Kinematics of surface currents at the northern margin of the Gulf of Cadiz

Abstract. The subtidal surface water circulation at the northern margin of the Gulf of Cadiz, at the southern extremity of the Iberian upwelling system, is described based on validated hourly high frequency radar measurements from 2016 to 2020. Statistical analyses (mean, standard deviation, eccentricity and empirical orthogonal functions) are applied to the dataset, which is completed with ADCP time-series from multiple moorings at 5 inner-shelf stations and ERA5 wind. Off the shelf, the main circulation pattern consists of a slope current, best developed in summer when north-westerlies dominate, in particular at the most exposed western region. Other mechanisms than upwelling must contribute to this flow in order to explain its seasonal persistence. The slope circulation reverses for regional wind events with east component > 10 m.s^-1, approximately. On the shelf, currents are mainly alongshore and balanced. The circulation is generally continuous along the coast, except for weak (< 0.1 m.s^-1, broadly) poleward flows. In the latter case, the flow tends to remain equatorward near Cape Santa Maria. In winter, coastal poleward flows often extend over the entire margin and are mainly wind-driven. In summer, these flows generally consist of coastal counter currents (CCCs) with poleward direction opposed to the one of the slope current. The CCCs are associated with significant cyclonic recirculation, strongest at West, where a transient eddy is shortly observed for weak wind stress. This circulation develops after periods of strong north-westerlies, supporting that CCCs result from the unbalance of a regional along-shore pressure gradient.