UPSC Express

Regular movement of a large mass of ocean water in fairly defined direction over great distances is known as ocean current. They origin and movement is influenced by a multitude of factors including planetary winds, coriolis forces, gravity, temperature, salinity etc.

The currents are categorised as warm or cold on the basis of relative temperature w.r.t the surrounding water.

• Examples of warm currents (which bring warm water into cold water areas): Kuroshio current, gulf stream
• Examples of cold currents (which bring cold water into warm water areas):  Oyashio Current, Labrador current.

Factors affecting ocean currents of the world:

a) Factors related to atmosphere:

• Planetary winds: The leading role in ocean currents is played by frictional drag of surface water by planetary winds. Most of the currents of the world tend to follow the direction of planetary winds. For example, within the tropics, the trade winds blow from the east and hence, north equatorial and south equatorial currents flow east to west. The impact of planetary winds is most prominently seen in Indian Ocean, where there is a seasonal reversal of the monsoon winds and generates the Somali current.
• Others: Other factors including atmospheric pressure, rainfall and rate of evaporation also affect the flow of currents. .

b) Factors related to earth’s rotation:

• As the earth rotates from west to east, the water near equator tends to move from east to west.
• Earth’s rotation on its own axis generates the Coriolis force, due to which the moving water turns and follows an elliptical path known as gyre. In these gyres, water circulates clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.

c) Set of factors related to ocean:

• Temperature difference and salinity difference also affect the ocean current movement.
• Salinity: Water having high salinity is heavier and it subsides. As a result, light water with less salinity rushes towards the area of high salinity and a current is formed. For example, Mediterranean Sea is more saline and on the surface, water moves from Atlantic to the Mediterranean.
• Temperature: Similarly, equatorial water becomes lighter due to heating while water at the poles becomes heavy due to cooling. As a result, a difference in sea levels in created and a surface current is created from equatorial regions towards the poles.

d) Modifying factors:

• Shape of coastline and oceanic bottom topography also affect the currents. For example, the South equatorial current splits up as the Brazilian current at the Brazilian coast.

Ocean currents are important phenomenon which help in circulation of the ocean water and are essential for regulating the temperature, salinity and nutrient flow throughout the ocean.