Mounds of circulating water in the Pacific, Indian and Atlantic oceans are known as 'subtropical gyres'. Ocean currents circulate around the gyres, clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The ocean surface at the centres of these gyres is up to a metre above mean sea level.
Ocean surface current systems are driven by winds. Friction between wind and the water surface sets the upper layer of ocean into motion. The subtropical gyres are driven mainly by easterly trade winds near the equator and westerly winds at higher latitudes.
Warm boundary currents, such as the East Australian current, flow polewards on the western boundaries of the major oceans. These currents are among the fastest and deepest of the ocean currents, and distribute heat to the polar regions. The cool currents flowing towards the equator on the eastern sides of the oceans are wider, shallower and slow moving.
The cold Antarctic circumpolar current, driven by westerly winds, circles Antarctica. This current extends deeply, carrying more water than any of the other currents, and is the only current to flow around the globe. It has an important role in distributing water and heat between the Indian, Pacific and Atlantic oceans.