Oceans and seas

Agulhas Current

June 5, 2011, 4:10 pm

The Agulhas Current is the western boundary current in the Indian Ocean south of 30oS. The southern Agulhas Current flows southwestward as a narrow jet along a steep continental slope, and is normally pinned to within 10 to 15 kilometres of its mean position at latitudes 28.5–3oS. The current generally effluxes from the southern part of the Mozambique Channel. caption A schematic of the currents around South Africa. To the left (west) is the South Atlantic Ocean, while on the right (east) is the Indian Ocean. The Agulhas Current flows southwestward along the east coast of South Africa in the Indian Ocean. From Boebel et al., 2003.

Large meanders – called the Natal pulse – can sometimes occur within this region. These extend an average of 170 km offshore with downstream propagation rates of about 21 centimeters/second (cm/s), with the rates decreasing to five cm/s as the continental shelf broadens near 34oS. At this point the current separates from the coast and continues southwestward along the Agulhas Bank, where many meanders, plumes and eddies exist.

The maximum transport of the Agulhas occurs in the vicinity of Agulhas Bank, where transport estimates range from 95 to 136 Sv. The core of the current has been defined as where surface velocities exceed 100 cm/s with the core averaging about 34 km wide with a mean peak speed of 136 cm/s (with a greatest peak speed of 245 cm/s).

At around 36oS the Agulhas leaves the continental shelf and develops oscillations of increasing amplitude, eventually retroflecting back toward the Indian Ocean in the region of 16–20oE as the Agulhas Return Current. The retroflection loop usually encloses a pool of Indian Ocean surface water south of Africa whose temperature is more than 5o warmer than South Atlantic surface water at similar latitudes. The core of the Return Current infrequently passes over the Agulhas Plateau.

Marine Ecosystem

The region’s mangroves, seagrass beds, and coral reefs reflect high degrees of biodiversity and endemism (for instance the Zanzibar butterfly fish, the African butterfly fish, the ear-spot angelfish, and the tail-barred parrotfish). South Africa’s De Hoop Nature Reserve is important to several species of dolphins and the southern right whale. South Africa has several species of sea turtles, porpoises, jackass (black-footed) penguins, great white sharks, and abalone. The Agulhas Current LME is considered a Class II, moderately productive (150-300 grams of Carbon per square meter per year (gC/m2-yr)), ecosystem . Nutrient enrichment and mixing is current-associated within the Agulhas Current. Several studies have suggested that the Agulhas Current is responsible for the dispersal of the early life history stages of various fish species. There are notable occurrences of clupeoid larvae (pilchard, round herring and anchovy) and scombrid larvae (tuna and chub mackerel). There is a significant role of estuaries along the eastern continental southern African coastline in providing sheltered areas for juvenile organisms in this high energy marine environment. 

See Also

Further Reading

  • Peter Saundry. 2011. Seas of the world. Topic ed. C.Michael Hogan. Ed-in-chief Cutler J.Cleveland. Encyclopedia of Earth
  • R. G. Peterson and L. Stramma. Upper-level circulation in the south atlantic. Prog. Oceanog., 26:1–73, 1991.
  • J. R. E. Lutjeharms and I. J. Ansorge. The Agulhas Return Current. Journal of Marine Systems, 30: 115–138, 2001.
  • Boebel, O., Rossby, T., Lutjeharms, J., Zenk, W., Barron, C., 2003. Path and variability of the Agulhas Return Current. Deep-Sea Research II 50, 35–56.
Glossary

Citation

Baum, S. (2011). Agulhas Current. Retrieved from http://www.eoearth.org/view/article/149919

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