Equatorial Pacific Ocean Currents

• Under the influence of prevailing trade winds [tropical easterlies], the north equatorial current and the south equatorial current start from the eastern pacific (west coast of Central America) and traverses a distance of 14,500 km moving from east to west.

Counter equatorial current

• This raises the level of western pacific (near Indonesia and Australia) ocean by few centimeters. And this creates a counter-equatorial current which flows between the north equatorial current and the south equatorial current in west-east direction.

Three factors aid the formation of Counter-Equatorial current

1. Piling up of water in the western pacific due to trade winds.
2. The presence of doldrums (equatorial low pressure belt) in between the north equatorial current and the south equatorial current. Doldrums are narrow regions with calm (lower) atmospheric conditions. Such conditions aid the backward movement of piled up western pacific waters.
3. Piling of water in the western part of oceans due to rotation of earth (this is a very general point).

• The north equatorial current turns northward off the Philippines to form the Kuroshio current. Most of it lies in the sub-tropical high pressure belt and its northern part is under the influence of westerlies.

Oyashio Current and Okhotsk current

• There are two more cold currents in the northern Pacific, Oyashio flows across the east coast of Kamchatka Peninsula to merge with the warmer waters of Kuroshio, and the Okhotsk current flows past Sakhlain Islands to merge with the Oyashio current off Hokkaido (Northern Japanese Island).

North-Pacific current

• From the south-east coast of Japan, under the influence of prevailing westerlies, the Kuroshio current turns eastwards and moves as the North-Pacific current, reaches the west coast of North America, and bifurcates into two.

Alaska and Californian current

• The northern branch flows anti-clockwise along the coast of British Columbia and Alaska and is known as the Alaska current. The water of this current is relatively warm as compared to the surrounding waters in this zone.
• The southern branch of the current moves as a cold current along the west coast of USA and is known as the Californian current. The Californian current joins the north equatorial current to complete the circuit.

East Australian current

• Following the pattern in the northern hemisphere, the south equatorial current flows from east to west and turns southwards as the East Australian current. It then meets the South pacific current near Tasmania which flows from west to east.

Peru current or Humboldt Current

• Reaching the south-western coast of South America, it turns northward as the Peru current. It is a cold current, which finally feeds the south equatorial current, thus completing the great circuit.
• And the zone where Peru Cold current meets the warm equatorial ocean waters is an important fishing zone. Why? Read the topic below.

Phytoplankton and Fishing

Mixing zones of Cold and Warm Ocean Currents [Grand Banks] and cold water upwelling zones [Peru coast] are the most productive fishing grounds on earth. Why?

• Phytoplankton are the primary producers in the marine food chain and hence they are called the ‘grass of the sea’.
• Phytoplankton are predominantly microscopic, single celled organisms.
• Some species of algae are large, multicellular and live on the ocean bottom. However, they are insignificant players in the marine ecosystem compared to the phytoplankton as they only inhabit a narrow zone around the coast.

Why are cold and warm current mixing zones the good fishing grounds? Why are tropical waters highly unproductive?

• Algae and other plants are able to photosynthesize to produce organic material from inorganic nutrients.
• And the organic material forms the building block for all animals higher up in the food chain.
• Almost all biomass in the ocean is derived from the phytoplankton and to a lesser extent the benthic algae (found on the bottom of a sea or lake).
• However, there is a fundamental problem phytoplankton in the open ocean have to face. They need both sunlight and nutrients (such as nitrate and phosphate) to be able to photosynthesize.
• Sunlight is only available in the uppermost layers.
• During photosynthesis, the nutrients are quickly used up by phytoplankton so they are not available for long periods in the upper layers under normal circumstances.

This is indeed the case in tropical waters, and as a result they are very unproductive.

• To escape this problem the seawater needs to be mixed regularly to bring the nutrient rich deep waters up to the sunlight zone where the phytoplankton can grow.

This is one of the reasons why cold and warm currents convergence zones [mixing happens – the collision of currents causes mixing] and upwelling zones are very productive.

Atlantic Ocean Currents

• The behavior of the Atlantic Ocean Currents is quite significant because of their influence on the climate of North-western Europe, climate of North-Western Africa and fishing in the Grand banks region.

Equatorial Atlantic Ocean Currents

• Under the influence of prevailing trade winds [easterly 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. And this creates a counter-equatorial current which flows between the north equatorial current and the south equatorial current in west-east direction.

Antilles current

• 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.
• There is a rise in water level in the Mexican Gulf because of large amounts of water brought by the Mississippi river and branches of north and south equatorial currents.

Gulf Stream and North Atlantic Drift

• Antilles current creates a current that flows out through the Strait of Florida as Florida current, which mixes with Antilles current from the south.
• This combined current moves along the east coast of USA and is known as the Florida current upto 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.
• Here, westerly movement of North Atlantic Drift is due to the influence of westerlies.

Norwegian current

• 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.
• Norwegian current is very important as it keeps ocean to the north of Norway partly free from ice and also moderates the extremes of climate. It is because of this current, Russia is able to move cargo in summers through Arctic ocean (Barents Sea).
• 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.

Grand Banks-Richest Fishing Grounds on Earth

• The two cold currents—East Greenland current and the Labrador current—flow from the Arctic Ocean into the Atlantic Ocean.
• The Labrador current flows along part of the east coast of Canada and meets the warm Gulf Stream.
• The confluence of these two currents, one hot and the other cold, produce the famous fogs around Newfoundland.
• As a result of mixing of cold and warm waters, one of the world’s most important fishing grounds is created.[Explained in the previous post: Pacific Ocean Currents – Fishing and Phytoplankton]

• 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 (a part of it flows in Anatilles Current and other into Gulf of Mexico), 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 at about latitude 35°S (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.

Benguela current

• 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

  • Indian ocean is half an ocean, hence the behavior of the North Indian Ocean Currents is different from that of Atlantic Ocean Currents or the Pacific Ocean Currents.
  • Also, monsoon winds in Northern Indian ocean are peculiar to the region, which directly influence the ocean surface water movement [North Indian Ocean Currents].

  Indian Ocean Currents and Monsoons

  • The currents in the northern portion of the Indian Ocean change their direction from season to season in response to the seasonal rhythm of the monsoons. The effect of winds is comparatively more pronounced in the Indian Ocean.

  Winter Circulation

  • Under the influence of prevailing trade winds [easterly trade winds], the north equatorial current and the south equatorial current start from the south of Indonesian islands, moving from east to west.
  • This raises the level of western Indian (south-east of horn of Africa) ocean by few centimeters. And this creates a counter-equatorial current which flows between the north equatorial current and the south equatorial current in west-east direction.
  • The north-east monsoons drive the water along the coast of Bay of Bengal to circulate in an anti-clockwise direction.
  • Similarly, the water along the coast of Arabian Sea also circulate in an anti-clockwise circulation.

   

  Summer Circulation – North Equatorial Current Counter-Equatorial Current are Absent

  • In summer, due to the effects of the strong south-west monsoon and the absence of the north-east trades, a strong current flows from west to east, which completely obliterates the north equatorial current. Hence, there is no counter-equatorial current as well.
  • Thus, the circulation of water in the northern part of the ocean is clockwise during this season.

   

  Southern Indian Ocean Currents – Agulhas current, Mozambique current, West Australian current

  • The general pattern of circulation in southern part of the Indian Ocean is quite similar to that of southern Atlantic and Pacific oceans. It is less marked by the seasonal changes.
  • The south equatorial current, partly led by the corresponding current of the Pacific Ocean, flows from east to west.
  • It splits into two branches, one flowing to the east of Madagascar known as Agulhas current and the other between Mozambique and Western Madagascar coast known as Mozambique current.
  • At the southern tip of Madagascar, these two branches mix and are commonly called as the Agulhas current. It still continues to be a warm current, till it merges with the West Wind Drift.
  • The West Wind Drift, flowing across the ocean in the higher latitudes from west to east, reaches the southern tip of the west coast, of Australia.
  • One of the branches of this cold current turns northwards along the west coast of Australia. This current, known as the West Australian current, flows northward to feed the south equatorial current.