Currents are movement of oceanic water in a fairly defined direction under the influence of various forces.
The origin and nature of movement of currents are related to various factors:
1. Gravitational forces cause vertical movement of ocean waters. Greater values of gravitational forces cause the sinking and subsidence of water at the poles.
image source: askabiologist.asu.edu/sites/default/files/resources/articles/marine_biome/Surface-Current-Gyres.jpg
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2. Coriolis forces cause the ocean currents to be deflected right in the Northern Hemisphere, e.g., Gulf Stream and Kuroshio Current; and left in the Southern Hemispheres.
3. Differences-^in atmospheric pressure. High pressure causes lowering of the sea level artd low pressure causes the sea level to rise. Water moves from low pressure to high pressure area.
4. Winds and frictional forces. A steady blowing wind, through friction, brings about the surface flow in the direction of the wind, which in turn, exerts certain amount of friction and drags the lower layer with it thus, the entire water after sometime moves horizontally in the direction of the wind.
Trade winds, for example, cause the flow of Equatorial Currents from east to west and Westerlies from west to east cause Gulf Stream and Kuroshio Currents.
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i. Winds cause divergence and convergence and upwelling and downwelling of the ocean waters.
ii. The most important downwelling area is east of Greenland and Weddell Sea.
iii. Upwelling areas are those where the cold water from below comes on to the surface because the surface water has been blown away by offshore winds.
iv. Examples of currents generated by upwelling are California Current, Humboldt Current, Canary Currents, Benguela Current and Western Australian Current all on the western side of the continents.
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v. The only upwelling area toward the eastern side of the continent is off Somali-forming cool Somali Current.
5. Winds lead to piling of water. Piling is caused by wind blowing constantly in one direction along a land barrier or partially enclosed seas the piling of water under the stress of N’. Equatorial Current in the Gulf of Mexico has led to evolution of Equatorial Counter Currents.
6. Density differences are caused by evaporation, precipitation, melting of snow and meeting of river. Evaporation causes an increase in salinity and density and causes the water to sink inviting less dense water to take its place. Example includes movement of water from the Atlantic Ocean to Mediterranean Sea.
i. Currents can also be generated where the river meets the sea.
ii. Melting of snow feed the Labrador Current.
7. Ocean current once generated get modified by direction and shape of the coastline.
For example, the Equatorial Current is bifurcated by the obstruction of Brazilian coast and one branch flow as Gulf Stream while the second branch as Brazilian current.