Rapid freshening of the upper ocean in the South China Sea since the early 1990s

Ocean salinity is of dynamic and thermodynamic importance in physical oceanography and plays an important role in modulating ocean and climate variability. Analyses of the observations reveal that the upper-ocean water in the South China Sea (SCS) had a rapid freshening trend since the early 1990s. Salinity in the SCS above 100 m (S_SCS) increased slightly (~0.06) in 1980-1992, while it decreased ~0.24 psu in 1993-2012, with a negative trend of -0.012 psu yr^-1. The maximum freshening occurred in the surface layer west of the Luzon Strait and gradually became smaller from northeast to southwest and against depth indicating the important influence of the Kuroshio intrusion. Quantitative analysis of salinity budget above 100 m in the SCS suggested that weakening Kuroshio intrusion is the leading factor controlling the S_SCS freshening, while increased air-sea freshwater flux and river discharge also play a minor role. Based on GODAS (Global Ocean Data Assimilation System) model output, the Luzon Strait transport (LST) above 100 m decreased with a negative trend of -0.12 Sv yr^-1 (1 Sv=10⁶ m³ s^-1) from 1993 to 2012, corresponding to a freshening trend of the S_SCS at -0.011 psu yr^-1. The LST and S_SCS changes are all closely related to the Pacific Decadal Oscillation (PDO). It is demonstrated that due to the PDO shifting from the warm phase to the cool phase since the early 1990s, the strength of the Kuroshio intrusion into the SCS weakened markedly, thereby resulting in the pronounced freshening of the SCS water.