Currents

Currents- Horizontal Movement

• Surface ocean currentsare driven by global wind systems that are fueled by energy from the sun.
  • These currents transfer heat from the tropics to the Polar Regions, influencing local and global climate.
  • The warm Gulf Stream originating in the tropical Caribbean, for instance, carries about 150 times more water than the Amazon River.
• Deep Ocean currents are caused by differences in water density, resulting from the variability of water temperature (thermo) and salinity (haline). This process is known as thermohaline circulation.
  • In cold regions, when sea water freezes, it leaves behind salt, increasing the density of the surrounding seawater. This sinks, making way for lighter, warmer water, thereby starting the ‘global conveyer belt’.

• The general movement of the currents in the northern hemisphere is clockwise and in the southern hemisphere is anti-clockwise.
• This is due to the Coriolis force which is a deflective force and follows Ferrel’s law.
• A notable exception to this trend is seen in the northern part of the Indian Ocean where the current movement changes its direction in response to the seasonal change in the directionof monsoon winds.
• The warm currents move towards the cold seas and cold currents towards the warm seas.
• In the lower latitudes, the warm currents flow on the eastern shoresand cold on the western shores [food for imagination].
• The situation is reversed in the higher latitudes. The warm currents move along the western shores and the cold currents along the eastern shores.
• At the surface, aspects of wind-driven circulation cause the gyres(large anticyclonic current cells that spiral about a central point) to displace their centres westward, forming strong western boundary currents against the eastern coasts of the continents, such as the Gulf Stream–North Atlantic–Norway Current in the Atlantic Ocean and the Kuroshio–North Pacific Current in the Pacific Ocean.
• In the Southern Hemisphere the counterclockwise circulation of the gyres creates strong eastern boundary currents against the western coasts of continents, such as the Peru (Humboldt) Current off South America, the Benguela Current off western Africa, and the Western Australia Current.
• The Southern Hemisphere currents are also influenced by the powerful eastward-flowing circumpolar Antarctic Current, which separates the Southern Ocean from the Atlantic, Pacific, and Indian oceans.
• It is a very deep, cold, and relatively slow current, but it carries a vast mass of water, about twice the volume of the Gulf Stream.
• The Peru and Benguela currents draw water from this Antarctic current and, hence, are cold.
• The Northern Hemisphere lacks continuous open water bordering the Arctic and so has no corresponding powerful circumpolar current, but there are small cold currents flowing south through the Bering Strait to form the Oya and Anadyr currents off eastern Russia and the California Current off western North America; others flow south around Greenland to form the cold Labrador and East Greenland currents.
• The Kuroshio–North Pacific and Gulf Stream–North Atlantic–Norway currents move warmer water into the Arctic Ocean via the Bering, Cape, and West Spitsbergen currents.

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• There are a variety of factors that affect how ocean currents (water in motion) are created, including a combination of two or more factors.
• The different types of currents (referred to as surface or thermohaline, depending on their depth) are created by, among other things, wind, water density, the topography of the ocean floor and the coriolis effect. 

Primary Forces Responsible for Ocean Currents: 

Insolation

• • Heating by solar energy causes the water to expand. That is why, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes.
  • This causes a very slight gradient and water tends to flow down the slope. The flow is normally from east to west.

Wind (atmospheric circulation)

• • Wind blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course.
  • Winds are responsible for both magnitude and direction [Coriolis force also affects direction] of the ocean currents. Example: Monsoon winds are responsible for the seasonal reversal of ocean currents in the Indian ocean.
  • The oceanic circulation pattern roughly corresponds to the earth’s atmospheric circulation pattern.
  • The air circulation over the oceans in the middle latitudes is mainly anticyclonic [Sub-tropical High Pressure Belt] (more pronounced in the southern hemisphere than in the northern hemisphere due to differences in the extent of landmass). The oceanic circulation pattern also corresponds with the same.
  • At higher latitudes, where the wind flow is mostly cyclonic [Sub-polar Low Pressure Belt], the oceanic circulation follows this pattern.
  • In regions of pronounced monsoonal flow [Northern Indian Ocean], the monsoon winds influence the current movements which change directions according to seasons.

Gravity:

• • Gravity tends to pull the water down to pile and create gradient variation.

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.

Shape of the Ocean basins:

• • Another major factor that determines the direction of surface currents is the shape of ocean basins.
  • When a surface current collides with land, it changes the direction of the currents. Imagine pushing the water in a bathtub towards the end of the tub.
  • When the water reaches the edge, it has to change direction.

Secondary Forces Responsible for Ocean Currents: 

• • Temperature difference and salinity difference are the secondary forces.
  • Differences in water density affect vertical mobility of ocean currents (vertical currents).
  • Water with high salinity is denser than water with low salinity and in the same way cold water is denser than warm water.
  • 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 slowly 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.

Vertical movements, often referred to as upwelling and downwelling, exhibit much lower speeds, amounting to only a few metres per month. As seawater is nearly incompressible, vertical movements are associated with regions of convergence  and divergence in the horizontal flow patterns.

Upwelling

• Water that has greater density usually sinks to the bottom.
• However, in the right conditions, this process can be reversed.
• Denser water from the deep ocean can come up to the surface in an upwelling.
• Generally, an upwelling occurs along the coast when wind blows water strongly away from the shore.
• As the surface water is blown away from the shore, colder water from below comes up to take its place.
• This is an important process in places like California, South America, South Africa, and the Arabian Sea because the nutrients brought up from the deep ocean water support the growth of plankton which, in turn, supports other members in the ecosystem.
• Upwelling also takes place along the equator between the North and South Equatorial Currents.

Desert formation

• • Cold ocean currents have a direct effect on desert formation in west coast regions of the tropical and subtropical continents.
  • There is fog and most of the areas are arid due to desiccating effect (loss of moisture).

Rains

• • Warm ocean currents bring rain to coastal areas and even interiors. Example: Summer Rainfall in British Type climate.
  • Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes. This results in warm and rainy climates. These areas lie in the western margins of the subtropical anti-cyclones.

Moderating effect

• • They are responsible for moderate temperatures at coasts. [North Atlantic Drift brings warmness to England. Canary cold current brings cooling effect to Spain, Portugal etc.]

Drizzle

• • Mixing of cold and warm ocean currents create foggy weather where precipitation occurs in the form of drizzle [Newfoundland].

Climate

• • Warm and rainy climates in tropical and subtropical latitudes [Florida, Natal etc.],
  • Cold and dry climates on the western margins in the sub-tropics due to desiccating effect,
  • Foggy weather and drizzle in the mixing zones,
  • Moderate clime along the western costs in the sub-tropics.

Tropical cyclones

• • They pile up warm waters in tropics and this warm water is the major force behind tropical cyclones.

Effects on climate and weather: Ocean currents have significant impact on movement to heat energy and moisture between the oceans and the atmosphere as they circulate water worldwide.

• • For example, the Gulf Stream is a warm current which originates in the Gulf of Mexico and moves north towards Europe. It keeps the sea surface warm, which keeps Western Europe warmer than other areas at similar latitudes.
  • The North Atlantic drift and Kuroshio warm currents bring in rainfall along the western coast of Europe and eastern coast of Japan respectively.
  • Labrador, Kurile and Falkland cold currents are responsible for heavy snowfall in the affected areas during winters.
  • The Benguela and Peru currents have caused the existence of the Kalahari Desert along the Western coast of South Africa and the Atacama Desert along the South American coast respectively.
  • The Humboldt Current is a cold current moving off the coast of Chile and Peru.  It keeps the coast cool and the northern part of Chile arid. It leads to extremely productive waters. However, when El Nino disturbs the current, Chile’s climate is also altered.

Effects on marine life: Ocean currents are important for the distribution of world’s sea life as they transport nutrients, planktons, etc  from one area to another which favours the growth of fishes. The mixing of warm and cold currents lead to abundant fish catch in the coastal countries.

• • For example, ocean currents have enriched the famous fish catch areas of the North Sea, Grand Bank and George Bank near Nova Scotia, Peruvian coast, Eastern Pacific near Japan, etc.

Ease & challenges of Navigation: Since ancient times, sailors have used the knowledge of ocean currents and winds to ease navigation in sea routes around the coasts and across the globe.

• • Understanding of currents is essential to augment the speed of ships, reduce the shipping costs and fuel consumption and also to avoid obstacles such as icebergs and debris.
  • Ice-free port in higher latitudes: The Arctic port of Murmansk in Russia is ice free all year as the seas near Murmansk are warmed by relatively warm Atlantic water that flows into the Barents Sea via the Norwegian Current and North-Cape Current.
  • The convergence of cold and warm ocean currents causes dense fogs which creates obstacles in navigation.

Transport of Debris: Ocean currents also move the debris around the world which in turn leads to formation of icebergs as well as trash Islands.

• • For example, the Labrador Current and Falkland currents bring icebergs from Arctic and Antarctic oceans respectively into the shipping lanes in the north and South Atlantic oceans respectively.

Energy Independence: Ocean currents are emerging as a possible source of alternative energy for the coastal countries. As dense water carries an enormous amount of energy that can be captured and converted into usable form through use of water turbines.