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| The '''Agulhas Current''' {{IPAc-en|ə|ˈ|ɡ|ʌ|l|ə|s}} 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]], with an estimated net transport of 100 Sverdrups, as comparable western boundary currents transport less, ranging from the [[Brazil Current]] (16.2 [[Sverdrup]]s) to the [[Kuroshio]] (42 Sverdrups).<ref>{{cite journal | author = Bryden, HL., Beal, LM., Duncan, LM. | title = Structure and transport of the Agulhas Current and its temporal variability | journal = Journal of Oceanography | volume = 61 | pages = 479–492 | year = 2003 | url = http://www.springerlink.com/content/l6880hj337438qq7/ }}</ref>
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| [[File:agulhas sst.png|thumb|Mean sea surface temperature map of the Agulhas Current for 2009. Note the separation of the current from the African coast as a warm tongue of water south of [[Cape Agulhas]], as it retroflects into the [[Indian Ocean]].]]
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| ==Physical properties==
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| The sources of the Agulhas Current are the [[East Madagascar Current]] (25 [[Sverdrup]]s), the [[Mozambique Current]] (5 Sverdrups) and a reticulated part of the Agulhas Current itself (35 Sverdrups).<ref>{{cite journal | author = Stramma, L. and Lutjeharms, J. | title = The flow field of the subtropical gyre in the South Indian Ocean into the Southeast Atlantic Ocean: a case study. | journal = Journal of Geophysical Research | volume = 99 | pages = 14053–14070 | year = 1997 | url = http://www.agu.org/pubs/crossref/1997/96JC03455.shtml }}</ref> 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 [[Continental shelf|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.<ref name="Siedler">Siedler, G., Church, J., and Gould, J. "Ocean Circulation & Climate", Academic Press, 2001.</ref>
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| ===Retroflection===
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| In the southeast [[Atlantic Ocean]] 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. | |
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| ===Agulhas leakage=== | |
| It is estimated that as much as 15 Sv of [[Indian Ocean]] 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 [[Thermohaline circulation|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]].<ref name="Siedler" />
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| ===Filaments===
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| Surface water filaments are estimated to account for up to 13% of the total salt transport from the Agulhas Current into the [[Benguela Current]] and [[South Atlantic Gyre]]. Due to surface dissipation, these filaments are not believed to significantly contribute to inter-basin heat flux.<ref name="Siedler" />
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| ===Eddies===
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| Where the Agulhas turns back on itself the loop of the retroflection pinches off periodically, releasing an [[eddy (fluid dynamics)|eddy]] 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<math>\cdot</math>10<sup>6</sup> kg/s and heat at a rate of 0.045 PW.<ref name="Siedler" />
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| ==Biological properties==
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| [[File:agulhas chla.png|thumb|Mean chlorophyl-a concentration map of the Agulhas Current for 2009. Note the high productivity water in the Agulhas Retroflection.]]
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| ===Primary production===
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| The Agulhas acts as an oceanic [[convergence zone]]. Due to [[continuity equation|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 production|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.<ref name="Mann">Mann, K.H., Lazier, J.R. "Dynamics of Marine Ecosystems", Third Edition, Blackwell Publishing, 2006</ref>
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| ===Impact of rings===
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| [[Warm core ring]]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]] 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).<ref name="Mann" />
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| 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.
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| ==See also==
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| * [[Agulhas Bank]]
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| * [[Sardine run]]
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| * [[Rogue wave (oceanography)]]
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| ==References==
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| {{Reflist}}
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| ==Bibliography==
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| * Steve Baum and C.Michael Hogan. 2011. [http://www.eoearth.org/article/Agulhas_Current?topic=49523 ''Agulhas Current''. Eds P.Saundry & C.J.Cleveland, Encyclopedia of Earth. National Council for Science & the Environment]. Washington DC
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| == External links ==
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| * [http://oceancurrents.rsmas.miami.edu/indian/agulhas.html Agulhas current]
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| * [http://oceanmotion.org/html/impact/conveyor.htm Ocean Motion: Ocean Conveyor Belt] (including Agulhas current role)
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| * [http://www.pbs.org/wnet/nature/episodes/sharkland/introduction/1942/ "Sharkland"] (2007), a ''[[Nature (TV series)|Nature]]'' documentary about sharks and the Agulhas Current.
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| * [http://www.asclme.org/ UNDP/GEF Agulhas and Somali Current Large Marine Ecosystems (ASCLME) Project]
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| {{Ocean}}
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| {{coord|30|00|S|35|00|E|source:kolossus-svwiki|display=title}}
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| [[Category:Marine ecoregions]]
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| [[Category:Ocean currents]]
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| [[Category:Rogue wave incidents]]
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