Basin‐scale wind‐forced dynamics of the seasonal southern South China Sea gyre

Abstract

Using observed wind‐forcing, the seasonal development and disappearance of the near‐surface anticyclonic gyre in the southern South China Sea is modeled, along with associated dynamic features. Forced by monsoon winds, the ocean circulation in the South China Sea alternates between a single cyclonic gyre during the winter northeast monsoon and a double gyre, cyclonic/anticyclonic in the northern/southern South China Sea, during the summer southwest monsoon, reversing the western boundary current along Vietnam with each shift of the monsoon regime. During the summer southwest monsoon, the poleward western boundary current converges with the equatorward western boundary current of the northern gyre, turning offshore to create an eastward jet and flanking dipole recirculation near 13°N. The development and timing of the eastward jet and recirculations are consistent with large‐scale basin dynamics. The subsequent transition to the winter northeast monsoon regime destroys the recirculation cells and eastward jet. Employing high‐resolution 1.5‐layer and 2.0‐layer numerical models, we demonstrate the applicability of large‐scale dynamics to the deeper central basin of the South China Sea, specifically showing that the gyre‐scale circulation governs the development of the eastward jet and dipole recirculations. The destruction of the recirculations by the monsoon winds each autumn is also modeled. This effort highlights the dominance of monsoon wind‐forcing in the South China Sea's basin‐scale dynamics.