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

• A freshening trend of salinity in the SCS was found since the early 1990s.
• Weakening Kuroshio intrusion is the leading factor controlling the freshening.
• Decadal change of the salinity in the SCS was modulated by the PDO.

Ocean salinity is often used as a dynamical tracer for investigating the Kuroshio intrusion into the South China Sea (SCS). In this study, we found that the upper-ocean water in the SCS had a freshening trend since the early 1990s. Salinity in the upper 100 m of the SCS (SSCS) decreased by ~0.24 psu in the period 1993–2012, yielding a negative trend of −0.012 psu yr−1. The maximum freshening occurred in the surface layer west of the Luzon Strait, and freshening gradually lessened from northeast to southwest and with depth, indicating the important influence of the Kuroshio intrusion. Quantitative analysis of salinity budget from the surface to 100 m depth in the SCS suggests that the weakened Kuroshio intrusion is the leading factor controlling the SSCS freshening, while the increased air-sea freshwater flux plays a minor role. Based on GODAS (Global Ocean Data Assimilation System) model output, the Luzon Strait transport (LST) in the upper 100 m decreased in 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 SSCS at a rate of −0.011 psu yr−1. Both the LST and SSCS changes are closely related to the Pacific Decadal Oscillation (PDO). Our findings demonstrate that the strength of the Kuroshio intrusion into the SCS weakened markedly since the PDO phase shifted in 1990s, which resulted in the pronounced freshening of the SCS water.