Why are South China Sea waters warmer in the south than in the north during winter?

The South China Sea is warmer in the south than in the north during winter primarily due to the influence of latitude on solar radiation and the resulting temperature distribution of the ocean surface.1

In general, ocean surface temperatures are higher at lower latitudes because they receive more consistent solar heating throughout the year compared to higher latitudes. During the winter months, the northern portion of the South China Sea experiences cooler temperatures—approximately 70 °F (21 °C)—as it is situated at a higher latitude and is more directly influenced by cold air masses moving off the Asian continent. In contrast, the southern portion remains warmer, at approximately 81 °F (27 °C), due to its proximity to the equator, where the seasonal variation in solar heating is significantly less pronounced.12