Seasonal and interannual variability of coastal upwelling along northwest Africa and Portugal from 1981 to 1991

Abstract

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°N and north of 25°N and the almost permanent upwelling between 20°N and 25°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°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 Niño in the Pacific Ocean.

The meridional temperature gradient in the eastern North Atlantic during MIS 11 and its link to the ocean–atmosphere system

“…1). These changes are related to the meridional migration of the Intertropical Convergence Zone (ITCZ) (e. g. Wooster et al, 1976;Nykjaer and van Camp, 1994). Because the trade wind intensity affects both upwelling of cold waters and lateral water advection from the relatively cold Canary Current in the north, changes over longer timescales should be captured by our SST proxies (Hughes and Barton, 1974;Fischer et al, 1996).…”

Geochemical distribution patterns as indicators for productivity and terrigenous input off NW Africa

“…Off Cape Blanc, almost permanent upwelling prevails and ''giant filaments'' (Schemainda et al, 1975;Mittelstaedt, 1991;Van Camp et al, 1991;Nykjaer and Van Camp, 1994) leading to the highest chlorophyll concentrations in the surface water (Fig. 1).…”

“…The development and growth of the larger, permanent filaments seems to be related to changes in topography such as headlands (Strub et al, 1991). Large filaments such as those found at Cape Ghir, Cape Yubi, Cape Bojador, Cape Blanc and Mauritania (20.81N) are observable as colder jets and meanders extending several hundreds of kilometres offshore (Van Camp et al, 1991;Nykjaer and Van Camp, 1994;Barton et al, 1998;Davenport et al, 1999Davenport et al, , 2002Barton and Arístegui, 2004). Although upwelling occurs mostly on the shelf, these filaments play an important role in carbon cross-shelf transport and export to the deep ocean as has been described by Strub et al (1991), Gabric et al (1993) and Barton et al (1998).…”

“…A characteristic feature of the upwelling zone is the formation of filaments, which transport the cooler upwelled water offshore (Van Camp et al, 1991;Nykjaer and Van Camp, 1994). The development and growth of the larger, permanent filaments seems to be related to changes in topography such as headlands (Strub et al, 1991).…”

Marine Radiocarbon Reservoir Effect of Coastal Waters Off Cape Verde Archipelago

“…The southward extension of the northeast tradewinds also induces upwelling in Cape Verde coastal waters (Nykjaer and Van Camp 1994). The northeast tradewinds bring upwelled waters from the NACW across Cape Blanc, resulting in a winter band of high coastal upwelling that extends to the Cape Verde Islands ).…”