Agulhas Current

The Agulhas Current /əˈɡʌləs/ is the western boundary current of the southwest Indian Ocean. It flows down 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 (Sv, millions m3/s), as western boundary currents at comparable latitudes transport less — Brazil Current (16.2 Sv), Gulf Stream (34 Sv), Kuroshio (42 Sv).[1]

Benguela and Agulhas Currents
The courses of the warm Agulhas current (red) along the east coast of South Africa, and the cold Benguela current (blue) along the west coast. The Agulhas Current is formed by the confluence of the warm Mozambique and East Madagascar Currents, which meet south-west of Madagascar (not shown in the diagram). The cold Benguela Current originates from upwelling of water from the cold depths of the Atlantic Ocean against the west coast of the continent. The two currents do not "meet" anywhere along the south coast of Africa.

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).[2] 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.[3] The current reaches its maximum transport near the Agulhas Bank where it ranges between 95-136 Sv.[4]

The core of the current is defined as where the surface velocities reaches 100 cm/s (39 in/s), which gives 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).[4]

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 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.[5] 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 a strong inshore counter-current.[6]

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.[7]

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

Agulhas Current NLOM
Agulhas rings are peeled off the Agulhas Current in the Agulhas Basin where it retroflects back into the Indian Ocean.

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. It should be noted that a small amount of the Agulhas Leakage joins the North Brazil Current, carrying Indian Ocean water into the North Atlantic Subtropical Gyre.[3] 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.[8]

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.[3]

Where the Agulhas turns back on itself the loop of the retroflection pinches off periodically, releasing an eddy into the South Atlantic Gyre. This "Agulhas Ring" enters the flow of the Benguela Current or is advected northwestward across the South Atlantic where it joins the South Equatorial Current, where they dissipate into the larger background currents. These anticyclonic warm core rings are estimated to have a transport of 3-9 Sv each, in total injecting salt at a rate of 2.5106 kg/s and heat at a rate of 0.045 PW.[3]

Paleoclimate

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 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).[9]

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, Goldstein & Skrivanek 2012 analysed cores in 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.[10] Simon et al. 2013, however, noted that changes in temperature and salinity in 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.[11]

Rogue waves

The south-east coast of South Africa is on the main shipping route between the Middle-East and Europe/the U.S. and several large ships sustain major damage because of rogue waves in the area where these waves occasionally can reach 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.[12]

Agulhas Undercurrent

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.[13] 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 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.[14]

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.[14] NADW rounds the southern tip of Africa after which the major part (9 Sv) flow eastward and 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.[15]

The periodicity of the meanders and Natal pulses of the Agulhas is matched by the Agulhas Undercurrent.[14] 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 the Agulhas moves offshore, and finally returns to normal.[15]

Biological properties

Agulhas chla
Mean chlorophyl-a concentration map of the Agulhas Current for 2009. Note the high productivity water in the Agulhas Retroflection.

Primary production

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 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.[16]

Impact of rings

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 the South Atlantic. It can also be noted that the size of phytoplankton in Agulhas Rings tends to be smaller than in the surrounding water (around 20 µm in diameter).[16]

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.

See also

References

Notes

  1. ^ Bryden, Beal & Duncan 2003, Discussion, p. 491
  2. ^ Stramma & Lutjeharms 1997, Abstract
  3. ^ a b c d Siedler, Church & Gould 2001, pp. 310–313
  4. ^ a b Baum 2014
  5. ^ Jackson et al. 2012
  6. ^ Leber & Beal 2012
  7. ^ Leeuwen, Ruijter & Lutjeharms 2000, Abstract
  8. ^ Schiele 2014
  9. ^ Simon et al. 2013, Introduction, pp. 101-103
  10. ^ Franzese, Goldstein & Skrivanek 2012
  11. ^ Simon et al. 2013, Conclusions, p. 110
  12. ^ Forsberg & Gerber 2012
  13. ^ RSMAS 2005
  14. ^ a b c Beal 2009, Abstract, Introduction, pp. 2436-2437
  15. ^ a b Beal 2009, Discussion and summary, pp. 2448-2449
  16. ^ a b Mann & Lazier 2006

Sources

Coordinates: 30°00′S 35°00′E / 30.000°S 35.000°E

Agulhas Bank

The Agulhas Bank (, from Portuguese for Cape Agulhas, Cabo das Agulhas, "Cape of Needles") is a broad, shallow part of the southern African continental shelf which extends up to 250 km (160 mi) 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.

Agulhas Return Current

The Agulhas Return Current (ARC) is an ocean current in the South Indian Ocean. The ARC contributes to the water exchange between oceans by forming a link between the South Atlantic Current and the South Indian Ocean Current. It can reach velocities of up to 4 knots (7.4 km/h; 4.6 mph) and is therefore popular among participants in trans-oceanic sailing races.

Benguela Current

The Benguela Current 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 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 depth in turn fuel high rates of phytoplankton growth, and sustain the productive Benguela ecosystem.

Cape Agulhas

Cape Agulhas (; Portuguese: Cabo das Agulhas [ˈkaβu ðɐz ɐˈɣuʎɐʃ], "Cape of the Needles") is a rocky headland in Western Cape, South Africa.

It is the geographic southern tip of the African continent and the beginning of the dividing line between the Atlantic and Indian Oceans according to the International Hydrographic Organization.Historically, the cape has been known to sailors as a major hazard on the traditional clipper route. It is sometimes regarded as one of the great capes. It was most commonly known in English as Cape L'Agulhas until the 20th century. The town of L'Agulhas is located near to the cape.

Cape Peninsula

The Cape Peninsula (Afrikaans: Kaapse Skiereiland) is a generally rocky peninsula that juts out into the Atlantic Ocean at the south-western extremity of the African continent. At the southern end of the peninsula are Cape Point and the Cape of Good Hope. On the northern end is Table Mountain, overlooking Cape Town, South Africa. The peninsula is 52 km long from Mouille point in the north to Cape Point in the south.

The Peninsula has been an island on and off for the past 5 million years, as sea levels fell and rose with the ice age and interglacial global warming cycles of, particularly, the Pleistocene. The last time that the Peninsula was an island was about 1.5 million years ago. Soon afterwards it was joined to the mainland by the emergence from the sea of the sandy area now known as the Cape Flats. The towns and villages of the Cape Peninsula and Cape Flats now form part of the City of Cape Town Metropolitan Municipality.

The Cape of Good Hope is sometimes given as the meeting point of the Atlantic and Indian Oceans. Thus the west coast of the Peninsula is invariably referred to as the "Atlantic Coast", but the eastern side is known as the "False Bay Coast". It is at Cape Point (or the Cape of Good Hope) that the ocean to the south is often said to be divided into the Atlantic Ocean to the west, and the Indian Ocean to the east. However, according to the International Hydrographic Organization agreement that defines the ocean boundaries, the meeting point is at Cape Agulhas, about 200 km (120 mi) to the southeast.Similarly, Cape Point is not the fixed "meeting point" of the cold Benguela Current, running northwards along the west coast of Africa, and the warm Agulhas Current, running south from the equator along the east coast of Africa. In fact the south flowing Agulhas Current swings away from the African coastline between about East London and Port Elizabeth, from where it follows the edge of the Continental shelf roughly as far as the southern tip of the Agulhas Bank, 250 km (155 miles) south of Cape Agulhas. From there it is retroflexed (turned sharply round) in an easterly direction by the South Atlantic, South Indian and Southern Ocean currents, known as the "West Wind Drift", which flow eastwards round Antarctica. The Benguela Current, on the other hand, is an upwelling current which brings cold, mineral-rich water from the depths of the Atlantic Ocean to the surface along the west coast of Southern Africa. Having reached the surface it flows northwards as a result of the prevailing wind and Coriolis forces. The Benguela Current, therefore, effectively starts at Cape Point, and flows northwards from there, although further out to sea it is joined by surface water that has crossed the South Atlantic from South America as part of the South Atlantic Gyre. Thus the Benguela and Agulhas currents do not strictly "meet" anywhere, although eddies from the Agulhas current do from time to time round the Cape to join the Benguela Current.

East Madagascar Current

The East Madagascar Current 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 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.

Geostrophic current

A geostrophic current is an oceanic current in which the pressure gradient force is balanced by the Coriolis effect. The direction of geostrophic flow is parallel to the isobars, with the high pressure to the right of the flow in the Northern Hemisphere, and the high pressure to the left in the Southern Hemisphere. This concept is familiar from weather maps, whose isobars show the direction of geostrophic flow in the atmosphere. Geostrophic flow may be either barotropic or baroclinic. A geostrophic current may also be thought of as a rotating shallow water wave with a frequency of zero. The principle of geostrophy is useful to oceanographers because it allows them to infer ocean currents from measurements of the sea surface height (by combined satellite altimetry and gravimetry) or from vertical profiles of seawater density taken by ships or autonomous buoys. The major currents of the world's oceans, such as the Gulf Stream, the Kuroshio Current, the Agulhas Current, and the Antarctic Circumpolar Current, are all approximately in geostrophic balance and are examples of geostrophic currents.

Günter Dietrich

Günter Dietrich (November 15, 1911, Berlin – October 2, 1972, Kiel) was a German oceanographer. He was the first to describe the Agulhas Current in detail, he provided essential contributions to the understanding of bottom water exchange in the North Atlantic and he shaped marine research in Germany after World War II.

Johann Lutjeharms

Johann Reinder Erlers Lutjeharms (13 April 1944 - 8 June 2011), was a leading South African marine scientist and an authority in the oceanography of the Agulhas Current, who authored a definitive work on the subject, The Agulhas Current (Springer 2006). His main field of investigation was the large-scale circulation patterns of the oceans adjacent to southern Africa and their influence on weather and climate, and he participated in 17 research cruises and was responsible for a further 48 projects undertaken on such cruises.

List of marine bony fishes of South Africa

This is a sublist of the List of marine fishes of South Africa for bony fishes recorded from the oceans bordering South Africa.

This list comprises locally used common names, scientific names with author citation and recorded ranges. Ranges specified may not be the entire known range for the species, but should include the known range within the waters surrounding the Republic of South Africa.

List ordering and taxonomy complies where possible with the current usage in Wikispecies, and may differ from the cited source, as listed citations are primarily for range or existence of records for the region.

Sub-taxa within any given taxon are arranged alphabetically as a general rule.

Details of each species may be available through the relevant internal links. Synonyms may be listed where useful.

Osteichthyes (), popularly referred to as the bony fish, is a diverse taxonomic group of fish that have skeletons primarily composed of bone tissue, as opposed to cartilage. The vast majority of fish are members of Osteichthyes, which is an extremely diverse and abundant group consisting of 45 orders, and over 435 families and 28,000 species. It is the largest class of vertebrates in existence today.

The group Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). The oldest known fossils of bony fish are about 420 million years old, which are also transitional fossils, showing a tooth pattern that is in between the tooth rows of sharks and bony fishes.

Madagascar Current

The Madagascar current is an oceanic current in the west Indian Ocean.

The Madagascar current is split into two current the North Madagascar Current (NMC) and the East Madagascar Current (EMC). The south easterly trade winds, the South Equatorial Current (SEC) and the South Indian Ocean (SIO) flow westward when it reaches Madagascar’s east coast the flow splits into the NMC and EMC. Both currents redistribute mass and heat along the stream current system along Madagascar’s coast. The North Madagascar Current flows into the South Equatorial Current just North of Madagascar and is directed into the Mozambique Channel, this connects to the gyre’s equatorial currents into the Agulhas Current off the coast of Southeastern Africa.The Northern Indian Ocean lies within a large anticylonic supergyre, northern Madagascar lies between this gyre and a cyclonic gyre in the northern Indian Ocean. There are eddies that originate in the Mozambique Channel and in the southern region of Madagascar that can affect the timing of the ring formation of the retroflection of the Agulhas Current.

Mozambique Channel

The Mozambique Channel (French: Canal du Mozambique, Malagasy: Lakandranon'i Mozambika, Portuguese: Canal de Moçambique) is an arm of the Indian Ocean located between the Southeast African countries of Madagascar and Mozambique. The channel is about 1,600 km (1,000 mi) long and 419 km (260 mi) across at its narrowest point, and reaches a depth of 3,292 m (10,800 ft) about 230 km (143 mi) off the coast of Mozambique. A warm current, the Mozambique Current, flows in a southward direction in the channel, leading into the Agulhas Current off the east coast of South Africa.

Nils Bang

Nils Daniel Bang (13 September 1941 – 2 December 1977) was a South African oceanographic scientist who was a pioneer in the study of the fine structure of coastal upwelling systems. In March 1969, Bang initiated, planned and executed South Africa's first truly multi-ship oceanographical research operation, the Agulhas Current Project, along the current's length. Although the research was conducted on a limited budget and with rudimentary equipment, Bang's studies using thousands of closely spaced bathythermograph readings were later corroborated by satellite imagery and airborne radiation thermometry.In the field of physical oceanography, in the fine structure of coastal upwelling systems, Bang—along with W.R.H (Bill) Andrews and Larry Hutchings, his counterparts in biological oceanography—produced work that was acclaimed in their field. Bang's work shed light on the dynamics of the interleaving water masses of the frontal zone in coastal upwelling systems and the meandering of the front.At the time of his death, aged 36, Bang was acting head of the Physical Oceanography Division of the National Research Institute for Oceanology at the Council for Scientific and Industrial Research (South Africa).

North Madagascar Current

The North Madagascar Current is an Ocean current near Madagascar. The Madagascar current is split into two current the North Madagascar Current and the East Madagascar Current (EMC). The North Madagascar Current (NMC) flows into the South Equatorial Current just North of Madagascar and is directed into the Mozambique Channel, this connects to the gyre’s equatorial currents into the Agulhas Current off the coast of Southeastern Africa.

Retroflect

Retroflection is the movement of an ocean current that doubles back on itself.

Sardine run

The sardine run of southern Africa occurs from May through July when billions of sardines – or more specifically the Southern African pilchard Sardinops sagax – spawn in the cool waters of the Agulhas Bank and move northward along the east coast of South Africa. Their sheer numbers create a feeding frenzy along the coastline.

The run, containing millions of individual sardines, occurs when a current of cold water heads north from the Agulhas Bank up to Mozambique where it then leaves the coastline and goes further east into the Indian Ocean.

In terms of biomass, researchers estimate the sardine run could rival East Africa's great wildebeest migration. However, little is known of the phenomenon. It is believed that the water temperature has to drop below 21 °C in order for the migration to take place. In 2003, the sardines failed to 'run' for the third time in 23 years. While 2005 saw a good run, 2006 marked another non-run.The shoals are often more than 7 km long, 1.5 km wide and 30 metres deep and are clearly visible from spotter planes or from the surface.

Sardines group together when they are threatened. This instinctual behaviour is a defence mechanism, as lone individuals are more likely to be eaten than large groups.

Taonius pavo

Taonius pavo is a species of glass squid found in the Atlantic Ocean. Its exact geographic distribution is uncertain, but it may extend to the southwestern Indian Ocean through the Agulhas Current.

Wave–current interaction

In fluid dynamics, wave–current interaction is the interaction between surface gravity waves and a mean flow. The interaction implies an exchange of energy, so after the start of the interaction both the waves and the mean flow are affected.

For depth-integrated and phase-averaged flows, the quantity of primary importance for the dynamics of the interaction is the wave radiation stress tensor.

Wave–current interaction is also one of the possible mechanisms for the occurrence of rogue waves, such as in the Agulhas Current. When a wave group encounters an opposing current, the waves in the group may pile up on top of each other which will propagate into a rogue wave.

Whaling in Madagascar

Whaling in Madagascar is currently banned on a commercial level in compliance with sanctuary regulations. Despite erratic weather conditions, there is a history of overhunting sperm whales, humpback whales, and Bryde's whales within the surrounding waters of Madagascar. In an attempt to allow native populations to recuperate from these operations, the region about Madagascar was included within the Indian Ocean Whale Sanctuary by the International Whaling Commission.

Whaling records from this region have provided data used to justify scientific and historic discoveries. For example, Japanese and Filipino operation logs justified the existence of the Omura's Whale (Balaenoptera omura) in 2003.

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