Southeastern Tropical Atlantic Coastal Dynamics and Sea Surface Temperature affected by River Discharge: insights from modelling.

The Eastern Boundary Upwelling system of the Southeastern Tropical Atlantic (SETA) is of great socioeconomic importance for local communities since it supports highly productive fisheries and diverse marine ecosystems. Comprehending the local processes that shape the physical characteristics of this system is thus crucial. The SETA is characterized by a strong meridional sea surface temperature (SST) gradient and is influenced by a large freshwater input from land mainly due to Congo River discharge. Here, we use high-resolution ocean model sensitivity experiments to show the impacts of the freshwater discharge from rivers on the mean state SST and the dynamics of this coastal region. By comparing experiments with and without river discharge we find that the freshwater presence increases the mean state coastal SST by up to 0.9ºC from 6ºS to 25ºS, while reducing the SST by more than 1ºC from 6ºS to 3ºS. These changes are associated with a halosteric effect of an elevated sea surface due to the lower sea surface salinity, leading to strong pressure gradients that drive upwelling and downwelling processes north and south of the Congo River mouth at 6ºS, respectively. Alongshore horizontal temperature advection also related to the sea surface height gradients plays likewise an important role in warming (cooling) the SST mean state south (north) of 6ºS. Ultimately, the change in coastal currents pushes the meridional temperature gradient further south. These results highlight the influence of freshwater input on SST and ocean surface dynamics, particularly relevant in the context of projected climate change, suggesting a future increase in Congo River discharge.