Current

Currents are large-scale water movements that occur everywhere in the ocean. The surface currents are driven by winds, while deep subsurface currents are driven by density differences in the ocean water. Ocean currents transport heat from the equator toward the poles, thereby partially equalizing surface temperatures over the earth. Ocean currents, winds, and weather patterns are closely linked. Currents can affect the food chain by transporting nutrients and plankton from one area to another. Fish congregate in high plankton areas to feed, attracting larger predators such as tuna, birds and marine mammals (and humans!). Wind Driven Currents The ocean and atmosphere of the earth are heated unevenly by the sun. More heating takes place at the equator than at the poles. This difference in temperature at the equator and the poles causes warm air to rise along the equator, and cold air to sink at the poles. Rising and sinking air creates wind, as adjacent air masses move in response..

At the North and South Poles, ocean water is cooled by the polar ice caps

and by the lack of sun. Very cold, dense water sinks and flows along the

bottom of the ocean toward the equator. Antarctic bottom currents flow past

the equator into the northern hemisphere. These polar bottom currents are

very slow moving. It may take 600 years for Antarctic bottom water to reach

into the northern hemisphere. This very cold water is full of oxygen and is the

primary source of oxygen in the deep sea.

At the equator, waters warmed by the tropical sun rise, expand, and flow

out away from the equator. Remember that the atmosphere is moving in much

the same pattern, also due to unequal heating by the sun. The world's oceans travel in well-defined circular patterns called currents which flow like rivers. When the atmosphere pushes over the surface of the ocean some of the energy goes to forming waves while the rest goes to pushing the water in the direction of the wind. North of the equator currents bend to the right; south of the equator, they bend to the left. This is called the Coriolis effect. Winds, continents and the Coriolis effect make currents flow around the oceans in huge loops called gyres.
Energy from the sun also causes currents to flow. Water near the equator is heated more than water at middle latitudes causing a surface flow toward the poles. Where two currents meet, the colder water sinks pushing warmer water up to the surface.
Read more at :

http://earth.usc.edu/~stott/Catalina/Oceans.html