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

Abstract

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 100m of the SCS (SSCS) decreased by ~0.24psu in the period 1993–2012, yielding a negative trend of −0.012psuyr−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 100m 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 100m decreased in a negative trend of −0.12Svyr−1 (1Sv=106m3s−1) from 1993 to 2012, corresponding to a freshening trend of the SSCS at a rate of −0.011psuyr−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.