Ocean Currents

Download 36.95 Kb.
Size36.95 Kb.

Ocean Currents

An ocean current is a continuous, directed mass flow of seawater generated by winds and by water circulation that is produced by water-density differences. Depth contours, shoreline configurations and interaction with other currents influence a current's direction and strength. A deep current is any ocean current at a depth of greater than 100m.

Ocean currents can flow for great distances, and together they create the great flow of the global conveyor belt which plays a dominant part in determining the climate of many of the Earth’s regions. Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than any other region at the same latitude. Another example is Lima, Peru, where the climate is cooler (sub-tropical) than the tropical latitudes in which the area is located, due to the effect of the Humboldt Current.

Various ocean currents: The ocean water does not remain stationary in one place. The water of all the oceans and seas are flowing either as surface or under current. Hence, the ocean currents are divided into three types on the basis of the location of the oceans. These are :

(a) The Atlantic Ocean Currents;

(b) The Pacific Ocean Currents;

(c) The Indian Ocean Currents.

The Atlantic Ocean Currents

The continents of Europe and Africa are in the east of Atlantic while North America and South America are in the west. The equator has divided the Atlantic Ocean into north and south part. So, the currents can be divided into two parts.

A. South Atlantic Ocean Currents; and

B. North Atlantic Ocean Currents.

A. South Atlantic Ocean Currents

1. The Antarctic Current: The cold waters from the Antarctic move under the influence of the strong westerly wind from west to east. This is known as the Antarctic current and it enters into the Atlantic by the south of South America.

2. Benguela Current: The branch of the Antarctic current being deflected near the Cape of Good Hope turns to the north and flows by the west side of South Africa. This current is known as the Benguela current. This is a cold current and It turns westward under the influence of the south-east trade winds and joins the South Equatorial current.

3. South Equatorial Current: The South Equatorial Current originates from the extended part of the Benguela current. This current marches towards north-west up to the equator being influenced by the rotation of the earth and the south-east trade winds. This current is known as the South Equatorial Current as it flows to the south of the equator. This is warm current.

4. Brazil Current: The branch of South Equatorial current known as Brazil current which flows through the east coast of Brazil to the south-west. The current is warm since it originates from the warm current and flows through the tropical region, crossing the Tropic of Capricorn, the current turns eastward gradually under the influence of westerly wind and meets the Antarctic current.

5. Falkland Current: A branch of the Antarctic current on entering the Atlantic Ocean turns north and flows northward along the coast of Falkland Island and Argentina. This is known as Falkland Current and it is a cold current.

B. North Atlantic Ocean Currents

1. North Equatorial Current: The warm currents flowing from east to west along the north of the equator under the influence of the rotation of the earth and the northeast trade winds is known as the North Equatorial Current. This is warm current. After crossing the Mid-Atlantic, the northern branch of the South Equatorial current meets the North Equatorial current. The North Equatorial current is divided into two branches. The first branch flows into the Gulf Stream taking a turn to the north. The second branch flows first into the Caribbean Sea and then into the Gulf of Mexico.

2. The Equatorial Counter Current: Between the North and South Equatorial currents, there is a weak current flowing from west to east and is known as the Equatorial Counter Current. This is a warm current.

3. The Gulf Stream: The Gulf Stream, practically, has originated from the extended part of South and North Equatorial currents. This mixed current enters into the Caribbean Sea and is divided into two branches being obstructed by the islands. One of the branches enters into the Gulf of Mexico. The rush of water from the Mississippi river of the United States of America enhances the speed of this current and flows through the narrow strait of Florida to North Atlantic. It is known as the Gulf Stream since it has originated in the Gulf of Mexico. The extent of the Gulf Stream varies from 64 to 80 kilometres at the entrance of Florida strait, the depth is 914 metres, the average speed is 8 kilometres per hour and the temperature is 30° Celsius. The colour of the current is deep blue.

At the Mid-Atlantic, the depth and the temperature of the current comes down while the width gets increased and this current flows here at a speed of 2.5 kilometres per hour. While flowing north-east along the east coast of the United States of America, the Gulf Stream is divided into three branches at the mid of the North Atlantic due to the influence of the westerlies.

(a) North Atlantic Stream;

(b) West Greenland Current; and

(c) Canaries Current.

(a) North Atlantic Stream: The first branch of the Gulf Stream being called North Atlantic Stream flows along the coast of West Europe, the British Islands and Norway into the North Sea as a warm current.

(b)West Greenland Current: The warm West Greenland current, the second branch of Gulf Stream curving northward proceeds along the south of Iceland and Greenland and then flows through Davis Strait between Greenland and Baffin Island to the north.

(c) Canaries Current: The third branch of Gulf Stream known as the Canaries current turning southward flows along Portugal and the west coast of West Africa. The current is divided into two branches. The first branch curving south-west under the influence of Trade Wind flows into North Equatorial Current. This is a cold current.

4. Guinea Current: The Guinea current, the second branch of Canaries Current, flows along the coast of Guinea of West Africa to the south up to the Equator. Then joining the Equatorial Counter Current, it flows into the Bay of Guinea. This is a cold current. The flow of different currents and cross currents along the sides of North Atlantic Ocean has caused in the centre of the ocean an area of stagnant sea often full of drifted branches of plants, grass, sea-weeds etc. and is called the Sargasso Sea.

5. Labrador Current: Two cold currents from the North Ocean flows along the east and west of Greenland into the Atlantic Ocean. These two currents join in the north of Labrador Peninsula and taking the name of the Labrador Currents. It flows along the east coast of Newfoundland and the United States of America to the south. The Labrador Current flowing south to New York gets deflected by the warm Gulf Stream. The deep blue water of the Gulf Stream and the green water of the Labrador flow side by side in the opposite direction and the border of these two cross currents is called the Cold Wall.

Pacific Ocean Currents

Pacific Ocean currents follow the general pattern of those in the Atlantic. The North Equatorial Current flows westward in the general area of the northeast trades, and the South Equatorial Current follows a similar path in the region of the southeast trades. Between these two, the weaker North Equatorial Counter current sets toward the east, just north of the equator.

Kuroshio: After passing the Mariana Islands, the major part of the North Equatorial Current curves somewhat toward the northwest, past the Philippines and Taiwan. Here it is deflected further toward the north, where it becomes known as the Kuroshio, and then toward the northeast past the Nansei Shoto and Japan, and on in a more easterly direction. Part of the Kuroshio, called the Tsushima Current, flows through Tsushima Strait, between Japan and Korea, and the Sea of Japan, following generally the northwest coast of Japan. North of Japan it curves eastward and then southeastward to rejoin the main part of the Kuroshio. The limits and volume of the Kuroshio are influenced by the monsoons, being augmented during the season of southwesterly winds, and diminished when the northeasterly winds are prevalent.

The Kuroshio (Japanese for “Black Stream”) is so named because of the dark colour of its water. It is sometimes called the Japan Current. In many respects it is similar to the Gulf Stream of the Atlantic. Like that current, it carries large quantities of warm tropical water to higher latitudes, and then curves toward the east as a major part of the general clockwise circulation in the Northern Hemisphere. As it does so, it widens and slows, continuing on between the Aleutians and the Hawaiian Islands, where it becomes known as the North Pacific Current. As this current approaches the North American continent, most of it is deflected toward the right to form a clockwise circulation between the west coast of North America and the Hawaiian Islands called the California Current. This part of the current has become so broad that the circulation is generally weak. Near the coast, the south-eastward flow intensifies and average speeds are about 0.8 knot. But the flow pattern is complex, with offshore directed jets often found near more prominent capes, and pole ward flow often found over the upper slope and outer continental shelf. It is strongest near land. Near the southern end of Baja California, this current curves sharply to the west and broadens to form the major portion of the North Equatorial Current. During the winter, a weak counter current flows north-westward, inshore of the south-eastward flowing California Current, along the west coast of North America from Baja California to Vancouver Island. This is called the Davidson Current.

Off the west coast of Mexico, south of Baja California the current flows south-eastward during the winter as a continuation of part of the California Current. During the summer, the current in this area is north-westward as a continuation of the North Equatorial Counter current. As in the Atlantic, there is in the Pacific a counter clockwise circulation to the north of the clockwise circulation. Cold water flowing southward through the western part of Bering Strait between Alaska and Siberia, is joined by water circulating counter clockwise in the Bering Sea to form the Oyashio. As the current leaves the strait, it curves toward the right and flows south-westerly along the coast of Siberia and the Kuril Islands. This current brings quantities of sea ice, but no icebergs. When it encounters the Kuroshio, the Oyashio curves southward and then eastward, the greater portion joining the Kuroshio and North Pacific Current. The northern branch of the North Pacific Current curves in a counter-clockwise direction to form the Alaska Current, which generally follows the coast of Canada and Alaska. When the Alaska Current turns to the southwest and flows along the Kodiak Island and the Alaska Peninsula, its character changes to that of a western boundary current and it is called the Alaska Stream. When this westward flow arrives off the Aleutian Islands, it is less intense and becomes known as the Aleutian Current. Part of it flows along the southern side of these islands to about the 180th meridian, where it curves in a counter clockwise direction and becomes an easterly flowing current, being augmented by the northern part of the Oyashio. The other part of the Aleutian Current flows through various openings between the Aleutian Islands, into the Bering Sea. Here it flows in a general counter clockwise direction.

The southward flow along the Kamchatka peninsula is called the Kamchatka Current which feeds the southerly flowing Oyashio. Some water flows northward from the Bering Sea through the eastern side of the Bering Strait, into the Arctic Ocean.

The South Equatorial Current, extending in width between about 4°N latitude and 10°S, flows westward from South America to the western Pacific. After this current crosses the 180th meridian, the major part curves in a counter clockwise direction, entering the Coral Sea, and then curving more sharply toward the south along the east coast of Australia, where it is known as the East Australian Current. The East Australian Current is the weakest of the subtropical western boundary currents and separates from the Australian coast near 34°S. The path of the current from Australia to New Zealand is known as the Tasman Front, which marks the boundary between the warm water of the Coral Sea and the colder water of the Tasman Sea.

The continuation of the East Australian Current east of New Zealand is the East Auckland Current. The East Auckland Current varies seasonally: in winter, it separates from the shelf and flows eastward, merging with the West Wind Drift, while in winter it follows the New Zealand shelf southward as the East Cape Current until it reaches Chatham Rise where it turns eastward, thence merging with the West Wind Drift. Near the southern extremity of South America, most of this current flows eastward into the Atlantic, but part of it curves toward the left and flows generally northward along the west coast of South America as the Peru Current or Humboldt Current.

Indian Ocean

  • Agulhas Current

  • East Madagascar Current

  • Equatorial Counter Current

  • Indonesian Through-flow

  • Leeuwin Current

  • Madagascar Current

  • Mozambique Current

  • Somali Current

  • South Australian Counter Current

  • South Equatorial Current

  • Southwest and Northeast Monsoon Drift (or Indian Monsoon Current)

  • West Australian Current

  • West Wind Drift

Arctic Ocean

  • East Greenland Current

  • Norwegian Current

  • Beaufort Gyre (water or ice flow)

  • Transpolar Drift (water or ice flow)

Significance: Knowledge of surface ocean currents is essential in reducing costs of shipping, since travelling with them reduces fuel costs. In the sail-ship era, knowledge of currents was even more essential. A good example of this is the Agulhas Current, which long prevented Portuguese sailors from reaching India. Even today, the round-the-world sailing competitors employ surface currents to their benefit. Ocean currents are also very important in the dispersal of many life forms.

Ocean currents are important in the study of marine debris, and vice versa. These currents also affect temperatures throughout the world. For example, the current that brings warm water up the north Atlantic to northwest Europe stops ice from forming by the shores, which would block ships from entering and exiting ports, the currents have a decisive role in influencing the climate of the regions they flow through. The cold currents that flow from the polar and sub-polar regions bring in a lot of plankton.

Lecture Delivered by: Prof. Khurshid Anwar

Download 36.95 Kb.

Share with your friends:

The database is protected by copyright ©ininet.org 2022
send message

    Main page