occur during August, when temperatures are close to 24 °C. Comparing the TC-time and TS-time diagrams for the same area, it can be observed that the highest chlorophyll concentration from the TC-time diagrams, during the SW Monsoon, is not only related to a low temperature, but also to low salinity values. The meridional component of pseudostress wind and wind magnitude play a major role on temperature variability along the Arabian coast, during the SW Monsoon. Conclusion: The application of time diagram technique has successfully identified the upwelling and the Arabian Sea Surface Water (ASSW) along Arabian and Somali coasts, as well as the geographical distribution of the seasonal cycles of temperature, salinity, and chlorophyll.
... The Southern Ocean (locations below 40°S) is made up of a complex of currents (Caccavo et al., 2018;Thompson et al., 2018). According to the positions of three fronts (the Subantarctic Front, Polar Front, and Continental Water Boundary), the Southern Ocean is divided into four distinct circular zones (Subantarctic, Polar Frontal, Antarctic and Continental Zone from north to south) around the Antarctic continent (Tomczak and Godfrey, 1994;Patmore et al., 2019). Previous studies have shown that the Subantarctic and Antarctic Zones have different zooplankton (including tintinnids) and phytoplankton compositions, divided by the Polar Front (David, 1955;Kane, 1966;Deacon, 1982;Liang et al., 2020). ...
Information on tintinnid horizontal distribution in the Antarctic Continental Zone is scarce. During the summer of 2019/2020, tintinnid diversity and horizontal distribution in surface waters were investigated in the Ross Sea and Amundsen Sea polynya. Eight tintinnid species were found and the dominant species showed obvious horizontal distribution characteristics. In the Ross Sea, three tintinnid community groups were identified. Cymatocylis cristallina and Laackmanniella prolongata (group I) were dominant species and were mainly distributed in stations closer to the coast than were species in the other two groups. Codonellopsis gaussi (group II) and Cy. convallaria (group III) were mainly distributed in nearshore and offshore stations, respectively. In the Amundsen Sea polynya, the dominant species Cy. cristallina, L. prolongata and Salpingella faurei (group I) were mainly distributed in stations closer to the coast than were species in the other two groups. Cy. convallaria (group III) was mainly distributed in offshore stations. The distribution area where C. gaussi and C. cristallina were found in high abundance and abundance proportion of loricae with protoplasts was divided by the approximate boundary of the Antarctic Slope Front Current and Coastal Current in the Ross Sea. The highest abundance proportion in the Ross Sea was the 32-36 μm lorica oral diameter (LOD) size class (75.7%), and the 36-40 μm LOD size class (56.0%) was found in the Amundsen Sea polynya. Temperature-salinity-plankton diagrams of the two seas revealed that temperature may be the main reason for species distribution. Our results contribute to a better understanding of horizontal distribution of the microbial food web, and serve as a baseline for future studies of pelagic community change in the Antarctic Continental Zone.
The cheilostome bryozoan Cauloramphus magnus is common in the rocky intertidal habitat from southeastern Alaska to northern Japan. We examined its phylogeography by analyzing 576 bp of the mitochondrial COI (cox1) gene sequenced for 298 colonies from 16 localities in northern Japan. A maximum-likelihood phylogeny detected three main clades (A, B, C). Clades A and B occurred throughout the study area but differed in frequency, haplotype diversity, and haplotype distribution; each resolved into three divergent subclades (AI-III, BI-III). Clade A shared none among 15 haplotypes between the Pacific and Sea of Japan sides of Hokkaido. In contrast, Clade B (29 haplotypes) was thrice as common as Clade A among samples, with haplotype B28 common on both sides. Divergent Clade C (nine haplotypes) was detected only at Rumoi. K2P divergences of 12.3-28.3% among Clades A-C suggest these are distinct biological species, a conclusion supported by different inferred evolutionary histories. A bPTP species delimitation analysis indicated nine phylogenetic species among the sequences included in our phylogeny (AI-III, BI-III, C, and one specimen each from Alaska and the Commander Islands), with K2P divergences of 3.9-6.5% among subclades in A or B. Statistical and principal components analyses suggested weak morphological differentiation between Clades A + B and C, although overlapping ranges of measurements preclude identification to clade; these three clades are morphologically cryptic. For taxonomy, we suggest retaining the name C. magnus for lineages within this species complex across its range, followed by a clade designation, if known.
The paper analyzes the Russian doctrine of international law and gives a critical assessment of its state, because the problems of Antarctica traditionally belong to international maritime law. However, the author defines the concept of international Antarctic law as an intersectoral international legal institution with a certain independence within the framework of international territorial law. It is noted that the problems of Antarctica have gone beyond the cognitive interest of the academic community, moving into the context of a clash of geopolitical interests among states, which caused the attempts of five states to establish their sovereignty by means of sectoral division. The Antarctic Treaty of 1959 established a notification procedure for the establishment of Antarctic stations, the modern administrative regime is provided by the activities of institutional mechanisms. With about 100 Antarctic stations and bases opened by States, their international legal status remains uncertain. The problem has acquired particular importance because Russian stations are located in the sectors of territorial claims, which generates competitive processes. By concluding bilateral agreements with States that have put forward territorial claims, it is possible to partially neutralize the competition of jurisdictions in certain areas of cooperation. In the documents adopted by the Antarctic Treaty Consultative Meetings (ATC), attention is paid only to the problems of the station placement and issues of their verification during inspection activities. In order to fill the gap, the author proposed the definition of the Antarctic station as a legal enclave of the State, which is subject to the exclusive jurisdiction of the founding State and establishes immunity from the jurisdiction of other states. The maintenance and development of Russia’s presence in Antarctica is ensured by Russian Antarctic legislation. In order to develop the Russian expeditionary infrastructure (including Antarctic stations and field bases), it is important to develop and adopt a regulation on the status of the Russian Antarctic station, in connection with which measures to improve legislation are proposed.
New Zealand is located in the southwest Pacific Ocean and is surrounded by a complex system of boundary currents that vary on a variety of time scales, with important impacts on weather, primary productivity, and fisheries. While various observational and modeling studies have shed light on many of the characteristics of New Zealand's ocean circulation, this study provides a comprehensive, quantitative, seamless, 3‐dimensional characterization of the region's boundary current circulation and ocean heat content. We use a 28 yr long hydrodynamic model simulation that accurately represents the mean and variability of the ocean circulation to characterize and quantify the temporal and spatial variability across the region. We show that low‐frequency variability in boundary current transport and upper ocean heat content are correlated over the entire New Zealand oceanic region. Oceanic eddies dominate heat content variability at intra‐annual scales in the northeast of the region, while on the west and southeast coasts heat content varies predominantly at interannual scales controlled by large‐scale changes in the subtropical and sub‐Antarctic fronts. The model shows a significant downstream strengthening and deepening of the boundary currents off northeast New Zealand which, if confirmed by observations, could suggest a significant new understanding of this Western Boundary Current system. This study presents a region‐wide characterization of the temporal and spatial variability of ocean currents and heat content, and their interconnections, providing a vital basis for understanding climate change impacts and the increasing occurrence of marine heatwaves.
The oxygen isotope method is probably the most widely used proxy of paleotemperature determination in the fossil record. The relationship as first proposed by Urey (1947) suggests that the ratio of ¹⁸ O to ¹⁶ O in the calcitic shells of fossils is proportional to temperature. This was subsequently confirmed by empirical studies (Epstein et al, 1951, Emiliani, 1954; 1955). However, Shackleton (1967), suggested on the basis of co-variance of benthonic and planktonic foraminifera, that the δ ¹⁸ O composition of seawater varied only as a function of glacial ice growth and decay. However, more recent studies have shown that there is still a residual temperature component in the δ ¹⁸ O variability of deep waters.
This study investigated the relationship between the distribution of modern ostracod biofacies and environmental factors in Lützow–Holm Bay, off Cape Darnley, and off Totten Glacier in East Antarctica. We collected study samples from water depths of 219 to 987 m by the 61st Japanese Antarctic Research Expedition. Nineteen species belonging to 13 genera and 47 species belonging to 31 genera of ostracods were found in three samples from Lützow–Holm Bay and ten samples from off Totten Glacier, respectively. We found no ostracods in the samples off Cape Darnley. Q-mode cluster analysis reveals four ostracod biofacies (A to D). Antarctiloxoconcha frigida (Neale, 1967) and Australicythere polylyca (Müller, 1908) were common under the influence of cold water in the upper bathyal zone (biofacies A to C). The genus Krithe was the most abundant taxon in biofacies D with low dissolved oxygen and high-water temperature (0.38°C, 34.66, and 5.0 ml/L, respectively), indicating the presence of warm deep seawater, i.e., modified Circumpolar Deep Water. Thus, we have checked the relationships between the ostracod assemblages and the environmental parameters analyzed in Lützow–Holm Bay and off Totten Glacier, and so strengthened the previous ostracod and environmental data.
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https://bioone.org/journals/paleontological-research/volume-27/issue-2/PR210033/Relationship-between-Modern-Deep-Sea-Ostracods-and-Water-Mass-Structure/10.2517/PR210033.short
University of Sri Jayawardenapura, Sri Lanka
Nitrous oxide (N2O) is a biogenic trace gas that has a significant role in global climate change, stratospheric chemistry and in the ocean nitrogen cycle. Its concentration in the ambient air has increased to the current value of 330 ppbv from 275 ppbv (pre-industrial period). The oceans are thought to account for 25-30 % of global N2O emissions. However, the biogeochemical pathways resulting in its formation are not well known. Two microbial pathways, nitrification and denitrification, dominate N2O production with their N2O source product varying with oxygen availability. There is a paucity of N2O data for many oceanic regions, and hence the global budget of N2O is not fully closed. This thesis describes the N2O distribution and its changes with AOU and nutrients along the selected regions in the Southwest Pacific Ocean (SWP) and Northeastern Arabian Sea (NEAS).
The comparison of the oxygen minimum zones (OMZs) in NEAS with minimum oxygen concentrations of > 10 μM and the SWP Ocean with minimum oxygen concentrations > 130 μM reveals significant differences in the N2O cycling of both the regions, which is reflected in N2O saturations, dual isotope ratios and isotopomers.
At coastal Otago Continental Shelf, N2O distribution was the highest during spring; [N2O], and saturations varied with MSTW > Neritic > SASW. In late autumn, an inverse trend in the distribution of N2O was observed. At the surface, saturations varied between 110 % - 130 % in spring, and it decreased below 100 % during autumn. The results indicate that the Otago coastal region is a source of atmospheric N2O. At the SWP open ocean stations, the minimum [N2O] was always found in the surface layer, with average N2O saturation values of 101 ± 1 % (winter), and 103 ± 1 % (spring) in the STSW, and 102.5 ± 0.5 % in the SASW. These values are similar to the global oceanic mean values (103.5%), derived by Bange et al. (2008).
At the NAES, surface mixed layers were poorly oxygenated (20 – 120 µM) relative to the SWP, with a strong oxygen minimum zone (OMZ) present below the mixed layer (25 - 1000 m). The N2O water column distribution showed a single peak structure, with only one broad maximum at mid-depths. The surface saturations are 2 - 4 times higher than the SWP saturations at NAES. N2O sea to air (Fs-a) fluxes indicates that the SWP and NEAS is a source of N2O to the atmosphere, though the extent of the fluxes varies regionally and seasonally.
In SWP, below the surface mixed layer [N2O] varied with depth. In the upper thermocline [DO] decreased below that of the surface water whereas [N2O] increased. Beneath the upper thermocline [N2O] in the AAIW increased coincident with an increase in [DO] except at the subantarctic SWP. The maximum [N2O] was found in the CPDW where DO was the minimum. At NEAS N2O saturation were 220 - 630 % in intermediate water (ICW) and 330-390 % in AAIW.
A [DO] vs [N2O] inverse relationship and ∆N2O vs AOU positive correlation observed in the SWP as evidence for nitrification as the major formation pathway of N2O. Positive correlations between ∆N2O and nitrate (NO3-) provides further evidence for the nitrification process being the primary source of N2O. For the NAES, ∆N2O vs AOU and ∆N2O vs nitrate suggests formation primarily via nitrification.
Stable isotopes and isotopomers of N2O provided more insight into the N2O formation pathways. The depletions in δ 15Nbulk and δ18O in the SWP surface mixed layer, minima in the subsurface, and enrichment at the bottom suggest nitrification, except in the subsurface 200-500 m. The NAES dual isotopes reflect the major role of nitrification especially in the surface and in the OMZ. These different oxygen isotope results suggest oxidation of hydroxylamine (NH2OH) followed by nitric oxide (NO) oxidation (during nitrification) at all depths in the SWP (except at 200-500 m) and NEAS. To examine the formation processes, Δ18O was also determined (δ18ON2O - δ18O of DO). Δ18O was almost constant at all depths for SWP waters, while it showed a minimum (roughly 9 ‰ lower than waters above and below) at 200-500 m except in subantarctic SWP waters. This observation proves the additional contribution to N2O source from nitrifier denitrification at 200-500m in the SWP (except in the subantarctic) and throughout the OMZ in the NEAS. The intramolecular distribution of isotopomers of 15N in N2O and S.P were also supportive of these findings.15N isotope labelled incubation experiments using 15NH4Cl and K15NO3 for the selected stations of Otago Continental Shelf transect also indicated that ammonium oxidation is the major process responsible for the production of N2O
The variability of Indian Ocean shallow meridional overturning circulation (SMOC) is studied using the century long ocean reanalysis simple ocean data assimilation (SODA) data. Though SMOC exhibits stronger southward transport during boreal summer, it displays stronger variability during boreal winter. The spectrum analysis of winter SMOC index reveals presence of highest amplitude between 5 to 7 years at 95% confidence level, suggesting the dominance of intra-decadal SMOC variability. The robustness of intra-decadal SMOC variability is also confirmed in different ocean reanalysis data sets. Composite analysis of filtered upper Ocean Heat Content, sea level, thermocline depth and Sea Surface Temperature anomalies for strong (weak) SMOC years show negative (positive) anomaly over north and East of Madagascar. Correlation analysis, of filtered SMOC index and sea level pressure (zonal winds) over the India Ocean, found significant negative (positive) correlation coefficient north of 40 °S (around 10 °S) and significant positive (negative) correlation coefficient over the 45 °S to 70 °S (20 °S to 50 °S and north of 5 °S). This meridional pattern of correlation coefficient for sea level pressure, manifesting the out of phase relationship between sub-tropics and high latitude mean sea level pressure, resembles with Southern Annular Mode (SAM). We conclude that the intra-decadal variability of mean sea level pressure leads to zonal wind variation around 10 °S modulating SMOC, which in turn affects the upper ocean thermal properties in the east and north of Madagascar. This study for the first time brought out coherent intra-decadal evolution of SAM and SMOC during boreal winter.
We have measured Cd/Ca ratios of several benthic foraminiferal species and studied benthic foraminiferal assemblages on two cores from the northern Indian Ocean (Arabian Sea and northern Bay of Bengal, BoB), in order to reconstruct variations in intermediate-water circulation and paleo-nutrient content since the last deglaciation. Intermediate water Cdw records estimated from the benthic Cd/Ca reflect past changes in surface productivity and/or intermediate–bottom-water ventilation. The benthic foraminiferal assemblages are consistent with the geochemical data. These results suggest that during the last deglaciation, Cdw variability was primarily driven by changes in intermediate-water properties, indicating an enhanced ventilation of intermediate–bottom water masses during both Heinrich Stadial 1 and the Younger Dryas (HS1 and YD, respectively). During the Holocene, however, surface primary productivity appears to have influenced Cdw more than intermediate water mass properties. This is evident during the early Holocene (from 10 to 6 cal ka) when benthic foraminiferal assemblages indicate that surface primary productivity was low, resulting in low intermediate-water Cdw at both sites. Then, from ∼ 5.2 to 2.4 cal ka, surface productivity increased markedly, causing a significant increase in the intermediate-water Cdw in the southeastern Arabian Sea and the northeastern BoB. The comparison of intermediate-water Cdw records with previous reconstructions of past Indian monsoon evolution during the Holocene suggests a direct control of intermediate-water Cdw by monsoon-induced changes in upper-water stratification and surface primary productivity.
The choice of vertical mixing scheme in ocean models plays an important role in modeling the surface and subsurface circulation and the vertical structure. This work performs a comparative study between K‐profile parametrization (KPP) and k‐ϵ mixing schemes for a regional domain in the Bay of Bengal (BoB) using the Modular Ocean Model version 5 (MOM5). It is observed that sea surface temperature (SST) and the mixed layer depth (MLD) show significant improvement with the k‐ϵ mixing scheme. Energetic analysis shows that changes in the viscous dissipation and turbulent buoyancy flux are the primary reason for improvement with k‐ϵ. The overestimation of viscous dissipation in the KPP scheme is corrected by k‐ϵ, resulting in a deeper mixed layer closer to observations. The tendency of buoyancy flux to retain stability in the water column also results in a better representation of SST in k‐ϵ. Overall, we conclude that the k‐ϵ mixing scheme works better for the BoB region.
The Arabian Sea (AS) is known to be one of the most productive regions of the world ocean with very high organic carbon (OC). The very high OC in the sediments of the eastern AS (western margin of India) are well debated with hypothesis revolving around high primary productivity and preservation due to the oxygen minimum zone (OMZ). Studies on sediment OC from the AS date back to the 1930s. The papers published from 1960 to 1970 were mostly focused on fisheries and carbon productions. From 1970 to 1980, papers published were on the data collected during International Indian Ocean Expedition (IIOE), with less focus on the western margin of India. The researchers observed high OC in the sediments covering large areas and suggested productivity as the major reason. It was only after the 1980s that OC preservation was related to the OMZ. After 1990 there came two strong schools of thought, where one supported productivity and the other preservation. Subsequently, factors such as slope width and downslope transport were introduced. Studies on OC content in sediment cores started only after the 1990s aiming to look into changes in the OC content and to infer past productivity. The major observation on OC distribution in sediment cores was that the OC content during Glacial and interglacial times is not uniform throughout the Arabian Sea. The richness of OC in sediment cores affected the extent and intensity of early diagenesis is in sediments within the OMZ and those in the oxygenated waters. High OC was observed to have an effect on magnetic minerals in sediment cores. Here, we compile the studies on OC distribution from the AS, especially the western margin of India, and put together the processes responsible for high OC in the sediments. We suggest that the Arabian Sea is complex in physiology, hydrodynamics, and circulation (atmospheric and ocean) patterns that lead to multifaceted conditions involving differential productivity and accumulation and, preservation and productivity are insufficient to explain the OC distribution.
The renewal of the deep North Atlantic by the various overflows of the Greenland-Scotland ridges is only one manifestation of the convective and mixing processes which occur in the various basins and shelf areas to the north: the Arctic Ocean and the Greenland, Iceland, and Norwegian seas, collectively called the Arctic Mediterranean. The traditional site of deep ventilation for these basins is the Greenland Sea, but a growing body of evidence also points to the Arctic Ocean as a major source of deep water. This deep water is relatively warm and saline, and it appears to be a mixture of dense, brine-enriched shelf water with intermediate strata in the Arctic Ocean. The deep water exits the Arctic Ocean along the Greenland slope to mix with the Greenland Sea deep water. Conversely, very cold low-salinity deep water from the Greenland Sea enters the Arctic Ocean west of Spitsbergen. Within the Arctic Ocean, the Lomonosov Ridge excludes the Greenland Sea deep water from the Canadian Basin, leaving the latter warm, saline, and rich in silica. In general, the entire deep-water sphere of the Arctic Mediterranean is constrained by the Greenland-Scotland ridges to circulate internally. Therefore it is certain of the intermediate waters formed in the Greenland and Iceland seas which ventilate the North Atlantic. These waters have a very short residence time in their formation areas and are therefore able to rapidly transmit surface-induced signals into the deep North Atlantic.