Cyclonic surface circulation in the northern coast of the Gulf of Cádiz: insights from Lagrangian drifters and high-resolution models.

This study presents direct in situ observational evidence of a cyclonic circulation cell east of Cape Santa María (CSM), in the northern Gulf of Cádiz (GoC). Three Lagrangian drifters deployed in October 2022 revealed a coherent cyclonic circulation cell with a characteristic diameter of approximately 40-45 km, consistently estimated from drifter trajectories and relative vorticity. Their trajectories showed alternating eastward and westward flows, modulated by wind variability, bathymetry, and mesoscale dynamics. Satellite observations showed a cooler, chlorophyll-a enriched core, consistent with upwelling and retention of enriched surface water masses within the circulation cell. High-resolution WRF atmospheric simulations indicated alternating easterly and westerly wind regimes that were associated with reversals in coastal circulation. Westerly winds between 18th and 24th October produced positive Ekman pumping east of CSM, creating conditions favourable to the intensification of cyclonic circulation. The IBI ocean model reproduced the main structure of the circulation cell structure, including flow accelerations near the shelf edge, and indicated upward vertical motions within the cyclonic circulation cell. A Lagrangian particle experiment suggested that the cyclonic cell favours both retention and offshore export of surface waters, with residence times of up to approximately 16 days. Overall, this study highlights the role of interactions between atmospheric forcing, bathymetry, and mesoscale dynamics in controlling small-scale surface circulation in the GoC, and underscores the value of integrating drifter observations, satellite data, and numerical models to characterize coastal dynamics.