Seasonal variability of the equatorial undercurrent termination and associated salinity maximum in the Gulf of Guinea

The termination of the Equatorial Undercurrent (EUC) in the eastern equatorial Atlantic during boreal summer and fall, and the fate of the associated saline water masses, are analyzed from in situ hydrological and currents data collected during 19 hydrographic cruises between 2000 and 2007, complemented by observations from Argo profiling floats and PIRATA moorings, and from a numerical simulation of the Tropical Atlantic Ocean for the period 1993–2007. An intense variability of the circulation and hydrological properties is evidenced from observations in the upper thermocline (24.5–26.2 isopycnal layer) between June and November. During early boreal summer, saline water masses are transported eastward in the upper thermocline to the African coast within the EUC, and recirculate westward on both sides of the EUC. In mid-boreal summer, the EUC weakens in the upper thermocline and the equatorial salinity maximum disappears due to intense mixing with the surface waters during the upwelling season. The extra-equatorial salinity maxima are also partially eroded during the boreal summer, with a slight poleward migration of the southern hemisphere maximum until late boreal summer. The upper EUC reappears in September, feeding again the eastern equatorial Atlantic with saline waters until boreal spring. During December–January, numerical results suggest a second seasonal weakening of the EUC in the Gulf of Guinea, with a partial erosion of the associated equatorial salinity maximum.