Interannual scales of variation of pigment concentrations from coastal zone color scanner data in the Benguela Upwelling system and the Subtropical Convergence zone south of Africa

Abstract

South African oceanographers were engaged in collecting hydrographic and biological sea truth data in order to calibrate the coastal zone color scanner (CZCS) measurements from the Nimbus 7 satellite over the Benguela Upwelling region near Cape Town, South Africa, during the period 1978 to 1981 (Shannon, 1985). We give a brief overview of this research and then concentrate on a more recent analysis of level III CZCS data obtained from NASA for the region 10–60°S, 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 concentrations of chlorophyll (5 mg m−3) typical of other upwelling systems in the world ocean occurred in the Benguela shelf region, and the data show a strong interannual signal in the 7 years of composited data from 1978 to 1985. Two distinct regimes were found in the Benguela Upwelling system, the seasonal variations of pigment concentration in the northern and southern Benguete regions being out of phase. In the southern ocean, levels of chlorophyll were generally low (0.15 mg m−3), with the strongest signal (1.5 mg 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 levels of chlorophyll found in this region are 10 times those of the typical open southern ocean. There is a very clear interannual signal in the CZCS data for this Subtropical Convergence region, which has a low value in 1979, rises to a maximum in 1981, and then decreases to another low value in 1985. There appears to be no clear 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.