Spatial and temporal variability of chlorophyll concentrations from nimbus-7 coastal zone colour scanner data in the Benguala upwelling system and the sub-tropical convergence region south of Africa

Master Thesis


Permanent link to this Item
Journal Title
Link to Journal
Journal ISSN
Volume Title

University of Cape Town

South African oceanographers were engaged in collecting hydrographic and biological sea truth data in order to calibrate the CZCS measurements from the NIMBUS-7 satellite over the Benguela Upwelling region and along the east coast of South Africa during the period 1978 to 1981. A brief overview of the CZCS validation programme and its application to the South African marine environment is given, followed by an analysis of level-Til CZCS data obtained from NASA for the region 10° - 60°S, and 10° - 100°E. This area includes the Benguela Upwelling system on the continental shelf, and the Southern Ocean with the Subtropical Convergence zone south of Africa. High annual values (5mg m⁻³) of chlorophyll occurred in the Benguela shelf region, typical of other upwelling systems in the world ocean, and the data shows a strong interannual signal in the seven years of composited data from 1978-1985, with maxima in 1982. Two distinct regimes were found in the Benguela Upwelling system, the seasonal variations of pigment concentration in the northern and southern Benguela regions being out of phase. In the Southern Ocean, the values of chlorophyll were generally low (0.15mg m⁻³) with the strongest signal (1.5mg m⁻³) found at the southern border of the Agulhas retroflection region and its frontal boundary with the colder subantarctic water to the south. The high values of chlorophyll found in this region are ten times the typical open Southern Ocean values. There is a clear interannual signal in the CZCS data for this Subtropi£al Convergence region, which has a low value in 1979 rising to a maximum in 1981 and then decreasing to another low value in 1985. There appears to be no pronounced seasonal variation in the Subtropical Convergence data. Reasons for the strong signal in the surface chlorophyll concentrations at the front between the Agulhas Return Current and the Southern Ocean are discussed, and it is shown that the Agulhas Plateau sets up a topographic Rossby wave in the Agulhas Return Current, which can be clearly identified in the CZCS signal. The large expanse of the Subtropical Convergence region is found as able to sustain a standing stock of phytoplankton similar in magnitude to that on the Benguela shelf, for limited periods of time. A brief analysis of sea surface temperature versus chlorophyll concentration shows the relationship between the two parameters to take the form of an inverted parabola, having a temperature window within which maximum chlorophyll concentrations are found.

Bibliography: pages 68-74.