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| Ocean currents across seas (Photo: Windows to the Universe) |
Ocean currents are large masses of surface water that circulate in regular patterns around the oceans. Those flowing from lower latitudes to higher latitudes have a higher surface temperature and are warm currents. Those flowing from higher to lower latitudes are cold currents.
Ocean currents follow a fixed pattern of movement, which is controlled or influenced by the following set of factors.
I. Factors in relation to the earth’s nature
a. Gravitational force
b. Deflective force by the earth’s rotation
II. Ex-oceanic factors
a. Atmospheric pressure and its variation
b. Wind and frictional force or planetary winds
c. Precipitation
d. Nature of evaporation and insolation [amount of solar radiation falling onto a particular area of the earth]
III. Sub-oceanic factors
a. Pressure gradient
b. Temperature differences
c. Salinity
d. Density
e. Melting of ice
IV. Other factors modifying the ocean currents
a. Direction and shape of the coastlines
b. Seasonal variation
c. Bottom topography or configuration
The gravitational pull increases towards the poles. This pull has two effects on the flow of ocean water. First, it compresses water body a bit more towards the poles making a level-gradient fro ocean water and as a result water moves towards the poles.
Secondly, the pull creates a centripetal force towards the earth’s centre but as the ocean floor obstructs water to reach destination, the ocean water starts moving horizontally in its attempt to reach the destination [the vector resolution rule].
The earth’s rotation deflects freely moving objects including ocean currents to the right. In the northern hemisphere, this is a clockwise direction, e.g. the circulation of the Gulf Steam and Canary current. In the southern hemisphere, this is anti-clockwise direction, e.g. Brazilian current, and the West Wind Drift.
Over the regions of greater atmospheric pressure, the level of the sea is found to be lowered and vice versa due to compressional phenomenon. Therefore, ocean water flows from higher level to lower level, that is, from low air pressure region to high air pressure zone. For example, Canary current flows from sub-polar low air pressure zone to sub-tropical high air pressure region.
Prevailing winds are perhaps the most influential factors of the flow of the ocean water. Trade Winds move equatorial waters westward and warm the eastern coasts of the continents and resultantly force equatorial waters to move towards the poles when obstructed by continental masses. For example, the NE Trade Winds move the North Equatorial Current and its derivatives, the Florida Current and the Gulf Stream Drift to warm the southern and eastern coasts of the US.
Similarly, the SE Trade Winds drive the South Equatorial Current which warms the eastern coast of Brazil as the warm Brazilian Current. The westerlies of of the temperate latitude result in a north-easterly flow of water in the northern hemisphere, e.g. the movement of North Atlantic Drift.
In the southern hemisphere, westerlies drive the West Wind Drift all around the globe and where obstructed by continental mass give rise to distinct ocean currents — the Peruvian Current towards equator off South America, Benguela Current off South Africa and West Australian Cold Current off Australia.
The strongest evidence of prevailing winds impacting the ocean current flow is seen in the North Indian Ocean. Here, the direction of currents changes completely with the direction of the monsoon winds.
Main ocean currents and the relevant prevailing are as follows:
|
Ocean Currents |
Prevailing Winds |
|
North Equatorial Warm Current |
NE Trade Winds |
|
South Equatorial Warm Current |
SE Trade Winds |
|
Counter-equatorial Warm Current |
Counter-equatorial Westerlies |
|
North Atlantic Drift |
Westerlies |
|
North Pacific Current |
Westerlies |
|
West Wind Drift |
Westerlies |
Ocean regions receiving excessive additional water on account of greater amount of precipitation serve as the source regions of ocean currents, for example, the movement of equatorial warm currents to higher latitudes. The Gulf Stream, the Brazilian Current, East Australian Current, Kuro Siwo Current have their source in the equatorial ocean water due to excessive rains.
Differences in the distribution patterns of insolation and amount of evaporation over oceans lead to formation of a current. Insolation and evaporation at an oceanic place work in tandem and where its combined effect is less than the supply of water/precipitation, the current flow starts from there. Thus, current flows from equatorial zone to the temperate zone which also receives current from the polar zone due to the same reason.
Sub-oceanic pressure gradient also causes the flow of ocean water. But it is more important for vertical movement than the horizontal movement. However, temperature difference does play a major role. As warm air is lighter and rises higher than cooler air, ocean water gathers more along equatorial zone and as a consequence, the warm equatorial current/water moves along the surface slowly towards the poles. The heavier cold water of the polar region creeps slowly along the bottom of the sea towards the equator.
Salinity of ocean water varies from place to place. Waters of high salinity are denser than waters of low salinity. Hence, waters of low salinity flow on the surface of waters of high salinity or towards the regions of high salinity. At the bottom of the ocean, the flow is reversed. Hence, the Atlantic waters enter the Mediterranean Sea at surface and return at the bottom.
Density being the sum total of temperature, pressure and salinity directly controls an ocean current. Low density waters flow towards high density waters’ zone. Hence, waters from mid and low latitudes flow towards high latitudes. Similarly, warm waters flow towards cold waters.
Melting of ice adds fresh volume of water to the ocean in higher latitudes and as a result, it flows towards low latitudes, e.g. cold currents emerging from the Artic region — Labrador and Oya Siwo are cold currents.
Costal form modifies the direction of a current to some extent. The westward movement of the South Equatorial Current is obstructed by the Cape San Roque of South America bifurcating it into the Caribbean Stream and the Brazilian Current.
Changing seasons also bring some impact of current flow worldwide in terms of latitudinal shift of ocean currents. But the most remarkable impact is seen in the Indian Ocean where there is complete reversal of ocean current under the impact of the monsoonal winds of different seasons.
Bottom topography of ocean also determines the direction of ocean currents, especially the impact of mid-oceanic ridge is more pronounced. It is very apparent in the Atlantic Ocean where North Equatorial Current, whose normal range of flow is 0-12-degree latitude, is deflected northward even up to 25-degree latitude due to the presence of the mid-oceanic ridge.
It is therefore obvious that the origin and maintenance of ocean currents are influenced by a large number of factors but the most interesting part of this phenomenon is that none of the factors can be studied or taken alone as far as ocean currents’ flow is concerned.
