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

摘要

The North Pacific Tropical Water (NPTW), characterized by subsurface highsalinity, is observed in the South China Sea (SCS) and is often used as anindicator of the water intrusion from the northwestern Pacific into the SCS.Based on the assimilation product from a global high-resolution HybridCoordinate Ocean Model (HYCOM) from 2008 through 2013, this studyinvestigates the seasonal variability of subsurface high-salinity water(SHSW) in the northern SCS and its relationship with the North EquatorialCurrent–Kuroshio circulation system. Results show that the obvious seasonalvariability of the SHSW appears at about 100–200 m in depth. It extends asfar west as southeast of Hainan, reaching its volume maximum (minimum) inJanuary (May). The seasonal variance contribution (seasonal varianceaccounting for the entire variance) is 0.38 in the period we considered,albeit with significant annual variance in other years. Further analysisshows that the changes in high-salinity water volume are highly correlatedwith the shift in the North Equatorial Current bifurcation latitude (NECBL),which reaches its northernmost point in December and its southernmost pointin May. Due to the large-scale wind changes in the Pacific, the Luzon Straittransport (LST) weakens (strengthens) when the NECBL shifts to the south(north) during summer (winter), which results in the reduced (enhanced) SHSWintrusion from the northwestern Pacific into the northern SCS. It is alsofound that, on a seasonal timescale, the Kuroshio transport (KT) does notvary in phase with NECBL, LST and SHSW, indicating that the KT changes areprobably not the governing factor for the seasonal variability of SHSW in thenorthern SCS.