Coastally trapped eddies in the north of the Gulf of Guinea

Cyclonic eddies generated downstream of Cape Palmas and Cape Three points have been suggested to contribute to the coastal upwelling along the northern coast of the Gulf of Guinea. A numerical analysis using a high-resolution model is used to investigate the mesoscale activity and the coastal upwelling generation processes. An eddy detection and tracking tool is applied to altimeter and model data, showing good agreement between these data sets. About two cyclonic eddies per year with an average radius of 60 km were identified downstream of both capes. These cyclonic eddies have an average lifetime of about 60 days during the major coastal upwelling period (boreal summer) and an eastward propagation. These cyclonic eddies are shallow, energetic (their relative vorticity can reach 3 times the earth's rotation), and dimensionless parameters show that they are in an eddy shedding regime. Mean flow interactions and barotropic instabilities associated with capes are their main generation processes. An idealized experiment is conducted in order to analyze the effect of capes on eddy generation and on coastal upwelling. It reveals that these cyclonic eddies generated downstream of capes are not the process responsible for this coastal upwelling. This experiment also suggests that the cyclonic eddies are the cause of the westward and coastal Guinea Counter Current that is associated with a transfer of energy from eddy kinetic to the mean flow.