Agulhas Current
Encyclopedia
The Agulhas Current is the Western Boundary Current of the southwest Indian Ocean
. It flows down the east coast of Africa
from 27°S to 40°S. It is narrow, swift and strong. It is even suggested that the Agulhas is the largest western boundary current in the world ocean, as comparable western boundary currents transport less, ranging from the Brazil Current
, 16.2 Sverdrup
s), to the Kuroshio, 42 Sverdrups.
(25 Sverdrup
s), the Mozambique Current
(5 Sverdrups) and a reticulated part of the Agulhas Current itself (35 Sverdrups). The net transport of the Agulhas Current is estimated as 100 Sv. The flow of the Agulhas Current is directed by the topography
. The current follows the continental shelf
from Maputo
to the tip of the Agulhas Bank
(Cape Agulhas
). Here the momentum of the current overcomes the vorticity balance holding the current to the topography and the current leaves the shelf.
the current retroflect
s (turns back on itself) in the Agulhas Retroflection due to shear interactions with the strong Antarctic Circumpolar Current
. This water becomes the Agulhas Return Current, rejoining the Indian Ocean Gyre
. It is estimated that up to 85 Sv (Sverdrup
s) of the net transport is returned to the Indian Ocean
through the retroflection. The remaining water is transported into the South Atlantic Gyre
in the Agulhas Leakage. Along with direct branch currents, this leakage takes place in surface water filaments, and Agulhas Eddies.
water is leaked directly into the South Atlantic. 10 Sv of this is relatively warm, salty thermocline
water, with the remaining 5 Sv being cold, low salinity Antarctic Intermediate Water
. Since Indian Ocean
water is significantly warmer (24-26°C) and saltier than South Atlantic water, the Agulhas Leakage is a significant source of salt and heat for the South Atlantic Gyre
. This heat flux is believed to contribute to the high rate of evaporation in the South Atlantic, a key mechanism in the Meridional Overturning Circulation
. It should be noted that a small amount of the Agulhas Leakage joins the North Brazil Current
, carrying Indian Ocean
water into the North Atlantic Subtropical Gyre.
and South Atlantic Gyre
. Due to surface dissipation, these filaments are not believed to significantly contribute to inter-basin heat flux.
into the South Atlantic Gyre
. This "Agulhas Ring" enters the flow of the Benguela Current
or is advected northwestward across the South Atlantic where it joins the South Equatorial Current
, where they dissipate into the larger background currents. These anticyclonic warm core ring
s are estimated to have a transport of 3-9 Sv each, in total injecting salt at a rate of 2.5
106 kg/s and heat at a rate of 0.045 PW.
. Due to mass continuity
this drives surface waters down, resulting in the upwelling
of cold, nutrient rich water south of the current. Additionally, the convergence tends to increase the concentration of plankton in and around the Agulhas. Both of these factors result in the area being one of enhanced primary productivity
as compared to the surrounding waters. This is especially notable in the Agulhas Retroflection waters, where chl-a concentrations tend to be significantly higher than the surrounding South Indian Ocean and South Atlantic Ocean waters.
s are known to have lower primary productivity than surrounding cold waters. Agulhas Rings are no exception, and have been observed to carry waters with low chlorophyll-a
concentration water into the South Atlantic. It can also be noted that the size of phytoplankton
in Agulhas Rings tends to be smaller than in the surrounding water (around 20 µm in diameter).
Agulhas Rings have also been observed as removing larval and juvenile fish from the continental shelf. This removal of young fish can result in a reduced Anchovy
catch in the Benguela
system if a ring passes through the fishery.
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
. It flows down the east coast of Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
from 27°S to 40°S. It is narrow, swift and strong. It is even suggested that the Agulhas is the largest western boundary current in the world ocean, as comparable western boundary currents transport less, ranging from the Brazil Current
Brazil Current
The Brazil Current is a warm water current that flows south along the Brazilian south coast to the mouth of the Río de la Plata. This current is caused by diversion of a portion of the Atlantic South Equatorial Current from where that current meets the South American continent...
, 16.2 Sverdrup
Sverdrup
The sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...
s), to the Kuroshio, 42 Sverdrups.
Physical properties
The sources of the Agulhas Current are the East Madagascar CurrentEast Madagascar Current
The East Madagascar Current is an oceanic flow feature near Madagascar. It flows southward from 20S on the east side of Madagascar to the southern limit at Cape Saint Marie and subsequently feeds the Agulhas Current...
(25 Sverdrup
Sverdrup
The sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...
s), the Mozambique Current
Mozambique Current
The Mozambique Current is an ocean current in the Indian Ocean, usually defined as warm surface waters flowing south along the African east coast in the Mozambique Channel, between Mozambique and the island of Madagascar....
(5 Sverdrups) and a reticulated part of the Agulhas Current itself (35 Sverdrups). The net transport of the Agulhas Current is estimated as 100 Sv. The flow of the Agulhas Current is directed by the topography
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
. The current follows the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
from Maputo
Maputo
Maputo, also known as Lourenço Marques, is the capital and largest city of Mozambique. It is known as the City of Acacias in reference to acacia trees commonly found along its avenues and the Pearl of the Indian Ocean. It was famous for the inscription "This is Portugal" on the walkway of its...
to the tip of the Agulhas Bank
Agulhas Bank
The Agulhas Bank is a broad, shallow part of the southern African continental shelf which extends south of Cape Agulhas before falling away steeply away to the abyssal plain....
(Cape Agulhas
Cape Agulhas
Cape Agulhas is a rocky headland in the Western Cape, South Africa. It is the geographic southern tip of Africa and the official dividing point between the Atlantic and Indian oceans...
). Here the momentum of the current overcomes the vorticity balance holding the current to the topography and the current leaves the shelf.
Retroflection
In the southeast Atlantic OceanAtlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
the current retroflect
Retroflect
Retroflect, meaning "to turn back on itself", is an oceanographic term used to describe the movement of an ocean current which doubles back on itself.-Usage history:...
s (turns back on itself) in the Agulhas Retroflection due to shear interactions with the strong Antarctic Circumpolar Current
Antarctic Circumpolar Current
The Antarctic Circumpolar Current is an ocean current that flows from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and, at approximately 125 Sverdrups, the largest ocean current...
. This water becomes the Agulhas Return Current, rejoining the Indian Ocean Gyre
Indian Ocean Gyre
The Indian Ocean Gyre, located in the Indian Ocean, is one of the five major oceanic gyres.- External links:*...
. It is estimated that up to 85 Sv (Sverdrup
Sverdrup
The sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...
s) of the net transport is returned to the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
through the retroflection. The remaining water is transported into the South Atlantic Gyre
South Atlantic Gyre
The South Atlantic Gyre is the southern branch of the subtropical gyre in the south Atlantic. This gyre is heavily influenced by northwesterly winds that drive a broad eastward drift, which makes it difficult to distinguish between the northern boundary of the subtropical gyre and the southern...
in the Agulhas Leakage. Along with direct branch currents, this leakage takes place in surface water filaments, and Agulhas Eddies.
Agulhas leakage
It is estimated that as much as 15 Sv of Indian OceanIndian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
water is leaked directly into the South Atlantic. 10 Sv of this is relatively warm, salty thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
water, with the remaining 5 Sv being cold, low salinity Antarctic Intermediate Water
Antarctic Intermediate Water
Antarctic Intermediate Water is a cold, relatively low salinity water mass found mostly at intermediate depths in the Southern Ocean. The AAIW is formed at the ocean surface in the Antarctic Convergence zone or more commonly called the Antarctic Polar Front zone...
. Since Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
water is significantly warmer (24-26°C) and saltier than South Atlantic water, the Agulhas Leakage is a significant source of salt and heat for the South Atlantic Gyre
South Atlantic Gyre
The South Atlantic Gyre is the southern branch of the subtropical gyre in the south Atlantic. This gyre is heavily influenced by northwesterly winds that drive a broad eastward drift, which makes it difficult to distinguish between the northern boundary of the subtropical gyre and the southern...
. This heat flux is believed to contribute to the high rate of evaporation in the South Atlantic, a key mechanism in the Meridional Overturning Circulation
Thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
. It should be noted that a small amount of the Agulhas Leakage joins the North Brazil Current
North Brazil Current
North Brazil Currents is a term of the complexion of Atlantic Southern equatorial current and the Atlantic equatorial counter current, Guinea current. These stream in opposite directions, and fluctuate in their strength, thus making it very difficult to sail fast in the respective area.- See also...
, carrying Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
water into the North Atlantic Subtropical Gyre.
Filaments
Surface water filaments are estimated to account for up to 13% of the total salt transport from the Agulhas Current into the Benguela CurrentBenguela Current
The Benguela Current is the broad, northward flowing ocean current that forms the eastern portion of the South Atlantic Ocean gyre. The current extends from roughly Cape Point in the south, to the position of the Angola-Benguela Front in the north, at around 16°S. The current is driven by the...
and South Atlantic Gyre
South Atlantic Gyre
The South Atlantic Gyre is the southern branch of the subtropical gyre in the south Atlantic. This gyre is heavily influenced by northwesterly winds that drive a broad eastward drift, which makes it difficult to distinguish between the northern boundary of the subtropical gyre and the southern...
. Due to surface dissipation, these filaments are not believed to significantly contribute to inter-basin heat flux.
Eddies
Where the Agulhas turns back on itself the loop of the retroflection pinches off periodically, releasing an eddyEddy (fluid dynamics)
In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object...
into the South Atlantic Gyre
South Atlantic Gyre
The South Atlantic Gyre is the southern branch of the subtropical gyre in the south Atlantic. This gyre is heavily influenced by northwesterly winds that drive a broad eastward drift, which makes it difficult to distinguish between the northern boundary of the subtropical gyre and the southern...
. This "Agulhas Ring" enters the flow of the Benguela Current
Benguela Current
The Benguela Current is the broad, northward flowing ocean current that forms the eastern portion of the South Atlantic Ocean gyre. The current extends from roughly Cape Point in the south, to the position of the Angola-Benguela Front in the north, at around 16°S. The current is driven by the...
or is advected northwestward across the South Atlantic where it joins the South Equatorial Current
South Equatorial Current
The South Equatorial Current is a significant Pacific, Atlantic, and Indian Ocean current that flows east-to-west between the equator and about 20 degrees south. In the Pacific and Atlantic Oceans, it extends across the equator to about 5 degrees north....
, where they dissipate into the larger background currents. These anticyclonic warm core ring
Warm core ring
A warm core ring is a type of mesoscale eddy which breaks off from the Gulf Stream, creating an independent circulatory system of warm water. This type of system is theorized to have helped turn Hurricane Katrina into such a strong hurricane....
s are estimated to have a transport of 3-9 Sv each, in total injecting salt at a rate of 2.5
106 kg/s and heat at a rate of 0.045 PW.Biological properties
Primary production
The Agulhas acts as an oceanic convergence zoneConvergence zone
Convergence zone usually refers to a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions....
. Due to mass continuity
Continuity equation
A continuity equation in physics is a differential equation that describes the transport of a conserved quantity. Since mass, energy, momentum, electric charge and other natural quantities are conserved under their respective appropriate conditions, a variety of physical phenomena may be described...
this drives surface waters down, resulting in the upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
of cold, nutrient rich water south of the current. Additionally, the convergence tends to increase the concentration of plankton in and around the Agulhas. Both of these factors result in the area being one of enhanced primary productivity
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...
as compared to the surrounding waters. This is especially notable in the Agulhas Retroflection waters, where chl-a concentrations tend to be significantly higher than the surrounding South Indian Ocean and South Atlantic Ocean waters.
Impact of rings
Warm core ringWarm core ring
A warm core ring is a type of mesoscale eddy which breaks off from the Gulf Stream, creating an independent circulatory system of warm water. This type of system is theorized to have helped turn Hurricane Katrina into such a strong hurricane....
s are known to have lower primary productivity than surrounding cold waters. Agulhas Rings are no exception, and have been observed to carry waters with low chlorophyll-a
Chlorophyll a
Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary...
concentration water into the South Atlantic. It can also be noted that the size of phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
in Agulhas Rings tends to be smaller than in the surrounding water (around 20 µm in diameter).
Agulhas Rings have also been observed as removing larval and juvenile fish from the continental shelf. This removal of young fish can result in a reduced Anchovy
Anchovy
Anchovies are a family of small, common salt-water forage fish. There are 144 species in 17 genera, found in the Atlantic, Indian, and Pacific Oceans. Anchovies are usually classified as an oily fish.-Description:...
catch in the Benguela
Benguela
Benguela is a city in western Angola, south of Luanda, and capital of Benguela Province. It lies on a bay of the same name, in 12° 33’ S., 13° 25’ E...
system if a ring passes through the fishery.
External links
- Agulhas current
- Ocean Motion: Ocean Conveyor Belt (including Agulhas current role)
- "Sharkland" (2007), a NatureNature (TV series)Nature is a wildlife television program produced by Thirteen/WNET New York. It has been distributed to United States public television stations by the PBS television service since its debut on October 10, 1982. Some episodes may appear in syndication on many PBS member stations around the U.S. and...
documentary about sharks and the Agulhas Current. - UNDP/GEF Agulhas and Somali Current Large Marine Ecosystems (ASCLME) Project