Decadal variation and trend of the upper layer salinity in the South China Sea from 1960 to 2010

Ocean salinity is an essential measurable indicator of water cycle and plays a crucial role in regulating ocean and climate change. Using Simple Ocean Data Assimilation (SODA) reanalysis product, substantial decadal variability of the salinity in the upper layer of the South China Sea (SCS) from 1960 to 2010 was examined. Results show that a decadal variation of the upper layer salinity is clear. The upper layer (100 m) waters are found to freshen from 1960, become saltier during 1975 to 1995, and freshen again from 1995 to 2010. The strongest anomalies appear in the northeastern, northern, and northwestern regions in the three periods, respectively. The salinity variation trends become weaker below the upper layer, except the salinification trend in the northern SCS extends to at least 300 m during the salinification period. Diagnosis of the salinity budget over the top 100 m shows that during the first freshening period horizontal advection, vertical advection, and surface freshwater forcing all contribute to salinity freshening, and horizontal advection is relatively larger. The contribution of horizontal advection and surface freshwater forcing to the positive salinity anomaly is comparable, while the vertical advection is the secondary factor in the salinification period. Horizontal advection, especially zonal advection, plays a crucial role during the second freshening period. Moreover, horizontal advection is more important than that in the first freshening period. In addition, the contribution of horizontal advection is mainly in zonal direction controlled by Kuroshio intrusion. Further analysis shows the upper-layer salinity variations have a Pacific Decadal Oscillation (PDO)-like signal, with freshening during the negative PDO years, and salinification during the positive PDO years. PDO mainly influences the upper-layer salinity changes through both atmospheric bridge and oceanic bridge.