Abstract
In attempt to study the spatial differences in autumnal equatorial upwelling intensity relative to bio-productivity in the eastern equatorial Atlantic, oceanographic in situ data along regions 10°W, 0°E, 2.5°E and 6°E, collected in the Gulf of Guinea during autumn cruise carried out in September 2007 were analyzed. The surface mean values for temperature along 10°W, 0°E, 2.5°E and 6°E are 25.02, 26.15, 26.88 and 25.60°C, respectively. There was eastward weakening of the equatorial undercurrent from 10°W until its complete disappearance at 6°E. The highest concentration of nitrate recorded at the surface at 10°W was attributed to the shoaling pycnocline observed at this region. The surface mean values for nitrate along 10°W, 0°E, 2.5°E and 6°E are 0.37, 0.09, 0.04 and 0.04 μmol.kg-1, respectively. In response to the shoaling pycnocline at 10°W, corresponding to the highest concentration of chlorophyll fluorescence was recorded at this region. The surface mean values for chlorophyll fluorescence along 10°W, 0°E, 2.5°E and 6°E are 0.34, 0.05, 0.07 and 0.08 mg/m3, respectively. Contributions to equatorial upwelling by the equatorial undercurrent were the strongest and mostly expressed along 10°W. Profiles for apparent oxygen utilization and chlorophyll fluorescence gave indications that biological response to surface enrichments within the equatorial bands was highest at region 10°W. Vertical sections for studied parameters were unsuggestive of westward advection from 6°E to 10°W within the equatorial band, and this signifies the important role of vertical processes in equatorial enrichment at 10°W during boreal autumn. Key words: Vertical mixing, zonal advection, equatorial undercurrent, equatorial upwelling, Gulf of Guinea, nutrients, phytoplankton biomass, surface enrichment.  
Highlights
IntroductionInter-annual variability of this under-sampled and poorly understood region in the tropical Atlantic ocean has been the goal of several measurement programs
Atlantic equatorial upwelling has been considered of great significance for the important role it plays in biological productivity of this region (Voituriez and Herbland, 1977; Grodsky et al, 2008; Nubi et al, 2016)
Equatorial upwelling is associated with the Atlantic cold tongue (ACT) that appears near 10°W (Caniaux et al, 2011)
Summary
Inter-annual variability of this under-sampled and poorly understood region in the tropical Atlantic ocean has been the goal of several measurement programs. Atlantic equatorial upwelling has been considered of great significance for the important role it plays in biological productivity of this region (Voituriez and Herbland, 1977; Grodsky et al, 2008; Nubi et al, 2016). Equatorial upwelling is associated with the Atlantic cold tongue (ACT) that appears near 10°W (Caniaux et al, 2011). Its appearance has been correlated with the seasonal increase of the southern hemisphere trade-winds (Hastenrath and Lamb, 1978) and the northward migration of the inter-tropical convergence zone (ITCZ) (Picaut, 1983; Colin 1989; Waliser and Gautier, 1993)
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