Causes for the recent increase in sea surface salinity in the north-eastern Gulf of Guinea

In situ sea surface salinity (SSS) observations showed an increase >0.5 over the period 2002-2009 in the Gulf of Guinea, off the Niger Delta. Observed changes in the Niger River runoff were not consistent with this increase in SSS, but the increase was reproduced in a regional numerical simulation with climatological river runoff. The simulated mixed-layer salinity budget was used to identify the mechanisms responsible for the increase. When comparing the period 2002-2009 with the period 1993-2001, significant changes in the salt budget were identified. The increase in SSS in the more recent period appeared to be driven by changes in the atmospheric freshwater flux, mainly attributed to a regional decrease in precipitation. Horizontal advection partly compensated for the effect of freshwater flux through changes in zonal currents and zonal SSS gradients.