N.K. Chauhan 
Ocean currents are the continuous, predictable, directional movement of seawater driven by earth's rotation, gravity, wind (Coriolis Effect), temperature and water density.
Ocean water moves in two directions - horizontally and vertically.
Horizontal movements are referred to as Waves and Currents, while vertical changes are called Tides, Upwellings.
An ocean current flows for great distances and together they create the global conveyor belt, which plays a dominant role in influencing the temperature and determining the climate of the regions through which they travel.
There are two type of Ocean Currents:
1) Surface Currents (Surface Circulation) are the upper 400 meters of the ocean and make up about 10 per cent of all the water in the ocean.
2) Deep Water Currents (Thermohaline Circulation) move around the ocean basins by density driven forces and gravity and make up the other 90 per cent of the ocean.
Ocean currents are influenced by two types of forces:
1) Primary forces
Primary forces like heat, wind, gravity, coriolis force initiate the movement of water.Heat causes the water to expand, this makes a very slight gradient and water tends to flow down the slope - the flow is normally from east to west.
When air moves across the ocean's surface, it pulls the top layers of water with it through friction.
Surface ocean currents are driven by consistent wind patterns that persist throughout time over the entire globe, such as the jet stream.
Gravity tends to pull the water down to pile of water against the pressure gradient.
But the Coriolis Force intervenes and cause the water to move to the right (in the northern hemisphere) around the mound of water.
These large mounds of water and the flow around them are called Gyres.
This produce large circular currents in all the ocean basins.
Due to Coriolis force which follows Ferrel's law, the general movement of ocean currents in the northern hemisphere is in the clockwise and in the southern hemisphere it is in the anti-clockwise direction.
A notable exception to this trend is seen in the northern part of the Indian Ocean where the current movement changes its direction in response to the seasonal change in the direction of monsoon winds.
2) Secondary forces
Secondary forces like temperature and salinity difference influence the currents to flow.Temperature difference
The difference in heating of the Sun at the equator and the poles causes a difference in the temperature of ocean water.At the equator, since the temperature is higher the ocean water gets heated up and expands.
This makes the warm water lighter and hence rises while at the poles, cold water is denser and sinks.
Warm water from the equator slowly moves along the surface towards the poles, while the cold water from the poles slowly creeps along the bottom of the sea towards the equator.
Salinity difference
The density of water also depends on its salinity and the salinity of water varies from place to place.Waters of low salinity flow on the surface, while waters of high salinity flow at the bottom.
Differences in water density affect vertical mobility of ocean currents (vertical currents).
| Cold Ocean Current | Details |
|---|---|
| Humboldt (Peruvian) Current | flows north along the western coast of South America |
| Kurile (Oya shio Current) | subarctic, flows south and circulates 'counterclockwise' in western North Pacific Ocean |
| California Current | moves southward along the western coast of North America |
| Antarctic Circumpolar Current | flows clockwise around Antarctica, keeps warm ocean waters away from the Antarctica |
| Labrador Current | flows from the Arctic Ocean to south along the coast of Labrador |
| Canary Current | part of the North Atlantic Gyre, flows southwest along N-W Africa and joins the 'Atlantic North Equatorial Current' |
| Eastern Greenland Current | runs south, off the eastern coast of Greenland |
| Benguela Current | flows northward along western coast of Africa, part of South Atlantic Ocean gyre |
| Falkland Current | flows northward along southern coast of Argentina, opposite to 'Brazil current' |
| Northeast Monsoon Current | North Indian Ocean |
| Somali Current | seasonal change in direction, along the coast of Somalia and Oman |
| Western Australian Current | flows northward parallel to the western coast of Australia |
| South Indian Ocean Current | South Indian Ocean |
| Warm Ocean Current | Details |
|---|---|
| North Equatorial Current | flows from east to west in both "Pacific" and "Atlantic" Ocean, between 10° N and 20° N |
| Kuroshio Current (Black or Japan Current) | begins east of the "Philippines" then flows in a north-east-ward direction past "Taiwan" and "Japan" |
| Tsushima Current | the northeastward-flowing branch of the Kuroshio along the west coast of Japan |
| North Pacific Current | flows west-to-east between 30 and 50 degrees north in the Pacific Ocean |
| Alaskan Current | southwestern alongside the west coast of the North American |
| Equatorial Counter Current | flows west-to-east at about 3-10°N in the "Atlantic", "Indian" and "Pacific" oceans, between "north & south equatorial current" |
| El Nino Current | Central & East-Central Equatorial Pacific |
| South Equatorial Current | flows from east to west in "Indian", "Pacific" and "Atlantic" Ocean, between 5° N and 20° S |
| East Australian Current | southward flowing along eastern coast of Australia |
| Florida Current | in "Caribbean Sea", from the Straits of Florida along the southeastern coast of the US, joins Gulf Stream |
| Gulf Stream | originates in the Gulf of Mexico and follows the eastern coastlines of the United States |
| Norwegian Current | in Barents Sea (Arctic Ocean), northeastwards along the coast of Norway |
| Irminger Current | westward off the southwest coast of Iceland |
| Antilles Current | branch of the Atlantic North Equatorial Current, northward east of the Antilles and joins the Florida Current |
| Brazilian Current | flows south along the Brazilian south coast |
| Mozambique Current | flowing south along the African east coast, between Mozambique and Madagascar |
| Madagascar Current | split into two - North & East Current, in "west Indian Ocean" |
| Agulhas Current | flows south along the east coast of Africa |
The North East & South West Monsoon Drift
The Indian Monsoon Current, also called Monsoon Drift are surface current of the northern Indian Ocean.During winter, the flow of the upper ocean is directed westward from near the Indonesian Archipelago to the Arabian Sea.
During the summer, the direction reverses, with eastward flow extending from Somalia into the Bay of Bengal.
The seasonally reversing open ocean currents that pass south of India are referred to as the Winter Monsoon Current (Northeast Monsoon Current) and the Summer Monsoon Current (Southwest Monsoon Current).
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In the Indian Ocean the place of a "north equatorial current" is taken by the Monsoon Current.There is, however, an Indian South Equatorial Current flowing westerly with the trades north of latitude 22° S, it divides to form the East Africa Coastal Current, moving northward, and a south-flowing stream.
