Ocean Currents

Q.”Temperature,salinity and density differences in ocean water are the prime causes of ocean water circulation.” elaborate (30/11) Q. How atmospheric conditions influence and control ocean water circulation.(Self)

The general movement of a mass of oceanic water in a definite direction is called ocean current which is more or less similar to water streams (rivers) flowing on the land surface of the earth.

CONTROLLING FACTORS

1.Factors related to the rotation of the earth

• Rotation of earth creates coriolis force which deflects the general direction of ocean currents.

2.Factors related to the oceans

a.Temperature difference:

• High temperature in the equatorial region results in expansion of water particles due to which water density decreases .Similarly, in polar areas due to low temperature water density increases.
• Thus, due to expansion of volume, water moves from equatorial region to polar region.
• There is movement of ocean water below the water surface in the form of subsurface current from colder polar areas to warmer equatorial areas in order to balance the loss of water in the equatorial areas.
• Thus, surface currents and subsurface currents form a complete circulatory system of ocean water.
• The Gulf Stream and kuroshio warm currents moving from equator towards north are examples of such Currents.

b.Salinity difference:

• Oceanic salinity affects the density of ocean water and density variation causes ocean currents. Salinity increases the density of ocean water if two areas having equal temperature are characterized by wearing salinity the area of high salinity will have greater density than the area of low salinity. The denser water sinks and moves as sub surface currents whereas less saline water moves towards greater saline water as surface current.For example, the current flowing from the Atlantic ocean to the Mediterranean sea via Gibraltar Strait is caused because of salinity difference.

c.Density Difference:

• In fact, difference in the density of ocean water is the main cause for the movement of ocean water as ocean currents.Water density depends on a number of factors e.g. temperature, salinity, pressure etc.

3.Factors related to Atmosphere

a.Air pressure

• Areas of high atmospheric pressure are characterized by low volume of water and thus, lowering of water level. Similarly areas of low atmospheric pressure has higher water level. Thus, water moves as surface current from the areas of higher water level( low pressure areas) to lower water level areas (high pressure areas)

b.Winds

• Prevailing or planetary winds blowing on water surface also moves water in its direction due to  friction. Most of the ocean currents of the world follow the direction of prevailing winds for example equatorial current flows westward under the influence of  trade winds. Similarly, there is a seasonal change in the direction of current in the Indian Ocean due to seasonal change in the direction of monsoon winds.

c.Rainfall and evaporation

• Due to high rainfall and low evaporation water level Rises. Similarly due to low rainfall and and how evaporation water level decreases .Thus, surface current develops between both areas.
• Evaporation and rainfall are also related to  salinity and density.Low evaporation and high rainfall lowers the salinity and the reduces water density because of this sea level Rises. In just opposite condition, similarly,water level false.Thus, a surface current develops.Ex- warm currents moving north from equator; cold current moving south from poles.

4.Modifying factors

a.Direction, shape and configuration of coast lines

b.Bottom reliefs

• Generally, the ocean currents while crossing over sub-marine ridge are deflected to the right in the Northern hemisphere and to the left in the southern hemisphere.
• According to Ekman the ocean currents tend to follow the bottom reliefs in the middle and high latitudes but they are independent of in in the low latitudes

c.Seasonal variations

EFFECTS OF OCEAN CURRENTS

1.Modification in coastal climate

• Ocean current affect the temperature and precipitation in the coastal areas.
• European type climate in western coast of Europe is due to North Atlantic warm current which keeps coasts warmer even in the winter season.
• Gulfstream increases the temperature of eastern coast of USA during summer resulting in heat waves.
• Similarly cold currents like Labrador and Falkland result in heavy snowfall in the affected areas.
• Warm currents results in increased amount of precipitation for example North Atlantic warm current result in rainfall on the Western coast of Europe
• Cold current results in decreased precipitation.For example, Kalahari Desert on the West Coast of South Africa is due to cold Benguela current.

2.Effects on fishing

• Ocean currents transport plankton from one area to the other area which are food for fishes. Ex- Gulf Stream carries plankton from Mexican Gulf to the coast of newfoundland and North Western Europe creating major fishing ground.
• Also convergence of cold and warm currents result in upwelling which brings nutrients on the surface resulting into breeding ground of fishes.Ex- convergence of labrador current and gulf stream near Newfoundland.

3.Effects on trade and navigation.

• It determined major ocean routes for the navigation of  ships in ancient times but presently power motors ship do not depend on it.
• Fog due to convergence of cold and warm currents create navigation problems.
• Large icebergs brought by cold currents damage ships.Ex- Labrador and Falkland cold currents.

ATLANTIC OCEAN CURRENTS

North Atlantic currents

• Under the influence of prevailing trade winds,the north equatorial current and the south equatorial current start from the eastern Atlantic (west coast of Africa), moving from east to west.

• This raises the level of western Atlantic (north of the Brazil bulge) ocean by few centimeters which  creates a counter-equatorial current flowing between the north equatorial current and the south equatorial current in west-east direction
• The south equatorial current bifurcates into two branches near Cape de Sao Roque (Brazil).
• Part of the current enters the Caribbean Sea along with north equatorial current into the Mexican Gulf, while the remainder passes along the eastern side of the West Indies as the Antilles current.
• Combined current moves along the east coast of USA and is known as the Florida current up to the Cape Hatteras and as the Gulf Stream beyond that.
• Near the Grand Banks, the Gulf Stream mixes with cold Labrador and East Greenland currents and flows eastward across the Atlantic as the North Atlantic Drift under influence of westerlies.
• The North Atlantic Current breaks up into two branches on reaching the eastern part of the ocean.
• The main current, continuing as the North Atlantic Drift, reaches the British Isles from where it flows along the coast of Norway as the Norwegian current and enters the Arctic Ocean..
• The southerly branch flows between Spain and Azores as the cold Canary current.
• This current finally joins the north equatorial current completing the circuit in the North Atlantic.
• The Sargasso Sea, lying within this circuit, is full of large quantities of seaweed and is an important geographical feature.
  

South Atlantic ocean

• In the South Atlantic Ocean, the south equatorial current, flowing from east to west, splits into two branches near Cape de Sao Roque (Brazil).
• The northern branch joins the north equatorial current whereas the southern branch turns southward and flows along the South American coast as the warm Brazil current.
• The south flowing Brazil current swings eastward (due to westerlies) to join the West Wind Drift flowing from west to east.
• A small branch of West Wind Drift splits and flows between Argentinian coast and Falkland Islands and this current is called as Falkland cold current.
• It mixes with warm Brazil current at the southern tip of Brazil.
• A branch of the South Atlantic splits at the southern tip of Africa and flows along the west coast of South Africa as the cold Benguela current, which joins the south equatorial current to complete the circuit.

INDIAN OCEAN CURRENTS(Asked Last year)

PACIFIC OCEAN CURRENTS