The Agulhas Current ( / ə ˈ 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).
35-605: 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
70-551: A duration of 10 days, an optimal period for biological production. It is estimated that the annual biomass production in the Benguela system is 4.7×10 gC/y , making the Benguela system 30 to 65 times more productive per unit area than the global ocean average. While upwelling promotes abundant primary and secondary production in the upper parts of the water column and near the coast, deeper waters with limited oxygen exchange create hypoxic areas called oxygen minimum zones at
105-485: 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 1,800 m (5,900 ft) 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. NADW rounds the southern tip of Africa after which the major part (9 Sv) flow eastward and
140-629: A height of more than 30 m (98 ft). 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 800 m (2,600 ft), there is an Agulhas Undercurrent which flows equatorward. The undercurrent is 2,000 m (6,600 ft) deep and 40 km (25 mi) wide and can reach 90 cm/s (35 in/s) at 1,400 metres (4,600 ft), one of greatest speeds observed in any current at this depth, but it also displays
175-608: A much larger offshore bulge, known as Natal pulses (NP). Natal pulses move along the coast at 20 km (12 mi) per day. An ACM can bulge up to 20 km (12 mi) and a NP up to 120 km (75 mi) from the current's mean position. The AC passes 34 km (21 mi) offshore and an ACM can reach 123 km (76 mi) offshore. When the AC meanders, its width broadens from 88 km (55 mi) to 125 km (78 mi) and its velocity weakens from 208 cm/s (82 in/s) to 136 cm/s (54 in/s). An ACM induces
210-803: A smaller part (2 Sv) northward through the Agulhas Undercurrent and into the Natal Valley (the basin between South Africa and the Mocambique Plateau); remnants of NADW has been observed in the Mozambique Basin and Channel . The undercurrent is more leaky than the Agulhas above, resulting in a relatively well-mixed composition of water masses — at intermediate depth there is a mixture of Antarctic Intermediate Water and Read Sea Water . The periodicity of
245-575: A specific ocean current is a stub . You can help Misplaced Pages by expanding it . Benguela Current The Benguela Current / b ɛ ŋ ˈ ɡ ɛ l ə / is the broad, northward flowing ocean current that forms the eastern portion of the South Atlantic Ocean gyre . The current extends from roughly Cape Point in the south, to the position of the Angola-Benguela Front in the north, at around 16°S . The current
280-501: 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
315-584: A transport of 3-9 Sv each, in total injecting salt at a rate of 2.5 ⋅ {\displaystyle \cdot } 10 kg/s and heat at a rate of 45 TW . Since the Pleistocene , the buoyancy of the South Atlantic thermocline and the strength of the Atlantic meridional overturning circulation has been regulated by the shedding of warm, saline Agulhas Rings. The Agulhas leakage affects
350-463: Is an oceanic flow feature near Madagascar . It flows southward from 20°S on the east side of Madagascar to the southern limit at Cape Saint Marie and subsequently feeds the Agulhas Current . Its flow is complicated by large cyclonic and anticyclonic eddies . The East Madagascar Current has a controlling role in the western boundary current of the southwest Indian Ocean together with
385-782: 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
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#1732766133505420-591: Is driven by the prevailing south easterly trade winds. Inshore of the Benguela Current proper, the south easterly winds drive coastal upwelling , forming the Benguela Upwelling System. The cold, nutrient rich waters that upwell from around 200–300 m (656–984 ft) depth in turn fuel high rates of phytoplankton growth, and sustain the productive Benguela ecosystem. Source waters for the Benguela include cold upwelled waters from
455-533: 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
490-514: Is well-defined, with many temporary and seasonal eddies and meanders. There is however a well-defined thermal front between the waters associated with the Benguela Upwelling System and those of the eastward flowing Atlantic currents which are not deflected northward by the African continent. The icy Benguela and the warm, south-flowing Agulhas current do not meet off the Cape of Good Hope (see diagram on
525-476: 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 100 cm/s (39 in/s), which gives
560-552: 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
595-594: 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. Where
630-697: The Mozambique Current . The mean speed of the East Madagascar Current varies between 0.2–0–9 m/s (0.66–0.00–29.53 ft/s) with a peak in spring and the lowest point in summer. It creates a high-pressure area and affects the Indian Monsoon . The East Madagascar Current is intense and narrow and retroflects into the central Indian Ocean south of Madagascar similarly to the Agulhas Current south of South Africa. 23°00′S 51°00′E / 23.000°S 51.000°E / -23.000; 51.000 This article about
665-668: 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. 30°00′S 35°00′E / 30.000°S 35.000°E / -30.000; 35.000 East Madagascar Current The East Madagascar Current
700-718: 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
735-465: 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
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#1732766133505770-458: The Agulhas moves offshore, and finally returns to normal. The Agulhas acts as an oceanic convergence zone . Due to mass continuity this drives surface waters down, resulting in the upwelling of cold, nutrient rich water south of the current. Additionally, the convergence tends to increase the concentration of plankton in and around the Agulhas. Both of these factors result in the area being one of enhanced primary productivity as compared to
805-501: The Agulhas turns back on itself the loop of the retroflection pinches off periodically, releasing an eddy into the South Atlantic Gyre. These "Agulhas Rings" enter the flow of the Benguela Current or are advected northwestward across the South Atlantic where they join the South Equatorial Current , where they dissipate into the larger background currents. These anticyclonic warm core rings are estimated to have
840-485: The Atlantic thermocline on a decadal timescale and over centuries it can change the buoyancy of the Atlantic thermocline and therefore the formation rates of North Atlantic Deep Water (NADW). 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
875-628: The Benguela Niño, warm, salty waters from the Angola Current move southward, between 15°S and 25°S . This slab of warm salty water extends to 150 km (93 mi) offshore and to 50 m (164 ft) depth. Heavy rains, changes in fish abundance, and temporal proximity to the Pacific El Niño have been observed; however, the causes and effects of the Benguela Niño are not well understood. One research team has shown that
910-420: The Benguela system, phytoplankton growth requires a period of upwelling followed by a period of stratification and relatively calm waters. The phytoplankton bloom usually lags the upwelling event by 1 to 4 days and blooms for 4 to 10 days. In order for zooplankton to have a continuous food supply, the phytoplankton blooms must not occur too far apart. Pulses of upwelling in the Benguela system regularly have
945-631: 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. Simon et al. 2013 , however, noted that changes in temperature and salinity in
980-399: The coastal shelf and upper coastal slope. The Benguela oxygen minimum zone starts around a depth of 100 m and is a few hundred meters thick. Bacteria that use sulphur rather than oxygen, called sulphur-reducing bacteria , reside in the oxygen minimum zone. The most abundant fishes in the Benguela system are Sardinops and Engraulis . The Southern African pilchard ( S. s. ocellatus ),
1015-416: The core an average width of 34 km (21 mi). The mean peak speed is 136 cm/s (54 in/s), but the current can reach 245 cm/s (96 in/s). 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
1050-671: The depths of the Atlantic Ocean close inshore, joined further off-shore by nutrient poor water that has crossed the Southern Atlantic from South America as part of South Atlantic Gyre . Eddies from the warm South Indian Ocean Agulhas current along South Africa's east coast come round the Cape of Good Hope from time to time to join the Benguela current. The Benguela current is 200 to 300 km wide and widens further as it flows north and northwest. Its western, seaward edge
1085-439: The meanders and Natal pulses of the Agulhas is matched by the Agulhas Undercurrent. More research is needed but observations seem to indicate that during a meander event the Agulhas moves first onshore, then offshore, and finally onshore again, first weakening then strengthening 10-15 Sv. At the same time the undercurrent is first squeezed offshore and weakened when the Agulhas moves onshore, then strengthened and forced upward when
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1120-463: The right, above), but there is a body of water off the South African south coast, east and particularly west of Cape Agulhas that consists of eddies from both currents, so that offshore water temperatures along the south coast of Africa vary chaotically. Northward winds along the coast result in Ekman transport offshore and upwelling of nutrient-rich deep water to the euphotic zone . The intensity of
1155-518: The surrounding waters. This is especially notable in the Agulhas Retroflection waters, where chlorophyll-a concentrations tend to be significantly higher than the surrounding South Indian Ocean and South Atlantic Ocean waters. 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
1190-411: The upwelling event is determined by wind strength. Variations in wind strength result in pulses of upwelling, which propagate to the south along the coast with speeds of 5 to 8 m/s. The pulses are similar to a Kelvin wave , except on a scale of 30 to 60 km instead of 1000 km, and can propagate around the Cape depending on wind systems. Pulses of upwelling induce biological production. In
1225-660: Was intensely fished in the 1950s and peaking in 1968 with landings over 1.3 million tons. Since then, fishery of the Sardinops has declined and fishery of the Southern African anchovy ( Engraulis capensis ) has taken over. Similar to the Pacific El Niño , a thick slab of warm, nutrient-poor water enters the northern part of the Benguela upwelling system off the Namibia coast about once per decade. During
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