Seasonal and interannual variability of SMOS, SMAP and Argo sea surface salinity at the top 10 river mouths worldwide

Abstract:

Large rivers are key components of the land-ocean branch of the global water and biogeochemical cycles. River discharges can have an important influence on several physical, biological, optical and chemical processes in the coastal ocean. It is therefore of importance to routinely monitor the dispersal patterns of river plumes at time scales from weekly, monthly, seasonally to interannually.

Sea Surface Salinity (SSS) has been extensively used to trace major river plumes into the ocean. On one hand, in-situ measurements are sparse in time and space making it challenging to study the seasonal to interannual variability of river plumes, regions with sharp horizontal and vertical gradients. In addition, ocean models and assimilation products typically use a time-mean or seasonal climatology of river discharge to represent river forcing and relax SSS towards a seasonal climatology, which in turn, suppresses the interannual variations of SSS. On the other hand, satellite measurements provide SSS observations at improved spatiotemporal resolution. SSS observations are currently available from the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA Soil Moisture Active Passive (SMAP) satellite missions.

The main objective is to study the consistency of the two current SSS satellite missions, SMOS and SMAP, the consistency of two Argo gridded products as well as the consistency between Argo gridded and satellite SSS products at different time scales, from monthly to interannual in major river plumes. We show that SSS from SMOS and SMAP satellites have good consistency in depicting seasonal and interannual SSS variations near major river mouths. Two in-situ products under-estimate the seasonal and interannual variation of SSS substantially within a few degrees of the river mouth. The under-estimation of SSS variability near river mouths by in-situ products is mostly associated with high-discharge season, attributable to the limited in-situ sampling of the river plumes. This work has implications for the scientific community regarding the use of in-situ based gridded products to study river plumes or other areas of sharp horizontal and vertical salinity gradients and it has implications for the modelling community on the importance of satellite SSS observations near river mouth.