Turbulent Coherent Structures Near Coastal Capes

A numerical study aimed at investigating the conditions under which different flow regimes appear near coastal capes is presented. The impacts of the regimes are also quantified in terms of integral quantities like mixing, current transport and form drag. Idealized and realistic numerical simulations are run both in barotropically and baroclinically-driven systems. The realistic cases model the Western Adriatic Current (WAC) in the Adriatic Sea. In both cases, the turbulent state of the flow is controlled in first approximation by the Burger number, Bu. When a steady barotropic and geostrophic current impinges on a triangular idealized cape, vertical movements are strong for Bu \u3c 0.1 and pronounced lee waves can be found downstream of the obstacle. For 0.1 less than or equal to Bu \u3c 1, fluid parcels flow more around the obstacle than over it. Flow separation occurs and small tip eddies start to shed. For Bu greater than or equal to 1, tip eddies merge to form larger eddies in the lee of the cape. Flow regimes are also strongly dependent on the obstacle slope alpha when Bu greater than or equal to 1. Flow regime diagrams in the Bu-alpha space are determined. A baroclinic current as the WAC becomes unstable in absence of wind as it separates from the coast for the presence of capes along its path. Downwelling favorable winds narrow and thicken the coastal buoyant current, raising Bu above a critical value and suppressing baroclinic instabilities. Upwelling favorable winds enhance instabilities via the opposite mechanism. With downwelling winds waters mix but remain relatively fresh (S less than or equal to 38), while most of the freshwater signal is lost with upwelling winds. The along-shore transport increases with downwelling winds while it decreases and can even reverse with upwelling winds. The form drag calculated across the obstacles in the different simulations is at least twice the magnitude of skin friction. In barotropic conditions it increases with increasing Bu and decreasing alpha and an empirical parametrization in the Bu-alpha space is put forth. Across the Gargano Promontory, more symmetric pressure fields are observed with downwelling winds; the form drag decreases as a result. The opposite is registered with upwelling winds