Seasonal and interannual variations of sea surface salinity associated with the Mississippi River plume observed by SMOS and Aquarius

•Observations of seasonal & interannual SSS changes in Mississippi and River plume•Interannual SSS can be as large as seasonal SSS anomalies in Mississippi River plume.•River discharge is the major forcing, evaporation-precipitation plays a minor role•Significant implications to ocean modeling/forecast and hypoxic zone monitoring

We use sea surface salinity (SSS) from ESA's Soil Moisture and Ocean Salinity (SMOS) and NASA's Aquarius/Satelite de Aplicaciones Cientificas (SAC)-D missions to study seasonal and interannual variations of SSS in the Gulf of Mexico near the Mississippi River outflow during 2010-2014. A seasonal cycle is clearly observed in SSS, with an SSS maximum during December-April and an SSS minimum in July-August. The averaged magnitude of the seasonal variation is about 4 pss. Interannual changes are also observed, with the lowest SSS in summer 2011 and the highest SSS in summer 2012 and a difference of 4.2 pss. Therefore, the magnitude of interannual changes can be comparable to that of the seasonal cycle. Our analysis suggests that the seasonal variations of SSS near the Mississippi River mouth are consistent with variations in Mississippi River discharge. Evaporation minus precipitation and ocean dynamics play a minor role. River discharge also contributes to the interannual variation of SSS, especially during summer 2011 and to some extent the highest summer minimum in 2012 in terms of timing, but not magnitude. For the latter, ocean dynamics and E-P also contribute. The implications of the findings to ocean modeling and hypoxic zone monitoring are also discussed.