Original Articles

Climatology and Variability of Sea Surface Temperature and Surface Chlorophyll in the Benguela and Agulhas Ecosystems As Observed by Satellite Imagery


Abstract

A climatology of satellite-derived sea surface semperature (SST) and surface chlorophyll a concentration (Chl), and their associated variability at time-scales from weeks to years, was constructed for the Benguela and Agulhas ecosystems. Global area coverage data at 4.5 km spatial resolution from both AVHRR and SeaWiFS sensors were used to assemble the climatology, from weekly and 5-day time-series respectively. The SST data series spanned 18 years (1982–1999), the Chl data the period September 1997–April 2002. The dominant pattern in the annual SST is the cold upwelled water on the western continental shelf of South Africa and Namibia. SST was high at the Angola-Benguela Front (15–17°S) and on the Agulhas Bank, the northern and southern extremities of the upwelling region. Monthly SST variability was high in both regions, except for the Lüderitz and Cape Columbine-Cape Peninsula upwelling zones, where variability was low. The western Agulhas Bank exhibited a clear seasonal pattern of warm surface water in summer and cool surface water in winter, with an amplitude of 2.5°C. A band of high Chl (>5–10 mg m−3) was apparent close to the coast from the Angola-Benguela Front to Cape Town, but there was a well-defined relative minimum at Lüderitz. On the South Coast, highest Chl (>3 mg m−3) was between Cape Agulhas and Port Elizabeth, in the form of a plume moving offshore. In contrast to SST, Chl variability was mainly at intramonthly intervals, although variability was particularly high north of the Angola-Benguela Front in summer, and at various upwelling sites in winter.

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