Influence of the Gulf of Guinea Islands on the Atlantic Equatorial Undercurrent Circulation

In the easternmost portion of the Gulf of Guinea, Bioko Island marks the beginning of an island chain that stretches NE-SW to the Equator, where Sao Tome Island sits in the path of the Equatorial Undercurrent (EUC). In this study, we explore the meso-to-large-scale effects of local flow-topography interactions that escalate from the EUC encounter with the Gulf of Guinea islands. A mean shipboard Acoustic Doppler Current Profiler section captures the EUC as a strong subsurface jet that ultimately hits Sao Tome. Motivated by these observations, we ran two ocean general circulation model simulations that differ by the presence versus absence of the Gulf of Guinea islands. Diagnostics of salinity and potential vorticity (PV) from these simulations show that the EUC bifurcates at 6 degrees E, triggering mesoscale activity that spreads the EUC waters. On both sides of Sao Tome, the EUC branches roll up into eddies that propagate westward. These low-PV anticyclones carry high salinity through the tropical Atlantic, introducing PV anomalies along eddy corridors mirrored by the Equator. The formation of such eddies is affected by diapycnal and isopycnal mixing and friction, and their distribution between hemispheres is intrinsically related to the location of Sao Tome. The eddies are modulated at both seasonal and interannual scales. A strong EUC generates numerous and stronger eddies, while seasonal equatorial upwelling indirectly hinders their formation. Convergence of the EUC upstream of Sao Tome reveals intense downwelling and freshening, whereas divergence and upwelling are associated with the EUC negotiating the island.