Sea surface salinity interannual variability in the western tropical Atlantic: An ocean general circulation model study

Interannual variability of sea surface salinity in the northwest tropical Atlantic is investigated in several ocean general circulation model eddy-permitting simulations using different air-sea freshwater forcing. The only simulation able to reproduce some features of the observed variability from 1989 to 1993, in particular in the northwest South American region up to 20degreesN, east of the Lesser Antilles, is one with no relaxation to a salinity climatology. The model variability results from advection of freshwater originating from the large South American continental shelf river runoff and has a seasonal cycle associated with the displacement of the intertropical convergence zone: Anomalies develop from March to May and then propagate to a larger area within the next months with the horizontal currents (where both geostrophic and Ekman currents contribute). Later on, from July to October, the anomalies are damped through entrainment across the mixed layer base. The vertical diffusion through the mixed layer base does not weaken when anomalously fresh surface water is present (i.e., no barrier layer effect). According to the European Centre for Medium-Range Weather Forecasts ERA15 reanalysis, air-sea freshwater fluxes have little impact on the large (over 0.5 practical salinity scale) sea surface salinity (SSS) variability in this region. In the model, eddy variability plays little role in the interannual SSS variability, maybe because of inadequate resolution resulting in too weak and too few North Brazil Current eddies.