Mamadou Thiam

The Atlantic Equatorial Undercurrent (EUC) exhibits substantial variability in its vertical position, with implications for equatorial ocean dynamics and air–sea interactions. We investigate the oceanic and atmospheric conditions associated with anomalously deep and shallow EUC core-position events using monthly SODA oceanic and Twentieth Century Reanalysis (20CR) atmospheric data. The EUC core position is defined as the velocity-weighted vertical center of the eastward subsurface current, and events are identified from standardized position anomalies. We identify 95 independent deep and 84 shallow events, with mean core positions of 87.7 ± 4.1 and 58.1 ± 3.4 m, respectively. Despite this difference, mean maximum EUC velocities are similar (1.15 ± 0.14 and 1.13 ± 0.16 m s⁻¹). Composite analyses show contrasting reorganizations of the equatorial current system, vertical shear, temperature, SST, and SSH. Deep events exhibit stronger westward flow above the EUC, enhanced positive vertical shear, negative temperature, SST and SSH anomalies, and a steeper east–west thermocline slope than shallow events. SSH anomalies propagate eastward at approximately 1.2 m s⁻¹, consistent with Kelvin-wave-like signals. Contrasting wind-stress and sea-level-pressure anomalies further indicate distinct coupled ocean–atmosphere states rather than simple opposite phases of the same variability.

L'étude du couplage océan-atmosphère dans la région de l'Atlantique Tropical Nord-Est (ATNE) a longtemps été négligé car la variabilité du climat dans cette zone et les régions avoisinantes, en particulier celle des précipitations au Sahel, est surtout contrôlée par la circulation atmosphérique de grande échelle. Mais les travaux de recherche présentés ici montrent que ce couplage est non négligeable, et explorent l'influence de la température de surface océanique (Sea Surface Temperature, SST) sur le transport d'humidité de l'océan Atlantique vers le Sénégal et les précipitations. Nous avons commencé par identifier le mode principal caractérisant la variabilité des précipitations de juillet à septembre au Sénégal sur une période de 40 ans (1979-2018). En utilisant trois valeurs mensuelles par an (juillet, août et septembre), l'indice dérivé des données d'observation de CRU représente les deux tiers de la variance totale, partagés entre l'intermensuel (données mensuelles d'été) et l'interannuel. Ce mode a été comparé au mode identique obtenu avec les précipitations sur l'ensemble du Sahel : les deux modes sont significativement corrélés (R=0.7) et partagent plus de 50 % de leur variance intermensuelle / interannuelle. Cependant, les régressions linéaires sur les anomalies de SST montrent que ce mode sénégalais est nettement moins influencé par les téléconnexions atmosphériques telles que l'oscillation australe El Niño (El Niño Southern Oscillation, ENSO) dans le Pacifique, le nord de l'Atlantique ou la Méditerranée, contrairement au Sahel. En revanche, il semble être très lié à la SST de l'ATNE. Dans la deuxième partie de la thèse, nous avons alors utilisé les données issues des réanalyses ERA5 pour examiner les signaux de SST, de vent, de pression et de transport d'humidité en ATNE précédant une augmentation des précipitations au Sénégal. Les résultats montrent l'existence possible d'un mécanisme de rétroaction régionale très intéressant : un à deux mois avant l'augmentation des précipitations, une fluctuation de la circulation atmosphérique à grande échelle provoque un ralentissement des alizés de nord-est, entraînant la formation d'une anomalie chaude de SST en ATNE. En juillet-août-septembre (JAS), l'augmentation des précipitations au Sénégal coïncide également avec une augmentation de la convergence du transport d'humidité de basse couche en provenance de l'Atlantique. Ce signal est lié à une intensification du West African Westerly Jet (WAWJ) un peu au nord de sa position moyenne (10°N), correspondant à une réponse géostrophique aux fluctuations du gradient méridien de pression. Comme ces fluctuations pourraient découler d'un ajustement hydrostatique aux gradients méridiens au sud de l'anomalie de SST, la variabilité des précipitations au Sénégal pourrait donc être significativement influencée par celle de la SST en ATNE via ce mécanisme. Dans la dernière partie, nous avons examiné à une échelle plus fine l'hypothèse d'une possible rétroaction entre la SST et le vent de surface en ATNE. Malgré une variabilité moindre de la SST dans cette région, un motif cohérent est identifié, où une anomalie chaude de la SST résulte de fluctuations de la pression en Atlantique Nord. Cette anomalie chaude conduit à une baisse de la pression locale, expliquant une augmentation subséquente du vent de surface dans la région du West African Westerly Jet (WAWJ). Cette rétroaction négative entre la SST et le vent de surface s'étend sur une à deux semaines, offrant une explication plausible aux signaux précédemment observés à l'échelle mensuelle. Ce mécanisme influence la variabilité intrasaisonnière à l'interannuelle des précipitations au Sénégal : il sera nécessaire par la suite de mesurer son importance via une modélisation forcée, puis de vérifier qu'il est correctement représenté dans les modèles de prévision opérationnels.

This study analyzes air–sea interaction in the northeast tropical Atlantic (NETA) at intraseasonal time‐scales, focusing on the feedback between sea‐surface temperature (SST) and surface winds in July, August, and September, using observational data from 2009 to 2020 (Reynolds SST and advanced scatterometer surface winds) and European Centre for Medium‐Range Weather Forecasts Reanalysis v5 data from 2000 to 2020. The analysis reveals that anomalies in the northeastern trade winds over the NETA, often associated with an African easterly wave, result in warmer SST when winds weaken and cooler SST when they intensify. In turn, this SST anomaly modulates surface wind anomalies, generating a negative feedback loop that lasts for 2–3 weeks. A secondary positive feedback loop at the northern edge of the SST anomaly contributes to this prolonged persistence and results in a slow northward migration of the coupled SST–sea‐level‐pressure gradient pattern. A warm SST anomaly between 5° N and 15° N therefore alters the propagating pattern of easterly waves by creating a negative pressure anomaly, enhancing moisture transport convergence off the West African coast and intensifying precipitation. It potentially contributes to the seasonal northward migration of the intertropical convergence zone from early to mid boreal summer. This study highlights the importance of better understanding these air–sea interaction processes and their representation in operational forecasting models to improve weather forecasts in the western Sahel region.

This study examines 40 years of monthly precipitation data in Senegal (1979–2018) using Climatic Research Unit observations and ERA5 reanalyzes, aiming to understand the influence of oceanic and atmospheric factors on Senegal's precipitation in July, August and September (JAS). The variability of Senegal's precipitation is first compared with that of the broader Sahel region: although they share a significant portion of their variance, Senegal appears more closely related to the Northeastern Tropical Atlantic (NETA) Sea Surface Temperature (SST). A detailed examination of this region reveals that Senegal's increased precipitation is linked to the northward shift of the InterTropical Convergence Zone, consistent with numerous previous studies. Over the continent, this shift corresponds to a northward shift of the African Easterly Jet (AEJ) and, consequently, the mesoscale convective systems (MCSs) responsible for most precipitation. It seems primarily driven by the northward shift of the Heat Low. Over the ocean just west of Senegal, there is a comparable shift of the AEJ, accompanied by an increase in low‐level moisture transport convergence within the West African Westerly Jet (WAWJ) which explains the majority of the increase in JAS precipitation in Senegal. This phenomenon is triggered by a negative pressure anomaly in the NETA, located above a positive SST anomaly: we suggest that the latter is the origin of the former, forming a feedback mechanism that potentially significantly influences Senegal's precipitation. The mechanism involves a geostrophic adjustment of the WAWJ to the southern gradients of the SST anomaly.

This study examines 40 years of monthly precipitation data in Senegal (1979-2018) using CRU observations and ERA5 reanalyses, aiming to understand the influence of oceanic and atmospheric factors on Senegal’s precipitation in July, August and September (JAS). Comparing Senegal’s precipitation variability with the broader Sahel region, it emerges that Senegal’s precipitation is more closely associated with the Northeastern Tropical Atlantic (NETA) Sea Surface Temperature (SST). The increased Senegal’s precipitation is linked to the northward shift of the InterTropical Convergence Zone (ITCZ), consistent with numerous previous studies. Over the continent, this shift corresponds to a northward shift of the African Easterly Jet (AEJ) and, consequently, the Mesoscale Convective Systems responsible for most precipitation. It seems primarily driven by the northward shift of the Heat Low.Over the ocean west of Senegal, there is a comparable shift of the AEJ, accompanied by increased low-level moisture transport convergence within the West African Westerly Jet (WAWJ). This phenomenon is triggered by a negative pressure anomaly in the NETA, located above a positive SST anomaly: we suggest that the latter is the origin of the former, forming a feedback mechanism that potentially significantly influences Senegal’s precipitation. The mechanism involves a geostrophic adjustment of the WAWJ to the southern gradients of the SST anomaly. To validate the NETA SST feedback’s role in Senegal’s precipitation, further investigations using daily data or regional atmospheric models are recommended. The findings hold potential for enhancing seasonal forecasting capabilities.

Mechanisms of the interannual variability of upwelling onset dates on the Petite-Cote of Senegal are investigated using daily NEMO model data outputs and NOAA ocean surface temperature observations (OISST). We first determined the phenology (onset dates, end dates, duration and occupied area of the upwelling) over the Petite-Côte located in south of the Senegalese coast. Our results show that upwelling in the Petite-Côte starts on average on 03-December (with a standard deviation of 13 days) and ends on average on 13-June (with a standard deviation of 11 days). Upwelling lasts, on average, 6 months in the region. The maximum upwelling intensity is noted on 15-May and the maximum occupied area exhibits a February-May plateau. We then performed a composite analysis based on the earliness of the upwelling setup dates to understand the mechanisms involved. Our results show that for the earliest upwelling years, we note a strengthening of the vertical velocities 3 weeks before the onset of the upwelling but we note especially an abnormal intensification of the coastal jet. The latter seems to be the initiator of the early onset of the upwelling in the region. These reinforcements are associated with a significant decrease in surface temperature (SST) and an intensification of northerly trade winds. Key words: Upwelling, phenology, Senegalese coast, petite-côte. Mechanisms of the interannual variability of upwelling onset dates on the Petite-Cote of Senegal are investigated using daily NEMO model data outputs and NOAA ocean surface temperature observations (OISST). We first determined the phenology (onset dates, end dates, duration and occupied area of the upwelling) over the Petite-Côte located in south of the Senegalese coast. Our results show that upwelling in the Petite-Côte starts on average on 03-December (with a standard deviation of 13 days) and ends on average on 13-June (with a standard deviation of 11 days). Upwelling lasts, on average, 6 months in the region. The maximum upwelling intensity is noted on 15-May and the maximum occupied area exhibits a February-May plateau. We then performed a composite analysis based on the earliness of the upwelling setup dates to understand the mechanisms involved. Our results show that for the earliest upwelling years, we note a strengthening of the vertical velocities 3 weeks before the onset of the upwelling but we note especially an abnormal intensification of the coastal jet. The latter seems to be the initiator of the early onset of the upwelling in the region. These reinforcements are associated with a significant decrease in surface temperature (SST) and an intensification of northerly trade winds. Key words: Upwelling, phenology, Senegalese coast, petite-côte.