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The structure of incipient coastal counter currents in South Portugal as indicator of their forcing agents

Luciano De Oliveira Júnior, Erwan Garel, Paulo Relvas

The alongshore subtidal water circulation along the South Portugal inner shelf is characterized by the temporal alternation of equatorward (i.e., broadly eastward) flows related to coastal upwelling processes and poleward (i.e., broadly westward) Coastal Counter Currents (CCCs). The objective of this study is to get insights about the main drivers of CCCs based on kinematic parameters describing the structure of the flow at the moment it changes direction. The parameters are derived from an extensive bottom-mounted ADCP dataset (16 deployments; 34,121 hourly records) collected at a single mooring (23 m water depth). Results show that the so-called incipient flows present contrasted general patterns whether they turn from equatorward to poleward or the opposite. Complementary observations at a nearby station indicate that these characteristics are spatially consistent along the studied area. Although 70% of CCCs are generated under favourable wind conditions (Levanter), these flows generally develop through the bed layer, in particular in summer. Hence, the Levanter wind - expected to promote flow setup through the surface layer - is not the main driver of CCCs in most cases. The general structure of incipient CCCs strongly suggests that the dominant force competing with the wind stress is an alongshore pressure gradient (APG). Furthermore, the maximum equatorward flow magnitude before CCCs setup is significantly correlated with the following (poleward) acceleration of incipient CCCs near the bed. Such relation is consistent with the development of CCCs due to the unbalance of an APG (produced during active upwelling) when wind relaxes. This process is further supported by an analysis of the depth-averaged momentum equation which suggests that the coastal circulation is mainly driven by linear dynamics in the region.

Keywords: Coastal flow; Upwelling circulation; Flow reversal; Alongshore pressure gradient; Gulf of Cadiz