How sea surface salinity variability contributes to ocean turbulent heat fluxes during Benguela Niños and Niñas

Sea Surface Temperatures (SST) in the Southeastern Tropical Atlantic Ocean off Angola and Namibia feature pronounced variability on interannual time scales with impacts on the marine ecosystem and rainfall over Southwest Africa. Extreme warm and cold events, so called Benguela Niños and Niñas, are typically remotely forced by wind changes in the western equatorial Atlantic and subsequent Kelvin and coastal trapped wave propagation. Local processes such as coastal wind variations, air-sea heat flux anomalies and freshwater anomalies can additionally drive or amplify the events. Freshwater and thereby salinity anomalies, which have only recently been discussed as a local forcing, are mostly related to anomalous discharge of the Congo river.

In this study, we use an extensive in-situ data set in an attempt to quantify the impact that these sea surface salinity variations have on the mixed-layer turbulent heat fluxes and consequently on Benguela Niños and Niñas. We find that the impact occurs via the changes in stratification with fresh anomalies leading to stronger surface layer stratification, which reduces the mixing with cold waters from below, thus enhancing SSTs. Comparing the 1995 Benguela Niño that featured very low salinity values with the 1997 Benguela Niña that was accompanied by high surface salinity, the mixed layer turbulent heat loss was found to be three times lower in the former case than in the latter. In general, interannual variations in surface salinity, dominated by salt advection, strongly impact the heat exchange between the ocean surface and subsurface layer off Angola in early boreal spring when the Congo river discharge is at its seasonal maximum.