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.