How do monsoons change surface currents in the South China Sea?

In the South China Sea, monsoons are the primary drivers of sea-surface currents and the exchange of water with adjacent bodies of water. The direction of these currents reverses seasonally in response to the prevailing wind patterns:12

• Winter: During the winter months, the monsoon winds blow from the northeast, causing the surface flow to move generally toward the southwest.1
• Summer: During the summer, the monsoon winds shift to blow predominantly from the southwest. This drives the surface water toward the northeast, with the strongest flow occurring along the coast of Vietnam at speeds reaching up to 3.5 miles (5.6 km) per hour.1

Additionally, the summer monsoon influences local hydrology by displacing surface water away from the coast. This process triggers upwelling, which brings colder, nutrient-rich water to the surface off the coast of central Vietnam. In August, surface water also flows into the South China Sea from the Java Sea through the Karimata and Gelasa straits.1