Impact of the Sea Surface Temperature in the North-Eastern Tropical Atlantic on Precipitation Over Senegal

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, spanning 40 years of monthly precipitation data in Senegal, explores the intricate relationship between oceanic and atmospheric factors shaping precipitation patterns from July to September. The increased summertime precipitation in the Western Sahel is primarily of continental origin, associated with the northward shift of mesoscale convective systems linked to lower pressure in the Sahara. However, over the ocean west of Senegal, there is also an increase in inland moisture transport that explains a significant part of the intensified precipitation from July to September in Senegal. This transport is reinforced by a low-pressure system over the ocean, potentially caused by warmer sea surface temperatures between 10 degrees N and 20 degrees N off West Africa. This close connection between Senegal's precipitation and ocean surface temperature in the Northeastern Tropical Atlantic could help enhance crucial seasonal forecasts for agricultural planning, the economy, and food security in West Africa. Wet summers in Senegal are preceded by La Nina events and warming in the Mediterranean but also by warming in the Northeastern Tropical Atlantic Moisture transport convergence within a stronger West African Westerly Jet (WAWJ) explains this increase in precipitation Feedback between the North Tropical Atlantic surface temperature and atmospheric pressure is proposed to explain this WAWJ acceleration