• Ocean currents are the continuous, predictable, directional movement of seawater.
• These movements are caused and influenced by various forces and are like river flows in oceans.
• Ocean water moves in two directions: horizontally and vertically.
• Horizontal movements are referred to as currents.
• Vertical changes are called upwellings or downwellings.
• Ocean currents have a significant impact on humankind and the biosphere due to their influence on climate.

Types of Ocean Currents

• Based on depth:
  • Ocean currents can be classified based on their depth as surface currents and deep water currents.
  • Surface currents constitute about 10% of all the water in the ocean, and these waters are the upper 400 meters of the ocean.
  • Deep water currents make up the other 90% of the ocean water and move around the ocean basins due to variations in density and gravity.
  • The density difference is a function of different temperatures and salinity.
  • Deep waters sink into the deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase.
• Based on temperature:
  • Ocean currents can be classified based on temperature as cold currents and warm currents.
  • Cold currents bring cold water into warm water areas, usually found on the west coast of continents in low and middle latitudes and on the east coast in high latitudes in the Northern Hemisphere.
  • Warm currents bring warm water into cold water areas, usually found on the east coast of continents in low and middle latitudes and on the west coast of continents in high latitudes in the Northern Hemisphere.

Forces Responsible For Ocean Currents

• Primary Forces:
  • Influence on insolation:
    • Heating by solar energy causes water to expand
    • Near the equator, ocean water is about 8 cm higher in level than in middle latitudes due to this expansion
    • A slight gradient is created, causing water to flow down the slope
    • The flow is typically from east to west
  • Influence of Wind (atmospheric circulation):
    • Wind causes movement of water on the surface of the ocean.
    • Friction between the wind and water surface affects the movement of water.
    • Winds determine the magnitude and direction of ocean currents, along with the Coriolis force.
    • Monsoon winds cause seasonal reversal of ocean currents in the Indian Ocean.
    • The oceanic circulation pattern corresponds to the earth’s atmospheric circulation pattern.
    • Anticyclonic air circulation over oceans in middle latitudes results in anticyclonic oceanic circulation pattern.
    • Cyclonic wind flow in higher latitudes results in cyclonic oceanic circulation pattern.
    • Monsoonal flow affects current movements in regions of pronounced monsoons.
  • Influence of Gravity:
    • Gravity tends to pull the water down to pile and create gradient variation.
  • Influence of Coriolis force:
    • The Coriolis force intervenes and causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere.
    • These large accumulations of water and the flow around them are called Gyres. These produce large circular currents in all the ocean basins. One such circular current is the Sargasso Sea.
• Secondary Forces:
  • Temperature and salinity differences are secondary forces affecting ocean currents.
  • Density differences in water affect vertical mobility of ocean currents.
  • Water with high salinity is denser than water with low salinity.
  • Denser water tends to sink, while relatively lighter water tends to rise.
  • Cold-water ocean currents occur when the cold water at the poles sinks and moves towards the equator.
  • Warm-water currents travel out from the equator along the surface, flowing towards the poles to replace the sinking cold water.

Important Ocean Currents

Causes of Ocean Currents

• Planetary winds:
  • Planetary winds are a major cause of ocean currents.
  • The winds blow continuously in a particular direction and drag the surface water due to friction.
  • This leads to the formation of ocean currents.
  • Most ocean currents follow the direction of prevailing or planetary winds.
  • Equatorial currents flow westward due to northeast and southeast trade winds.
  • North Atlantic Drift in the Atlantic and the North Pacific current in the Pacific move in the northeast direction under the influence of westerlies.
• Variation on sea water temperatures:
  • The ocean temperatures vary significantly in both horizontal and vertical distribution.
  • The water is warmer at the equator than at the poles.
  • High temperatures and more rainfall at the equator decrease the water density.
  • Lighter water from the equatorial region moves towards colder and denser water in polar areas.
• Variation in water salinity:
  • Salinity is the saltiness of an ocean.
  • The amount of salts in seawater varies from one part of the ocean to another.
  • High salinity water moves below the water of low salinity, generating ocean currents from areas of low salinity to areas of high salinity.
  • There is a marked variation in the salinity of the Atlantic Ocean and the Mediterranean Sea.
  • Ocean currents flow from the Atlantic Ocean to the Mediterranean Sea because of this variation.
• Rotation of the Earth:
  • The Earth rotates on its axis from west to east.
  • This rotation causes a deflective force known as the Coriolis force.
  • Coriolis force deflects the general direction of the winds and ocean currents.
  • The currents flowing from the Equator towards the North and South Poles are deflected to their right in the Northern Hemisphere and towards their left in the Southern Hemisphere.
  • The counter-equatorial currents are also the result of the rotation of the Earth.
• Configuration of coastline:
  • The shape and configuration of coastlines affect the direction and movement of ocean currents.
  • Obstruction of equatorial current by Brazilian coast leads to bifurcation into two branches, the Northern Branch is called the Caribbean current while the Southern branch is called the Brazilian current.
  • In the Indian Ocean, the monsoon currents follow the coastlines closely.
  • 

Desert Formation and Ocean Currents

Major hot deserts are located between 20-30 degree latitudes and on the western side of the continents. Why?

• Hot deserts are arid due to the effects of off-shore Trade Winds.
• Major hot deserts are located on the western coasts of continents between latitudes 15° and 30°N and S.
• Hot deserts lie along Horse Latitudes or the Sub-Tropical High-Pressure Belts where the air is descending, which is unfavorable for precipitation.
• Rain-bearing Trade Winds blow off-shore and the Westerlies blow outside the desert limits.
• Winds reaching the deserts blow from cooler to warmer regions, lowering their relative humidity and making condensation almost impossible.
• There is scarcely any cloud in the continuous blue sky and the relative humidity is extremely low, decreasing from 60 percent in coastal districts to less than 30 percent in the desert interiors.
• On western coasts, the presence of cold currents gives rise to mists and fogs by chilling the oncoming air. This air is later warmed by contact with the hot land, and little rainfall occurs.
• The desiccating effect of the cold Peruvian Current along the Chilean coast is so pronounced that the mean annual rainfall for the Atacama Desert is not more than 1.3 cm.