Seasonal structures of upper layer circulation in the southern South China Sea from in situ observations

Abstract

Conductivity-temperature-depth and acoustic Doppler current profiler data from six cruises spanning 1989 to 1999 are used to investigate the seasonal structure of the upper layer circulation in the southern South China Sea (SCS). The surveys were made during winter, late spring, summer, and late fall. More detailed structures not presented in previous studies are found. In summer the upper layer circulation of the southern SCS is dominated by an anticyclonic gyre with a strong eastward flow on its northern border, which originates off the southeast coast of Vietnam: the Summer Southeast Vietnam Offshore Current. In winter a stronger cyclonic gyre exists in the western portion of the southern SCS and a weaker anticyclonic circulation in the eastern portion. At the juncture of these two gyres, there is a strong northward upwind flow, called the Winter Natuna Off-Shelf Current. In late spring the anticyclonic gyre begins to form in the northwest, and a trace of the Natuna Off-Shelf Current can still be observed, especially subsurface in the southern part of the study area. Mesoscale eddies are active in this period. In late fall the winter-type pattern begins to appear. The Natuna Off-Shelf Current and the cyclonic gyre in the northwest part of the southern SCS emerge, and the summer anticyclonic gyre vanishes. In the southeast the currents are basically toward the northeast with an anticyclonic trend, revealing a multi-eddy feature. These upper ocean currents appear consistent with the wind forcing.