The five major ocean gyres

A gyre is a large system of ocean currents moving in a circle. Gyres are caused by the Coriolis effect. Because the Earth is rotating, ocean currents in the northern hemisphere tend to move in a clockwise direction and currents in the southern hemisphere move in an anti-clockwise direction.

The Coriolis force acts most strongly on the wind, and the wind then creates a torque that tends to spin the ocean currents.