The main patterns of water movement in the oceans result from the Earth's rotation and the gravitational interaction between the Earth, Moon and Sun.
The ocean's surface currents are created and driven by the winds on the Earth's surface, which themselves result from differences in the temperature of the air between the equator and the poles.
The Earth's rotation subjects the movements of air and water on its surface to a force that causes them to move in a clockwise fashion in the Northern Hemisphere, and in an anticlockwise motion in the Southern Hemisphere. This is known as the Coriolis Effect.
Surface currents move very much slower than the prevailing winds that create them. The faster ocean currents travel at up to 9.25km/h and the slower ones at about 1.85km/h. But the currents do not move in exactly the same direction as the winds because of the Coriolis Effect.
Landmasses also deflect currents and cause water to travel in large circular paths called gyres. The principal gyres move in a clockwise direction in the North Atlantic and North Pacific oceans and anticlockwise in the South Atlantic and South Pacific oceans, following the Coriolis Effect.
The North Atlantic gyre is composed of three major currents - the Gulf Stream, the Canary Current and the North Equatorial Current.
The gyre in the northern Indian Ocean changes direction with the seasons because of the seasonal change of the monsoon winds. In winter the wind blows from the north-east, causing an anticlockwise circulation in the ocean; in the summer it blows from the south-west, creating a clockwise current. This is known as the Monsoon effect.
The currents on the western sides of gyres move water away from the equator, as do the Gulf Stream and the Brazil Current in the Atlantic, and they are warm. They gradually lose their heat and those currents that return water to the equator, such as the Canary and Benguela currents, are cooler.
Water in the center of gyres is almost stationary. The warm currents on their western sides are narrow and flow quite quickly, whereas the cold currents on the eastern sides are much slower and occur in broad bands. Surface currents extend to less than 350m deep, but they still transport large volumes of water. The Gulf Stream, for example, probably contains a hundred times as much water as there is in all the Earth's rivers.
When water is turned away from landmasses, a phenomenon called upwelling occurs. To replace the surface water blown away from the coastline, water moves up from deeper, colder layers and decreases the temperature of the surface water. Ocean currents can thus moderate climate by cooling hot regions, and sometimes by warming cool ones. The harbors of Norway, for example, are kept ice-free by the North Atlantic Drift, which is warmed by the Gulf Stream.
In addition to their climatic influences, ocean currents have a profound effect on marine plants and animals - and, indirectly, on some terrestrial forms of life. All marine animals ultimately depend for food on small photosynthetic plankton which, in the relatively light upper layers of the oceans, use sunlight to convert inorganic substances into food, giving off oxygen as a byproduct.
Slightly denser than seawater, the plankton slowly sink but are brought back to the surface by turbulent upwelling; they can also be carried long distances by ocean currents. Many marine animals feed on plankton and therefore tend to follow their food source as it is moved by the sea which, in turn, determines the movements of those animals that eat the plankton-feeders. This interdependence of predator and prey continues through numerous marine species and may eventually involve terrestrial forms of life that feed on marine creatures.
Unlike most ocean currents, there is one that flows all the way around the world - the Antarctic Circumpolar Current. Just north of the Antarctic landmass there are no land barriers and so the Westerlies can drive the current eastwards without being obstructed.
Near the equator, the winds are not very strong and a narrow weak current - the Equatorial Counter Current - flows eastwards between the westward flowing North and South Equatorial Currents.
Currents also exist in ocean waters four or five kilometers below the surface. These are not caused by the wind but by differences in density of the water. Denser water flows over the ocean floor beneath warmer, less dense water. The density of water depends, in turn, on its temperature and salinity; cold water has greater salinity and is denser than warmer water.
Deeper currents often flow in the opposite direction to the surface currents, and they usually travel more slowly. The layer of water below the surface current moves with the top layer because of the friction between the two, but because its speed is slightly slower than the layer above, the Coriolis force moves it farther to the right or left of the wind's direction. This process continues down through successive layers, in what is known as the Ekman Spiral, each layer decreasing in velocity until, at a depth of about 100m, the water flows at 180° to the wind's direction and at about 4 per cent of the speed of the surface current.
In the Straits of Gibraltar, for example, the surface current flows eastwards into the Mediterranean Sea, but a deeper, more saline current flows beneath it, out into the Atlantic Ocean. A deep current also exists below the Gulf Stream, which flows southwards along the sea bed in the opposite direction to the Gulf Stream at the surface. Marcia Malory thanks to Best Aus Casinos website for help during preparation of the article.