Satellite-based evidence of upwelling separation off NW Iberia

A high-resolution image sequence of sea surface temperature (SST) and Chlorophyll-a (Chl- a ), together with numerical model solutions, is used to study the spatio-temporal variability of the two variables under intermittent upwelling-favourable winds. It is shown that the evolution of the cross-shore SST and Chl- a profiles over the shelf is linked to the intensity, duration and temporal separation between the wind events. The model's realistic representation of the cross-shore SST supports the interpretation that the observed variability is governed, in the inner-shelf, by the offshore separation of upwelling divergence and, over the mid-shelf, by offshore Ekman transport and mesoscale circulation. The observation of an alongshore low SST/low Chl- a band, bounded by the 30 m and 50 m isobaths, for the days of maximum wind stress, matching the model's solution for the outcrop of colder subsurface waters, constitute a satellite-based evidence of upwelling separation from the coast. The results are in close agreement with previous works on upwelling in shallow waters, straight coastline and gentle slope, but were not yet reported in the study area off NW Portugal. This evidence prompts for the need to use high-resolution (<1 km) numerical models/imagery to properly assess the inner-shelf circulation in the region, and the effects on the marine ecosystem, namely the offshore transport of marine organisms or pollutants. • Satellite image sequence reveals upwelling details off North-West Iberian inner shelf. • Peak Ekman divergence revealed by a low chlorophyll-α alongshore band. • Observation of the offshore progression of peak Ekman divergence for strong wind. • High resolution needed to properly model the inner-shelf circulation in the region.