The South Atlantic connects the North Atlantic, Indian, Pacific and Southern Ocean circulations by channeling the upper and lower limb of the thermohaline circulation and being part of the wind-driven Southern Hemisphere supergyre. Local air-sea fluxes, interior mixing, and inter-basin exchange processes such as Agulhas leakage influence its northward heat and salt transport, with potential implications for the strength and stability of the Atlantic Meridional Overturning Circulation. Moreover, the South Atlantic features intricate regional ocean circulation patterns, such as the Benguela Upwelling System and the Brazil-Malvinas confluence zone. These highly productive ecosystems sustain diverse marine life and are of fundamental importance for regional fisheries. It is crucial to understand how natural variability and climate change alter South Atlantic dynamics and ecosystems. However, in situ observations are often too sparse in time to robustly infer trends, and model simulations are still showing contrasting trends.
We invite contributions that advance our understanding of the physical and biogeochemical processes governing South Atlantic regional dynamics, inter-basin exchanges, extremes and global impacts. These may cover short (e.g., seasonal) to very large (e.g., millennial) timescales and originate from observational, modelling, and paleo proxy work as well as from interdisciplinary approaches. We aim to promote discussions on future inclusive South Atlantic observing and modelling strategies.
The upper cell of the Atlantic meridional overturning circulation (AMOC) comprises a deep lower limb exporting North Atlantic deepwater (NADW) southward, fed by a shallow upper limb carrying interocean waters northward from the southern South Atlantic and thereby enabling continued NADW production. On decadal and longer timescales, high-latitude density anomalies affecting the production of NADW are expected to generate coherent overturning transport changes across latitudes in the Atlantic basin. This process calls for compensating effects in upstream components of the northward AMOC upper limb, wherein upper-ocean meridional transports are adjusted according to flow continuity principles, tending to even out mass imbalances.
Besides the South Atlantic functioning as a mediator of interocean exchanges that are crucial for driving and maintaining the AMOC, the subtropical South Atlantic is the only region where the total northward component of the AMOC upper limb is subjected to ocean interior dynamics along its interhemispheric trajectory — before being settled over the Atlantic western boundary, a narrow crossroad along which it proceeds up to the subpolar North Atlantic by delineating the American coastline. This results from the AMOC upper limb being incorporated into the anticyclonic South Atlantic subtropical gyre (SASG) at its origins. By crossing the subtropical South Atlantic and meeting the South American coast, the AMOC upper limb is ultimately decoupled from the SASG and placed over the South Atlantic western boundary — as the westward flow along the SASG northern boundary bifurcates meridionally, redistributing waters between these two major large-scale circulation systems.
This work aims to demonstrate that the singular dynamic setting of the South Atlantic general circulation, compared to its North Atlantic counterpart, makes it more vulnerable to large AMOC changes. The findings from two recent studies based on transient deglacial simulation results will be discussed, which suggest that the upper-ocean flow redistributions taking place over the South Atlantic western boundary are highly responsive to AMOC changes under pre-industrial paleocean dynamics, and thus have the potential to provide future insights into the degree and timescales over which overturning in the North Atlantic impacts adjacent ocean basins and vice versa.
This model-based perspective elucidates the link between the South Atlantic western boundary current system and the AMOC through the establishment of meridional connectivity of AMOC variability. By definition, the steady-state AMOC system is meridionally coherent. Questions that naturally arise are: How is AMOC meridional coherence transiently modulated? And how will this modulation process evolve under modern climate change conditions? These questions are particularly relevant for upcoming research efforts dedicated to understanding how the South Atlantic circulation is to be shaped by climate change.
How to cite: Marcello, F., Wainer, I., M. de Mahiques, M., and C. Bicego, M.: South Atlantic upper-ocean flow redistributions and their connection to overturning in the North Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3852, https://doi.org/10.5194/egusphere-egu25-3852, 2025.
We investigate mesoscale eddy effects on the ventilation timescales and pathways of South Atlantic Antarctic Intermediate Water (AAIW) using a high-resolution 1/10° eddy-resolving ocean model (Parallel Ocean Program) combined with a Lagrangian particle tracking algorithm (OceanParcels). AAIW sequesters a significant amount of anthropogenic carbon along its ventilation pathways through the eddy-rich Southern Ocean. The distribution of subduction zones along dynamic sections of the Antarctic Circumpolar Current, as well as the Malvinas Confluence Zone, indicates an influence of mesoscale eddies on ventilation, in addition to the zonally uniform Ekman transport. To identify eddy effects, we perform particle backtracking from the South Atlantic AAIW interior to the mixed layer, both with eddy-resolving model output velocity and its mean state, where mesoscale eddy velocities are absent. We characterise mean ages and ventilation pathways for South Atlantic AAIW originating from subduction zones located around the Drake Passage, the South Atlantic, and the South Indian Ocean. Eddy effects increase the contribution of Drake Passage waters to the South Atlantic AAIW and reduce mean age estimates, attributable to accelerated advection together with deeper and more southward subduction below the mixed layer, near the Malvinas Confluence Zone. We highlight the role of eddies in the ventilation of the South Atlantic pycnocline, which, if accurately represented, increase estimates of ventilation rates and strengthen cold water inflow, enhancing Southern Ocean carbon and heat uptake in ocean models.
How to cite: Schäfers, S., Griesel, A., and Chouksey, M.: Role of mesoscale eddies in ventilation pathways of South Atlantic AAIW using Lagrangian backtracking, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10739, https://doi.org/10.5194/egusphere-egu25-10739, 2025.
Agulhas Current meanders, also known as Natal Pulses, are the dominant modes of variability within the Agulhas Current. These meanders significantly impact local hydrological dynamics and ecosystems. Previous observations and model outputs suggest that the influence of these meanders on Agulhas ring shedding is limited, while their effect on the Agulhas retroflection remains unclear. Models also imply that the Southern Ocean supergyre has more influence on Agulhas leakage variability than upstream drivers. Here, we developed an algorithm based on 28 years of daily-averaged Global Ocean Physics Reanalysis data with a spatial resolution of 0.083° × 0.083°. Using current velocities and sea surface height anomalies (SSHA), we tracked Agulhas Current meanders from the Agulhas Current Time-series Experiment (ACT) region to the Agulhas Return Current. Specifically, we examine the influence of the meanders on the westmost extent, i.e. minimum longitude, of the Agulhas retroflection on the basis that westwards excursions of the Agulhas current increase the probability of leakage over a longer path through this region. This is compared with the potential influence of the Subtropical Front (STF) which is the southmost front of the Southern Ocean supergyre. Our algorithm detects between 1-6 meanders per year in the ACT region, with an increasing trend over the 28-year period. These are more than reported by recent studies. More work is required to understand why we identify more meanders in the reanalysis than was found from in situ and satellite observations, however the increasing trend in the number of meanders is similar. The meanders correspond to negative SSHA propagating along the mean Agulhas Current path that is well defined down to Agulhas Bank at 22O E. The maximum lagged correlation coefficient between the minimum longitude of the Agulhas retroflection and 22O E SSHA is –0.23, which is three times higher than that between the minimum longitude of the retroflection and the subtropical front. This suggests that meanders exert a stronger influence on the zonal movement of the Agulhas retroflection compared to the STF. Additionally, the 10-day lagged correlation coefficient of SSHA between the Agulhas retroflection and the return current is 0.18, indicating that Agulhas meanders also have significant impact on the return current. These findings support the hypothesis that Agulhas meanders play an important role in shifting the position of the Agulhas retroflection. Finally, we quantify heat loss along the lagrangain trajectories in the Agulhas retroflection region to evaluate the likelihood of greater leakage for longer trajectories.
How to cite: Shen, S., Lenn, Y.-D., Donohue, K., and Beal, L.: Agulhas Meanders vs. Subtropical Front: Influence on Retroflection path over 28 years, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-358, https://doi.org/10.5194/egusphere-egu25-358, 2025.
Agulhas Rings are anticyclonic, warm-core eddies that play a crucial role in the exchange of water masses between the Indian and Atlantic Oceans. Formed at the Agulhas Retroflection near the southern tip of Africa, these rings constitute an essential component of the global thermohaline circulation, transporting ocean properties such as heat, salt, and energy. Their movement and property transfer significantly influence regional climate systems and large-scale ocean dynamics. It is well established that Agulhas Rings differ significantly in their characteristics. However, certain types, such as subsurface-intensified Agulhas Rings, remain remarkably understudied and are poorly represented in most ocean models. Investigations of these features often require high-resolution observational and modelling approaches. In this study, we present high-resolution glider observations from March/April 2021 and 2022 of two types of Agulhas Rings (surface- and subsurface-intensified), highlighting their distinct characteristics. Both glider campaigns utilized a microstructure probe to enable detailed observations of energy dissipation and diapycnal mixing. Our analysis reveals that major differences between the two eddy types occur near the eddy centre, where the subsurface-intensified Agulhas Ring exhibits elevated energy dissipation (log10(ε)= -7 W kg-1) and diapycnal mixing (log10 (κρ)= -4 m2s -1) beneath the surface mixed layer. Further analysis shows that mixing length scales (up to 18 km), are also elevated within the sub-surface intensified eddy, suggesting enhanced vertical and lateral distribution of ocean properties. These findings indicate a faster decay of the sub-surface intensified eddy and thus suggest a more local impact compared with the potentially longer-lived surface intensified eddy. By highlighting the distinct oceanic and energetic characteristics of surface- and subsurface-intensified Agulhas Rings, this study contributes to a better understanding of their role in influencing thermohaline structure and redistributing energy. These findings provide valuable insights that can support the development of more accurate parameterizations in future ocean models.
How to cite: Oelerich, R., Walter, M., and Bachmayer, R.: Energy dissipation and mixing within surface- and sub-surface intensified Agulhas Rings, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1410, https://doi.org/10.5194/egusphere-egu25-1410, 2025.
The Cape Basin is a highly dynamic region subjected to ocean ventilation and deepwater mass formation. Flowtopography interactions along the Agulhas Retroflection produce standing meanders with elevated Eddy Kinetic Energy and modified frontal structures. While numerical studies have shown that these features enhance deep water mass formation and tracer stirring, observational data on these processes have been limited due to their fine spatial and temporal scales. This study combines high-resolution Seaglider measurements of Apparent Oxygen Utilization (AOU) with backscatter data to map ventilation pathways. Results demonstrate the transport of low AOU values to depth via advection and stirring along isopycnals, as well as across-isopycnal transport near ocean fronts with strong buoyancy gradients and elevated diapycnal spiciness curvature. These findings provide critical observational evidence for the role of submesoscale processes in deep water mass formation and their broader implications for global climate dynamics and ocean circulation.
How to cite: Koets, R., Swart, S., du Plessis, M., and Donohue, K.: Drivers of ventilation in the CapeBasin using Apparent OxygenUtilization (AOU) as a tracer., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-429, https://doi.org/10.5194/egusphere-egu25-429, 2025.
Despite recognition of the South Atlantic Ocean as a significant CO₂ sink, the variability of carbon fluxes (FCO₂) at small temporal and spatial scales remains poorly understood. This gap is especially evident in transitional regions like the Cape Basin, where mesoscale oceanic features and localized atmospheric processes strongly influence ocean-atmosphere CO₂ exchange. Current observational and modeling approaches lack the resolution to capture these fine-scale fluctuations, potentially biasing global CO₂ flux estimates. To address this, our study examines short-term (1–10 days) and small-scale (0.1–10 km) drivers of CO₂ flux variability in the Cape Basin using 2-month high-resolution time-series data from a Wave Glider during late summer. By decomposing the carbon flux equation and applying Reynolds decomposition, we show that wind-driven gas transfer velocity (Kw) dominates the periods of enhanced FCO2, accounting for approximately 78% of flux variability on average in the Cape Basin. A secondary contribution of 11% comes from changes in the air-sea pCO₂ gradient (ΔpCO₂). . However, during localized periods - over the course of hours to days - mesoscale eddies and fronts enhance the ΔpCO₂ to the order of 60 µatm. During moderate winds (5 m s-1 < U10 < 15 m s-1), this sets ΔpCO2 as the dominant driver in FCO2 variability (>50%). However, when U10 < 15 m s-1, Kw dominates variability of FCO2 irrespective of ΔpCO₂. These findings underscore the importance of small-scale ocean processes in CO₂ exchange, their nuanced relationship with wind speed - dominated by large scale extratropical cyclones in the Cape Basin - and the need for high-resolution observations to improve global flux estimates.
How to cite: Ruiz Gomez, G., Swart, S., Du plessis, M., and Nicholson, S.-A.: Drivers of CO2 flux variability in the Cape Basin, South Africa., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-364, https://doi.org/10.5194/egusphere-egu25-364, 2025.
Small-scale variability is essential to understanding ocean circulation, air-sea interactions, and biogeochemical processes. Yet, current satellite-derived sea surface salinity (SSS) data can only resolve features larger than 40 km. This study aims to capture smaller scale variability (≤25 km) by reconstructing SSS data from satellite observations. The focus is on the Brazil-Malvinas Confluence (BMC), a biologically productive region characterized by intense mesoscale structures and associated with strong SSS gradients primarily due to the discharge of the La Plata River. A Lagrangian reconstruction method is employed to advect satellite SSS fields by altimetric geostrophic currents to capture smaller-scale details. These reconstructed fields are validated against in-situ salinity measurements from thermosalinographs. Results show that the reconstructed fields successfully capture smaller scale features observed in the region. This approach seeks to enhance the effective resolution of SSS data, overcoming the limitations of current satellite observations.
How to cite: Combes, V., Martí-Solana, C., and Barceló-Llull, B.: Detecting Salinity Fronts From Satellite Observations in the Brazil-Malvinas Confluence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1530, https://doi.org/10.5194/egusphere-egu25-1530, 2025.
The Eastern Boundary Upwelling system of the Southeastern Tropical Atlantic (SETA) is of great socioeconomic importance for local communities since it supports highly productive fisheries and diverse marine ecosystems. Comprehending the local processes that shape the physical characteristics of this system is thus crucial. The SETA is characterized by a strong meridional sea surface temperature (SST) gradient and is influenced by a large freshwater input from land mainly due to Congo River discharge. Here, we use high-resolution ocean model sensitivity experiments to show the impacts of the freshwater discharge from rivers on the mean state SST and the dynamics of this coastal region. By comparing experiments with and without river discharge we find that the freshwater presence increases the mean state coastal SST by up to 0.9ºC from 6ºS to 25ºS, while reducing the SST by more than 1ºC from 6ºS to 3ºS. These changes are associated with a halosteric effect of an elevated sea surface due to the lower sea surface salinity, leading to strong pressure gradients that drive upwelling and downwelling processes north and south of the Congo River mouth at 6ºS, respectively. Alongshore horizontal temperature advection also related to the sea surface height gradients plays likewise an important role in warming (cooling) the SST mean state south (north) of 6ºS. Ultimately, the change in coastal currents pushes the meridional temperature gradient further south. These results highlight the influence of freshwater input on SST and ocean surface dynamics, particularly relevant in the context of projected climate change, suggesting a future increase in Congo River discharge.
How to cite: Costa Aroucha, L., Lübbecke, J., Brandt, P., Schwarzkopf, F., and Biastoch, A.: Southeastern Tropical Atlantic Coastal Dynamics and Sea Surface Temperature affected by River Discharge: insights from modelling., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1289, https://doi.org/10.5194/egusphere-egu25-1289, 2025.
Cold-water corals and sponges form iconic and globally occurring benthic communities, provide important habitats for a diverse associated fauna and thrive in environmental conditions with often large temporal and spatial variations in near-bottom currents, food availability and other environmental parameters. We investigate the variability of near‐bottom currents and physical processes from simulations with a nested hydrodynamic modelling framework at two seamounts rich in benthic fauna along the Northeast Walvis Ridge, Valdivia Bank and Ewing Seamount. Our aim is to obtain new insights on physical drivers of observed occurrences and distribution of benthic suspension feeders (cnidarians and sponges) in this data‐poor area. We use dynamic downscaling of high-resolution implementations of the ROMS-AGRIF model in combination with high-resolution bathymetry and open boundary forcing from the basin-scale model INALT20 and the OSU inverse tidal model to explore the fine-scale physical processes and mechanisms that potentially drive a continuous or episodic food supply to the benthic communities. Over a three-year period, we analysed how near-bottom currents vary in space and time and assess potential connections between the distribution of filter-feeding fauna and the surrounding physical marine environment. We identified a close link between flow dynamics, internal tide dynamics and faunal species distributions. We propose that physical processes such as kinetic energy dissipation and internal wave dynamics could serve as functional indicators of food supply and particle encounter rates in future species distribution and habitat suitability models for important deep-sea taxa, such as those that represent vulnerable marine ecosystems. Our results also show little impact of mesoscale eddies from the Agulhas Leakage as they propagate north-westward into the southeast Atlantic along a well-defined corridor, which only occasionally extends as far north as the Valdivia Bank and Ewing Seamount.
How to cite: Mohn, C., Schwarzkopf, F. U., Jiménez García, P., Orejas, C., Huvenne, V. A. I., Schumacher, M., Pérez-Rodríguez, I., Sarralde Vizuete, R., López-Abellán, L. J., Dale, A. C., Devey, C., Hansen, J. L. S., Møller, E. F., and Biastoch, A.: Dynamics of Near-Bottom Currents in Cold-Water Coral and Sponge Areas along the Walvis Ridge., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16401, https://doi.org/10.5194/egusphere-egu25-16401, 2025.
Simple box models of the Atlantic Meridional Overturning Circulation (AMOC) often rely on ad-hoc scaling laws that link AMOC strength to the meridional density gradient. In contrast, Global Climate Models (GCMs) provide more comprehensive simulations but demand substantial computational resources. To evaluate the validity of these scaling laws, we develop a simplified AMOC model that represents the circulation as a geostrophically balanced flow confined to the western boundary of the Atlantic basin. Basin-wide pressure gradients are driven by mixing along continental boundaries and wind-driven upwelling in the Southern Ocean. We explore the limiting cases of quasi-adiabatic and diffusive overturning circulations, deriving corresponding scaling laws for AMOC strength. Given the critical influence of these scaling laws on AMOC stability, we examine how stability depends on the dominant driving mechanism—either diffusive mixing or adiabatic upwelling. This analysis aims to identify GCM biases that could significantly affect AMOC stability and must be addressed to accurately assess the risk of an AMOC collapse in the coming century.
How to cite: Vanderborght, E. and Dijkstra, H.: A Mechanical Model for the Inter-Hemispheric Overturning Circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16235, https://doi.org/10.5194/egusphere-egu25-16235, 2025.
The ocean is a chaotic system where the presence of mesoscale eddies makes the understanding of basin or global scale flows difficult. However, the pressures on the continental shelves (boundary pressures) are only weakly influenced by eddies and, in most places, directly reflect the global scale processes. In this study, we use depth-integrated boundary pressure as a constraint to estimate the total upwelling in the Indian and Pacific oceans. The two main factors that determine the difference in the depth-integrated pressure between the east Pacific and the east Atlantic are the winds and the upwelling. Calculations from reduced-order theoretical models and diagnostics from 1/12th degree NEMO simulation show that, given the winds and boundary pressures, we can infer upwelling/downwelling in each ocean basin with errors of order 1 Sv. We apply this method using observations near eastern boundaries to put constraints on the total upwelling in the Pacific and Indian oceans.
How to cite: Gururaj, S., Hughes, C., and Bingham, R.: Constraining the overturning in the Pacific and Indian oceans by using boundary pressures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11518, https://doi.org/10.5194/egusphere-egu25-11518, 2025.
The Atlantic meridional overturning circulation (AMOC) is a key feature of the global ocean circulation and has a big impact on regional weather and global climate. To project future AMOC variability, it is vital to understand and properly assess the past variability. The AMOC transport and in particular its geostrophic component is measured at several latitudes by specific observing systems. However, the transport can be calculated using multiple combinations of instruments which complicates both the comparison of the observed AMOC at different latitudes and to model transports.
Here, we present results from a systematic comparison of methods to compute the upper branch of the AMOC at 11°S, utilizing data from the Tropical Atlantic Circulation and Overturning at 11°S (TRACOS) array. The TRACOS array comprises 10 years of observations, including data from Pressure Inverted Echo Sounders, tall moorings, and ship sections at the eastern and western boundaries as well as supplementary data from Argo floats and satellites. By subsampling the observational setup in two ocean models (INALT20 and VIKING20X, both based on NEMO), we quantify uncertainties in AMOC transport estimates on different time scales.
We find that bottom pressure measurements, despite being prone to sensor drifts, effectively capture the seasonal variability. The largest potential source of error lies in the choice of vertical structure between the measurement points. Longer-term variability assessments based on moored density measurements require particularly high vertical resolution in the upper 500m. For both methods, the error associated with replacing the eastern boundary data with a climatological seasonal cycle is small, indicating low uncertainty resulting from loss of instruments in that region. We also consider uncertainties in the Ekman transport, which has a magnitude of about one-third of the geostrophic transport at 11°S. Ekman transport estimates vary by a few Sverdrups depending on the choice of wind stress product or drag coefficient used. All in all, we find that the TRACOS array can assess AMOC signals but we also show potential improvements in the array design to reduce uncertainties regarding longer-term variability.
How to cite: Hans, A. C., Hummels, R., Brandt, P., Juricke, S., and Schwarzkopf, F.: Uncertainty evaluation of the AMOC transport calculation at 11°S, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13885, https://doi.org/10.5194/egusphere-egu25-13885, 2025.
The circulation of the tropical Atlantic is a complex superposition of thermohaline and wind-driven flows. The zonally integrated meridional flow is associated with the Atlantic Meridional Overturning Circulation (AMOC) — a major component of the global climate system. In the tropics, the northward, upper branch of the AMOC flow is superimposed by the shallower overturning associated with the wind-driven Subtropical cells (STC).
The western boundary observing system of the TRACOS (Tropical Atlantic Circulation & Overturning at 11°S) array consists of four tall moorings monitoring the strong western boundary current system – more specifically, the North Brazil Undercurrent (NBUC) and the Deep Western Boundary Current (DWBC) as part of the AMOC, the STC, and the wind-driven gyre system. More than 15 years of moored current meter observations, collected between 2000 and 2004 and continuously since 2013 within the NBUC and DWBC, are analyzed for their variability across different time scales. NBUC and DWBC transports are calculated from alongshore velocity data by regressing current meter observations onto variability patterns. These patterns are obtained from 15 ship sections, derived by combining underway vessel mounted ADCP measurements with on-station lowered ADCP measurements to provide full-depth alongshore velocity fields. The NBUC transport is dominated by a seasonal signal with only minor interannual variations and no obvious trend, while the DWBC is dominated by strong intra-seasonal variability induced by the passage of deep eddies. Despite these deep eddies being evident throughout the DWBC transport time series, longer term variability is also becoming evident as the time series is prolonging.
How to cite: Hummels, R., Brandt, P., Dengler, M., and Hans, A. C.: Variability of the Western Boundary Current System at 11°S, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13966, https://doi.org/10.5194/egusphere-egu25-13966, 2025.
The Cape Basin is a turbulent region off the west coast of South Africa where warm and salty Indian Ocean waters mix with cooler, fresher South Atlantic waters. This injection of heat and salt is known as Agulhas leakage and has been tied to the Atlantic Meridional Overturning Circulation’s (AMOC) strength, stability, and variability. Paleoclimate data and model hindcasts point to an increase in Agulhas leakage as the climate warms, but quantifying real-world leakage is very difficult owing to its turbulent nature. This is further complicated by the basin’s four dynamical regions: eddies, eddy-eddy interactions, topography, and filamentation which can influence and transform the water masses through the water column. Their properties and circulation are critical to understand because they can influence where and how this leakage is transported into the South Atlantic. Using data from the ARGO float array, we identified six water masses in the region using neutral density and calculated their heat and salt transports along the leakage corridor. Tropical Surface Waters (TSW, <25.5), Subtropical Surface Waters (STSW, 25.5-26), South Atlantic Subtropical Mode Waters (SASTMW, 26-26.5), and the Upper North Atlantic Deep Waters (UNADW, 27.92-28.08) have a combined heat and salt transport of 6.85x10-11 PW and 11.8 kg/s respectively into the leakage corridor. South Indian Mode Waters (SICW, 26.5-27) and the Intermediate Waters (IW, 27-27.92) have a combined heat and salt transport of 3.36x10-11 PW and 47.6 kg/s respectively through the leakage corridor. As a result, approximately 30% of heat and 80% of salt are transported into the South Atlantic primarily from the SICW and the IW masses. Previous studies have primarily focused on the intermediate water masses as the contributor, yet our results show that Mode Waters can have a significant impact on Agulhas leakage’s transport and variability. To improve our understanding of Agulhas leakage and its impact on the AMOC, we must turn to improving our understanding on the basin’s seasonal variability and its potential mode water formation.
How to cite: Sampson, R., Beal, L., and Novelli, G.: Water Mass Transformation in the Cape Cauldron, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1135, https://doi.org/10.5194/egusphere-egu25-1135, 2025.
The Cape Basin off the western coast of South Africa is characterized by rich mesoscale and submesoscale variability generated by the shedding of eddies and filaments from the Agulhas Retroflection. These features carry warm, salty water into the cooler, fresher South Atlantic Ocean. Thermohaline interleaving is common in this region due to strong lateral water-mass gradients and the presence of stirring processes. These intrusions are an important pathway to water-mass transformation because they result in an increased surface area over which diapycnal mixing can work to homogenize water property contrasts. We present the first observational study that can track interleaving features at high vertical and horizontal resolution over distances of O(100 km) in the Cape Basin by using diapycnal spiciness curvature to detect intrusions within data collected by the Wire Flyer towed profiling vehicle and EM-APEX profiling floats. These datasets show interleaving features present throughout thermocline waters (σ = 26.0 – 27.5), but their scales and slopes vary significantly. Wire Flyer sections highlight strong differences in interleaving characteristics and generation mechanisms over distances of 10s of km. Several of the transects exhibit signatures of internal waves, which appear to modulate the interleaving structure. EM-APEX floats provide an alternative, semi-Lagrangian sampling perspective and show the persistence of interleaving features over time scales of 1-15 days. We conclude that the thermohaline variability in this region is primarily driven by mesoscale stirring, although double diffusion may be acting to grow the features after their formation. This study showcases the variety of physical processes existing at different length and time scales that contribute to the formation and structure of interleaving in the Cape Basin.
How to cite: Grose, L., Donohue, K., and Roman, C.: Patterns of thermohaline interleaving in the Cape Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3338, https://doi.org/10.5194/egusphere-egu25-3338, 2025.
Eastern Boundary Upwelling Systems, driven by wind-induced Ekman transport, bring cold, nutrient-rich deep waters to the surface, making them hotspots of biological activity with significant economic, ecological, and social value. Also contributing to the upwelling of deep waters, is the Ekman pumping, which is created by cyclonic surface wind stress curl (WSC). WSC further influences local circulation and the formation of upwelling hotspots, such as those found downwind of prominent capes. Accurately representing these processes is critical for predicting future changes in upwelling and their impacts on marine productivity, in particular for the Southern Benguela Upwelling (SBU), a region characterised by a complex coastal geometry and orography. Using a fine resolution (1 km), curvilinear grid regional numerical model of the SBU, based on CROCO, this work highlights the sensitivity of upwelling processes to the fine scale spatial wind variability, by using two different wind products which differ by their resolution: ERA5 (~30 km) and WASA3 (~3 km). The lack of coastal wind drop-off in the CROCO-ERA5 simulation results in more intense nearshore and less intense offshore upwelling than observed in the CROCO-WASA3 simulation. These differences in the spatial structure of the coastal upwelling impact the coastal circulation. The inshore branch of the equatorward flowing Benguela Jet is shifted offshore in the CROCO-WASA3 run, while a poleward coastal jet emerges with intermittency. A better understanding of these upwelling and current structures could provide new insights into the generation of harmful algal blooms in the SBU.
How to cite: Flügel, R., Fearon, G., Herbette, S., Treguier, A. M., and Veitch, J.: The Role of Fine-Scale Winds in Upwelling and Coastal Circulation in the Southern Benguela Upwelling System, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10536, https://doi.org/10.5194/egusphere-egu25-10536, 2025.
Living benthic foraminifera were studied in the highly productive West African upwelling system off Walvis Bay, Namibia. The Benguela Current System, influenced by the prevailing southeastern trade winds, governs the region's oceanography and is significantly impacted by climate variability. This results in seasonal upwelling, leading to high primary productivion and the formation of a Diatomaceous Mud Belt (DMB). Inside this DMB anoxic and/or hypoxic conditions prevail between 150 and 450 m depths, impacting benthic foraminiferal communities (Inthorn et al., 2006). Our aims were to establish the vertical distribution patterns of foraminiferal species in the sediment (microhabitats) at each sampling station and to examine the ecological responses of foraminifera to varying oxygen conditions.
Sediment samples from NIOZ cruises 64PE449 and 64PE450 obtained by multicorer were analysed for living foraminifera identified by Rose Bengal staining. Subsamples were collected at 0.5 cm intervals from the top 2 cm of each sediment core and at 1 cm intervals from 2 to 10 cm depth. Isotopic analysis (δ¹³C) was conducted using IR-MS on cleaned specimens (20–45 µg) after ultrasonic washing to remove clays, using NFHS1 and NBS19 standards for calibration.
At the two deeper stations, at 750 and 324 m depth, bottom water oxygen concentrations were 105 and 44 µMol/L, respectively. The benthic foraminiferal assemblages were diverse and were largely limited to the top 1.5 cm of the sediment. Interspecific differences in microhabitat were limited.
At the six stations positioned around the 100 m isobath, oxygen concentrations varied between 3 and 20 µMol/L. The faunal diversity was much lower, with only five species being recorded: Bulimina elongata, Bolivina pacifica, Fursenkoina complanata, Nonionella stella, and Virgulinella fragilis. The assemblages were always strongly dominated by one or two species. At these stations, faunal penetration into the sediment was much larger, at some stations until 10 cm, again with rather limited interspecific differences in microhabitat. Remarkably, at three stations one or more conspicuous density maxima were found at depth in the sediment. This suggests at present, repetitive deposition of cm thiick sediment deposits takes place, burying living foraminiferal assemblages, which remain preserved, and stained by Rose Bengal, for some time in the deeper sediment layers.
A key finding is the significantly lower δ¹³C values observed in Virgulinella fragilis compared to co-occurring species at similar depths, in accordance to Bernhard, 2003., attributed to bicarbonate release during sulphate reduction, indicating environments with high sulphate reduction rates. Further investigation will explore these metabolic and biomineralization variations and their relationship to foraminiferal assemblages, environmental parameters, and overall ecosystem functioning within the West African upwelling system. As δ¹³C is measurable in fossil tests, it gives hope in future to interpret outlying overly negative δ¹³C values as probable anoxic metabolic pathway in case of fossil foraminifera.
How to cite: Prasetyo, D. G., Báldi, K., Jorissen, F. J., Pacho, L., Jan De Nooijer, L., and Reichart, G.-J.: Living Foraminifera Assemblage of the West African Upwelling System, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-933, https://doi.org/10.5194/egusphere-egu25-933, 2025.
The Argentine Basin hosts a unique oceanic feature: the Zapiola Anticyclonic Circulation (ZAC) located above a sedimentary deposit. Taking advantage of a high-resolution (1/12º) global ocean reanalysis (GLORYS12) we examine the ZAC over 27 years (1993-2019). The mean ZAC is bottom-intensified with bottom currents reaching 0.10 m s-1. The ZAC volume transport ranges from -18.5 Sv to 268 Sv with a mean of 122 Sv. The strong negative peaks correspond to occasional ZAC collapses. During large transport events (>195.4 Sv) the ZAC shows a well defined coherent gyre. Strong transport events are associated with high eddy kinetic energy (EKE) at the periphery of the ZAC (especially to the west and south). In contrast, during weak transport events (<49.8 Sv), EKE increases at the center of the ZAC and decreases at the ZAC periphery.
A weak ZAC is more permeable to external mesoscale structures. Each weak event features a cyclonic eddy at the center of the ZAC carrying subantarctic cold and fresh waters.
The ZAC exhibits a multi-year modulation, with periods of 4-5 years (1993-1997, 1998-2003 and 2004-2009) of low salinity corresponding to low transport, and high salinity to high transport.
Over the last 27 years, transport time series exhibit a significant negative trend of -15 Sv.decade-1 associated with a negative trend in EKE (-0.015 (m/s)2.decade-1) to the north west of the ZAC. Waters in the Zapiola region become warmer and saltier in the first 2000 m of the water column because of the southward migration of the subtropical front.
How to cite: Poli, L., Artana, C., Provost, C., Sirven, J., and Le Blanc-Pressenda, R.: Collapses, maxima, multi-year modulation and trends of the Zapiola Anticyclonic Circulation: insights from Mercator Reanalysis., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2324, https://doi.org/10.5194/egusphere-egu25-2324, 2025.
