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 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).
Physical properties
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 95-136 Sv.
The core of the current is defined as where the surface velocities reaches , which gives the core an average width of . The mean peak speed is , but the current can reach .
Agulhas meanders and Natal pulses
As the Agulhas Current flows south along the African east coast, it tends to bulge inshore frequently, a deviation from the current's normal path known as Agulhas Current meanders (ACM). These bulges are occasionally (1-7 times per year) followed by a much larger offshore bulge, known as Natal pulses (NP). Natal pulses move along the coast at per day. An ACM can bulge up to and a NP up to from the current's mean position. 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.
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, 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.
Agulhas leakage and rings
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. Before reaching the Caribbean Sea, this leakage gets heated up by the sun around the equator, and, when finally joining the Gulf Stream, this warm and salty water contributes to the formation of deep water in the North Atlantic.
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.