Ocean currents are caused mainly by prevailing surface winds.
Surface winds cause water to move horizontally across the ocean surface, creating currents. These winds are influenced by a range of factors, including the rotation of the earth, the shape of the continents, and the distribution of land and sea.
The rotation of the earth also plays a role in ocean currents, as it creates a force known as the Coriolis effect. This effect causes the direction of the wind and currents to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
However, prevailing surface winds are the main driver of ocean currents. These winds are created by differences in air pressure in the atmosphere, which in turn are caused by the heating and cooling of the earth's surface. The movement of these surface winds, along with the Coriolis effect, creates a circular pattern of ocean currents known as a gyre.
While the amount of evaporation from the oceans and the shape of the ocean can also influence ocean currents, they are not the main causes. Evaporation can lead to the formation of denser, saltier water, which can sink and create vertical currents, while the shape of the ocean can create areas of upwelling and downwelling. However, these processes are secondary to the role of surface winds in driving ocean currents.