The Indo-Pacific warm water pool in boreal winter shows a conspicuous gap over the South China Sea (SCS) where sea surface temperature (SST) is considerably lower than over the oceans both to the west and east. The formation mechanisms for the climatology and interannual variability of SCS SST in boreal winter are investigated using a suite of new satellite measurements. The winter SCS is divided into two parts by the axis of the maximum northeasterly monsoonal winds. The positive wind curl in the southeastern half of the ocean drives a cyclonic gyre circulation in the deep basin. As its western boundary current, an intense southward flow is found south of Vietnam on the continental slope separating the Sunda Shelf to the west and the deep SCS basin to the east. This slope current exceeds 0.5 m s-1 in speed and advects cold water from the north. This cold advection results in a distinct cold tongue in the winter SST climatology. Both the slope current and the cold tongue are strongest in November to February. This winter cold tongue displays considerable interannual variability that is highly correlated with eastern equatorial Pacific SST. In an El Ni¿o the winter monsoon weakens, causing the SCS ocean circulation to spin down. The reduced western boundary current and its thermal advection result in a warming in the SCS winter cold tongue. Both SST variance and its correlation with the El Ni¿o-Southern Oscillation peak along the climatological cold tongue indicate that ocean dynamics are an important player in SCS climate variability.