When the wind blows against the sea surface, it drives the flow of surface seawater horizontally. But do you know that wind's ability is not limited to this? It can also trigger the vertical movement of seawater, causing seawater to rise and sink in certain sea areas, forming so-called upwelling and downwelling currents? This situation is particularly obvious in coastal waters. What is going on? It turns out that in coastal waters, the movement of seawater is not only driven by wind, but also affected by another important factor-geostrophic deflection force. Geostrophic deflection force refers to the deflection force generated by the earth's rotation from west to east. It causes all moving objects in the northern hemisphere to deflect to the right, and all moving objects in the southern hemisphere to deflect to the left. If in the northern hemisphere, a person stands on the shore and faces the sea, when the wind blows from right to left, parallel to the coast for a long time, under the influence of the geostrophic deflection force, the wind will make the surface seawater leave the coast. At this time, the lower seawater near the shore will rise to replenish it, and the flowing seawater is called upwelling. There are obvious upwelling in many sea areas on the earth, such as the upwelling along the Oregon coast in the North Pacific, the upwelling along the Peruvian coast in the South Pacific, and the upwelling along the Somali coast in the Indian Ocean. Famous wind-driven upwelling also exists along the coast of Zhoushan Island in the East China Sea and the eastern coast of Hainan Island in China.
On the contrary, in the northern hemisphere, if the wind blows from left to right, sea water will accumulate and sink ashore, and a downflow will occur. For example, when the northeasterly wind blows in winter, downcurrents will occur in the coastal areas of the Yellow Sea, East China Sea, and South China Sea in China. Another cause of sea water rising and sinking is atmospheric vortices. In open seas, an atmospheric vortex moving counterclockwise can cause the seawater in the affected sea area to rise. The winter upwelling that occurs in the southern part of the Bashi Strait in the South China Sea near the Philippine Islands is a typical example. In contrast, a clockwise atmospheric vortex causes the sea to fall.