Browsing by Author "Penven, Pierrick"

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    Climate change impact on ecosystems of Prince Edward Islands: role of oceanic mesoscale processes
    (2019) Asdar, Sarah; Ansorge, Isabel; Penven, Pierrick; Deshayes, Julie
    The subantarctic Prince Edward Islands (PEIs, 47◦S-38◦E) are classified as isolated and hostile regions, in which the terrestrial and marine ecosystems are relatively simple and extremely sensitive to perturbations. The island’s location, between the Subantarctic Front (SAF) and the Polar Front (PF), bordering the Antarctic Circumpolar Current (ACC) provides an ideal natural laboratory for studying how organisms, ecological processes and ecosystems respond to a changing climate in the Southern Ocean. Recent studies have proposed that climate changes reported at the islands may correspond in time to a southward shift of the ACC and in particular of the SAF. This southward migration in the geographic position of major ocean fronts is likely to coincide with dramatic changes in the distribution of species and total productivity of this region. However, there are other sources of variability in the hydrodynamic conditions around the PEIs: upstream of the islands, at the South West Indian Ridge (SWIR), a region of high eddy kinetic activity produces mesoscale features that directly irrigate the PEIs and may impact their marine environment. Based on satellite altimetry in that region, the positions of the SAF and PF were found to be highly variable at interannual and monthly time scales. They also revealed a significant long-term southward trend which was highlighted at the Southern Ocean scale. The mesoscale activity also showed an interannual and intra-annual variability and a decrease in eddy kinetic energy over 24 years was observed in the region. At a more local scale, we highlighted that the archipelago’s environment was impacted by the mesoscale features produced at the SWIR. The temperature, the mixed layer and velocities recorded between the islands were clearly affected by the eddies passing in the vicinity of the PEIs. Moreover, a large signal dominating the main current time series appeared to be a tidal signal, another important driver of variability of the circulation in between the two islands. On a second hand, an idealised model configuration was designed for the PEIs region to study the mesoscale eddy properties and the physical mechanisms of their formation at the ridge. The Eddy Available Potential Energy revealed a maximum of energy around 800 m depth, confirming the deep reaching characteristic of the eddy originated in the region and suggested the presence of a local energy source at this depth. This eddies activity was shown to be the result of a combination of barotropic and baroclinic instabilities occurring at the ridge. Finally, we investigated on the potential consequences of a southward shift of the SAF in the region of the islands. Because the model was idealised, it allowed us to simulate an SAF southward shift by shifting the initial and boundary conditions. The main result was the clear decrease of mesoscale activity in the region which could potentially impact the ecosystems of the PEIs.
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    Environmental influences on banana shrimps of the Sofala Bank, Mozambique Channel
    (2015) Malauene, Bernardino Sergio; Moloney, Coleen L; Roberts, Michael J; Marsac, Francis; Penven, Pierrick; Lett, Christophe
    The Sofala Bank in the western Mozambique Channel is an essential habitat for shallow-water penaeid shrimps. It supports an important multi-sector and -species fishery, with Fenneropenaeus indicus and Metapenaeus monoceros (banana shrimp) being the two main target species. Over the past decade this valuable resource has been declining, which has been attributed to environmental changes, but no conclusive evidence has been found. This PhD thesis aims to understand the interactive roles of biophysical processes on recruitment of banana shrimps, par ticularly their larvae on the Sofala Bank. It is hypothesized that shrimp larvae can be advected offshore by passing mesoscale eddies to regions where they are unable to survive and are thus lost. In the absence of both physical and biological observations, a modelling approach is used. A high-resolution, nested, coastal, Regional Ocean Modeling System (ROMS) of the Sofala Bank is developed. In general, the model agrees well with available observations and literature. The ROMS outputs and self-organizing map analysis indicate that the shelf circulation, structure and river plumes are strongly influenced by the highly energetic o_shore eddy activity. A biophysical, individual-based model (IBM) coupled to the ROMS was developed for early life stages of banana shrimps on the Sofala Bank. The IBM uses spawning patterns identified from analyses of both commercial and research survey data. Simulations indicate that shrimp larvae are lost offshore by entrainment in mesoscale eddies at inter- and intra-annual scales and eddies therefore are unlikely to produce a continuous declining in the catch. In contrast, these eddies induce onshore transport of larvae, promoting coastal settlement, compared with periods without eddies. Locations for simulated larval coastal settlement are identified: northern, central and southern. The roles of tides and larval diel vertical migration in influencing simulated larval settlement success was not conclusive; further research considering a selective tidal stream is needed. Simulated larvae were sensitive to low lethal temperature and river plumes. A conceptual model for the Sofala Bank circulation and another for the banana shrimps, early life history dynamics are proposed based on the results of this thesis.
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    Equilibrium dynamics of the Benguela system : a numerical modelling approach
    (2009) Veitch, Jennifer Anne; Shillington, Frank; Penven, Pierrick
    The Regional Ocean Modelling System (ROMS) is used to systematically investigate equilibrium conditions and seasonal variations of the Benguela system, including both the large-scale flow regime as well as the coastal upswelling regime. A shelf-edge poleward flow exists in the northern Benguela region and is driven primarily by the wind-stress curl via the Sverdup relation. As such, it is strongly seasonal and is most intense during spring and summer when the wind-stress curl is most negative. The poleward flow deepens as it moves southward and between 25-27° much of it veers offshore due to the nature of of the wind-stress curl. In the mean state, the Benguela Current is characterized by two streams: the more inshore stream is topographically controlled and follows the run of the shelf-edge. The offshore stream is driven by nonlinear reactions of passing Aghulas rings and eddies and does not have a striking seasonal signal. The model simulates all seven of the major upswelling cells within its domain.
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    The influence of ocean ridges on the circulation to the south of the Mozambique channel and Madagascar
    (2008) Halo, Issufo Ferrão Mário; Ansorge, Isabelle Jane; Penven, Pierrick; Lutjeharms, Johann R E
    The Mozambique Channel and the region south of Madagascar are dominated by high variability of the oceanic flow due to the ubiquitous presence of mesoscale eddies. The bottom topography of this region has several shallow ridges. The water flowing through the Mozambique Channel propagates southwards predominantly as a train of anti-cyclonic eddies, moving towards the Agulhas Current. South of Madagascar, dipolar vortices regularly propagate in a south-westward direction. Their deep extent favours interaction with the shallow bathymetry of the Davie, Mozambique and Madagascar Ridges. The role of the Madagascar Ridge on the mesoscale circulation is investigated using altimetric observations and model simulations.
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    Modelling the oceanic circulation in the Delagoa Bight
    (2018) Cossa, Obadias J; Reason, Chris; Pous, Stéphane; Penven, Pierrick
    The ROMS is used to investigate the oceanic circulation in the Delagoa Bight region, near the southwestern end of the Mozambique Channel. The model is initially configured at a horizontal resolution of 1/10° (9.9km) over the domain 30.1-43°E,22.13-30.8° S. Subsequent configurations nested an inner grid of resolution 1/30° (3.3km) over the region 32.43-36.43°E,24.09-27.71°S Several sensitivity experiments were performed with and without the inner grid or with or without tidal forcing. Using only the outer coarse resolution grid, the first experiment (DELAGI) does not include tidal forcing whereas tides are included in the second (DELAG-II) experiment. DELAG-III and DELAG-IV both use the inner higher resolution grid but exclude and include tidal forcing respectively. The model was evaluated against observations, namely the WOA, Pathfinder SST and AVISO SSH. The results showed that ROMS adequately resolves the oceanic features in the region, namely the pathways of the anticyclonic eddies from the northern Mozambique Channel and from Madagascar, and the instances when the DBLE is present or absent. The model is also able to reproduce the main water masses and their sources in the region. Water masses found in the centre of the Bight enter through the northeastern sector, either by intrusion of pulses or instabilities of the southwards flowing current. When the DBLE is well established, upwelling is likely to contribute to the water masses in the lower layers. The transport of water towards the Bight from the east was found to be less than that from the north. The model also succeeds in representing the thermocline structure of the DBLE but it fails to capture the local salinity maximum. When tidal forcing is included, the speed of the flow close to the coast increases. The model also revealed the influence of the Inhambane Cyclone on the Delagoa Bight as well as on the region to its south. This cyclone, which is generated in the flow near Inhambane, is similar to Natal Pulses which occur in the Agulhas Current. An eddy detecting and tracking system was used with both the model outputs and VISO SSH to determine the statistics of the DBLE, namely its dimensions, amplitudes and life-times. A maximum radius of 59.52 km, life span of 126 days and an amplitude of 27.27 cm were found. It was also demonstrated that this feature is generated northeast of the Bight. When the inner grid was included in the simulations, anticyclonic features were generated within the Bight with a maximum diameter of 85.4 km and life span of 12 day. These anticyclonic features dominate the circulation when the DBLE is absent (less than 30 percent of the total period of the simulation). The possibility of several cyclonic cores existing simultaneously in the Bight was also demonstrated. Two cores were found with life spans of more than 12 days. When the number of cores is greater than two, they tend to be short lived. Analysis of energy conversion rates showed that the generation of both the DBLE and the Inhambane cyclones is mainly by barotropic instabilities, although in both regions of their generation, weak baroclinic instabilities were also found.
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    Modelling wind-driven inter-ocean exchange in the greater Agulhas with the regional ocean modelling system
    (2014) Loveday, Benjamin; Reason, Chris; Penven, Pierrick
    Two Regional Ocean Modelling System configurations, AGIO and ARC112, are developed to investigate (1) the structure of the Agulhas leakage, (2) the dynamical link between the leakage and the Agulhas Current, and (3) the sensitivity of this link to changes in the regional wind field. Both configurations span the Indian Ocean and South East Atlantic Ocean (29° W - 115° E, 48.25° S - 7.5° N) at 1/4° resolution. ARC112 includes a two-way, AGRIF nested, 1/12° child domain, encapsulating the Agulhas retroflection (0° E - 40° E, 45.5° S - 29.5° S). Model evaluation shows that the basin-scale circulation patterns of the South Indian Ocean are appropriately captured. Western boundary transports match those derived from in situ hydrography, though source region fluxes exceed those observed. Both configurations exhibit inertially governed retroflections and produce Agulhas rings with eddy kinetic energy patterns consistent with those derived from altimetry. Improved topography in ARC112 yields a retroflection position and leakage value closer to observations. Dominant regional water masses are captured, but discrepancies in their distributions remain, especially in highly turbulent areas. The interannual variability of upper ocean heat content is well captured, and Indian Ocean dipole modes are appropriately expressed. Leakage is shown to be confined to the top 1500 m. Flux estimates, derived using complementary Eulerian passive tracer and Lagrangian virtual float techniques, converge where retroflection position is more accurate. Eddy flux, isolated using an Okubo-Weiss parameterisation, contributes only 1/3 to the total flux at the GoodHope line, with a 2:1 anticyclone to cyclone ratio. The remaining intra-ring flux occurs due to mixing between rings in the Cape Basin thermocline, which contains up to 50% Indian Ocean waters. Using a hybrid-criteria eddy-tracking scheme, ARC112i is shown to represent all three recently identified eddy paths, producing an accurate number of rings and cyclones with trajectories and radii that mirror observations, despite higher simulated speeds. A multi-decadal strengthening of the eddy component of Agulhas leakage is ascribed to increases in anti-cyclone speed and cyclone size. Linear changes in trade wind intensity, imposed through a series of idealised wind stress anomalies, concomitantly modulate Agulhas Current transport. The leakage flux response to changing western boundary current inertia is minimal, decreasing with higher resolution. Large changes in eddy kinetic energy are associated with small leakage anomalies, suggesting that the former is a poor leakage proxy. Initially, the leakage responds linearly to increasing westerly wind intensity, but increased mixing between the Agulhas Return Current and Antarctic Circumpolar Current reduces inter-basin flux as the latter adjusts. Consequently, it is suggested that Agulhas Current and leakage magnitude may, to a degree, vary independently, and that multi-decadal trends in the region may be a function of the wind forcing used. Equatorward shifts in the zero line of wind-stress curl drive a small leakage increase, counter to proposed palaeoceanographic mechanism where leakage is implied to reduce under these conditions.
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    The Mozambique Channel eddies : characteristics and mechanisms of formation
    (2012) Halo, Issufo F M; Penven, Pierrick; Ansorge, Isabelle Jane
    The model outputs and the satellite data have shown a number of cyclonic and anticyclonic eddies being generated within the Mozambique Channel. The eddies were identified using a newly designed eddy detection algorithm. The algorithm has shown that cyclonic eddies appeared to be more ubiquitous than the anticyclonic and had a single mode sizedistribution. On the other hand, the anticyclonic eddies seemed to have a favorite site for their formation, and have shown a bi-modal size-distribution.
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    Numerical ocean model study of the Agulhas Bank and the cool ridge
    (2008) Chang, Nicolette; Shillington, Frank; Penven, Pierrick
    The oceanic structure and circulation of the Agulhas Bank, the very wide continental shelf area off South Africa, has been explored in this thesis. The Agulhas Bank is a complex ocean region influenced by shelf processes as well as a nearby western boundary current, the Agulhas Current on its eastern margin and the Benguela Upwelling system on its western margin. In addition, a cold water feature, known as the cool ridge, has been observed on the Eastern Agulhas Bank. A consistent dynamical description is not available but it is commonly observed as a south-westerly flow of cold water in the upper water column, roughly following the 100m isobath and extending seawards off the coast. The formation of the cool ridge has also been investigated in this thesis by means of a numerical ocean model. Previous studies on the Agulhas Bank have been limited temporally and / or spatially. Thus, the Agulhas Bank as a whole has been inadequately sampled to provide a comprehensive representation. In order to remedy these deficiencies, the Regional Ocean Modeling System (ROMS) was used to study the Agulhas Bank dynamics. A large-scale model of the surrounding oceans, the SAfE (South African Experiment) configuration of the ROMS model, was used to force a one-way embedded finer-resolution model over the Agulhas Bank. This produced, 8 years of model data at an approximate horizontal resolution of 8km and 32 vertical terrain-following levels. Two main experiments were performed to understand the nature of the Agulhas Bank. Firstly, the "Reference Experiment" derived a seasonal ocean climatology of the Agulhas Bank. Secondly, the "No Agulhas Experiment" was carried out in an approach in which the Agulhas Current was removed from the shelf edge. A comparison of these two experiments yielded the influence of the Agulhas Current on the Agulhas Bank. The ROMS model was able to reproduce the main observed seasonal structure and circulation of the Agulhas Bank as well as the cool ridge. The Agulhas Bank showed marked seasonality, with its two-layer structure being significantly influenced by the Agulhas Current. The direct influence of the Agulhas Current on the Bank occurs on the Outer Agulhas Bank by the Agulhas Current itself or an Agulhas Current filament. Ekman veering by the interaction of the Agulhas Current with the bottom topography on the slope of the eastern Agulhas Bank advect cool water vertically onto the Bank. This strengthens the thermocline from below, in contrast to surface warming by solar insolation in summer and the Agulhas Current in winter. Cold waters, upwelled over the shelf edge, indirectly affect the greater Agulhas Bank by their advection by the predominantly westward mean currents. The most significant influence of the cold shelf-upwelled waters are in bringing cold waters to shallower depths over most of the Agulhas Bank. This may influence the waters that upwell at the coast, which on the eastern Agulhas Bank (without the Agulhas Current) are trapped under a thick warm surface layer.
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    The role of ocean dynamics within tropical Atlantic climate variability
    (2010) Burls, Natalie; Reason, Chris; Penven, Pierrick
    Capitalising on the vast knowledge gained in the tropical Pacific, tropical Atlantic variability is viewed from an energetics perspective and contrasted against the Pacific at both seasonal and inter-annual timescales. The character of remotely forced thermocline depth variability is evaluated using available potential energy which succinctly quantifies the response of the upper equatorial ocean to large scale wind forcing. To facilitate the Atlantic-Pacific comparison, available potential energy values are derived from the temperature and salinity fields of two global ocean models.
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    Seasonality of the Agulhas Current with respect to near- and far-field winds
    (2018) Hutchinson, Katherine; Penven, Pierrick; Beal, Lisa; Ansorge, Isabel; Hermes, Juliet
    The Agulhas Current plays a critical role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain poorly understood. Model studies predict an austral winter-spring maximum in poleward volume transport, whilst observations reveal an austral summertime (February-March) maximum. Here, the role of winds on Agulhas Current seasonality is investigated using shallow water models, satellite measurements, and a 23-year transport proxy based on observations. A one-and-a-half layer reduced gravity model is shown to successfully reproduce the seasonal phasing of the current. This seasonality is found to be highly sensitive to the propagation speed of Rossby waves, which determines the arrival time of the wind stress signal at the western boundary. By matching Rossby wave speeds to those observed using altimetry, an Agulhas Current with a maximum flow in February and a minimum flow in July is simulated, agreeing well with observations. Near-field winds, to the west of 35◦E, dominate this seasonality, as signals from more remote wind forcing dissipate due to destructive interference while crossing the basin. Local winds driving coastal upwelling/downwelling directly over the Agulhas cannot, alone, account for the observed seasonal phasing, as they force a NovemberDecember maximum and June minimum in flow. The seasonal response to Indian Ocean winds is also investigated using a barotropic (single layer) model with realistic topography. A barotropic adjustment cannot explain the observed Agulhas Current seasonality, predicting a wintertime maximum in transport. The results from the barotropic simulation are similar to previous model studies, where seasonality is dominated by a southward propagation of signals via the Mozambique Channel, suggesting that these models are too barotopic in their response to the winds. Findings from this study elucidate the role of near-field winds and baroclinic processes in determining the seasonality of the Agulhas Current.
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    Simulation of variability in the tropical Western Indian Ocean
    (2014) Manyilizu, Majuto Clement; Reason, Chris; Penven, Pierrick; Dufois, Francois
    The oceanic circulation and properties in the Tanzanian shelf region in the tropical western Indian Ocean have been studied in this thesis using a regional ocean model. The study investigated the influences of the Northeast Madagascar Current (NEMC) in the Tanzanian shelf waters at the annual cycle. Furthermore, the thesis examined the interannual variability of the sea surface temperature (SST) in the Tanzanian shelf region, and compares it with that offshore or with subsurface temperature. At the annual cycle, the westward-flowing NEMC advects relatively warm and fresh waters from the north of Madagascar towards the Tanzanian shelf region by interrupting the upwelled water from the Seychelles-Chagos ridge. At interannual timescales, the weakest interannual SST variations, which lie over the weak subsurface waters variations, occur in the coastal waters off Tanzania, where its variance is shared with waters to the north of Madagascar. Such SST variations are dominated by variability at about five year periods. The strongest interannual SST variations, which lie over the strongest subsurface temperature variations, occur offshore, being dominated by two periods, one at about 2.7 and the other near five years. The interannual variability of the region seems to be linked to El Niño- Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events, which induce changes in the thermocline and surface forcing in the region. Local surface heat flux exchanges driven by the anomalous shortwave radiation dominate the weakest interannual SST variability in the Tanzanian shelf region, with some contribution by the advection of heat anomalies from the NEMC. Further offshore, the strongest interannual variability of the SST is dominated by the thermocline variations induced by local Ekman pumping from local wind stress curl and by remote forcing from large-scale climate modes.|
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    Surface circulation in the KwaZulu-Natal Bight and its impact on the connectivity of marine protected areas
    (2022) Heye, Sonia; Krug, Marjolaine; Hart-Davis, Michael; Penven, Pierrick; Rouault, Mathieu
    The KwaZulu-Natal Bight is a small, coastal region along South Africa's north-east coast. It stretches from Richards Bay to Durban and has a wide shelf compared to the surrounding coastline. As a result, the Agulhas Current is forced offshore, allowing the formation of complex circulation features on the KwaZulu-Natal Bight's shelf that assist with recruitment and retention of marine organisms in this region. This study aims to gain a deeper understanding of the surface circulation within the KwaZulu-Natal Bight and its impact on the connectivity between several surrounding Marine Protected Areas. These include iSimangaliso, uThukela Banks, Aliwal Shoal and Protea Banks and the information about their connectivity contributes to the CAPTOR (Connectivity And disPersal beTween prOtected aReas) project. The aim of this study is met by using high-resolution CROCO model output over a 10-year period, in combination with particle tracking tools, wind and surface drifter data. According to the model's mean circulation, the KwaZulu-Natal Bight's surface currents have a strong south-westward flow on the continental shelf slope where the effects of the Agulhas Current are strongly felt, butare weak and variable on the shelf. Observed variabilities of the mean flow have no distinct seasonal pattern and include a north- eastward current that repeatedly dominates the shelf. It is referred to as the Natal Bight Coastal Counter Current, which originates within the semi-permanent Durban Eddy in the southern KwaZulu- Natal Bight, where it extends throughout the water column. The Natal Bight Coastal Counter Current stretches along the mid-shelf into the northern KwaZulu-Natal Bight, gradually becoming shallower, weaker and narrower. When anticyclonic eddies offshore of the Agulhas Current pass this region, they occasionally replace the Durban Eddy and its associated Natal Bight Coastal Counter Current with a southward flow on the KwaZulu-Natal Bight's shelf. Therefore, the circulation in the KwaZulu-Natal Bight appears to be primarily driven by perturbations at the Agulhas Current front. However, there is also some indication of a direct wind-driven influence in coastal waters inshore of the 50 m isobath. To investigate the impact of the KwaZulu-Natal Bight's circulation on the connectivity between the above-mentioned Marine Protected Areas, particle tracking tools are used. Virtual particles are released in each Marine Protected Area within the model, during multiple northward and southward KwaZulu-Natal Bight surface circulation events. Their pathways are tracked for 30 days and reveal an overall strong southward Marine Protected Area connectivity, which is driven by the Agulhas Current, while a northward connection is less commonly observed. The northward flow of the Natal Bight Coastal Counter Current increases the water retention within uThukela Banks, but it does not extend into iSimangaliso to establish a northward Marine Protected Area connection. However, when the Natal Bight Coastal Counter Current originates within Aliwal Shoal, it may result in a northward Marine Protected Area connection between Aliwal Shoal and uThukela Banks. In this study, the virtual particles represent passively drifting larvae that are buoyant. To make these simulations more realistic, the virtual particles should be able to sink and appropriate swimming behaviours could be considered. However, swimming abilities will likely be overpowered by the surrounding circulation and observations on these behaviours are difficult to make. Therefore, the passive dispersion used in this study to mimic their trajectories may be sufficient and provides valuable insight into the impact of the KwaZulu-Natal Bight's surface circulation on Marine Protected Area connectivity and larval dispersion. The virtual particle tracking tools used in this study are not limited to biological applications. Future studies could use them to investigate the path and accumulation regions of virtual pollutants, such as microplastics, to determine the regions in which clean-ups would be most effective.
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    Variability of coastal upwelling south of Madagascar
    (2018) Ramanantsoa, Heriniaina Juliano Dani; Krug, Marjolaine; Rouault, Mathieu; Penven, Pierrick
    Madagascar’s southern coastal marine zone is a region of high biological productivity which supports a wide range of marine ecosystems, including fisheries. This high biological productivity is attributed to coastal upwelling. The thesis seeks to characterise the variability of the coastal upwelling south of Madagascar. The first part of the thesis provides new insights on the structure, variability and drivers of the coastal upwelling south of Madagascar. Satellite remote sensing is used to characterize the spatial extent and strength of the coastal upwelling. A front detection algorithm is applied to thirteen years of Multi-scale Ultra-high Resolution (MUR) Sea Surface Temperatures (SST) and an upwelling index is calculated. The influence of winds and ocean currents as drivers of the upwelling are investigated using satellite, in-situ observations, and a numerical model. Results reveal the presence of two well-defined upwelling cells. The first cell (Core 1) is located in the southeastern corner of Madagascar, and the second cell (Core 2) is west of the southern tip of Madagascar. These two cores are characterized by different seasonal variability, different intensities, different upwelled water mass origins, and distinct forcing mechanisms. Core 1 is associated with a dynamical upwelling forced by the detachment of the East Madagascar Current (EMC), which is reinforced by upwelling favourable winds. Core 2 which appears to be primarily forced by upwelling favourable winds, is also influenced by a poleward eastern boundary flow coming from the Mozambique Channel. This intrusion of Mozambique Channel warm waters could result in an asynchronicity in seasonality between upwelling surface signature and upwelling favourables winds. The second part of the thesis focuses on the interaction between the intrusion of warm water from Mozambique channel and the upwelling cell in Core 2. Cruise datasets, satellite remote sensing observations and model data analyses are combined to highlight the existence of a coastal surface poleward flow in the south-west of Madagascar: the South-west MAdagascar iv Coastal Current (SMACC). The SMACC is a relatively shallow (Coastal Current (SMACC). The SMACC is a relatively shallow (<300 m) and narrow (<100km wide) warm and salty coastal surface current, which flows along the south western coast of Madagascar toward the south, opposite to the dominant winds. The warm water surface signature of the SMACC extends from 22◦S (upstream) to 26.4◦S (downstream). The SMACC exhibits a seasonal variability: more intense in summer and reduced in winter. The average volume transport of its core is about 1.3 Sv with a mean summer maximum of 2.1 Sv. It is forced by a strong cyclonic wind stress curl associated with the bending of the trade winds along the southern tip of Madagascar. The SMACC directly influences the coastal upwelling regions south of Madagascar. Its existence is likely to influence local fisheries and larval transportpatterns, as well as the connectivity with the Agulhas Current, affecting the returning branch of the global overturning circulation. The last part of the thesis provides a holistic understanding of the inter-annual variability of the upwelling cells associated with the multiple forcing mechanisms defined in the first two parts of this work. Results reveal that the upwelling cells, Core 1 and Core 2, have different inter-annual variabilities. Inter-annual variability of Core 1 is associated with the East Madagascar Current (EMC) while Core 2 is linked with the South-west MAdagascar Coastal Current (SMACC). Inter-annual changes in the EMC occur as a result of oscillations in the South Equatorial Current (SEC) bifurcation off Madagascar, while the inter-annual variability in the SMACC is influenced by the cyclonic wind stress curl inter-annual variability. The upwelling is also linked with global/regional climate modes. Both Cores are highly correlated with the Subtropical Indian Ocean Dipole (SIOD). Core 2 is also correlated to the Indian Ocean Dipole (IOD). Both cores are significantly correlated with the El Ni˜no-Southern Oscillation (ENSO) after 12 months lag.