Numerical simulation of the Kuroshio intrusion into the South China Sea by a passive tracer

Abstract

Owing to lack of observational data and accurate definition, it is difficult to distinguish the Kuroshio intrusion water from the Pacific Ocean into the South China Sea (SCS). By using a passive tracer to identify the Kuroshio water based on an observation-validated three-dimensional numerical model MITgcm, the spatio-temporal variation of the Kuroshio intrusion water into the SCS has been investigated. Our result shows the Kuroshio intrusion is of distinct seasonal variation in both horizontal and vertical directions. In winter, the intruding Kuroshio water reaches the farthest, almost occupying the area from 18 degrees N to 23 degrees N and 114 degrees E to 121 degrees E, with a small branch flowing towards the Taiwan Strait. The intrusion region of the Kuroshio water decreases with depth gradually. However, in summer, the Kuroshio water is confined to the east of 118 degrees E without any branch reaching the Taiwan Strait; meanwhile the intrusion region of the Kuroshio water increases from the surface to the depth about 205 m, then it decreases with depth. The estimated annual mean of Kuroshio Intrusion Transport (KIT) via the Luzon Strait is westward to the SCS in an amount of -3.86x10(6) m(3)/s, which is larger than the annual mean of Luzon Strait Transport (LST) of -3.15x10(6) m(3)/s. The KIT above 250 m accounts for 60%-80% of the LST throughout the entire water column. By analyzing interannual variation of the Kuroshio intrusion from the year 2003 to 2012, we find that the Kuroshio branch flowing into the Taiwan Strait is the weaker in winter of La Nina years than those in El Nino and normal years, which may be attributed to the wind stress curl off the southeast China then. Furthermore, the KIT correlates the Nino 3.4 index from 2003 to 2012 with a correlation coefficient of 0.41, which is lower than that of the LST with the Nino 3.4 index, i.e., 0.78.