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The following points highlight the ten main currents in the Atlantic Ocean. The currents are: 1. North Equatorial Current 2. South Equatorial Current 3. Counter-Equatorial Current 4. Gulf Stream 5. Canary Current 6. Labrador Current 7. Brazil Current 8. Falkland Current 9. South Atlantic Drift 10. Benguela Current.
1. North Equatorial Current (Warm):
Normally, the north equatorial current is formed between the equator and 10° N latitude. This current is generated because of upwelling of cold water near the west coast of Africa. This warm current is also pushed westward by the cold Canary current. On an average, the north equatorial warm current flows from east to west but this saline current is deflected northward when it crosses the mid-Atlantic Ridge near 15°N latitude.
It again turns southward after crossing over the ridge.
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This current, after being obstructed by the land barrier of the east coast of Brazil, is bifurcated into two branches e.g.:
(i) Antilles current, and
(ii) Caribbean current.
The Antilles current is diverted northward and flows to the east of West Indies islands, and helps i n the formation of Sargasso Sea eddy while the second branch known as the Caribbean current enters the Gulf of Mexico and becomes Gulf Sream (fig. 29.1).
2. South Equatorial Current (Warm):
South equatorial current flows from the western coast of Africa to the eastern coast of South America between the equator and 20° S latitude. This current is more constant, stronger and of greater extent than the north equatorial current. In fact, this current is the continuation of the Benguela current.
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This warm current is bifurcated into two branches due to obstruction of land barrier in the form of the east coast of Brazil. The northward branch after taking north-westerly course merges with the north equatorial current near Trinidad while the second branch turns southward and continues as Brazil warm current parallel to the east coast of South America. This current is basically originated under the stress of trade winds.
3. Counter-Equatorial Current (Warm):
The counter equatorial current flows from west to east in between the westward flowing strong north and south equatorial currents. This currents is less developed in the west due to stress of trade winds. In fact, the counter current mines with the equatorial currents in the west but it is more developed in the east where it is known as the Guinea Stream.
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The counter equatorial current carries relatively higher temperature and lower density than the two equatorial currents. Several ideas have been put forth to explain the origin of the counter equatorial current.
According to some scientists this current is originated because of the influence of equatorial westerlies which blow from west to east in the calm zone of the doldrum or in the convergence zone of the north-east and south-east trade winds. It is argued that south-west monsoon winds develop in the zone of equatorial calm (doldrum) during northern summers.
These equatorial westerlies drag the waters and force them to flow from west to east under their influence. This concept is disputed on the ground that the counter equatorial current is all year phenomenon. In other words, it flows throughout the year while the monsoon winds (say equatorial westerilies) in the equatorial calm zone disappear during winter season.
According to another view the counter equatorial current is originated due to piling up of immense volume of water because of the convergence of the two great equatorial warm currents near the coast of Brazil. The pilling up of water raises the water level and hence water flows eastward as compensation current up to the Gulf of Guinea.
4. Gulf Stream (Warm):
The Gulf Stream is a system of several currents moving in north-easterly direction. This current system originates in the Gulf of Mexico around 20 N latitude and moves in north easterly direction along the eastern coast of North America and reaches the western coasts of Europe near 70° N latitude.
This system, named Gulf Stream because of its origin in the Mexican Gulf, consists of:
(i) Florida current from the strait of Florida to Cape Hatteras,
(ii) Gulf Stream from Cape Hatteras to the Grand Bank, and
(iii) North Atlantic Drift (current) from Grand Bank to the Western European coast.
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(i) Florida Current:
Florida current is in fact, the northward extension of the north equatorial current. This current flows through Yucatan channel into the Gulf of Mexico, thereafter the current moves forward through Florida Strait and reaches 30° N latitude. Thus, the Florida warm current contains most of the characteristics of the equatorial water mass. The average temperature of water at the surface is 750F (240C) while the salinity is 360/00. The temperature never falls below 43.70F (6.50C) at 390N latitude.
The current becomes narrow while passing through the Florida Strait but thereafter its width increases and the current flows close to the coast. The current is about 30 nautical miles away from the coast near Augustine and it is 85 nautical miles off the coast near 15 N latitude but the current comes very close to the coast near Cape Hatteras where it is only 10-20 nautical miles away from the coast.
There is wide range of variation in the width of the Florida current at different places. Its width is 30 nautical miles in the Florida Strait, 60 nautical miles near Cape Canaveral and 120 to 150 nautical miles at Charleston. Further northward this current is joined by the Antilles current, a branch of the north equatorial current, near 30°N latitude.
The origin of Florida current is attributed to the piling up of immense volume of water in the Gulf of Mexico due to powerful trade winds. Thus, the water is forced to move out of Florida Strait. The annual average velocity of Florida current is about 72 miles per day but it becomes 100 to 120 miles per day in January and June. According to Wust the discharge of Florida current passing through Florida Strait is 26 million m3 per second or 100 billion tons of water passes per hour through Florida Strait.
(ii) Gulf Stream:
Gulf Stream was discovered for the first time by Ponce de Leon in the year 1513. The Florida current after having the water of Antilles current is known as Gulf Stream beyond Cape Hatteras. This current is very wide and warm and is separated from the Sargasso sea to its right (in the east) and relatively cold water near the coast to its left. The temperature of water near the coast ranges between 4° and 10°C.
This zone of cold water between the coast and the Gulf Stream is called cold wall. The existence of this cold wall of cold water near the eastern coast of the USA is attributed to many factors. Some scientists opine that strong westerly winds drive the warm waters of the coast eastward and cool waters of the cold Labrador current move southerward along the coast upto Cape Hatteras, while some scientists believe that the cold water of the Gulf of St. Lawrence is deflected southward along the eastern coast of the USA.
The Gulf Stream carries warm water northward into the cold water of high latitudes and thus modifies the weather conditions of the adjoining areas. The Gulf Stream generally follows the coast line but it is deflected eastward at 40° N latitude due to the influence of westerlies and deflective force of the earth’s rotation.
Further northward this current is divided into several branches known as the Delta of the Gulf Stream. The main north-easterly branch is still called Gulf Stream. There is wide range of variation in the velocity of the current. The average velocity in the open ocean is 10 to 15 miles per day. It attains the velocity of 72 miles per day near New York but it slows down to 30 miles per day further eastward.
The Gulf Stream loses its original characteristics near 40°N latitude because it mixes with the cold Labrador Current. This current transports 74 to 93 million m3 of water per second to the north of Chesapeake Bay. The inversion of temperature (warmer air above cool air) caused due to the convergence of warm Gulf Stream and cold Labrador Current near Newfoundland results in the formation of dense fogs which present effective obstructions in the navigation of ships.
(iii) North Atlantic Current:
The Gulf Stream is divided into many branches at 45° N latitude and 45°W longitude. All the branches are collectively called as North Atlantic Drift or current. (A) Northern branch moves north-eastward. It undergoes major changes because of mixing of cool water of the cold Labrador Current with its warm water. Though the temperature and salinity are significantly reduced yet it maintains its main characteristics as warm current.
The velocity of the current also decreases. This current is further divided into several minor branches. (a) One branch flows along the coast of Norway across Wyville Thompson Ridge and reaches the Norwegian Sea. (b) Second branch is known as Irminger current which flows north and north-westward upto the southern coast of Iceland, (c) Third branch moves towards the eastern coast of Greenland where it joins the Greenland current. (B) Eastern branch is comparatively warmer than the northern branch.
This branch flows in easterly direction and reaches the western coasts of France and Spain. This branch is also divided into several sub- branches. (a) One branch enters the Mediterranean Sea while (b) the other branch, known as Rennell current (named after scientist Rennell), enters the Bay of Biscay and flows up to the northern coasts of France and Spain.
Rennell current is further divided into sub- branches wherein one branch enters the English Channel while the other branch after flowing to the south of Ireland merges with the North Atlantic Current, (c) Third branch is the main branch which flows through the coasts of Spain, Azores etc. and reaches the western coast of Africa to join the cold Canary current.
The Gulf Stream system largely modifies the weather conditions of the eastern coasts of the USA and the western coasts of Europe. The temperatures of these coastal areas are 4°F higher than the average temperatures of their latitudes. Gulf Stream is responsible for unique characteristics of West European Type of Climate.
The temperature of the south-eastern and eastern USA becomes exceptionally high during summers because the winds coming from over the Gulf Stream bring more heat in these areas but the eastern coastal areas of the USA are not benefitted by the Gulf Stream during winter because the winds are off shore (from the land towards the Atlantic Ocean). The convergence of warm Gulf Stream and cold Labrador Current near Newfoundland causes inversion of temperature which results in the formation of dense fogs which hinder sea transport.
5. Canary Current (Cold):
The Canary current, a cold current, flows along the western coast of North Africa between Maderia and Cape Verde. In fact, this current is the continuation of North Atlantic Drift which turns southward near the Spanish coast and flows to the south along the Coast of Canaries Island. The average velocity of this current is 8 to 30 nautical miles per day. This current brings cold water of the high latitudes to the warm water of the low latitudes and finally merges with the north equatorial current. The Canary cold current ameliorates the otherwise hot weather conditions of the western coasts of North Africa.
6. Labrador Current (Cold):
The Labrador Current, an example of cold current, originates in the Baffin Bay and Davis Strait and after flowing through the coastal waters of Newfoundland and Grand Bank merges with the Gulf Stream around 50°W longitude. The flow discharge rate of the current is 7.5 million m3 of water per second. This current brings with it a large number of big icebergs as far south as Newfoundland and Grand Bank. These iceberges present effective hindrances in the oceanic navigation. Dense fogs are also produced due to the convergence of the Labrador cold current and the Gulf Stream near Newfoundland.
7. Brazil Current (Warm):
The Brazil current is characterized by high temperature and high salinity. This current is generated because of the bifurcation of the south equatorial current because of obstruction of the Brazilean coast near Sun Rock. The northern branch flows northward and merges with the north equatorial current while the southern branch known as the Brazil current flows southward along the east coast of South America upto 40°S latitude.
Thereafter it is deflected eastward due to the deflective force of the rotation of the earth and flows in easterly direction under the influence of the westerlies. The Falkland cold current coming from the south merges with the Brazil current near 40°S latitude.
8. Falkland Current (Cold):
The cold waters of the Antarctic Sea flows in the form of Falkland cold current from south to north along the eastern coast of South America upto Argentina. This current becomes most extensive and developed near 30⁰S latitude. This current also brings numerous icebergs from the Antarctic area to the South American coast.
9. South Atlantic Drift (Cold):
The eastward continuation of the Brazil current is called South Atlantic Drift. This current is originated because of the deflection of the Brazil warm current eastward at 40° S latitude due to the deflective force of the rotation of the earth. The South Atlantic Drift, thus, flows eastward under the influence of the westerlies. This current is also known as the Westerlies Drift or the Antarctic Drift.
10. Benguela Current (Cold):
The Benguela current, a cold current, flows from south to north along the western coast of South Africa. In fact, the South Atlantic Drift turns northward due to obstruction caused by the southern tip of Africa. Further northward, this current merges with the South Equatorial Current.
Sargasso Sea:
Introduction:
There is an anticyclonic circulation of ocean currents comprising the north equatorial current, the Gulf Stream and the Canary current in the North Atlantic Ocean. The water confined in this gyral is calm and motionless. Thus, the motionless sea of the said gyral is called Sargasso Sea which is derived from the Portuguese word ‘sargassum’ meaning thereby sea weeds. It may be pointed out that similar Sargasso Sea is not found in the South Atlantic Ocean.
Extent:
The extent of the Sargasso Sea is delineated on the basis of the extent of sea weeds and the gyral of ocean currents. According to Manner the Sargasso Sea is found between 20°-40°N latitudes and 35°-75° W longitudes. According to Wing the boundary is determined by 27°W longitude in the east, by 20°N longitude in the south, by 40°N latitude in the north and by the location of the Gulf Stream in the west.
Origin:
The origin of the Sargasso Sea is attributed to several factors:
(i) The sizeable portion of the waters of the North Atlantic Ocean is confined in the gyral system formed by the anticyclonic circulation of the North Equatorial current, the Gulf Stream and the Canary current and thus the confined water does not have any connection with remaining waters of the ocean. Thus, the confined water becomes calm and motionless.
(ii) The Sargasso Sea is located in the transition zone of the trade winds (N.E. Trades) and the westerlies. This zone is characterized by the subsidence of air from above and the resultant anticyclonic conditions. Thus, the anticyclonic conditions cause atmospheric stability and hence there are very feeble and calm winds due to which there is little mixing of confined water (sargasso sea) with the remaining waters of the North Atlantic Ocean.
(iii) The North Atlantic Ocean is less extensive between 20⁰-40⁰ N latitudes than other oceans in the same latitudes.
(iv) The confined waters become calm due to higher velocity of the North Equatorial Current and the Gulf Stream.
Main Characteristics:
The Sargasso Sea records the highest salinity (370/00,) of the Atlantic Ocean due to high temperature and evaporation. The salinity is also increased because of no mixing of the water of the Sargasso Sea with the remaining water of the North Atlantic Ocean. The mean annual temperature is 28°C.
The sea is covered with rootless sea weeds which obstruct navigation. There are contrasting opinions about the extent and origin of sargassum (sea weeds). According to one group the sea weeds grow along the banks of Azores and Bahamas and these are brought by the sea waves and winds.
According to another theory sea weeds grow in the Mexican Gulf and these are brought by the Gulf Stream to the Sargasso Sea. The third group believes that the sea weeds of the Sargasso Sea are floating plants without roots.