The South China Sea is a large semi-enclosed sea where the seasonal circulation is driven  by the asiatic monsoon system. Wind stress, intrusion of the Kuroshio near the Luzon Strait and the interaction between shelf currents and the bottom topography create short and long-lived anticyclonic and cyclonic gyres.

A longitude-time hofmoeller diagram on the 20°N latitude (here for  the 2002 year) gives an idea of the eddy activity for both the spatial and time components. The eddies circulate wesward. During February-March, cyclonic eddies are predominant while during August-December, anticyclonic eddy are the most widespread. A lower eddy activity is observed during May (explained by a weaker wind stress during this transition period).