Correlation between subsurface high-salinity water in the northern South China Sea and the North Equatorial Current–Kuroshio circulation system from HYCOM simulations

A Wang,Y Qi,W Zhuang,Y Du

doi:10.5194/os-11-305-2015

Abstract

Abstract. The North Pacific Tropical Water (NPTW), characterized by subsurface high salinity, is observed in the South China Sea (SCS) and is often used as an indicator of the water intrusion from the northwestern Pacific into the SCS. Based on the assimilation product from a global high-resolution Hybrid Coordinate Ocean Model (HYCOM) from 2008 through 2013, this study investigates the seasonal variability of subsurface high-salinity water (SHSW) in the northern SCS and its relationship with the North Equatorial Current–Kuroshio circulation system. Results show that the obvious seasonal variability of the SHSW appears at about 100–200 m in depth. It extends as far west as southeast of Hainan, reaching its volume maximum (minimum) in January (May). The seasonal variance contribution (seasonal variance accounting for the entire variance) is 0.38 in the period we considered, albeit with significant annual variance in other years. Further analysis shows that the changes in high-salinity water volume are highly correlated with the shift in the North Equatorial Current bifurcation latitude (NECBL), which reaches its northernmost point in December and its southernmost point in May. Due to the large-scale wind changes in the Pacific, the Luzon Strait transport (LST) weakens (strengthens) when the NECBL shifts to the south (north) during summer (winter), which results in the reduced (enhanced) SHSW intrusion from the northwestern Pacific into the northern SCS. It is also found that, on a seasonal timescale, the Kuroshio transport (KT) does not vary in phase with NECBL, LST and SHSW, indicating that the KT changes are probably not the governing factor for the seasonal variability of SHSW in the northern SCS.

Highlights

The South China Sea (SCS) is the largest marginal sea with fascinating physical processes in the northwestern Pacific
The subsurface high-salinity water (SHSW) is often used as the passive tracer of the North Pacific Tropical Water (NPTW) (Qu et al, 1999; Li and Wang, 2012) because of its unique water mass properties
This paper analyzes the distribution and seasonal variability of the SHSW in the northern SCS based on the highresolution Hybrid Coordinate Ocean Model (HYCOM) assimilation product from 2008 to 2013

Summary

Introduction

The South China Sea (SCS) is the largest marginal sea with fascinating physical processes in the northwestern Pacific. As for seasonal variation, Wyrtki (1961) firstly mapped the winter and summer distributions of surface salinity in the SCS using in situ observations. He found that, in winter, there is a high-salinity water tongue intruding into the SCS through the LS and extending far into southern Vietnam along the continental shelf, while in summer, the highsalinity water tongue retreats. When the northeastern monsoon fully develops in late autumn to winter, water mass from the Pacific enters the SCS along the continental margin south of China and travels a distance of hundreds of kilometers into the SCS basin, significantly affecting the water mass characteristics in the SCS.

Data and method

Spatial distribution and seasonal variation

Summary and discussion