Sea Surface Temperature Impact on Diurnal Cycle and Seasonal Evolution of the Guinea Coast Rainfall in Boreal Spring and Summer

ERA5 reanalyses and observations of convective clouds and precipitation are used over the northern Gulf of Guinea between 7 degrees W and 3 degrees E to study the influence of ocean surface temperature and the land-sea temperature gradient on Guinea Coast rainfall (GCR) in boreal spring and summer. Seasonal composites are calculated around two dates indexing the onset (T-ref) and demise (T-end) of the GCR. The T-ref date corresponds to the emergence of the equatorial upwelling in boreal spring, which "pushes" the zonal precipitation belt northward against the Guinea coast. The T-end date characterizes the emergence of the coastal upwelling in July, which is known to coincide with the beginning of the "Little Dry Season" that lasts until September. Along the Guinea Coast, the diurnal cycle of the air-sea temperature gradient controls precipitation through the land-sea breeze, which explains why precipitation reaches its maximum around noon over the ocean, and in the late afternoon over the continent. The emergence of the Guinea Coast upwelling in July induces a weakening of southerlies on a seasonal scale, and a weaker land breeze on a diurnal scale. It induces a decrease in the convergence of humidity transport across the coast and in coastal oceanic precipitation. Therefore, the GCR is seasonally controlled by the latitude of the maximum tropospheric water vapor content and the annual cycle of the West African monsoon, but the ocean surface temperature is responsible for the abruptness of its onset via the intensification of the equatorial upwelling around the end of May, and possibly of its demise as well via the emergence of the coastal upwelling by early July.