Interannual variability of Sea Surface Salinity in North-Eastern tropical Atlantic: influence of freshwater fluxes

​​​​​In tropical regions, the fresh water flux entering into the ocean originates primarily from precipitations and, in a lesser extent, from continental rivers. Nevertheless, at regional scale, river flows can have a significant impact on the surface ocean dynamics. Riverine fresh water modifies salinity, and therefore density, stratification and circulation in the surface layer. The eastern Southern North Tropical Atlantic (e-SNTA) region off Northwest Africa, with its particular coastline, relatively high cumulative river discharge and the vicinity of ITCZ, is a particularly interesting location to study the linkage between precipitations, river flows and Sea Surface Salinity (SSS). In particular, the effect of river flows interannual anomalies on SSS have been unexplored in this region.

In this work, we focus on the regional SSS interannual variability, and their relations to river discharge and rainfall. We quantify the impact of these forcings on surface salinity and dynamics, combining informations coming from the CROCO regional ocean model and from SSS remote sensing.  Several simulations forced by different interannual and climatological forcings are analyzed. We compare the simulated SSS with satellite (ESA CCI product), in-situ (Argo, ships and Melax mooring datasets), and Glorys reanalysis. The mixed layer salinity budget is investigated to better understand the dynamics driving SSS variability.

Overall, the simulations are in good agreement with the observations, with a slight statistical improvement in the river plume regions when using ISBA interannual runoff and IMERG precipitation. 
We find that interannual SSS variability depends on surface circulation, river discharge, precipitation and wind variability.  Strong anomalies are mostly linked to strong precipitation anomalies. The impact of river discharge is highly dependent on surface currents. This highlights the importance of properly constraining river runoff and precipitation to simulate realistic sea surface salinities. 

This study shows the value of satellite salinity data for validating ocean models, and highlights the potential contribution of future L-band radiometric missions for coastal ocean observation.