Seasonal and interannual changes in Intense Benguela Upwelling (1982-1999)

Monthly maps of remotely sensed sea surface temperatures derived from NOAA/AVHRR thermal images are used to describe changes in 'Intense Benguela Upwelling' during 1982-1999. The coastal area under investigation lies between 9-34degrees S and 8-20degrees E and the total area of cold water between the coast and the course of the 13 degreesC isotherm is considered to be an index of intense, active upwelling. It exhibits a decreasing trend over the study period of 18 years and some evidence for a quasi-cycle of about 27 months. Seasonal cycles are discussed for the total cold water area as well as for its mean alongshore and offshore extents. The main season of cold surface water was found to occur between July and September during the austral winter. It peaks in August with a mean area of about 30x10(3) km(2) and relaxes drastically during the rest of the year. The underlying process of intense coastal upwelling is regionally trapped in two coastal zones. These are centred around 26 and 29degrees S and reach a mean offshore extension of 210 and 130 km, respectively, to form giant upwelling filaments. The area of cold water drastically shrinks, roughly by a factor of two, during weak upwelling years but significantly expands by a factor of about 1.5 during strong years. Associated sea level changes along the south-west African coast were derived from measurements at four coastal stations between 23 and 34degrees S during 1982-1987. The first principal component describes about 63 % of the total sea level variance. The lowest sea levels were found in the prominent Luderitz cell near 26degrees S. On both the annual and interannual time scale, decreasing cold water areas are accompanied by increasing sea levels and vice versa. Mean seasonal cycles reveal that variations in the total cold water area tag behind those in the sea level along the entire south-west African coast by about 1 month. (C) 2001 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.