Effect of seasonal surface freshwater flux on sea surface temperature in the tropical Atlantic Ocean

Abstract

In addition to its relationship to evaporative cooling and the atmosphere's radiative balance, anomalies of freshwater flux at the ocean surface have a potential impact on the stratification of the tropical mixed layer. Under light wind conditions when the thermocline is deep, excess rainfall can cause a low‐salinity layer to form, inhibiting vertical convection and mixing. It has previously been hypothesized that under conditions where the heat entering the mixed layer is balanced by cooling at the base of the mixed layer, such as occurs in upwelling regions, this effect could raise sea surface temperatures (Miller, 1976). Conversely, excess evaporation in such a region could destabilize the mixed layer, enhancing convection and lowering sea surface temperature. Here we report on a numerical experiment to test these ideas. We employ a model of the seasonal tropical Atlantic circulation which includes full representation of salinity and heat transport. Two simulations are compared. In one the surface boundary conditions include complete hydrological forcing by river discharge, rainfall, and evaporation. In the other simulation the vertical flux of water at the surface is set to zero. We find that seasonal changes in net freshwater flux at the surface do alter sea surface temperature by ≈±0.1°C. This change is too small to significantly alter the exchange of heat across the surface of the ocean. However, the effect is larger in areas strongly affected by river discharge and at the base of the mixed layer.