The thermal state of the South China Sea (SCS) modulates the regional climate variability over Southeast Asia. Current in the SCS is an important factor impacting the thermal state of the SCS, but their relationship is not clearly understood. There is an asymmetry in the thermal effect of weak and strong SCS winter currents. Weak SCS winter currents favor stable warm advection of mean temperature by the anomalous horizontal velocity (i.e., advha), which drives the SCS into a warm phase. However, the cooling effect of strong SCS winter currents on the SCS is weak, due to small and variable negative advha. The basin-integrated advha is primarily set by meridional heat flux in the southern SCS, which is mainly determined by the western boundary current (WBC) anomaly. The eastern boundary current (EBC) anomaly with opposite direction of WBC anomaly acts to weaken the advha. In weak (strong) SCS winter current years, the windstress anomaly over the southern SCS is localized around the western (eastern) boundary, which induces weak (strong) EBC anomaly. Therefore, warm advha in weak SCS winter current years is large enough to drive the SCS into a warm phase. However, the negative advha in strong SCS winter current years is variable, which can be occasionally offset by positive advection of anomalous temperature by the mean horizontal velocity and then the SCS present a warm phase, such as the year of 1998. Thus, the strong SCS winter current exerts a weak cooling effect on the SCS.

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