The circulation in the South China Sea was thought to be mainly winddriven current controlled by monsoons. But recent observations reveal two currents in opposite directions, namely the northeastward South China Sea Warm Current (SCSWC) and the southwestward Kuroshio South China Sea Branch (KSCSB), flowing along the steep continental slope in the northern South China Sea. Surprisingly, the SCSWC flows against the strong northeast monsoon and becomes even stronger in winter.
This may be explained by a "thermodynamic model" based on the intrusion of the Kuroshio through the Bashi Strait. Warm waters extend into the South China Sea in a tongue shape formation along the isobaths of the continental slope and, therefore first, head forthwestwards and then bend southwestwards. Hence, the KSCSB is formed by "topography trapping" due to the conservation of potential vorticity. The baroclinic effect due to the strong temperature gradient thus developed by these warm waters against the cold waters near the coast of South China drives the SCSWC strong enough against the monsoon in winter.
Results of 3-D baroclinic (in contrast to barotropic) prognostic numerical experiments are adequate to prove not only these ideas qualitatively but also the existence of cold eddies behind the intruding warm water tongue. Nevertheless further quantitative investigations are required to compare with observations although the circulation in this area is very sensitive to many factors subjected to seasonal and interannual variations.