Seasonal flow patterns on the NW Portuguese shelf

This study aims to improve the understanding of seasonal circulation on the NW Portuguese continental shelf, located at the northern boundary of the Canary Current Upwelling System. The analysis is based on numerical simulations from the Regional Ocean Modelling System (ROMS), applied to the Western Iberia region, using a 15-year realistic dataset spanning 2004–2018. The investigation identifies key oceanic circulation features of the NW Portuguese shelf and the upper slope/outer shelf transition zone, providing new insights into the temporal evolution of flow patterns throughout the year. Circulation in the region is influenced by shoreline orientation, shelf geometry, and bathymetric features, such as the Porto and Aveiro canyons. Emphasis is placed on alongshore and cross-shelf transport patterns, including recurrent outer-shelf bottom eddies near the canyon’s southern sides and the persistence of a coastal poleward current on the inner shelf, even during the typical summer upwelling season. Special focus is given to the transitional months between the well-documented summer upwelling and winter downwelling regimes. Wind stress strength and direction, and the drop-off characteristics are highlighted as critical drivers of the monthly evolution of shelf flow patterns. This is particularly evident on the inner shelf, where circulation appears to be largely determined by the competition between these two effects, the first related with coastal upwelling/downwelling occurrence and the later with Sverdrup integrated poleward/equatorward transport.