The general movement of a mass of water in fairly defined direction under various forces internal as well as external is known as the ocean currents.
The ocean currents do not follow any straight route. They have been found to follow some zigzag courses, which do not have equal width everywhere. They are branched into smaller streams and at places many smaller branches unit to form broader streams. Due to these characteristics, the ocean currents are sometimes called ocean rivers.
Causes of the Origin of Ocean Currents
Temperature Differences. Temperature is low at the poles and high at Equator. The polar water cools and becomes heavy while that at the Equator warms and becomes lighter. The heavy water sinks and the warm water floats. This makes a difference in the level of the sea. To maintain the balance, the water from the equator moves towards the poles in the form of currents. To counter this, polar currents flow towards the equator under the surface.
Salinity Differences. Saline water is heavy and therefore subsides. To take its place, the less saline water surrounding it rushes towards the area of high salinity. To maintain the balance, a current of high saline water moves towards the area of lower salinity under the surface of sea.
Axial Relation of the Earth. The Earth rotates one rotation around its axis in 24 hours. This rotation is responsible for the origin of currents.
(1) Centrifugal Force. Every revolving body develops a centrifugal force at its centre and makes it move away from it. This force increases as the distance of the body increases from the centre of the Earth due to the centrifugal force. The centrifugal force at the Equator is greater than that at the poles because the Great circle at the time of revolution coincides with the equator. The variation of this force causes the equatorial water to move towards the poles.
(ii) Ferrel's Law. Ferrel studied the rotational influence of the earth on the courses of ocean currents and stated that the ocean currents tend to deflect towards the right hand in the northern hemisphere and towards the left in the southern hemisphere. This influence diverts the flow of ocean currents and creates branches from the main currents.
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