The Agulhas Return Current (ARC) is an

ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...

in the South

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...

. The ARC contributes to the water exchange between oceans by forming a link between the

South Atlantic Current The South Atlantic Current is an eastward ocean current, fed by the Brazil Current. That fraction of it which reaches the African coast feeds the Benguela Current. It is continuous with the northern edge of the Antarctic Circumpolar Current. Th ...

and the South Indian Ocean Current. It can reach velocities of up to and is therefore popular among participants in trans-oceanic sailing races.

Oceanography

The ARC originates from the

Agulhas Current The Agulhas Current () is the western boundary current of the southwest Indian Ocean. It flows south along the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong. It is suggested that it is the largest western boundary curren ...

, the

western boundary current Boundary currents are ocean currents with dynamics determined by the presence of a coastline, and fall into two distinct categories: western boundary currents and eastern boundary currents. Eastern boundary currents Eastern boundary currents are ...

of the Indian Ocean, at the

Agulhas Retroflection Agulhas (''needles'' in the Portuguese language — or ) may refer to: * Cape Agulhas, the southernmost point of Africa * L'Agulhas, a town near the Cape * Cape Agulhas Lighthouse, located at the Cape * Cape Agulhas Local Municipality, the municipa ...

south of Africa and flows east along the

Subtropical Front A subtropical front is a surface water mass boundary or front, which is a narrow zone of transition between air masses of contrasting density, air masses of different temperatures or different water vapour concentrates. It is also characterized by ...

, roughly around 39°S, north of the

Antarctic Circumpolar Current The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise (as seen from the South Pole) from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feat ...

. The Agulhas Current follows the continental shelf of the African east-coast, pass through the

Agulhas Passage The Agulhas Passage is an abyssal channel located south of South Africa between the Agulhas Bank and Agulhas Plateau. About wide, it connects the Natal Valley and Transkei Basin in the north to the Agulhas Basin in the south and is the only ne ...

until it leaves the

Agulhas Bank The Agulhas Bank (, from Portuguese for Cape Agulhas, ''Cabo das Agulhas'', "Cape of Needles") is a broad, shallow part of the southern African continental shelf which extends up to south of Cape Agulhas before falling steeply to the abyssal pl ...

and reaches the

Agulhas Basin The Agulhas Basin is an oceanic basin located south of South Africa where the South Atlantic Ocean and south-western Indian Ocean meet. Part of the African Plate, it is bounded by the Agulhas Ridge (part of the Agulhas-Falkland Fracture Zone) to ...

south of South Africa. From there it

retroflect Retroflection is the movement of an ocean current that doubles back on itself. Usage history More commonly used to describe the way the mammalian intestine or uterus might turn back on itself, retroflection was first used in an oceanographic sense ...

s almost completely back into the south Indian Ocean as the ARC, and only a smaller part, known as Agulhas leakage, is shed into the

South Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...

. The water mass loses a lot of heat at the Agulhas Retroflection, up to , while evaporation and precipitation change the composition of the upper layers. The ARC, therefore, has another composition than the Agulhas Current proper. It also loses velocity from and volume transport from 70 to 54 million m³/s. Furthermore, all traces of Indian Tropical Surface Water are gone. Having left the retroflection, the ARC meanders east between 36°S and 41°S. These meanders were described as

Rossby wave Rossby waves, also known as planetary waves, are a type of inertial wave naturally occurring in rotating fluids. They were first identified by Sweden-born American meteorologist Carl-Gustaf Arvid Rossby. They are observed in the atmospheres and ...

s in 1970 and are known to shed cold

eddies In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid beh ...

equatorward and enhance the

primary productivity In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...

at the Subtropical Front. The ARC makes a large, quasi-permanent northward meander around the

Agulhas Plateau The Agulhas Plateau is an oceanic plateau located in the south-western Indian Ocean about south of South Africa. It is a remainder of a large igneous province (LIP), the Southeast African LIP, that formed (Ma) at or near the triple junction wher ...

after which it loses more velocity and volume by leakage into the

South Indian subtropical gyre South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþaz ...

. Over the

Crozet Basin Crozet may refer to: * Crozet Islands, a sub-antarctic archipelago of small islands in the southern Indian Ocean, part of the French Southern Territories * Crozet, a commune in France of the Ain département, in France * Crozet, Virginia, a censu ...

the last remnants of the ARC are gone. As it enters the Crozet Basin at 53°E, the transport of the ARC is 35 Sv, most of which is recirculated northward before reaching the

Kerguelen The Kerguelen Islands ( or ; in French commonly ' but officially ', ), also known as the Desolation Islands (' in French), are a archipelago, group of islands in the subantarctic, sub-Antarctic constituting one of the two exposed parts of the ...

Amsterdam Amsterdam ( , , , lit. ''The Dam on the River Amstel'') is the Capital of the Netherlands, capital and Municipalities of the Netherlands, most populous city of the Netherlands, with The Hague being the seat of government. It has a population ...

Passage. The current east of the Crozet Basin, at 66°E-70°E, is called the South Indian Ocean Current and lacks the distinctive features of the ARC.

References

Notes

Sources

* * * * * * {{Refend Currents of the Indian Ocean