Generation mechanisms of mesoscale eddies in the Eastern Tropical North Atlantic Ocean.

The Eastern Tropical North Atlantic (ETNA; 14°N-22°N; 16°W-26°W) is characterized by a contrasting oceanic circulation with only sluggish flow in the offshore shadow zone region and energetic flow along the coast. During the upwelling season, the coastal mean flow is dominated by a poleward undercurrent beneath the equatorward coastal jet whereas during the relaxation period, a stronger poleward flow from the surface to about 250-m depth prevails. Given the sluggish interior flow, mesoscale eddies in the region originate primarily from the coastal current system, with both, surface- and subsurface-intensified eddies prevailing. In this study, the formation mechanisms of these eddies are analysed using a high-resolution (~1.5 km) configuration of GFDL’s modular ocean model (MOM) with realistic topography. The model reproduces the complex coastal flow structures well, and surface and subsurface eddies with characteristics comparable to observed eddies are formed. Processes associated with instabilities of the coastal current favour the development of meanders and eddies. The interaction between the mean flow and topographic slope induces enhanced anticyclonic vorticity and near-zero potential vorticity in the bottom boundary layer. Submesoscale vortices are generated, and eventually merged to form mesoscale eddies. From an energy budget perspective, eddy kinetic energy is found to be predominantly fed from the vertical buoyancy of the boundary current.