[show abstract][hide abstract] ABSTRACT: The Agulhas Cur rent plays a crucial role in the thermohaline circulation through its leakage into the South Atlantic. Under both past and present climates, the trade winds and westerlies could have the ability to modulate the amount of Indian - Atlantic inflow. Compelling arguments have been put forward suggesting that trade winds alone have little impact on the magnitude of Agulhas l eakage. Here, employing three ocean models for robust analysis - a global coarse resolution, a regional eddy - permitting and a nested high - resolution eddy - resolving configuration - and systematically altering the position and intensity of the westerly wind belt in a series of sensitivity experiments, it is shown that the westerlies, in particular their intensity, control the leakage. Leakage resp onds proportionally to the westerlies intensity up to a certain point. Beyond this, through the adjustment of the large - scale circulation, energetic interactions occur between the Agulhas Return Current and the Antarctic Circumpolar Current that result in a state where leakage no longer increases. This adjustment takes place within 1 to 2 decades. Contrary to previous assertions, our results further show that an equatorward (poleward) shift in westerlies increases (decreases ) leakage. This occurs due to the redistribution of momentum input by the winds. It is concluded that the reported present - day leakage increase could therefore reflect an unadjusted oceanic response mainly to the strengthening westerlies over the last few decades.
Journal of Physical Oceanography 07/2013; · 3.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: All climate models predict a freshening of the North Atlantic at high latitude that may induce an abrupt change of the Atlantic Meridional Overturning Circulation (hereafter AMOC) if it resides in the bistable regime, where both a strong and a weak state coexist. The latter remains uncertain as there is no consensus among observations and ocean reanalyses, where the AMOC is bistable, versus most climate models that reproduce a mono-stable strong AMOC. A series of four hindcast simulations of the global ocean at 1/12° resolution, which is presently unique, are used to diagnose freshwater transport by the AMOC in the South Atlantic, an indicator of AMOC bistability. In all simulations, the AMOC resides in the bistable regime: it exports freshwater southward in the South Atlantic, implying a positive salt advection feedback that would act to amplify a decreasing trend in subarctic deep water formation as projected in climate scenarios.
Geophysical Research Letters. 01/2013; 40(12):3069-3073.
[show abstract][hide abstract] ABSTRACT: For more than fifty years, it has been generally accepted by
oceanographers that the Deep Western Boundary Current (DWBC) is the
principal conduit of recently-convected Labrador Sea Water (LSW)
exported from the high-latitude North Atlantic to the equator.
Supporting this supposition is observational evidence that the waters of
the DWBC have consistently greater equatorward velocities, higher
concentrations of passive tracers, and younger ages compared to ocean
interior waters. However, recent observations and simulations of floats
launched in the DWBC in the Labrador Sea show that most water parcels
are quickly ejected from the DWBC and follow instead interior pathways
to the subtropics. Here, we show that tracer observations from the last
three decades are compatible with the existence of both DWBC and
basin-interior export pathways. From analyses of observational data and
model output, we find that equatorward transport in the basin interior
is consistent with the large-scale vorticity balance at mid-depth.
Furthermore, from the modeling analysis we show that despite higher,
localized concentrations of tracer and particles in the DWBC, only 5% of
particles released in the Labrador Sea are transported from the subpolar
to subtropical gyre via a continuous DWBC pathway. Thus, the interior
pathway is a significant contributor to LSW export.
Geophysical Research Letters 12/2012; 39(24):24606-. · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: Variability of Antarctic Intermediate Water salinity in the South
Atlantic is investigated on interannual and intradecadal timescales.
Novel observations of slow, westward propagating salinity anomalies in
Argo data are presented. The features have no corresponding signal in
temperature and are anomalous in density. Analysis of 40 years of model
output supports the existence of these westward propagating salinity
anomalies and indicates that they are typical occurrences in time. The
features are intensified in a latitude band around 30°S associated
with the propagation of Agulhas rings. However, the features are much
larger than Agulhas rings and occur on decadal timescales in the model.
They propagate westward with speeds of 2.3 cm/s in observations, and 1.7
cm/s in model data. They are more consistent with planetary waves than
with the advection of large-scale salinity anomalies. The observation of
these features has implications for the interpretation of salinity
anomalies, such as the linking of hydrological cycle changes to salinity
Geophysical Research Letters 05/2012; 39(10):10605-. · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: Vast amounts of methane hydrates are potentially stored in sediments
along the continental margins, owing their stability to low temperature
- high pressure conditions. Global warming could destabilize these
hydrates and cause a release of methane (CH4) into the water column and
possibly the atmosphere. Since the Arctic has and will be warmed
considerably, Arctic bottom water temperatures play a key role in the
fate of gas hydrates. A hierarchy of ocean/sea-ice models has been
studied to understand the impact of warm inflowing Atlantic water and
the exchange of the Atlantic with the Arctic Ocean on the modulation of
bottom water temperatures on interannual to decadal timescales. The
future evolution projected by a climate model was analyzed and confirmed
strongest impact on shallow regions affected by Atlantic inflow. The
resulting warming is spatially inhomogeneous. Within the next 100 years,
the warming affects 25% of shallow and mid-depth regions containing
methane hydrates. Release of methane from thawing hydrates in these
areas could enhance ocean acidification and oxygen depletion in the
[show abstract][hide abstract] ABSTRACT: The concept of a spatially continuous western boundary current in the Mozambique Channel
has historically been based on erroneous interpretations of ships’ drift. Recent observations
have demonstrated that the circulation in the Channel is instead dominated by anti-cyclonic
eddies drifting poleward. It has therefore been suggested that no coherent Mozambique
Current exists at any time. However, satellite and other observations indicate that a continuous
current – not necessarily an inherent part of Mozambique Eddies – may at times be found
along the full Mozambican shelf break. Using a high-resolution, numerical model we have
demonstrated how such a feature may come about. In the model, a continuous current is a
highly irregularly occurring event, occurring about once per year, with an average duration of
only 9 days and with a vertical extent of about 800 m. Surface speeds may vary from 0.5 m/s to
1.5 m/s and the volume flux involved is about 10 Sv. The continuous current may occasionally
be important for the transport of biota along the continental shelf and slope.
South African Journal of Science - S AFR J SCI. 01/2012;
[show abstract][hide abstract] ABSTRACT: The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments - backed by palaeoceanographic and sustained modern observations - are required to establish firmly the role of the Agulhas system in a warming climate.
[show abstract][hide abstract] ABSTRACT: The transport of warm and salty Indian Ocean waters into the Atlantic Ocean-the Agulhas leakage-has a crucial role in the global oceanic circulation and thus the evolution of future climate. At present these waters provide the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation (MOC). There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages and model studies that the amount of Agulhas leakage and its corresponding effect on the MOC has been subject to substantial change, potentially linked to latitudinal shifts in the Southern Hemisphere westerlies. A progressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing, but because of the sparse observational records it has not been possible to determine whether there has been a concomitant response of Agulhas leakage. Here we present the results of a high-resolution ocean general circulation model to show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to the change in wind forcing. The increased leakage has contributed to the observed salinification of South Atlantic thermocline waters. Both model and historic measurements off South America suggest that the additional Indian Ocean waters have begun to invade the North Atlantic, with potential implications for the future evolution of the MOC.
[show abstract][hide abstract] ABSTRACT: Data sets from satellite observations and a nested high-resolution model are used to study a source region of the Agulhas Current. Altimeter-derived geostrophic surface currents are averaged over varying periods, providing evidence of the persistence of flow patterns in the extension of the southern branch of the East Madagascar Current (SEMC). South of Madagascar, the SEMC separates into one branch toward the Agulhas Current and into a second branch retroflecting and connecting to the Subtropical Indian Ocean Countercurrent (SICC). Good agreement is found between long-term mean patterns of observational and model dynamic heights. Two basic modes are identified in the SEMC extension, with anticyclonic motion favoring retroflection in the northern Mozambique Basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the SEMC flowing westward along the southern Madagascar slope. A cross-correlation sequence between model SEMC transports and the modal changes in the extension region displays a correlation at about 1-month lag which agrees with eddy propagation time from the SEMC to the outflow region. Mean model SEMC transports are determined using floats released at 21°S, and the contribution of the SEMC to the SICC is obtained using floats injected at 55°E with the model running backward. Almost half of the SEMC volume transport contributes to the Agulhas system, and about 40% of SICC water originates from the SEMC.
Journal of Geophysical Research 01/2009; 114(C01005). · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: The skill of numerical Lagrangian drifter trajectories in three numerical models is assessed by comparing these numerically obtained paths to the trajectories of drifting buoys in the real ocean. The skill assessment is performed using the two-sample Kolmogorov–Smirnov statistical test. To demonstrate the assessment procedure, it is applied to three different models of the Agulhas region. The test can either be performed using crossing positions of one-dimensional sections in order to test model performance in specific locations, or using the total two-dimensional data set of trajectories. The test yields four quantities: a binary decision of model skill, a confidence level which can be used as a measure of goodness-of-fit of the model, a test statistic which can be used to determine the sensitivity of the confidence level, and cumulative distribution functions that aid in the qualitative analysis. The ordering of models by their confidence levels is the same as the ordering based on the qualitative analysis, which suggests that the method is suited for model validation. Only one of the three models, a 1/10° two-way nested regional ocean model, might have skill in the Agulhas region. The other two models, a 1/2° global model and a 1/8° assimilative model, might have skill only on some sections in the region.
[show abstract][hide abstract] ABSTRACT: Time series of transports in the Agulhas region have been constructed by simulating Lagrangian drifter trajectories in a 1/10 degree two-way nested ocean model. Using these 34 year long time series it is shown that smaller (larger) Agulhas Current transport leads to larger (smaller) Indian-Atlantic inter-ocean exchange. When transport is low, the Agulhas Current detaches farther downstream from the African continental slope. Moreover, the lower inertia suppresses generation of anti-cyclonic vorticity. These two effects cause the Agulhas retroflection to move westward and enhance Agulhas leakage. In the model a 1 Sv decrease in Agulhas Current transport at 32°S results in a 0.7 ± 0.2 Sv increase in Agulhas leakage
Geophysical Research Letters 01/2009; · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Agulhas system at the interface between the Indian and Atlantic Ocean is an important region in the global oceanic circulation
with a recognized key role in global climate and climate change. The simulation of the Agulhas system was performed by a high-resolution
regional model nested in a global coarse-resolution ocean model. It is shown that this model simulates all characteristics
of the Agulhas regime in a highly realistic manner. Due to the two-way coupling of both models the importance of the Agulhas
leakage on the large-scale thermohaline circulation was demonstrated.
[show abstract][hide abstract] ABSTRACT: Predicting the evolution of climate over decadal timescales requires a quantitative understanding of the dynamics that govern the meridional overturning circulation (MOC). Comprehensive ocean measurement programmes aiming to monitor MOC variations have been established in the subtropical North Atlantic (RAPID, at latitude 26.5 degrees N, and MOVE, at latitude 16 degrees N) and show strong variability on intraseasonal to interannual timescales. Observational evidence of longer-term changes in MOC transport remains scarce, owing to infrequent sampling of transoceanic sections over past decades. Inferences based on long-term sea surface temperature records, however, supported by model simulations, suggest a variability with an amplitude of +/-1.5-3 Sv (1 Sv = 10(6) m(3) s(-1)) on decadal timescales in the subtropics. Such variability has been attributed to variations of deep water formation in the sub-arctic Atlantic, particularly the renewal rate of Labrador Sea Water. Here we present results from a model simulation that suggest an additional influence on decadal MOC variability having a Southern Hemisphere origin: dynamic signals originating in the Agulhas leakage region at the southern tip of Africa. These contribute a MOC signal in the tropical and subtropical North Atlantic that is of the same order of magnitude as the northern source. A complete rationalization of observed MOC changes therefore also requires consideration of signals arriving from the south.
[show abstract][hide abstract] ABSTRACT: The Antarctic Circumpolar Current (ACC) carries water freely around the whole continent of Antarctica, but not without obstructions. Some, such as the Drake Passage, constrict its path, while others, such as mid-ocean ridges, may induce meandering in the current's cores and may cause the genesis of mesoscale turbulence. It has recently been demonstrated that some regions that are only relatively shallow may also have a major effect on the flow patterns of the ACC. This is here shown to be particularly true for the Conrad Rise. Using the trajectories of surface drifters, altimetry and the simulated velocities from a numerical model, we show that the ACC bifurcates at the western side of this Rise. In this process it forms two intense jets with speeds up to 25 cms-1 at the two meridional extremities of the Rise with a relatively stagnant water body (0 - 5 cms-1) over the Rise itself. Preliminary results from a recent cruise provide compelling support for this portrayal. This current configuration may have important implications for the ecology of the region.
Geophysical Research Letters 10/2008; 35(20). · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: 1] The quantification of inter-ocean leakage from the South Indian to the South Atlantic Ocean is an important measure for the role of the Agulhas system in the global thermohaline circulation. To explore the specific role of mesoscale variability (such as Agulhas rings and Mozambique eddies) in this process a high-resolution model (based on NEMO-ORCA) for the Agulhas region has been set up. It is nested into a global coarse-resolution model. The high-resolution nest captures all salient features of the greater Agulhas region, including the upstream perturbations of the Agulhas Current and Natal Pulses along the African coast. A comparison of the inter-ocean exchange in the high-resolution nest with its coarse resolution counterpart reveals that the latter significantly over-estimates the amount of water flowing into the Atlantic Ocean, demonstrating the need to explicitly simulate the mesoscale features. A sensitivity experiment that excludes the upstream perturbations revealed no difference in the amount of inter-ocean exchange. However, the realistic representation of Agulhas rings and their drift path into the South Atlantic depends on the simulation of those upstream perturbations.
Geophysical Research Letters 01/2008; 35. · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: 1] Analyses of sea surface height (SSH) records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the North Atlantic subpolar gyre during the 1990s. Here we report hindcast simulations with high-resolution ocean circulation models that demonstrate a close correspondence of the SSH changes with the volume transport of the boundary current system in the Labrador Sea. The 1990s-decline, of about 15% of the long-term mean, appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the North Atlantic Oscillation (NAO). The changes in the subpolar gyre, as manifested in the deep western boundary current off Labrador, reverberate in the strength of the meridional overturning circulation (MOC) in the subtropical North Atlantic, suggesting the potential of a subpolar transport index as an element of a MOC monitoring system.
[show abstract][hide abstract] ABSTRACT: Analyses of ocean observations and model simulations suggest that there have been considerable changes in the thermohaline circulation (THC) during the last century. These changes are likely to be the result of natural multidecadal climate variability and are driven by low-frequency variations of the North Atlantic Oscillation (NAO) through changes in Labrador Sea convection. Indications of a sustained THC weakening are not seen during the last few decades. Instead, a strengthening since the 1980s is observed. The combined assessment of ocean hydrography data and model results indicates that the expected anthropogenic weak- ening of the THC will remain within the range of natural variability during the next several decades.
[show abstract][hide abstract] ABSTRACT: Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean are an important component of the global thermohaline circulation. Evidence exists that the variability in these exchanges, on both meso- and longer time scales, may significantly influence weather and climate patterns in the southern African region and the significance of these regional ocean–atmosphere interactions is discussed. Observations of the inter-ocean exchange are limited and it is necessary to augment these with estimates derived from models. As a first step in this direction, this study uses an eddy-permitting model to investigate the heat and volume transport in the oceanic region south of Africa and its variability on meso, seasonal and inter-annual time scales. On the annual mean, about (standard deviation ) of heat flows west into the South Atlantic across 20°E (longitude of Cape Agulhas, the southernmost point of Africa), with just over (standard deviation ) flowing north into the South Atlantic across 35°S. The seasonal variations in this transport are about 10% at 35°S in the South Atlantic and around 20% through 20°E; the model value of for summer (standard deviation ranging from in January to in March) appears consistent with respective estimates of 0.51 and derived from two WOCE summer cruises southwest of Cape Town to 45°S in 1990 and 1993. Volume transports of the Agulhas Current section through 35°S in the SW Indian Ocean range from 58 to in summer/autumn to 64– in winter/spring. The model results suggest that the inter-ocean exchange south of Africa is highly variable on seasonal through to interannual scales. If this variability is also the case in the real ocean (and the limited observations suggest that this is so), then there are likely to be significant implications for climate.
Deep Sea Research Part II: Topical Studies in Oceanography. 01/2003;