What causes most surface ocean currents?

Surface ocean currents are caused by winds. Wind currents run in certain directions because the earth rotates. Winds go to the right in the northern hemisphere and to the left in the southern hemisphere.

Winds curve out from the equator as the earth turns, similar to the way wind bends around a hand thrust out a car window. This is called the Coriolis effect. These wind patterns blow the surface of ocean water; the top of the water pulls water below it along with it, resulting in surface ocean currents. There is not enough force to affect the water more than about 100 yards down.