# Agulhas Current

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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, with an estimated net transport of 100 Sverdrups, as comparable western boundary currents transport less, ranging from the Brazil Current (16.2 Sverdrups) to the Kuroshio (42 Sverdrups).[1]

## Physical properties

The sources of the Agulhas Current are the East Madagascar Current (25 Sverdrups), the Mozambique Current (5 Sverdrups) and a reticulated part of the Agulhas Current itself (35 Sverdrups).[2] 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.[3]

### Retroflection

In the southeast Atlantic Ocean the current retroflects (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 (Sverdrups) 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.

### 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 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.[3]

### Filaments

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.[3]

### Eddies

Where the Agulhas turns back on itself the loop of the retroflection pinches off periodically, releasing an 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 rings are estimated to have a transport of 3-9 Sv each, in total injecting salt at a rate of 2.5$\cdot$106 kg/s and heat at a rate of 0.045 PW.[3]

## Biological properties

Mean chlorophyl-a concentration map of the Agulhas Current for 2009. Note the high productivity water in the Agulhas Retroflection.

### Primary production

The Agulhas acts as an oceanic convergence zone. 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.[4]

### Impact of rings

Warm core rings 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).[4]

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.