Evolution of upwelling systems coupled to the long-term variability in sea surface temperature and Ekman transport

In the early 1990s it was hypothesized that the global warming process would produce an increase in sea–land temperature gradients and, subsequently, enhance the wind patterns responsible for coastal upwelling. Hence, an increase in the intensity of coastal upwelling was expected in the main upwelling ecosystems around the world. However, recently published analyses of the evolution of coastal upwelling processes have shown contradictory evidence. For this reason, time series of sea-surface temperature (SST) and the upwelling index I_w extracted from the NCEP/NCAR reanalysis project database and covering the last 6 decades were studied. The time series analyses focused on the northern part of the Canary Current System and included comparisons with upwelling systems off NW Africa, California, Benguela and Peru. Climatic indices, including the Atlantic Multidecadal Oscillation, Eastern Atlantic Pattern, Interdecadal Pacific Oscillation Index and North Atlantic Oscillation, were investigated to explain the variability found in the NCEP/NCAR time series. A general sea-surface warming and weakening of the upwelling intensity in the Iberian/­Canary and NW African regions were found and these have intensified in the last 4 decades. These trends were clearly observed in winter and autumn for both regions, and a weakening in the upwelling intensity was also detected in summer in the NW African region. The North Atlantic Oscillation and the Eastern Atlantic Pattern indices correlated with both SST and I_w, particularly in winter and spring, and also with both the Iberian/­Canary and NW African regions. No clear trend was found for the California region, while, conforming to the hypothesis, the Benguela region exhibited enhancement of upwelling, but only slight sea-surface warming. In contrast, the Peru region indicated a weakening of upwelling accompanied by marginal sea-surface warming.