Агульясское течение: характеристики и особенности

Western boundary current of the southwest Indian Ocean that flows down the east coast of Africa

The Agulhas Current (/ə/'/ɡ//ʌ//l//ə//s/) 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 current in the world ocean, with an estimated net transport of 70 sverdrups (70 million cubic metres per second), as western boundary currents at comparable latitudes transport less — Brazil Current (16.2 Sv), Gulf Stream (34 Sv), Kuroshio (42 Sv).

The sources of the Agulhas Current are the East Madagascar Current (25 Sv), the Mozambique Current (5 Sv) and a recirculated part of the south west Indian subgyre south of Madagascar (35 Sv). 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 (250 km south of 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 reaches its maximum transport near the Agulhas Bank where it ranges between 95 and 136 Sv. The core of the current is defined as where the surface velocities reach , which gives the core an average width of The mean peak speed is , but the current can reach The AC passes offshore and an ACM can reach offshore. When the AC meanders, its width broadens from to and its velocity weakens from to An ACM induces a strong inshore counter current. Large scale cyclonic meanders known as Natal pulses are formed as the Agulhas Current reaches the continental shelf on the South African east coast (i. e. the eastern Agulhas Bank off Natal). As these pulses moves along the coast on the Agulhas Bank, they tend to pinch off Agulhas rings from the Agulhas Current. Such a ring shedding can be triggered by a Natal pulse alone, but sometimes meanders on the Agulhas Return Current merge to contribute to the shedding of an Agulhas ring.

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, also known as the "West Wind Drift" despite referring to the ocean current rather than to the surface winds. This water becomes the Agulhas Return Current, rejoining the Indian Ocean Gyre. It is estimated that up to 85 Sv (Sv) 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.

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. A small amount of the Agulhas Leakage joins the North Brazil Current, carrying Indian Ocean water into the North Atlantic Subtropical Gyre. 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. The provenance of ocean sediments can be determined by analysing terrigenous strontium isotope ratios in deep ocean cores. Sediments underlying the Agulhas Current and Return Current have significantly higher ratios than surrounding sediments. Franzese et al. 2009 analysed cores in the South Atlantic deposited during the Last Glacial Maximum (LGM, 20 000 years ago), and concluded that the Agulhas leakage was significantly reduced. The trajectory of the current was the same during the LGM and that the reduced leakage must be explained by a weaker current. Furthermore, it can be predicted that a stronger Agulhas Current will result in a more eastward retroflection and an increased Agulhas leakage. , however, noted that changes in temperature and salinity in the Agulhas leakage is at least partly the result of variability in the composition in the current itself and can be a poor indicator of the strength of the leakage.

The south east coast of South Africa is on the main shipping route between the Middle East and Europe/the U. S. and even large ships have sustained major damage because of rogue waves in the area where these waves occasionally can reach a height of more than Some 30 larger ships were severely damaged or sunk by rogue waves along the South African east coast between 1981 and 1991.

Directly under the core of the Agulhas Current, at a depth of , there is an Agulhas Undercurrent which flows equatorward. The undercurrent is deep and wide and can reach at , one of greatest speeds observed in any current at this depth, but it also displays a great variance with a transport of 4.2±5.2 Sv. The undercurrent can represent as much as 40% of the Indian Ocean overturning transport. Below a separate layer of the undercurrent can be distinguished: the more coherent North Atlantic Deep Water (NADW) which transports an average of 2.3±3.0 Sv. The periodicity of the meanders and Natal pulses of the Agulhas is matched by the Agulhas Undercurrent.

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. 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.