There are two main kinds of ocean currents, those driven by the wind and the earth's rotation that affect the surface waters, and those driven by density that affect the deeper waters.
Although relatively consistent in chemical composition, the waters of the oceans are not at all constant in terms of their movements. The easily visible wave and tide movements affect the surface layers more strongly.
Surface currents tend to be driven by winds. Ocean winds are the catalyst between the hydrosphere and the atmosphere. They assist the exchange of heat, greenhouse gases (such as carbon dioxide) and moisture between the ocean and the air above it.
When water evaporates from the surface the salt molecules stay behind, causing the surface waters to become more dense. This heavier water will sink to the bottom, turning the ocean’s water over.
The earth’s movement creates a gyral pattern of air movement. These in turn create large circular current patterns that are clockwise in the northern oceans and counter-clockwise in the southern oceans. Many of the currents formed transport vast masses of water. They, in turn, contain significant amounts of latent heat. This is why these large current systems significantly effect global climate systems.
c.o.o.l. fact #1
It all started with the mail. Benjamin Franklin, a great scientist, inventor and founding father of the USA, was appointed joint Postmaster General by the English King for the North American colonies on August 10, 1753. He noticed that ships traveling from England to the colonies would take a week longer than those traveling the opposite course. He asked whalers about their experience and, in this way, learned there was a river in the sea, called the Gulf Stream, that flowed up the east coast of America and then east to England, helping those ships traveling to England, and slowing down those traveling to the colonies.
c.o.o.l. fact #4
Waves are most often caused by winds. But what causes the wind? The sun. Where the sun beats down directly on the equatorial ocean, the ocean water and atmosphere is hotter than at higher latitudes (north and south). The resulting temperature imbalance creates a poleward flow of heat that is redistributed by atmospheric winds moving heat away from the equator.
(cool facts taken from the c.o.o.l. web site)
The deep ocean circulation is based more on chemical and temperature factors, called the thermo-haline system. The deep ocean currents are driven by the sinking and rising of waters of different densities (due to temperature and salt concentration differences). In general heavier, very cold and salty water sinks near the poles, where it is replaced by “lighter”, warmer and less salty water from the mid-latitudes. Thus there are complex layers of water moving over and under one another in the depths of the ocean.
The deep sea circulation is critically important because it helps to mix the deep waters, bringing oxygen and nutrients to the deepest layers.
Read more about ocean currents
Read more about the structure of the ocean or about the physical challenges of the deep sea