Spatial and temporal seasonal trends in coastal upwelling off Northwest Africa, 1981-2012

•Coastal upwelling across NW Africa (1981-2012) analyzed using multiple data sources.•Strong evidence across multiple indices for a summer (JJA) increase above 20°N.•NAO appears as a dominant control across other seasons.•Discrepancies between different data products is critically explored.•A modified Bakun upwelling intensification hypothesis is presented for the region.

Seasonal coastal upwelling was analyzed along the NW African coastline (11-35°N) from 1981 to 2012. Upwelling magnitudes are calculated by wind speed indices, sea-surface temperature indices and inferred from meteorological station, sea-surface height and vertical water column transport data. A permanent annual upwelling regime is documented across 21-35°N and a seasonal regime across 12-19°N, in accordance with the climatology of previous studies. Upwelling regions were split into three zones: (1) the Mauritania-Senegalese upwelling zone (12-19°N), (2) the strong permanent annual upwelling zone (21-26°N) and (3) the weak permanent upwelling zone (26-35°N). We find compelling evidence in our various indices for the Bakun upwelling intensification hypothesis due to a significant coastal summer wind speed increase, resulting in an increase in upwelling-favorable wind speeds north of 20°N and an increase in downwelling-favorable winds south of 20°N. The North Atlantic Oscillation plays a leading role in modifying interannual variability during the other seasons (autumn-spring), with its influence dominating in winter. The East Atlantic pattern shows a strong correlation with upwelling during spring, while El Niño Southern Oscillation and Atlantic Multi-decadal Oscillation teleconnections were not found. A disagreement between observationally-based wind speed products and reanalysis-derived data is explored. A modification to the Bakun upwelling intensification hypothesis for NW Africa is presented, which accounts for the latitudinal divide in summer wind regimes.