NCERT Class 11 - Fundamentals of Physical Geography, Chapter 14

Ocean Currents of the World

An ocean current is a continuous flow of water in a definite direction, like a river within the sea. Currents carry heat from the equator towards the poles and cold water back, which makes them one of the great controls of world climate.

Causes and types

• Prevailing winds are the main cause of surface currents; the trade winds and the westerlies push water along and set up the great circular systems called gyres.
• Differences in temperature and salinity change water density and drive deep circulation - cold, salty water sinks and flows along the ocean floor.
• The Coriolis force deflects currents to the right in the northern hemisphere and to the left in the southern, which is why gyres turn clockwise in the north and anticlockwise in the south.
• The shape of the coastline and the position of land masses divert currents and split them into branches.
• Currents are warm when they flow from lower to higher latitudes and cold when they flow from higher to lower latitudes or rise from the depths.

Currents by ocean

• The North Indian Ocean is unique - its currents reverse direction with the monsoon, flowing clockwise in summer and anticlockwise in winter.
• Upwelling occurs where winds push surface water away from a coast and cold, nutrient rich water rises to replace it, as off Peru and off the west coast of Africa.

Effects of currents

• Climate - the North Atlantic Drift keeps the ports of north-western Europe ice free in winter, at latitudes where ports on the Canadian side are frozen; the cold Labrador current makes the Canadian coast far colder.
• Deserts - cold currents flowing along western coasts in the subtropics chill the air above them, preventing rain and helping to create coastal deserts such as the Atacama and the Namib.
• Fisheries - the world's greatest fishing grounds lie where a warm and a cold current meet, because the mixing brings nutrients to the surface; the Grand Banks off Newfoundland, where the Gulf Stream meets the Labrador current, is the classic example.
• Fog - the same meeting of warm and cold water produces dense fog, which makes such areas dangerous for shipping.
• Navigation - ships historically used currents and the winds above them to shorten voyages, and icebergs carried south by cold currents are a hazard.
• El Nino - a periodic warming of the eastern Pacific that weakens the cold Peru current, destroying the fishery and disturbing rainfall patterns far beyond, including a tendency towards a weaker Indian monsoon.