Aspects of the filament activity within the Benguela upwelling system
Date
1988-12
Authors
Stockton, P. L. (Philip Leslie)
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: The Benguela upwelling system off southern and south western Africa is a zone of strong and
extensive upwelling. Owing to the greater fIsh numbers found along the front between the
upwelling and South East Atlantic Ocean the frontal zone is a key element in the ecology of
the upwelling area. This discourse focuses on the perturbations of this front.
The major data source used was the daily Meteosat satellite imagery for the years 1978, 1979
and 1982 to 1985 in the visible and infrared wavebands. These data enable the investigation of
cross frontal activity for the entire Benguela Upwelling System at various spatial and temporal
scales. NOAA satellite infrared imagery was also used.
At the macroscale two upwelling zones are described. The fIrst is the upwelling core along the
coast which exhibits well-developed and persistent upwelling. Offshore of this is an area in
where the more transient fllament activity predominates. This outer zone is one of constant
change and presents highly variable frontal boundary locations. As far as the persistent
upwelling is concerned, the northern boundary closely approximates that suggested by
Shannon (1985) of l7oS. Cape Agulhas was the effective southern boundary of any regular
upwelling. The mean offshore extent of the outer upwelling is 270 kilometres off Liideritz and
45 kilometres off Cape Town.
An analysis of the seasonal location of the front shows that the greatest upwelling extent at
both Liideritz and Cape Town is observed in winter. Although the winter upwelling extent is
the greater of the two seasons, the summer frontal location at Cape Town, in turn, exhibits
remarkable stability. The upwelling off Liideritz, on the other hand, is prone to almost
constant frontal location fluctuations. .
Most of the variation occurs as a result of the growth and decay of fllaments. Filaments were
seen along the entire upwelling zone from Cape Agulhas to Cape Frio. On average, the
fllament sector was 270 kilometres wide off Liideritz. Between Cape Point and Cape' Agulhas
the southwards extent of the upwelling rarely exceeded 40 kilometres, while the maximum
fllament off Cape Point was about 200 kilometres. A fllament at Cape Point generally extends
about 40 kilometres westwards and the upwelling zone off Walker Bay stretches about 20
kilometres southwards, onto the Agulhas Bank. Along the coast between Cape Agulhas and
Cape Point the fllaments rapidly react to changes in the wind speed and direction. Mesoscale
weather systems are therefore important factors in determining fllament activity there. This
also true to some extent at Liideritz. The berg wind can induce rapid fllament growth off
Liideritz.
Despite an upwelling positive wind direction it is the wind speed that determines whether
upwelling will develop at Cape Town at all. At Liideritz the wind speeds appear to determine
the cross frontal temperature gradient. The greater the wind speed, the steeper the
temperature gradient. Cross frontal temperature gradient of between 0.020C and .006oC per
kilometre were calculated for Liideritz, which compares well with readings in the Californian
upwelling zone (Koblinsky et al. 1984).
In the way fllaments extend the length of the frontal zone greatly and the manner in which
these cold water streams react to the changing winds, they are complex upwelling frontal
features of great variability and importance in the Benguela upwelling system.
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Description
Thesis (M.A.)--Stellenbosch University, 1988.
Keywords
Upwelling (Oceanography) -- South Africa, Benguela Current, Dissertations -- Geography