Spatial features of the near-surface and midwater circulation patterns off western and southern South Africa and their role in the life histories of various commercially fished species

Abstract

Currents on the South African shelf have been measured on 13 cruises since an Acoustic Doppler Current Profiler (ADCP) was installed on F.R.S. Africana in ]989. Most previous descriptions of the West Coast (Benguela) system, including the baroclinic jet currents off the Cape Peninsula and Cape Columbine, the ]aller clearly diverging into a dominant offshore arm and a weaker inshore arm, have been confirmed. This baroclinic flow originates along the south-western margin of the Agulhas Bank. Measurements of the inshore features of the Agulhas Current have shown strong WSW -SSW flow aligned with the bathymetry, but with significant she]fedge north-eastward return flows. The velocity differences between the surface and depth reveal barotropic dominance in all inshore areas and over the central Agulhas Bank. The West Coast outer shelf displays variable baroclinic effects, reflecting the importance of frontal features and cross-shelf flow. SSW flow on the outer eastern Agulhas Bank appears responsible for an offshore loss of eggs from this region. The funnelling of anchovy eggs from the western Agulhas Bank to the West Coast was clear, but this "funnel" was found to have holes through which substantial loss of eggs offshore is also probable. The currents on the Agulhas Bank and the West Coast are discussed with regard to the life histories of various commercially fished species, in addition to anchovy. The study concludes that hypotheses relating environmental factors and recruitment need to consider the variety of potential options in terms of life history contained within the prevailing current system. Seder! 'n Doppler-Akoestiese Stroomprofileerder in 1989 op VNS. Africana aangebring is, is strome tydens 13 vaarte op die Suid-Afrikaanse vastelandsplat gemeel. Die meeste vorige beskrywings van die Weskus (Benguela)stelsel is bevestig, met inbegrip van die barokliniese snelstrome teenoor die Kaapse Skiereiland en Kaap Columbine en laasgenoemde se ooglopende vertakking in 'n oorheersende aflandige been en 'n swakker aanlandige been. Hierdie barokliniese vloei het sy oorsprong langs die suidwestelike grens van die Agulhasbank. Metings van die aanlandige kenmerke van die Agulhasstroom het sterk WSW tot SSW vloei vertoon in ooreenstemming met die batimetrie, maar met beduidende noordoostelike terugkeervloei aan die rand van die vastelandsplal. Verskille in vloeisnelheid tussen die oppervlak en dieper water dui op barotropiese oorheersing in aile aanlandige gebiede en oor die sentrale Agulhasbank. Die afIandige streek van die plat aan die Weskus word gekenmerk deur wisselvallige barokliniese gevolge wat die belang van frontale kenmerke en dwarsvloei oor die plat beklemtoon. SSW vloei op die buitenste oostelike Agulhasbank is blykbaar verantwoordelik vir seewaartse verlies van eiers uit hierdie streek. Daar is onteenseglik 'n tregtereffek op beweging van ansjoviseiers van die weste]ike Bank na die Weskus, maar die "tregter" het openinge waardeur ook aansienlike seewaartse eierverlies waarskynlik is. Vermelde strome op die A...

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

“…2a) are strongly sloped at 12"-13"E salinity front, suggesting that the front is marked by a northward flowing baroclinic jet. The surface geostrophic velocity relative to 1500 db reaches 50-60 cm s-' It is likely that the 12" to 13"E feature is the northward extension of the Good Hope Jet and Cape Columbine Jet (approximately 100 cm s-' found near the latitude of Cape Town as described by BANG and ANDREWS (1974), SHANNON (1985) andBOYD et al (1992) which also marks the offshore extent of the upwelling water. 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.…”

Interannual variation in the composition of the assemblages of medusae and ctenophores in St. Helena Bay, Southern Benguela Ecosystem

“…The upwelling front that develops may, if the SE winds are strongly sustained, eventually be coincident with the shelf-edge front that separates coastal waters from warm, chlorophyll-poor oceanic waters. Onshore winds result in the fairly rapid movement of the oceanic water over the shelf, which leads to an increase in SST there and downwelling (Shannon, 1985;Boyd et al, 1992). The system is characterized by much variability, and by rapid changes in the physical environment.…”

Three decades of research on the greater Agulhas Current

“…Modelling has suggested that this lee eddy is driven by the passing Agulhas Current, fed cyclonic vorticity from passing shear edge eddies and that it eventually escapes into the South Atlantic to join a plethora of eddies and rings in the Cape Basin west of Cape Town. The location and persistence of this lee eddy may interfere with the shelf edge jet (Boyd et al, 1992) supposedly present here most of the time.…”