Remote Sensing Observations of Sea Surface Salinity Variability in the South China Sea

The South China Sea (SCS) is the largest marginal sea in the southeastern Asia and has tremendous oceanographic and climatic significance. Understanding and monitoring Sea Surface Salinity (SSS) variability in the SCS is of crucial importance to understanding the SCS inter-ocean circulation and its role in climate and the hydrological cycle. Using newly available satellite observations of SSS, we provide, for the first time, a detailed and synoptic view of the spatiotemporal variability of SSS in the SCS and characterize dominant patterns of SSS variability. The results depict the SCS as a very dynamic region exhibiting variability over a broad range of scales, from intra-seasonal to inter-annual, with the seasonal cycle dominating (~42% of the total SSS variance). Using satellite and in-situ data we examine the surface freshwater budget in a few key regions found to be responsible for the observed variability. The budgets show significant complexity with different terms dominating regionally. For example, the seasonal evolution of the SSS in the vicinity of Mekong River estuary is largely controlled by the advection term, with the surface freshwater term playing a secondary role. The surface freshwaterbudget near Kalimantan suggests that both the surface freshwater flux and horizontal advection are fundamental to the seasonal evolution of SSS, but the effect of vertical entrainment is not negligible, particularly in boreal winter.On inter-annual time scales, significant basin-wide salinification of the surface layer occurred during 2015-2016 as a consequence of an upward net surface freshwater flux (E-P) following a strong El Nino event. The intra-seasonal variability is surprisingly very weak throughout the SCS (the standard deviation is smaller than 0.1 psu), except for a few regions near the coast where it is likely related to the intra-seasonal variability in monsoon rainfall and coastal runoff.