ÁlvarezSalgado et al.  Outwelling versus Upwelling in NW Spain ÁlvarezSalgado et al.  Outwelling versus Upwelling in NW Spain3

Coastal upwelling regions, which are affected by equatorward-wind variability, are among the most productive areas of the oceans. It has been suggested that global warming will lead to a general strengthening of coastal upwelling, with important ecological implications and an impact on fisheries. However, in the case of the Iberian upwelling, the long-term analysis of climatological variables described here reveals a weakening in coastal upwelling. This is linked to a decrease of zonal sea level pressure gradient, and correlated with an observed increase of sea surface temperature and North Atlantic Oscillation. Weakening of coastal upwelling has led to quantifiable modifications of the ecosystem. In outer shelf waters a drop in new production over the last 40 years is likely related to the reduction of sardine landings at local harbors. On the other hand, in inner shelf and Ria waters, the observed weakening of upwelling has slowed down the residual circulation that introduces nutrients to the euphotic layer, and has increased the stability of the water column. The drop in nutrient levels has been compensated by an increase of organic matter remineralization. The phytoplankton community has responded to those environmental trends with an increase in the percentage of dinoflagellates and Pseudonitzschia spp. and a reduction in total diatoms. The former favors the proliferation of harmful algal blooms and reduces the permitted harvesting period for the mussel aquaculture industry. The demise of the sardine fishery and the potential threat to the mussel culture could have serious socio-economic consequences for the region.

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.

[1] The barotropic and baroclinic nontidal circulations of the Ría de Vigo (NW Spain) are studied using the three-dimensional finite difference model HAMSOM (Hamburg Shelf Ocean Model). The external forcing acting on the system includes winds, freshwater inflow in the inner part of the ria and all over its surface, and heat exchange with the atmosphere. Modeled velocity is compared with data from an AANDERAA DCM12 acoustic Doppler current meter at one site of the ria, and predicted salinity and temperature fields contrasted with SEABIRD conductivity-temperature-depth data along the main channel of the ria. All predicted current, salinity, and temperature fields are in good agreement with experimental data. A new residual circulation pattern of the Ría de Vigo and its dependence on coastal winds is described. According to this pattern the typical estuarine two-layer circulation overlays in the outer part of the ria with a lateral circulation, resulting in a three-dimensional dynamics. The lateral circulation is induced by interaction between the southward (northward) alongshore coastal jet associated with upwelling (downwelling) winds and the topographic configuration of the ria.