The Sun heats the Earth's atmosphere unevenly, creating very warm, high-pressure areas around the equator, and cool, low-pressure areas toward the poles. As the warm air moves to low-pressure areas, wind is created.
Uninterrupted, wind patterns would run in straight lines from pole to pole. However, because of the earth's rotation, wind is forced right or left instead. This phenomenon is called the Coriolis Effect.
As wind blows across the ocean, it drags water along with it, creating strong currents. Because the wind's reach is limited, these surface currents only run about 200 meters (656 feet) deep.
Short-lived winds create temporary surface currents. Constant winds create more permanent currents that flow strongly on a fixed course.
Surface current patterns follow wind patterns to an extent, although they are also influenced by geography. Because of the Coriolis effect, surface currents in every ocean flow in large loops called gyres.
Gyres in the Northern hemisphere flow clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere.