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The average ship-drift derived surface velocity plot shows two major source regions for the Agulhas current: (1) from the Mozambique channel to the north and along the coast; and (2) from the east including a major contribution from the Madagascar current. The average SST image shows the Agulhas current bringing warm water poleward. On the average, the Agulhas current retroflects and returns eastward with part of the flow recirculating in the counter-clockwise flowing subtropical gyre and part of the flow feeding the Antarctic Circumpolar Current. There is also a component of the Agulhas current that feeds the Benguela current and advects relatively warm and salty water into the South Atlantic as part of the Global "conveyor belt" circulation. Click here for example plots of seasonal averages. | |
Although no mention of the Agulhas Current survives from his first voyage, during the second rounding by Vasco da Gama in 1497, ships logs make mention of a southward current near Algoa Bay (near present day Port Elizabeth) of such strength that the flotilla was set steadily back for three days (Steinberg, 2001). By the mid-1500's, the Portuguese knew enough about the Agulhas Current to remain well out to sea as they rounded the African horn on the way to India, but to remain near the coast, although not too close, on the voyage home. (Peterson, et al., 1996; Steinberg, 2001).
The Agulhas Current takes its name from the point of the cape, called Cabo das Agulhas (or Cape of Needles) by later Portuguese seafarers. There are two dominant views on why this name was chosen. The first claims that the sharp rocks and reefs offshore were often described as needles, which combined with the treacherous currents to claim many ships. Among Portuguese sailors, this cape also became known as the Graveyard of Ships. The alternative explanation contends that the name is derived from the discovery that at the tip of the Cape, the compass needle points due north with no deviation between true and magnetic.
The Agulhas Current is the western boundary current of the South Indian Ocean. It flows down the east coast of Africa from 27°S to 40°S (Gordon, 1985). The source water at its northern end is derived from Mozambique channel eddies (de Ruijter et al., 2002) and the East Madagascar Current, but the greatest source of water is recirculation in the southwest Indian Ocean sub-gyre (Gordon, 1985; Stramma and Lutjeharms, 1997). There are temporal and latitudinal variations in the depth, path, and transport of the current. From synoptic measurements, the Agulhas Current was found to extend throughout the water column in March, but in a later survey during June it was limited to the upper 2300 m (Donohue et al., 2000). Its depth tends to increase with latitude to offset the increase in planetary vorticity (Boebel et al., 1998). In addition, there is seasonal oscillation in the sea surface height variability of the Agulhas Current. It is at a maximum during the austral summer and at a minimum during the austral winter. The magnitude of this seasonal change is about 30% of the mean value (Matano et al., 1998).
The dominant mode of variability of the Agulhas Current is in the form of natal pulses (Bryden et al., 2003). These are large solitary meanders containing a cold-core cyclone on the inshore side of the current (Lutjeharms and Roberts, 1988). Natal pulses occur about 6 times per year and propagate downstream at approximately 10 km/day (Lutjeharms et al., 2003). The passage of nearly all natal pulses is followed by the spawning of an Agulhas ring (Van Leeuwen et al., 2000).
Like other western boundary currents, the Agulhas Current is quite fast. At the surface, it can reach maximum speeds of 200 cm s-1 (Boebel et al., 1998). Beal and Bryden (1999) examined the deep velocity structure by using Lowered Acoustic Doppler Current Profiles (LADCP) and found that their results were different from those of previous studies that used geostrophic estimates. Beal and Brydenfound that the level of no motion across the Agulhas Current displays a V-shaped pattern. They were also able to detect an Agulhas Undercurrent at 800 m depth. The undercurrent is directly beneath the surface core of the poleward flowing Agulhas Current, and it flows equatorward with maximum speeds near 30-40 cm s -1 (Beal and Bryden, 1999; Donohue et al., 2000).
As one of the major currents in the Southern Hemisphere, the Agulhas Current system transports large volumes of water. One of the earliest measurements of the geostrophic volume transport of this current came from Gordon (1985), who found it to be 67 Sv (1 Sv = 1 x 106 m3 s-1). Several years later, Toole and Warren obtained a much higher estimate 85 Sv. However, several researchers pointed out that the geostrophic reference level that Toole and Warren used did not resolve the counter-flowing Agulhas Undercurrent. Beal and Bryden (1999) found the geostrophic volume transport as referenced to LADCP to be 73 Sv, which was only 3% different from the direct LADCP transport estimate. Then, Donohue et al. (2000) attempted to refine previous transport calculations by removing barotropic tides and by estimating instrumental and sampling errors. The two LADCP sections that they used yielded a net southward transport of 78±3 and 76±2 Sv. The latest estimate comes from Bryden et al. (2003) who find an average volume transport, calculated from year-long moored current meter measurements of 69.7±4.3 Sv.
As the Agulhas Current reaches the southern tip of the continental shelf of Africa, it begins to turn toward the west. Once it reaches the Southern Ocean, the current retroflects, or turns back on itself, and flows eastward as the Agulhas Return Current (Quartly and Srokosz, 1993). The Agulhas Return Current flows eastward and exhibits a quasi-stationary meandering pattern of wavelength 500 km between 38° and 40° S. Its core width is about 70 km with an associated transport of 44±5 Sv in the upper 1000 m (Boebel et al., 2003).
On average, the Agulhas Retroflection has a loop diameter of 340 km and can be found between 16°E and 20°E (Lutjeharms and van Ballegooyen, 1988). Altimeter data suggest that during the austral winter months there is an early retroflection of the current near 25°E (Matano et al., 1998) and there is greater mesoscale variability (Quartly and Srokosz, 1992). However, satellite studies of the Agulhas Retroflection based on feature-tracking rather than area-averaging (Lutjeharms and van Ballegooyen, 1988; Goni et al., 1997; Quartly and Srokosz, 2002) find that "ring-shedding events" dominate the variability, which are found to be neither continuous, nor periodic. The retroflection gradually extends westward prior to ring-shedding and quickly retrogrades eastward after an Agulhas Ring is spawned. Upstream, the dominant mode of variability within the Agulhas Current is in the form of large, solitary meanders, known as Natal Pulses (Bryden et al., 2003). There is evidence that these meanders may prompt ring-shedding as they propagate downstream and interact with the retroflection loop (Leeuwen et al., 2000).
An interesting aspect of the Agulhas Retroflection is that it periodically sheds pinched-off anticyclonic rings 320 km in diameter at its westernmost extension. These rings enclose pools of relatively warm and saline Indian Ocean water whose temperature is more than 5°C warmer and and salinity 0.3 psu greater than South Atlantic surface water of similar density (Gordon, 1985). The rings keep their distinctive thermal characteristics as far west as 5°E and as far south as 46°S, and they drift into the South Atlantic at approximately 12 cm s -1 (Lutjeharms and van Ballegooyen, 1988). This warm-water link between the Atlantic and Indian oceans is likely to have a strong influence on global climate patterns (Gordon, 1985).
Van Ballegooyen et al. (1994) studied the Agulhas Retroflection region and counted 14 new rings over a 2-year period. They also found that the heat anomaly contained in a ring could be as much as 2.4 x 1020 J, and the salt anomaly could be as much as 13 x 1012 kg. Lutjeharms and Cooper (1996) went on to calculate that the heat flux into the South Atlantic could be 0.0075 PW per ring, and the estimated salt flux could be 13 x 1012 kg per ring. Although climatologically important exchange between the Atlantic and Indian Oceans occurs mostly via Agulhas rings, there are also Agulhas filaments that make a minor contribution when they occasionally escape into the South Atlantic. These filaments are present 56% of the time and are on average 50 km wide and 50 m deep. Each filament carries excess heat of about 3.5 x 10 19 J and excess salt amounting to about 1-5 x 1011 kg. Since most of the heat is rapidly lost to the atmosphere, the main contribution to interbasin exchange by the filaments is a 3-9 x 10 12 kg annual salt flux (Lutjeharms and Cooper, 1996).
Recent float and model experiments reveal that Agulhas Rings are as deep as 1200 m and salt and heat exchange at intermediate depths is important. They also show that the Agulhas retroflexion region not only spawns large (200 km) anti-cyclonic Agulhas Rings, but also smaller (120 km) cyclones (Boebel et al., 2003). The interaction of these cyclones and anti-cylcones results in vigorous mixing and stirring of Indian Ocean and Atlantic Ocean water masses to the northwest of the retroflexion within a region dubbed the "Cape Cauldron".