Circulation along the northern shelf of the Gulf of Cadiz from high-frequency radar observations

Abstract:

The northern shelf of the Gulf of Cadiz (NSGC), at the SW of the Iberian Peninsula, is limited at east by the strait Gibraltar and at west by the Portuguese branch of the Canary upwelling system. The Cape Santa Maria (CSM) separates a western and eastern shelf region with a relatively narrow and wide shelf, respectively. Although based on few observations, the shelf circulation is generally assumed to consist of one seasonal cyclonic cell in each region. The circulation is limited offshore by the so-called Gulf of Cadiz Current (GCC) which flows equatorward along the shelf slope. The objective of the present work is to clarify the surface shelf circulation at the NSGC and its interaction with the GCC. The research was based on a 3-year (2016-2019) time series of hourly high-frequency radar (HFR) total surface velocities with 2 km resolution. HFR data was validated using near surface current velocities from 8 ADCP deployments at 3 different locations along the coast (in water depth of about 23 m). The dataset (subtidal time series at grid nodes with > 60% of quality-checked data) was analyzed based on seasonal mean maps and Hovmöller diagrams along cross-shore transects. The results show that the eastern and western regions have distinct circulation patterns. A cyclonic cell is episodically observed during few days at west, only, when the prevailing upwelling favorable westerly winds relax in summer. Otherwise, the coastal circulation consists in temporal alternations of poleward and equatorward flows, also typical of the eastern region where they are more frequent. The GCC is clearly identified all year on the eastern shelf break. At west, it is better evidenced in winter and its offshore position and intensity vary with the intensity and direction of the coastal (i.e., inner shelf) alongshore flow. Under strong easterlies, the GCC generally shutdowns or reverses from equatorward to poleward. Overall these results demonstrate that the circulation in the study area is more complex than previously assumed. Ongoing work includes EOF analyses and vorticity calculations to resolve the spatio-temporal variability of the identified circulation patterns.