Phytoplankton pigment distribution and frontal structure in the subtropical convergence region south of Africa

Abstract

Satellite data from the Coastal Zone Colour Scanner (CZCS) and Advanced Very High Resolution Radiometer were examined to determine the relationship between the phytoplankton pigment distribution and sea surface temperature (SST) in the Subtropical Convergence (STC) region south of Africa. The structure of the temperature fronts and their role in phytoplankton pigment distribution was explored. The multichannel sea surface temperature (MCSST) data for 1981–1986 showed considerable interannual variability in both the position of the Agulhas retroflection and the extent of the Rossby wave in the Agulhas Return Current. Pronounced interannual variation in pigment levels was found in the CZCS data for early 1979, 1982 and 1983, with highest pigment levels in early 1982. These greater pigment levels were shown to be a function of increased frontal intensity of the STC front rather than of the Agulhas front, or of SST per se. The principal interannual differences were found in the region where a strong Agulhas front occurred to the north of the STC front. The Agulhas front appeared to play a role in limiting the spatial distribution of phytoplankton pigment. Time series of level-2 CZCS data supported the scenario of upstream Agulhas retroflection resulting in a pronounced Rossby wave in the Agulhas Return Current, and increased phytoplankton pigment levels in the STC region.