Surface currents are forced by frictional interaction between the ocean surface and the prevailing atmospheric wind. The wind imparts its momentum to the top layer of the ocean.In each ocean, the prevailing trade winds drive equatorial ocean currents from east to west. When these currents encounter land, they divide to flow north and south along the eastern borders of the continents. As they progress, they are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere by the Coriolis Effect (see this link for a detailed explanation), creating large vortices or gyres, which are very large horizontal cells of water circulation.