Analysis Of Characters And Mechanisms Of The Mixed Layer Salinity Variation In South China Sea

As a key component of the physical environment in the South China Sea (SCS),salinity variation plays a key role in modulating the hydrodynamic environment of theSCS, and will fundamentally influence the circulation and the climate of China.Therefore, it is important for deep understanding of salinity variability in the SCS atdifferent time scales. In this study, we used Empirical Mode Decomposition (EMD)method to analyze EN3salinity data in the SCS. The results show that there arenotable seasonal, interannual variability and long-term trend of salinity in the mixedlayer of SCS. Preliminary analysis and discussion for these variations were carried outin this study.Investigation of contribution of sea surface freshwater flux, advection andvertical mixing to the seasonal salinity variability in different areas shows that, thedominant factor in the northern SCS is evaporation-precipitation, and the secondaryfactor is advection; while in the southern South China Sea, the impact of advection ismore important than the evaporation-precipitation (E-P) in modulating the seasonalsalinity variability. The vertical mixing could facilitate the ascending tendency ofsalinity in wintertime, while block the descending tendency of salinity in summertime.However, its impacts are quite smaller than that of the net fresh water flux andadvection.The relationship between changing rate of salinity in the mixed layer of SCS, seasurface freshwater flux, salinity flux through different straits with the ENSO cycle inthe interannual time scales is studied. The significant correlation was revealedbetween the annual components and ENSO signals after decompose of every time series into their IMFs using EMD. The sea surface freshwater flux was found to be thekey contributor to the interannual variability of salinity, however, its influence to thespatial distribution of salinity annual variation in the mixed layer, with strong signalsin the northern part and weak ones in the southern and middle parts, is still not clear.The spatial distribution of interannual salinity variability is associated with ENSOphenomenon. During El Nino events, the westward anomaly of salinity flux in theLuzon Strait strengthen the salinity rising rate in the northern SCS, while the eastwardanomalies of salinity flux in the Mindoro and Balabac Straits weaken the ascending ofsalinity in the middle and southern SCS.The linear trend analysis of the salinity in the SCS shows the decreasing trendduring last30years from1979to2008, with decreasing rate of0.03psu/10yr. Thesalinity decreasing is very likely due to the weakening of upwelling intensity at thebottom of the mixed layer. During last30years, the warming of sea surfacetemperature (SST) increases the vertical stability of upper mixed layer, which reducesthe vertical mixing and prevent the water with high salinity from entering the mixedlayer. Further quantitative analysis is required to confirm this finding.