The initiation of oil extraction at the offshore Sangomar field (Senegal) raises significant concerns regarding the potential contamination of Marine Protected Areas (MPAs) which host essential habitats along the Senegalese coast. This study quantifies hydrodynamic connectivity between the extraction zone (13.7°N,17.6°W) and five MPAs (Saint-Louis, Cayar, Gorée, Sangomar, and Abéné) over 2012–2019 using Lagrangian particle tracking (ARIANE), driven by a high-resolution regional ROMS simulation or Copernicus SMOC surface currents. Results reveal pronounced spatial and temporal variability in connectivity, establishing a clear hierarchy of vulnerability among the MPAs.Gorée and Cayar face the highest exposure risks, with median connection times of 16–20 days, while Abéné exhibits minimal exposure. Connectivity peaks during the warm season (June–September), a period characterized by relaxed upwelling and coastal currents that favor northeastward advection. Conversely, connectivity is minimal during the upwelling season (November–May), which is dominated by an intense southward jet. Two-dimensional (2D) Lagrangian simulations constrained to the surface layer estimate transport volumes 1.5–3 times higher than those derived from three-dimensional (3D) simulations, which account for vertical particle movement throughout the water column. In the 3D configuration, 40% of particles initially released in the surface layer (0–4 m) exit this layer during their trajectory, highlighting the critical role of vertical processes in particle dispersion. An analysis of Stokes drift contributions, using two independent methods (ROMS currents with wind-derived components and the SMOC product), reveals a 5–15% increase in transport during high-swell episodes. This variability is attributed to complex wind-wave-current interactions. These findings provide a quantitative foundation for seasonal risk zoning and inform the prioritization of emergency response efforts, particularly inthe Gorée–Cayar sector during June–September. They also underscore the necessity of integrating 3D hydrodynamic modeling and Stokes drift into oil spill contingency protocols. The study offers actionable guidance for Senegal’s marine spatial planning and reinforces the importance of cross-border cooperation under the Abidjan Convention.

The Casamance estuary, located in West Africa (Senegal), is a unique hydrodynamic system characterized by a seasonal alternation between a classical (positive) and an inverse estuarine regime during which salinity may exceeds 40 psu. As previous studies have mainly focused on seasonal and interannual variations of surface salinity, this study presents the first measurements of the thermohaline vertical structure in the estuary, collected between 2022 and 2025 near the river mouth during 16 field campaigns conducted outside the rainy season. These observations reveal both the positive, transition and inverse phases of the estuarine regime. Density stratification in the surface layer (0–5 m) is dominated by vertical gradients of salinity accounting for 35-95% of the stratification in the positive regime and 47–100% in the inverse regime. The microtidal signal plays an important role in controlling stratification variability: stratification is more pronounced during neap tides than during spring tides, reflecting the effect of vertical mixing by bottom friction due to tidal currents. During periods of weak tidal currents, the effect of longitudinal wind on stratification becomes detectable. In the positive regime, down-estuary winds enhance stratification through axial wind straining. In the inverse regime, strong up-estuary winds promote stratification by spreading lighter oceanic waters over the denser estuarine waters. Transverse cross-sections reveal a persistent lateral salinity asymmetry, with salinities systematically higher on northern bank than on the southern bank, regardless of the prevailing estuarine regime.

Recent offshore hydrocarbon discoveries along the Senegalese–Mauritanian margin increase the need to quantify oil-spill risk under the highly dynamic circulation of the southern Canary Current upwelling system. We investigate seasonal pollutant dispersion along the Senegalese Grande Côte using Lagrangian particle-tracking experiments forced by outputs from the open-source hydrodynamical ocean model CROCO. The analysis focuses on the role of wind-driven circulation, Ekman transport, and upwelling variability in controlling cross-shore and alongshore transport pathways. Results show a strong seasonal coastal circulation variability. During the cold season (January–May), intensified northerly winds drive coastal upwelling and offshore Ekman transport, enhancing divergence of surface currents and promoting the export of particles away from the coast. This regime limits particle nearshore accumulation but favors broader offshore dispersion over the continental shelf. In contrast, during the warm season (June–September), weakened upwelling-favorable winds and the establishment of anticyclonic circulation north of the Cape induce onshore transport and coastal retention. Particle-release experiments reveal enhanced trapping and accumulation along the Grande Côte during this period. The Kayar region and the Cape Verde Peninsula exhibit relatively higher pollutant exposure during the cold season, whereas the inner continental shelf along the Grande Côte becomes particularly vulnerable during the warm season. These findings suggest that seasonal wind forcing and associated Ekman dynamics have a significant influence on the trajectories of oil spills. Incorporating this variability into contingency planning is essential, as the inner continental shelf of the Senegalese Grande Côte is a dynamically sensitive, high-exposure zone under the warm-season circulation regime.

L'estuaire du fleuve Casamance, au sud du Sénégal, est une zone d'intense activité économique mais son fonctionnement hydrodynamique demeure peu documenté. Depuis une période de recherche active dans les années 80, les études sont rares, entraînant un manque de données pour établir une base de référence de l'estuaire et suivre son évolution. Cet estuaire connaît d'importantes fluctuations de salinité : durant la saison sèche (novembre-juin), il s'inverse et sa salinité croit vers l'amont, atteignant 50 psu en saison sèche puis 20 psu en saison humide (juillet-septembre). L'objectif de cette thèse est d'étudier les fluctuations de salinité à l'aide d'un modèle hydrodynamique (CROCO) et de données in situ. La première partie de la thèse a pour objet l'étude de la marée dans l'estuaire. Afin de calibrer la friction du modèle, les hauteurs d'eau et les courants de marée simulés et observés sont comparés. Malgré un bon accord des hauteurs d'eau, les courants de marée simulés sont sous-estimés de 50 %. La circulation résiduelle Lagrangienne, principalement orientée vers l'amont, est beaucoup plus lente que la circulation résiduelle eulérienne orientée vers l'aval. Ces premiers résultats illustrent le besoin d'acquérir des séries temporelles de courant plus longues et des données bathymétriques dans la partie est de l'estuaire pour améliorer le modèle. La seconde partie se concentre sur les fluctuations de salinité. Des radiales longitudinales réalisées près de l'embouchure indiquent de forts gradients longitudinaux, une faible stratification verticale et une forte variabilité saisonnière. Deux configurations du modèle (CASAM1et CASAM200, respectivement 1 km et 200 m de résolution horizontale) sont forcées par la réanalyse ERA5 et le débit d'eau douce GloFAS. Pendant la saison des pluies, la baisse de salinité est influencée par le débit et la salinité de l'eau saumâtre imposés en amont. CASAM1 simule une forte variabilité inter-annuelle : en fin de saison sèche, le maximum de salinité dépend du cumul d'évaporation et du minimum de salinité de l'année précédente. CASAM200, malgré une hydrodynamique plus réaliste, sous-estime l'augmentation de salinité en saison sèche. L'analyse du bilan de salinité intégré sur la verticale révèle que l’augmentation en saison sèche est principalement causée par l'évaporation et l'advection, et compensée par le mélange horizontal. Cette thèse marque une première avancée dans la modélisation hydrodynamique de l'estuaire de Casamance. Elle propose des pistes d'amélioration comme l'extension du domaine modélisé vers l'amont et l'usage de modèles hydrologiques mieux adaptés à la région. Une collecte pluriannuel d'observations in situ permettra également de consolider les connaissances et de mieux valider le modèle.