Ocean currents are the continuous, predictable, directional movement of seawater driven by gravity, wind (Coriolis Effect), and water density. Ocean water moves in two directions: horizontally and vertically. Horizontal movements are referred to as currents, while vertical changes are called upwellings or downwelling.
TYPES OF CURRENTS
Based on depth:
| Surface currents | Sub Surface currents |
| Found up to 100 m of depth. | Found in deeper waters 1000(s) of meters below. |
| Constitute around 10% of the total water in ocean. | 90% of the total water. |
| Formed at lower latitudes as higher temperature reduces the density, expands the water and hence the water floats on the surface. | Mainly formed at higher latitudes as lower temperature increases the density and cause them to sink. |
Based on Temperatures:
| Warm Currents | Cold currents |
| Bring warm water into cold water areas | Bring cold Water into warm water areas. |
| Observed on the east coast of continents in thelow and middle latitudes (both hemispheres) | Found on the west coast of the continents in the low and middle latitudes (both hemispheres). |
| In the northern hemisphere they are found on the west coasts of continents in high latitudes. | Found on the east coast in the higher latitudes in the Northern Hemisphere. |
| Thermal expansion and floatation drive these currents. | Thermal contraction and sinking drive these currents. |
FACTORS AFFECTING THE CURRENTS
Originating factors:
- Earth’s rotation: the rotational force of the earth causes movement of ocean water near the equator in opposite direction to the west to east rotation of the earth and thus Equatorial currents are generated.
- Oceanic factors: Temperature, salinity and density differences are responsible for expansion, floatation and sinking of water.
- Atmospheric factors: Winds drive the ocean current in the direction in which they move. For E.g. Trade winds drive the ocean water toward west. Rainfall and evaporation create level differences and thus moves the water.
Modifying factors:
- Direction, shape and configuration of coastline- currents flow parallel to the coastline. Equatorial current after being obstructed by Brazilian coast gets bifurcated into two branches and then moves along the coast.
- Bottom reliefs: North Atlantic drift is deflected to the right when it crosses Wyville Thompson ridge.
- Coriolis effect: deflective force affects the direction. Currents flowing from north pole towards equator deflects towards right.
Seasonal changes: They alter the direction of motion. Ex. Indian ocean currents show regional shifts under the influence of monsoonal winds.
TEMPERATURE & SALINITY AFFECTING OCEAN CIRCULATION
Temperature and salinity affect the density of water, resulting in water moving up or down through the ocean layers and moving as currents around the ocean.
- Salinity increases the density of ocean water. This denser water sinks and moves as subsurface current whereas less saline water moves towards greater saline water as surface current. Ex: The current flowing from the Atlantic Ocean to the Mediterranean Sea via Gibraltar Strait is caused by salinity difference.
- Due to high temperature in the equatorial region the water density decreases because of greater expansion of water particles whereas the density of sea water becomes comparatively greater in the polar areas. Consequently, water moves due to expansion of volume from equatorial region of higher temperature to polar areas of relatively very low temperature.
As warm water flows northwards it cools and some evaporation occurs, which increases the amount of salt. Low temperature and a high salt content make the water denser, and this dense water sinks deep into the ocean. The cold, dense water slowly spreads southwards, several kilometers below the surface. Eventually, it gets pulled back to the surface and warms in a process called “upwelling” and the circulation is complete. Ex: Atlantic Meridional Overturning Circulation (AMOC)
OCEAN CURRENTS SIGNIFICANCE
Geomorphic:
- Ocean current along with waves erode, modify, and develop coastal as well as submarine landforms.
- Longshore currents carry along with themselves vast quantities of material and sediments.
- Currents help to move eroded debris and deposit it as silt, sand, and gravel along the coasts
- Desert formation: Cold ocean currents have a direct effect on desert formation in west coast regions of the tropical and subtropical continents.
Climatic:
- Manage global climate by transporting excessive heat from equator towards pole.
- Modify the coastal climate. Ex. Gulf stream brings heat to the North-western Europe and lead to moderate conditions.
- Phenomenon like El-Nino and La-Nina are closely associated with oceanic currents.
- Currents along with the wind lead to regional climatic changes. Ex. Currents along the Indian coast impact the monsoon.
- Rainfall and fogs are also strongly associated with oceanic currents.
Ecological:
- Distribute oceanic heat and balances the temperature conditions.
- Carry nutrients and food to organisms that live permanently attached in one place and carry reproductive cells and ocean life to new places. Ex. Upwelling Benguela current brings the nutrients to the coast of Africa.
- Oceanic gyres are known to trap pollutants thus causing garbage patches. Ex. Great Pacific Garbage Patch.
Economic:
- Commercial fishing grounds are formed where warm and cold current mix. Ex. Labrador current mixing with Gulf stream near newfoundland.
- Aids as well as hinders navigation. Current support the ships moving in the same direction but it blocks their way when they carry large amount of ice bergs along with them, especially the colder currents.
- Currents offer a vast potential to be transformed into energy sources.
Social:
- Play an important role in determining the settlements in a coastal region.
- Impact health of population by altering the climatic conditions
ROLE OF INDIAN OCEAN CURRENTS ON REGIONAL CLIMATE
Indian ocean affects the regional climate in their unique way. Their impact becomes more significant on the account of seasonal reversals in their direction.
- SW Monsoon current shifts the warm moisture towards Arabian sea and Bay of Bengal thus facilitates the precipitation during summer months across Indian subcontinent.
- Agulhas current being warm in nature increases the precipitation along the Eastern coast of South Africa.
- West Australian current being cold in nature reduces the rainfall over western coast of Australia.
- West wind drift keeps the warm currents away from the Antarctica and hence allows it to preserve its massive ice sheets.
