An agulhas ring in the South Atlantic ocean and its interaction with the Benguela upwelling frontal system

Rae, C. M. Duncombe; Shillington, F. A.; Agenbag, J. J.; Taunton-Clark, J.; Gründlingh, Marten L.

doi:10.1016/0198-0149(92)90011-h

Cited by 54 publications

(26 citation statements)

References 24 publications

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“…Clearly the eddy has entrained some upwelled water from the continental margin. VAN FOREEST et al (1984), LUTJEHARMS and STOCKTON (1987), LUTJEHARMS and MEEUWIS (1987), LUTJEHARMS et al (1991), DUNCOMBE-RAE et al (1992) and SHILLINGTON et al (1990SHILLINGTON et al ( , 1992 show that filaments of upwelled surface water are entrained by offshore eddies, though some of the eddies they describe are not intense enough to be firmly labeled as Agulhas Eddies. The R.R.S.…”

Section: Discovery 165bmentioning

confidence: 99%

“…BOYD et al (1992) b o serve that the Columbine Jet bifurcates north of Cape Columbine, with the dominant branch turning seaward and a weaker branch following the shelf break. Recognizing that the jet is likely to be highly convoluted as filaments of upwelled water protrude into the interior and perhaps not always intact as a continuous feature (VAN FOREEST et al, 1984;LUTJEHARMS and STOCKTON, 1987;LUTJE-HARMS et al, 1991;SHILLINGTON et al, 1990SHILLINGTON et al, , 1992DUNCOMBE-RAE et al, 1992), it is proposed that the offshore branch of the Columbine Jet accounts for the 12"-13"E front observed by the Discovery 165B section.…”

Section: Thermohaline and Oxygen Stratificationmentioning

confidence: 99%

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Tropical atlantic water within the Benguela upwelling system at 27°S

Gordon

Bosley

Aikman

1995

Deep Sea Research Part I: Oceanographic Research Papers

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Abstract-A CTD-O2 and ADCP section across the African Atlantic continental margin near 275, obtained during R.R.S. Discovery cruise 165B in May 1987, reveals the water mass structure and associated velocity field of the shelf and upper slope of the Benguela upwelling system. Continental shelf water upwelling within the Benguela Current is drawn from the 12°C (about 200 m) level. The upwelling water is drawn from oxygen depleted, tropical South Atlantic thermocline water that is advected along the shelf floor by a southward flowing subsurface current. Lower thermocline and intermediate water from the tropical South Atlantic are also observed flowing southward over the continental slope. Tropical Atlantic water generally resides north of the Angola-Benguela Front at 165. A narrow band of upwelled water is observed well seaward of the shelf, along the western edge of a large Agulhas eddy, indicating that Agulhas eddies play a role in stirring eastern boundary upwelled water into the ocean interior. These eddies also draw into the interior tropical Atlantic water found over the upper continental slope. The net transport between the 120 and 350 isobaths as measured by the ship-mounted ADCP, referenced to the sea floor, is 0.9 x lob m3 s-' to the south, with 1.6 x lob m3 SK' of southward flowing tropical Atlantic water and 0.7 x lob m3 s-l of northward flowing upwelled surface water. The tropical thermocline water mass advected to the south is not observed offshore within the northward flowing Benguela Current, in an unaltered state, thus the 0.9 x lob m3 s-' must feed shelf upwelling south of 27"s. implying a net offshore flux of upwelled water between Liideritz (26") and Cape Columbine (33"s).

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Section: Discovery 165bmentioning

confidence: 99%

Section: Thermohaline and Oxygen Stratificationmentioning

confidence: 99%

Tropical atlantic water within the Benguela upwelling system at 27°S

Gordon

Bosley

Aikman

1995

Deep Sea Research Part I: Oceanographic Research Papers

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“…The Agulhas Current lies close to shore in the north, moving progressively offshore as it travels westwards along the edge of the Agulhas Bank and then retroflects eastwards south of Cape Agulhas (Lutjeharms, 2006). This area of retroflection is very dynamic, and eddies, rings and filaments are regularly shed that may move northwards into the South Atlantic (Duncombe Rae, 1991), frequently interacting with the Benguela Current as they do so (Duncombe Rae et al, 1992). The Benguela Current tracks the edge of the shelf along the west coast of South Africa and Namibia, and is close inshore only at the major capes and peninsulas: Cape Point and Cape Columbine in South Africa, Lüderitz and Cape Frio in Namibia (Shannon, 1985).…”

Section: Introductionmentioning

confidence: 99%

Patterns in marine hydrozoan richness and biogeography around southern Africa: implications of life cycle strategy

Gibbons

Buecher

Thibault

et al. 2010

Journal of Biogeography

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Aim To examine patterns of marine hydrozoan richness around southern Africa and to test the hypothesis that patterns of biogeography become weaker with increasing dispersal ability.Location The coastline of southern Africa from 21°S, 14°E to 28°S, 33°E, extending from the intertidal zone seawards a distance of 200 nautical miles.Methods Published and unpublished information on the distribution of marine Hydrozoa was entered as presence/absence data onto a gridded coastline of the region. A similarity matrix between the species composition of grid squares was constructed using the Bray-Curtis index and visualized using non-metric multidimensional scaling ordinations. Separate analyses were conducted, and compared, on the three major life cycle groupings: holoplanktic, meroplanktic and benthic.Results Over 450 species of marine Hydrozoa have been reported from the region, and species richness increases eastwards, in a manner at odds with the distribution of sampling effort. There was a significant correlation between the geographic structures of the resemblance matrices generated for the three life cycle groupings. In other words, all three groups showed similar patterns of biogeography around the region, and these were broadly similar to those reported by others. However, there were differences between them that reflect the resolution at which the data were examined. At a level of 40% similarity, there was no biogeographic structure to the holoplanktic fauna, the meroplanktic taxa were simply sub-divided into cool-and warm-temperate/subtropical elements, and in the case of benthic taxa, the cool-water fauna was further split into a southern Namaqua and a depauperate northern Namib component. Even at a resolution of 70% similarity, the holopelagic taxa could be separated only into cool-temperate and warm-temperate/subtropical faunas.Main conclusions Holoplanktic taxa show comparatively less biogeographic structure than meroplanktic taxa, which in turn show less clearly defined biogeographic structure than benthic taxa. It is suggested that this is related to the interaction between oceanography and dispersive-stage duration. The role that the Agulhas Current plays in influencing the Benguela Current fauna is highlighted. This study has implications for conservation planning exercises based on protecting representative biotopes in different biogeographic regions.

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“…The northern boundary of the Benguela is where the continental margin narrows at 16(S, and is marked by the permanent warm AngolaeBenguela Front (Boyd, 1987). Agulhas rings (Duncombe-Rae et al, 1992), other eddy features (Lutjeharms and Matthysen, 1995), and filaments (Nelson et al, 1998) interact with the west coast shelf, causing water to be drawn offshore. A poleward undercurrent on the shelf and slope varies in strength and seasonal dependence (Nelson, 1989).…”

Section: Introductionmentioning

confidence: 99%

Chokka squid (Loligo vulgaris reynaudii) abundance linked to changes in South Africa's Agulhas Bank ecosystem during spawning and the early life cycle

Roberts¹

2005

ICES Journal of Marine Science

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Chokka squid biomass and catch are highly variable, likely owing to their links to changes in the ecosystem, which impact spawning and recruitment. A synthesis of basic ecosystem components for the domain in which chokka squid live (i.e. South Africa's west coast and Agulhas Bank) was prepared using published and new data. It included bottom temperature, bottom dissolved oxygen, chlorophyll, and copepod abundance. Alongshore gradients of these indicated that the main spawning grounds on the eastern Agulhas Bank are positioned where bottom temperature and bottom dissolved oxygen are optimal for embryonic development. This location, however, appears suboptimal for hatchlings because the copepod maximum (food for paralarvae) is typically on the central Agulhas Bank some 200 km to the west. Data on currents suggest that this constraint may be overcome by the existence of a net west-flowing shelf current on the eastern Agulhas Bank, improving survivorship of paralarvae by transporting them passively towards the copepod maximum. CTD data and a temporal analysis of AVHRR satellite imagery reveal the copepod maximum to be supported by a “cold ridge”, a mesoscale upwelling filament present during summer when squid spawning peaks. In situ sea surface temperature (SST) data used as a proxy for cold ridge activity demonstrate considerable interannual variability of the feature, especially during El Niño-Southern Oscillation events. Negative linear correlations between maximum summer SST (monthly average) and squid biomass the following autumn (r2 = 0.94), and annual catch (r2 = 0.69), support the link between the “cold ridge–copepod maximum” and the early life cycle of chokka squid, and holds promise for prediction.

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An agulhas ring in the South Atlantic ocean and its interaction with the Benguela upwelling frontal system

Cited by 54 publications

References 24 publications

Tropical atlantic water within the Benguela upwelling system at 27°S

Tropical atlantic water within the Benguela upwelling system at 27°S

Patterns in marine hydrozoan richness and biogeography around southern Africa: implications of life cycle strategy

Chokka squid (Loligo vulgaris reynaudii) abundance linked to changes in South Africa's Agulhas Bank ecosystem during spawning and the early life cycle

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