Aspects of the filament activity within the Benguela upwelling system

[摘要] ENGLISH ABSTRACT:The Benguela upwelling system off southern and south western Africa is a zone of strong andextensive upwelling. Owing to the greater fIsh numbers found along the front between theupwelling and South East Atlantic Ocean the frontal zone is a key element in the ecology ofthe 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, 1979and 1982 to 1985 in the visible and infrared wavebands. These data enable the investigation ofcross frontal activity for the entire Benguela Upwelling System at various spatial and temporalscales. NOAA satellite infrared imagery was also used.At the macroscale two upwelling zones are described. The fIrst is the upwelling core along thecoast which exhibits well-developed and persistent upwelling. Offshore of this is an area inwhere the more transient fllament activity predominates. This outer zone is one of constantchange and presents highly variable frontal boundary locations. As far as the persistentupwelling is concerned, the northern boundary closely approximates that suggested byShannon (1985) of l7oS. Cape Agulhas was the effective southern boundary of any regularupwelling. The mean offshore extent of the outer upwelling is 270 kilometres off Liideritz and45 kilometres off Cape Town.An analysis of the seasonal location of the front shows that the greatest upwelling extent atboth Liideritz and Cape Town is observed in winter. Although the winter upwelling extent isthe greater of the two seasons, the summer frontal location at Cape Town, in turn, exhibitsremarkable stability. The upwelling off Liideritz, on the other hand, is prone to almostconstant frontal location fluctuations. .Most of the variation occurs as a result of the growth and decay of fllaments. Filaments wereseen along the entire upwelling zone from Cape Agulhas to Cape Frio. On average, thefllament sector was 270 kilometres wide off Liideritz. Between Cape Point and Cape' Agulhasthe southwards extent of the upwelling rarely exceeded 40 kilometres, while the maximumfllament off Cape Point was about 200 kilometres. A fllament at Cape Point generally extendsabout 40 kilometres westwards and the upwelling zone off Walker Bay stretches about 20kilometres southwards, onto the Agulhas Bank. Along the coast between Cape Agulhas andCape Point the fllaments rapidly react to changes in the wind speed and direction. Mesoscaleweather systems are therefore important factors in determining fllament activity there. Thisalso true to some extent at Liideritz. The berg wind can induce rapid fllament growth offLiideritz.Despite an upwelling positive wind direction it is the wind speed that determines whetherupwelling will develop at Cape Town at all. At Liideritz the wind speeds appear to determinethe cross frontal temperature gradient. The greater the wind speed, the steeper thetemperature gradient. Cross frontal temperature gradient of between 0.020C and .006oC perkilometre were calculated for Liideritz, which compares well with readings in the Californianupwelling zone (Koblinsky et al. 1984).In the way fllaments extend the length of the frontal zone greatly and the manner in whichthese cold water streams react to the changing winds, they are complex upwelling frontalfeatures of great variability and importance in the Benguela upwelling system.

[发布机构] Stellenbosch University