Validation and application of soil moisture active passive sea surface salinity observation over the Changjiang River Estuary

Using sea surface salinity (SSS) observation from the soil moisture active passive (SMAP) mission, we analyzed the spatial distribution and seasonal variation of SSS around Changjiang River (Yangtze River) Estuary for the period of September 2015 to August 2018. First, we found that the SSS from SMAP is more accurate than soil moisture and ocean salinity (SMOS) mission observation when comparing with the in situ observations. Then, the SSS signature of the Changjiang River freshwater was analyzed using SMAP data and the river discharge data from the Datong hydrological station. The results show that the SSS around the Changjiang River Estuary is significantly lower than that of the open ocean, and shows significant seasonal variation. The minimum value of SSS appears in July and maximum SSS in December. The root mean square difference of daily SSS between SMAP observation and in situ observation is around 3 in both summer and winter, which is much lower than the annual range of SSS variation. In summer, the diffusion direction of the Changjiang River freshwater depicted by SSS from SMAP is consistent with the path of freshwater from in situ observation, suggesting that SMAP observation may be used in coastal seas in monitoring the diffusion and advection of freshwater discharge.