Characterization of Northwest African Coastal Upwelling Systems

Abstract. Coastal upwellings are critical for nutrient supply, biological productivity, socio-economic activities, local weather patterns and regional climate variability. This study investigated characteristics of the Northwest African coastal upwelling system, including the Senegal-Mauritania and Gulf of Guinea coastal upwelling regions. The spatial and temporal variability were analysed using physical and biogeochemical ocean variables from 1982 to 2022. The analysis focused on upwelling indices derived from sea surface temperatures (SSTs), wind stress and chlorophyll-a concentration during the boreal winterspring and summer, when upwelling peaks in the Senegal-Mauritania and Gulf of Guinea upwelling systems, respectively. Additional indicators such as the annual cycles of sea level anomaly, ocean surface current velocity, geostrophic balance, salinity, nitrate and dissolved oxygen in surface waters were also examined as upwelling indicators. Upwelling trends were investigated by comparing the results of different datasets. Results highlight the complex interplay of local and large-scale processes that influence the dynamics of coastal upwelling in Northwest Africa. The variability in these coastal upwelling regions was not only influenced by local and remote wind forcing, but also by large-scale climatic drivers such as El Niño Southern Oscillations (ENSO) and Atlantic Niño events. These climatic factors influence the intensity and frequency and duration of upwelling seasons. In the Senegal-Mauritania coastal upwelling, a strong relationship was observed between upwelling and the equatorial ENSO events from late winter to the early spring, with a notable reduction of upwelling intensity after the peak of El Niño in the equatorial Pacific. In the Gulf of Guinea, the summer upwelling intensity was affected by the equatorial Atlantic mode and often decoupled from local wind forcing, providing new insights into alternative upwelling drivers.