Surface currents on the ocean are due to the water being moved by winds that blow in certain patterns because of the earth rotating in its own axis, creating the Coriolis effect.
The Coriolis effect is the way everything that is freely moving across the earth's surface tends to curve as they move. Winds are able to move the top 400 meters of the ocean thus creating surface currents.
Due to the Coriolis effect, surface currents in the northern hemisphere oceans flow clockwise in large circular patterns called gyres, while surface currents in the southern hemisphere oceans flow anti-clockwise.
