The Earth is surrounded by two great oceans: an ocean of air and an ocean of water. Both are in constant motion, driven by the energy of the sun and the gravity of the Earth. Their motions are linked; the winds give energy to the sea surface and ocean currents are the result. The currents carry heat from one location to another, altering the Earth's surface temperature patterns and modifying the air above.

Out in the open sea, ocean waters are driven by two great wind systems. Close to the equator the Trade Winds blow the surface waters westward. In the temperate zone, the Westerlies blow the surface waters back toward the east. The result is that in each great ocean basin there is roughly circular movement of the surface waters. In the northern hemisphere these wind driven currents move clockwise and in the southern hemisphere they move counter clockwise. Both surface and deep-water currents affect the world's climate by moving cold water from the poles toward the tropics and vice versa.