- The ocean is a vast open expanse of sea water that covers about 71.4% [including inland water bodies] of the earth’s surface.
- The oceans never stand still.
- They keep moving constantly. Three different kinds of movements are found in ocean.
- These are: waves, tides, and ocean currents.
- Without moving forward like the river water, the up and down movement seen on the surface of ocean are generally known as waves.
- In this type of movement water itself does not move to and fro but rises and falls, (vertical movements) and comes to the same position after each wave.
- The alternate vertical rise and fall of the ocean waters twice in 24 hours are known as tides.
Definition of Ocean Currents
- A continuous and regular horizontal movement of ocean water in a fairly defined direction over a great distance on the surface or sub-surface of the ocean is called Ocean currents.
- The quantity of water carried by the ocean current is very large and its speed ranges between 2 to 10 km per hour.
- When ocean currents flow from low to high latitudes, these are warm currents; while those moving from high to low latitudes are cold currents.
Causes of Ocean Currents
The following are the main causes of the origin of ocean currents
A. Prevailing Winds
- Ocean are mainly caused by the permanently blowing planetary or prevailing winds.
- The planetary winds push the surface layers of the ocean water in front of them in a constant flow.
- If due to a seasonal change the direction of wind is changed, the currents also similar change in their direction.
- In tropical areas, equatorial currents moving along with winds from east to west. In temperate regions westerlies drive the sea water from west
- The greatest effect of seasonal changes is experienced by the Monsoon winds.
- A change from summer to winter reverses the direction of wind and the similar effect is seen on the directions of ocean currents.
B. Difference in Temperature
- Temperature is low at the poles and high at the 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 sets-up convectional currents in the ocean.
- Warm water of the equatorial regions are light and move along the surface towards the polar regions where they are cooled.
C. Difference in Salinity
- The salinity of ocean water varies from place to place.
- Water of high salinity are denser than water of low salinity hence water of low salinity flow on the surface of the water of high salinity while water of high at the bottom towards water of low salinity.
D. Rotation of The Earth
The rotation of the earth brings about a change in the direction of the ocean currents. These are deflected to the right in the northern hemisphere and to the left in the southern hemisphere.
E. Shape of Coast
- The shape and direction of the coasts also change the direction of the currents.
F. Centrifugal Force
- The centrifugal force at the equator is greater than that at the poles. With the variation of these forces, the equatorial water moves towards the poles.
G. Evaporation and Rainfall
- Evaporation reduces the amount of water and due to heavy rainfall, the surface of seawater rises but decreases its salinity and density.
H. Melting of ice
- The melting of ice raises the sea level and reduces salinity. By the combined effect of all these, ocean currents flow in opposite directions.
Characteristics of Ocean Currents
- The general movement of the currents in the northern hemisphere is clockwise and the Southern hemisphere is anti-clockwise. This is due to the Coriolis force which is a deflective force and follows Ferrel’s Law.
- The warm currents move towards the cold seas and cool currents towards the warm seas.
- In the lower latitudes, the warm currents flow on the eastern shores and cold on the western shores. The situation is reversed in the higher latitudes.
- Convergence along which the warm and cold currents meet divergence from which they move out in different directions also controls the currents.
- The shape and position of coasts play a vital role in guiding the direction currents.
- The currents flow not only at the surface but also below the sea surface.
Effects of Ocean Currents
A. Difference in temperature of the atmosphere
- The regions where warm ocean currents pass, become comparatively warmer, and the regions where cold currents pass, become comparatively cooler.
- Winds crossing warm ocean currents gather enough moisture and cause ample rainfall. Winds crossing cold ocean currents do not gather much moisture and hence do not cause much rainfall.
C. Fog and storms
- Regions, where warm and cold ocean currents pass by each other, remain foggy almost throughout the year and often face storms.
D. Parity of temperature
- The ocean currents help in temperature distribution between higher and lower latitudes, thus helping to maintain parity in the temperature distribution of the world.
E. Frost-free harbours
- The mouths of the harbours of the Temperate and Frigid regions can remain frost-free and usable in winters if hot currents pass through that region, increasing the temperature.
- Ships can move faster and easily along the ocean currents. The movement against the direction of the currents is difficult, time-consuming and costlier.
G. Sand bars
- When the warm and cold ocean currents meet, the icebergs being carried by the cold current melt and the sand, pebbles etc. that were within the iceberg get deposited at the base, creating sand bars. These are economically very useful, as the low depth of the sea near the sand bars attracts many fishes that are easily acquirable.
H. Business and commerce
- The sand bars are very useful for fishing and related trades. The meeting of the warm and cold currents helps in the growth of plankton, which is the favourite food of the fish. Hence, a huge catch of fish naturally leads to excellent business and commerce. Eg. Grand Bank near Newfoundland.