Agulhas Bank
The Agulhas Bank 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 plain.
It is the ocean region where the warm Indian Ocean and the cold Atlantic Ocean meet. This convergence leads to treacherous sailing conditions, accounting for numerous wrecked ships in the area over the years. However, the meeting of the oceans here also fuels the nutrient cycle for marine life, making it one of the best fishing grounds in South Africa.
Extent and characteristics
The Agulhas Bank stretches approximately along the African coast, from off Cape Peninsula to Port Alfred, and up to from it. The bank slopes down relatively steeply from the coast to about deep and reaches before dropping very steeply to on its southern edge.The shelf spans an area of with a mean depth slightly over. It is entirely within the exclusive economic zone of South Africa.
The National Spatial Biodiversity Assessment 2004 recognised 34 biozones nested within 9 bioregions. The National Biodiversity Assessment of 2011 replaced these ecozones and biozones with the terms ecoregions and ecozones. In 2011, the Agulhas Ecoregion was divided into four distinct ecozones: Agulhas inshore, Agulhas inner shelf, Agulhas outer shelf, and Agulhas shelf edge. 33 different benthic habitats types were identified on the Agulhas Bank.
Dozens of warm temperate reefs along the coast of the Agulhas Ecoregion span from below sea level. Many rocky sub-tidal reefs are of aeolianite or sandstone origin, but granite, quartzite and siltstone reefs are also present. The Agulhas reefs are heterogeneous and include several possible different sub-types. Some of these reefs are within protected areas, but only a few areas included prohibit fishing.
Oceanography
The Agulhas Bank is a natural boundary between ocean currents from the Atlantic Ocean, Indian Ocean, and Southern Ocean, resulting in some of the most turbulent waters found in the world's oceans.Agulhas Current
The Agulhas Current flows south along the eastern African coast and along the south-eastern edge of the bank. It then retroflects back into the Indian Ocean south-west of the bank. This retroflection results in intense eddy activities such as meanders, eddies, and filaments.In the upper watercolumn, the eponymous "Agulhas rings" or "Agulhas eddies" move warm and salty water into the large South Atlantic gyre, ultimately exporting it to the tropics; while in lower ocean layers water is transported in the opposite direction.
Upwellings
Cyclonic eddies are another source of edge upwelling, notably occurring to the west of Port Elisabeth. Plumes of warm surface water migrate onto the bank along its eastern edge, providing subtropical surface water from the Indian Ocean to the region.In summer, easterly winds can intermittently drive coastal upwelling along the southern South African coast.
The Agulhas Bank region is dominated by westerly winds, and most of the upwelling on the bank is therefore related to the interactions of the Agulhas Current on its eastern edge, but easterly winds do occur and can generate local upwelling cells.
As the current is diverted away from the coast, dynamic processes draw an Ekman layer of cooler water up shoreward from below the warmer shelf-edge flow. In spring and summer, at a depth of, a semi-permanent ridge of cold water is present on the eastern and central shelf.
In summer months, a mixture of subtropical water separated by thermoclines from cool waters forms, though with considerable seasonal variation. On the shelf, deeper waters exhibit characteristics of the central Indian Ocean in the east, and central Atlantic Ocean waters in the west.
Agulhas meanders and Natal pulses
As the Agulhas Current flows south along the African east coast, it is frequently diverted inshore, a deviation from the current's normal path known as Agulhas Current meanders. These bulges are occasionally followed by a much larger offshore bulge, known as Natal pulses. 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 decreases from to. An ACM induces a strong inshore counter-current.Large-scale cyclonic meanders from Natal pulses are formed as the Agulhas Current reaches the continental shelf on the eastern South African coast. As these pulses move along the Agulhas Bank, they tend to pinch off Agulhas rings. Such ring shedding can be triggered by a Natal pulse alone, but meanders on the Agulhas Return Current may merge to contribute to Agulhas ring formation.
Agulhas leakage and rings
Agulhas rings are large anticyclonic eddies or warm core rings of ocean water that are pinched off the Agulhas Current along the eastern edge of the Agulhas Bank from where they move into the South Atlantic. As the Agulhas Current reaches the east coast of South Africa, large solitary meanders known as Natal pulses form at irregular intervals. 165 days after the appearance of a Natal pulse, an Agulhas ring will form off Durban. Agulhas rings are among the largest eddies in the world and play an important role in the Agulhas Leakage, the transport of warm water from the Indian Ocean to the Atlantic Ocean, affecting global climate.The average diameter of these Agulhas rings is of, but they can grow to over 500 km. They extend down to the ocean floor, circulating at, and move into the South Atlantic at /day. Only half of the Agulhas eddies that leave the Cape Basin manage to cross the Walvis Ridge, and those that do tend to lose half their energy within the six months before reaching the ridge. The Agulhas rings transport an estimated 1-5 Sv of water from the Indian Ocean to the South Atlantic.
The Agulhas rings are thought to be of global climatic importance. Their delivery of warm water from the Indian to the Atlantic Ocean can control the rate of thermohaline overturning for the entire Atlantic, though other factors contribute in varying degrees to the inter-ocean exchanges in the region, including filaments from the Agulhas Current and intrusions of water from Antarctica. Cold, cyclonic eddies have been observed in the south-western Atlantic.
Based on model simulations, researchers have found that the interaction of the Agulhas Current and the eastern edge of the bank are responsible for Agulhas rings.
The provenance of ocean sediments can be determined by analysing terrigenous strontium isotope ratios in deep ocean core samples. Sediments underlying the Agulhas Current and Return Current have significantly differing ratios to those of surrounding sediments. Analyses of cores in the South Atlantic deposited during the Last Glacial Maximum, show that the Agulhas leakage was significantly reduced during the period. It has been hypothesised that the reason for this was a relatively stronger Agulhas Current, resulting in more eastward retroflection. However, analyses of such cores south of the African coast show that the trajectory of the current was the same during the LGM, and that the reduced leakage must be explained by a weaker current. Consequently, it can be predicted that a stronger Agulhas Current will result in its retroflection occurring more eastward, with an increased Agulhas leakage.
Benguela Current
Compared to the Agulhas Current, the Benguela Current on the west and south-west coast of Africa is more intense and steadier. Its dynamic southern upwelling system is driven by the prevailing northward winds that produce an intense off-shore Ekman transport. Most of this upwelling is concentrated to a few upwelling cells in the southern region: Namaqua, Cape Columbine, and Cape Peninsula. The wind is most intense from October to February, and the contrast in sea surface temperature between the open sea and the shelf is most prominent during summer.Coastal upwelling is also common on the western bank, but the more stable atmospheric condition results in larger cold water plumes that sometimes merge to form a continuous upwelling regime along the South African south-west coast. This upwelling zone is the southernmost extension of the Benguela Current Large Maritime Ecosystem. The Agulhas Current regularly flows around the southern tip of the bank and brings warm water to the western bank along the bank's western edge.
Regularly, the mesoscale eddies from the east interact with the Benguela upwelling system on the African west coast.
Deep water eddies
Flowing south along the South American continental slope, the Deep Western Boundary Current carries North Atlantic Deep Water into the South Atlantic. At about 8°S and at a depth of, the DWBC breaks into anticyclonic eddies during periods of strong meridional overturning circulation. One such NADW eddy was observed in 2003 and the researchers speculated that a deeply penetrating Agulhas ring pinched it off the NADW slope current. Spinning at, these deep-water eddies move around the southern tip of the Agulhas Bank and into the Indian Ocean. Most of the NADW flow meanders east around the Agulhas Plateau together with the surface Agulhas Return Current, but a smaller portion continues north along the African east-coast as the Agulhas Undercurrent.Of 89.5 Sv released from the North Atlantic, 3.6 Sv leaves the South Atlantic south of the Agulhas Bank. However, 0.9 Sv recirculate in the basin north of the Walvis Ridge for centuries, of which 50-90% end up flowing south of the Agulhas Bank within 300 years, increasing the net inter-oceanic exchange with 4.1-4.5 Sv.