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A numerical simulation of the tropical Atlantic Ocean indicates that surface cooling in upwelling zones of the Gulf of Guinea is mostly due to vertical mixing. At the seasonal scale, the spatial structure and the time variability of the northern and southern branches of the South Equatorial Current (SEC), and of the Guinea Current, are correlated with the timing and distribution of turbulent heat fluxes in the Gulf of Guinea. Through modulation of the velocity shear at the subsurface, these surface currents control the vertical turbulent exchanges, bringing cold and nutrient-rich waters to the surface. This mechanism explains the seasonality and spatial distribution of surface chlorophyll concentrations better than the generally accepted hypothesis that thermocline movements control the nutrient flux. The position of the southern SEC explains why the cold tongue and high chlorophyll concentrations extend from the equator to 4°°S in the southeastern part of the basin. [ABSTRACT FROM AUTHOR]