River discharge impacts coastal Southeastern Tropical Atlantic sea surface temperature and circulation: a model-based analysis

Abstract. The Southeastern Tropical Atlantic (SETA) coastal region sustains highly productive fisheries and marine ecosystems, thus having immeasurable socio-economic importance for West African countries. It is characterized by high sea surface temperature (SST) variability and freshwater input from land mainly due to Congo River discharge. In this study, using high-resolution ocean model sensitivity experiments, we show that the presence of low salinity waters from the river discharge increases the mean state SST in the SETA coastal fringe by about 0.26 °C on average and by up to 0.9 °C from south of the Congo River to the Angola-Benguela front (ABF). North of the Congo River up to about 4° S, this input significantly reduces the mean state SST by more than 1 °C. We demonstrate that the impact of river discharge on SST is associated with a halosteric effect, which modifies the sea surface height gradient and alters geostrophic currents, producing a southward (northward) coastal geostrophic flow, with an onshore (offshore) geostrophic component to the south (north) of the Congo River. Hence, advective warming (cooling) and downwelling (upwelling) are generated south (north) of the river mouth. Furthermore, the southward advection generated by the low salinity waters pushes the ABF further south. These results draw attention to the freshwater impact on SSTs and ocean surface dynamics, especially in the projected climate change scenario of continuously increasing land to ocean discharge.