SEASONAL AND INTERANNUAL VARIABILITY OF COASTAL UPWELLING ALONG NORTHWEST AFRICA AND PORTUGAL FROM 1981 TO 1991

The seasonal and interannual variability of coastal upwelling in the northwest African upwelling area for the period from 1981 to 1991 has been investigated using remote sensing data of sea surface temperature (SST) and wind data. Approximately 3400 individual SST images have been generated from National Oceanographic and Atmospheric Administration Global Area Coverage data and resampled to form a coherent cloud-free data set. From the satellite data set an SST upwelling index defined as the zonal temperature difference has been derived as a function of latitude and time. Wind data from the European Centre for Medium-Range Weather Forecasts have been used to determine a second upwelling index calculated as the fraction of the Ekman transport perpendicular to the coast. The two independent indices have been compared with previously published data of seasonal variability and have been examined for interannual variabilities. In general the two upwelling indices demonstrate the known seasonal variability which exists south of 20-degrees-N and north of 25-degrees-N and the almost permanent upwelling between 20-degrees-N and 25-degrees-N. Off Portugal and Cape Ghir, there are temporal shifts between the maximum winds and the maximum zonal temperature differences of 2 and 3 months, respectively, while at most other locations the indices are temporally well correlated. Interannual variabilities are greater where upwelling is seasonal compared to those regions where upwelling is dominant throughout the year. The strongest interannual variabilities are found south of 20-degrees-N where upwelling was particularly weak during the period from the end of 1982 to the beginning of 1984, coinciding with the exceptionally strong 1982/1983 El Nino in the Pacific Ocean.