The Agulhas Current is the western border of the southwest Indian Ocean. It flows on the east coast of Africa from 27 Â ° to 40 Â °. It's narrow, fast and powerful. It is suggested that this is the largest western boundary in the world of the oceans, with an estimated net transport of 70 Sverdrups (Sv, millions of m 3 /s), since western boundary flows in comparable latitudes carry less - 16.2 Sv), Gulf Stream (34 Sv), Kuroshio (42 Sv).
Video Agulhas Current
Physical properties
The sources of the Agulhas Current are the Eastern Madagascar Stream (25 Sv), the Mozambique Flow (5 Sv) and the recirculated portion of south-western sub-south India south of Madagascar (35 Sv). The net transport of Agulhas Current is estimated as 100 Sv. The current flow of Agulhas is directed by topography. The current follows the continental shelf from Maputo to the end of Agulhas Bank (250 km south of Cape Agulhas). Here the momentum of the current overcomes the balance of vortices that hold current to topography and the current leaves the shelf. Currently it reaches maximum transportation near Agulhas Bank where ranges between 95-136 Sv.
The core of the current is defined as where the surface velocity reaches 100 cm/s (39Ã, in/s), which gives an average core width of 34 km (21 mi). Average peak velocity is 136 cm/s (54 in/s), but currently it can reach 245 cm/s (96 in/s).
Agulhas meander and Christmas pulse
When the Flow of Agulhas flows south along the east coast of Africa, it tends to flood the coast frequently, a deviation from the normal path of the current known as Agulhas Current meanders (ACM). This bulge is sometimes (1-7 times per year) followed by a much larger offshore bulge, known as Christmas pulse (NP). Christmas pulses move along the coast at 20 km (12 mi) per day. An ACM can swell up to 20 km (12 mi) and NP up to 120 km (75 mi) from the current average position. AC passes 34 km (21 million) offshore and ACM can reach 123 km (76 million) offshore. When the AC meander, its width extends from 88 km (55 mi) to 125 km (78 mi) and its velocity weakens from 208 cm/sec (82 inches/second) to 136 cm/sec (54 inches/sec). ACM induces a strong backflow on land.
Large scale cyclone Meanders known as Christmas pulses are formed when Agulhas currently reach the continental shelf on the east coast of South Africa (ie East Agulhas Bank holiday Christmas). When these pulses move along the coast at Agulhas Bank, they tend to pinch the Agulhas ring from the Agulhas Current. Such a shedding ring can be triggered by a Christmas pulse only, but sometimes the meander at Agulhas Return Current joins to contribute to the shedding of the Agulhas ring.
Retroflection
In the southeastern Atlantic Ocean, the current retroflexion (retractable) in Retroflection of Agulhas due to shear interaction with Circumpolar Antarctic Circum, also known as "West Wind Drift" though refers to ocean currents rather than to the surface of the wind. This water became Agulhas Return Current, rejoining the Indian Ocean. It is estimated that up to 85 Sv (Sv) of clean transport is returned to the Indian Ocean via retroflexion. The remaining water is transported to Pilin South Atlantic at Agulhas Leak. Along with direct branch flows, these leaks occur in surface water filaments, and Agulhas Eddies.
Agulhas leaked and rang
It is estimated that 15 Sv waters of the Indian Ocean leak directly into the South Atlantic. 10 Sv is relatively warm, salty thermocline water, with the remaining 5 Sv cold, low salinity of Antarctic Intermediate Water. Since the Indian Ocean water is significantly warmer (24-26 Â ° C) and more salty than the South Atlantic water, Agulhas Leak is a significant source of salt and heat for the Southern Atlantic Pilin. These heat fluxes are believed to contribute to high evaporation rates in the South Atlantic, a key mechanism in the Meridional Overturning Circulation. It should be noted that a small number of Agulhas Leakage join the Northern Brazilian Stream, bringing the Indian Ocean water to the North Atlantic Subtropical Pilin. Before reaching the Caribbean Sea, this leak was exposed by the sun around the equator, and, when it finally joined the Gulf Stream, this warm and salty water contributed to the formation of deep water in the North Atlantic.
The water surface filament is estimated to account for up to 13% of total salt transport from Agulhas Current to Benguela Current and South Atlantic Gyre. Due to surface dissipation, this filament is not believed to contribute significantly to inter-basin heat flux.
Where Agulhas returns on its own, the retroflection loop is wedged periodically, releasing eddy to South Atlantic Gyre. This "Agulhas Ring" enters the flow of the Benguela Stream or is driven northwestward across the South Atlantic where it joins the Southern Equatorial Stream, where they disappear into a larger background stream. This anticyclonic warm core ring is estimated to have 3-9 Sv transport each, a total salt shot at a rate of 2.5
Since the Pleistocene, the South Atlantic thermocline's buoyancy and the circulating power of overturning the Atlantic have been governed by the shedding of warm and salty Agulhas Rings. Agulhas leaks affect the Atlantic thermocline on a decadal time scale and over the centuries it can alter the buoyancy of the Atlantic thermocline and therefore the rate of formation of North Deep North Atlantic (NADW).
The provenance of marine sediments can be determined by analyzing the ratio of terrigenous strontium isotopes in the deep-sea nucleus. The sediments underlying Agulhas Current and Return Current have a higher ratio of the surrounding sediments. Franzese, Goldstein & amp; Skrivanek 2012 analyzed the core in the South Atlantic that was stored during the Last Glacial Maximum (LGM, 20,000 years ago), and concluded that Agulha leaks were significantly reduced. The current path is equal to LGM and the reduced leakage must be explained by a weaker current. Furthermore, it can be predicted that a stronger Agulhas Current will result in more retroflexions to the east and an increased Agulha leak. Simon et al. 2013, however, notes that changes in temperature and salinity in Agulha leak at least partly result from variability in the composition within the stream itself and may be a poor indicator of leakage strength.
Nasty waves
South-east coast of South Africa is on the main cruise line between the Middle East and Europe/US and some large ships are severely damaged by the nasty waves in areas where these waves can sometimes reach heights of more than 30 m (98Ã, ft). About 30 larger vessels were heavily damaged or submerged by wicked waves along the east coast of South Africa between 1981 and 1991.
Agulhas Undercurrent
Directly below the core of the Agulhas Current, at a depth of 800 m (2,600 ft), there is a Flow Down Agulhas that flows the equator. The bottom current is 2,000 m (6,600 feet) deep and 40 km (25 miles) wide and can reach 90 cm/s (35 in/s) at 1,400 meters (4,600 feet), one of the largest speeds observed at any time at depth this, but also displays a large variance with 4.2 Â ± 5.2 Sv transport. The downstream may represent as much as 40% of the Indian Ocean overturns transport.
Below 1,800 m (5,900 ft) of separated layers from the lower currents can be distinguished: The more coherent Atlantic Sea (NADW) which transports an average of 2.3 Â ± 3.0 Sv. NADW circles the southern tip of Africa after the main part (9 Sv) flows eastward and the smaller part (2 Sv) north through Agulhas Undercurrent and into the Christmas Valley (basin between South Africa and Mocambique Plateau); the remains of NADW have been observed in the Mozambique Basin and Channel. The lower stream is more leaking than the Agulhas above, resulting in relatively well mixed water mass composition - in the middle depth there is a mixture between Intermediate Intermediate Water and Sea Water Read.
The meandiness period and the Christmas pulse of Agulhas are matched by Agulhas's Bottom Flow. More research is needed but observations seem to indicate that during the tortuous event Agulhas first moves land, then offshore, and finally overland again, weakening first then strengthening 10-15 Sv. At the same time, the undercurrent was first pressed offshore and weakened as Agulhas moved on land, then strengthened and forced upward when Agulhas moved abroad, and finally returned to normal.
Maps Agulhas Current
Biological properties
Primary production
Agulhas acts as a zone of ocean convergence. Because of this mass continuity it pushes the surface water downwards, resulting in a cold upwelling, nutrient-rich water south of the currents. In addition, convergence tends to increase the concentration of plankton in and around Agulhas. These two factors produce an area that is one of the primary productivity increases compared to the surrounding waters. This is especially important in Agulhas Retroflection waters, where a-chlorophyll concentration tends to be much higher than the surrounding Indian Ocean and the waters of the South Atlantic Ocean.
Ringer effects
The warm core ring is known to have lower primary productivity than the surrounding cold water. Agulhas Rings is no exception, and has been observed to carry water with chlorophyll-low water concentrations 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 (about 20 Ã,Âμm in diameter).
Agulhas Rings has also been observed as removing larvae and adolescent fish from the continental shelf. The removal of these young fish can result in a lower catch of fish in the Benguela system if the rings pass through the fishery.
See also
- Agulhas Passage
- Running Sardines
References
Note
Source
Source of the article : Wikipedia
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