The horizontal and vertical circulation of ocean waters are called currents. Major causes of ocean currents include wind friction, gravity, and variation in water density in different parts of the ocean.

The Role of Wind
The Role of WindSurface ocean currents are mainly caused by wind. The winds that most affect the oceans' currents are:

The Westerlies (40-50 degree latitudes) blow west to east.
The Trade Winds (20 degree latitudes) blow east to west.
Both of these winds are a result of warm air from the tropics moving to the poles and the Coriolis effect (that due to the rotation of the Earth, water moves clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere). Because these huge circular currents are blocked by continents, smaller (though still giant) ocean current circles called gyres are formed. 

The Role of Density
Differences in water density affect vertical ocean currents (movement of surface ocean water to the bottom of the ocean and movement of deep ocean water to the surface). 

Variations in water density are caused by variations in salinity (the amount of salt in water) and temperature. Saltier water is more dense than less salty water; cold water is more dense than warm water. Denser water tends to sink, while less dense water tends to rise. Cold-water currents occur as the cold water at the poles sinks and slowly moves toward the equator. Warm-water currents travel out from the equator along the surface, flowing toward the poles to replace the sinking cold water.

As these currents mix the oceans surface and deep waters, they help replenish the oxygen in the water.

Other Causes of Currents
Other causes of currents include tides, rain, runoff, and ocean bottom topography. Topography is the surface features of a place. Ocean topography includes slopes, ridges, valleys, and mountains! All these things are found at the bottom of the ocean, and can influence currents.

The Gulf Stream
The Gulf Stream is one of the strongest currents known. It moves along through the Gulf of Mexico, past the east coast of the United States and on to Northern Europe. Without the warm Gulf Stream, England and other places in Europe would be as cold as Canada.

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