Deviations of satellite-measured sea surface salinity caused by environmental factors and their regional dependence

Satellite measurements have largely improved sea surface salinity (SSS) observations in the recent decade, including Soil Moisture Ocean Salinity (SMOS), Aquarius/Sate ' lite de Aplicaciones Cientificas (SAC) -D, and Soil Moisture Active-Passive (SMAP). This study evaluates the deviations between the satellite SSS products and the in-situ observations (Delta S) and analyses their relationships with the environmental factors, including sea surface temperature (SST), precipitation, and wind speed. The level 3 satellite SSS generally correlates well with Argo -observed SSS but differs in the middle and high latitudes and the tropical convergence zones. The smallest Delta S appears in the subtropical open oceans, where the sea surface features warm SST, rare precipitation, and low wind speed. The variations in mean |Delta S| for the three satellites show a robust dependence on SST, precipitation, and wind, with large Delta S (outside 5%-95% interval) in particular tending to occur in areas of cold SST, heavy rainfall, and high wind speeds. The most pronounced SSS deviations occur in the middle and high latitudes (beyond 40 degrees) due to cold SST. In particular, heavy rainfall in the subpolar regions and strong westerly winds in the mid-latitudes also make the satellite SSS measurements less accurate. In the tropical convergence zones, large SSS deviations are mainly related to heavy rainfall, showing significant surface freshening. The fresh biases related to rainfall are more significant in Level 2 SSS than in Level 3, due to the short period of the convective rainfall. In addition, surface freshening is more obvious in low wind speed scenarios, showing that the fresh skin layer induced by rain is more common on the calm ocean surface.

Automated summary

Satellite measurements have greatly improved sea surface salinity (SSS) observations, but deviations still exist compared to in-situ measurements. These deviations are related to environmental factors such as sea surface temperature (SST), precipitation, and wind speed. Deviations are largest in middle and high latitudes, where the SST is colder. Heavy rainfall and strong westerly winds also contribute to larger deviations. In tropical convergence zones, large deviations are mainly caused by heavy rainfall and result in significant freshening. The Level 2 SSS measurements show stronger biases related to rainfall compared to Level 3. Surface freshening is more apparent in low wind speed scenarios.