Driving processes of seasonal cooling in the northern Gulf of Guinea coastal upwelling

This study investigates the main processes driving the seasonal coastal cooling in the northern Gulf of Guinea, with a particular focus on the Ivorian/Ghanaian upwelling system. Using a high-resolution (1/36°) ocean regional model, the analysis of the heat balance for the period 2008-2017 shows that the cooling processes in both regions are predominantly driven by vertical advection and vertical mixing, the latter playing a dominant role in the surface layer. In the Ivorian upwelling system, vertical mixing is attributed to the vertical shear of horizontal currents, enhanced by the Guinea Current in the presence of weak stratification at the base of the mixed-layer. Vertical advection is controlled by the seasonal variations in the vertical current. In the Ghanaian upwelling system, vertical mixing is also associated with vertical shear in the horizontal currents, here due to Ekman transport induced by local winds, but this time with moderate stratification compared to that observed offshore Côte d'Ivoire, at the base of the mixed-layer. Vertical advection is also attributed to seasonal variations in the vertical current. The results showcase that advection transports deep cold waters up to the base of the mixed-layer, where vertical diffusion acting as a relay mixes these waters with the warmer surface waters, thereby extending the cooling effect to the surface. This study shed light on the key cooling processes, providing a better understanding of its dynamics for optimal management of the marine ecosystem in this region.