The water in the ocean is never still. Like a river on land, it flows along a relatively fixed route for many years, which is called "ocean current". However, both sides of the river are land, and both sides of the ocean current are sea water.
In general, it is difficult to see with the naked eye. The largest ocean current in the world is hundreds of kilometers wide, thousands of kilometers long and hundreds of meters deep. The ocean currents are very large. Ocean currents don't all flow in one direction.
There are many reasons for the formation of ocean circulation. Wind, the location of the ocean, the distribution of land and sea, and the deflection force (called Coriolis force) produced by the earth's rotation have all exerted influence, which can be said to be the result of the comprehensive action of many factors. The wind can not only set off waves, but also blow seawater into streams.
The steady wind all the year round can form a lasting ocean current. The persistent equatorial airflow is formed by the easterly wind from the trade winds. The stability of west wind drift is attributed to the strong westerlies. Therefore, some people call the ocean surface current "wind current".
However, the "ring" formed by the ocean circulation can't record all the credit in the wind account, and the distribution of continents and the role of geostrophic deflectors play an important role. When the equatorial current reaches the western edge of the ocean all the way west, it is blocked by the mainland, and there are only two ways out. One is to return to the east coast by the original road, and the other is to turn.
However, because the "follow-up troops" are mighty and constantly coming in, it is impossible to return them all, so we have to separate a small team and sneak into the lower level to return, becoming an equatorial undercurrent; Most of the rest have to turn around and find another way to move on. Where do I turn? At this time, the geostrophic deflection helped it.
In the northern hemisphere, ocean currents turn right under the action of geostrophic bias and turn left in the southern hemisphere. Coupled with the obstruction of the mainland, the ocean current turned to the polar regions on a large scale.
On the way to the polar direction of the ocean current, the geostrophic force is not relaxed for a moment, and the tension is getting stronger and stronger. When the latitude is about 40 degrees, the strong westerly belt and geostrophic force form a combined force, which makes the ocean current become westerly and eastward. Similarly, when west wind drift reaches the east coast of the ocean, it will inevitably flow to the equator through the road, thus completing a big cycle.