Wind Stress Curl and Surface Circulation in the South China Sea and the Philippine Sea

Abstract

Wind stress curl is believed to be the main driving force for the circulation in the South China Sea. However, there has been no attempt to relate wind stress curl with surface currents. In this study, the variability of wind stress curl and surface currents were characterized using time-domain empirical orthogonal functions (EOF). The data used were the monthly average wind stress curl obtained from UWM COADS monthly time series for 1980-1989 and Richardson’s ship-drift derived surface currents. Surface current EOF analysis yields three dominant modes, which account for 50%, 13%, and 7% of the total variance. Mode 1 is dominated by the western boundary current system in the Pacific with no significant temporal variations. Mode 2 reflects reversal of the South China Sea western boundary flow and variations in Mindanao eddy associated with the reversing monsoons. Mode 3 magnitudes were maximum during the monsoon transition periods and were dominated by the circulation in the Celebes Sea. For the wind stress curl, the first two EOF modes explained 40% and 32% of the total variance, respectively. Mode 1 is dominated by the positive wind stress curl throughout the South China Sea during the northeast monsoon. Mode 2 is associated with wind stress curl distribution during the southwest monsoon. The seasonal variability of surface currents in the South China Sea was highlighted by the seasonal reversal of the western boundary currents consistent with the reversal of the wind stress curl in the southern part of the South China Sea. In the interior, the surface currents were mostly associated with Ekman drift, except in the central part where an eastward extension of the western boundary current was observed during the southwest monsoon.