Biological and physical signatures in the tropical and subtropical Atlantic

The variability of sea surface chlorophyll concentration in the tropical and subtropical Atlantic during the first year of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) imagery is examined. An Ocean General Circulation Model (OGCM) is used, along with TOPEX/Poseidon dynamic height observations and global gridded wind stress data sets, to explain the physical forcing of surface ocean color signals. Regions of high surface chlorophyll are strongly correlated with mesoscale and large-scale physical processes such as the strong upwelling off the west coast of Africa, the relatively high oceanic production within the Guinea Dome region, and the generation and propagation of large anticyclonic eddies along the coast of South America, north of the equator. The major river outflows (Amazon, Orinoco, and Congo) have strong signatures with plumes of apparently high Chi a in excess of 10 mg m(-3) near their deltas. The fall bloom in the eastern tropical Atlantic observed by the Coastal Zone Color Scanner (CZCS) was absent in 1997, whereas a bloom was observed in this region in July-September 1998, which was not observed by the CZCS, We attribute these apparent anomalies to the projection of the 1997-1998 El Nine event into the tropical Atlantic basin; these signals are correlated with sea surface temperature anomalies known to be associated with ENSO, The SeaWiFS images show that there are seasonal blooms within the hydrographic provinces of the Guinea and Angola domes. These hydrographic provinces are characterized by the dynamic uplift of the thermocline at the North Equatorial Current southern boundary (Guinea Dome) and the Benguela Current eastern boundary (Angola Dome). Within these domes, the Ekman pumping and transport are significant due to the strong trade winds at the surface. The Ekman drift plays a major role in the spreading of surface blooms, The spreading of the oceanic bloom at 12 degrees N, 30 degrees W, the Congo River plume, and the areal extent of the upwelling blooms off the coast of Africa, parallels the strength and extent of the Ekman surface drift. Upwelling, when broadly defined to include large scale vertical excursions of the thermocline, explains virtually all of the surface chlorophyll observations in excess of 0.5 mg m(-3), except in the river plumes.