Turbulent structures around capes

Prominent coastline features like headlands and capes force oceanic currents to separate from the coast. Flow separation is a complex and not fully-understood process and it gives rise to several oceanic regimes. These regimes affect the dispersion of dissolved pollutants, floating organisms, nutrients and sediments. From a dynamical perspective, they result in enhanced mixing, drag and dissipation.

Our research is focused on the conditions under which different flow regimes appear in the vicinity of capes. Realistic numerical simulations are aimed at the dynamics around two capes in Italy, the Gargano and Portofino promontories. We also perform idealized experiments which help shed light on the separation process in stratified and rotating environments.

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The Gargano Promontory lies in the middle portion of the Adriatic Sea (upper panel). Chlorophyll satellite images show that the dynamics around the Gargano are variable. Sometimes, the coastal current flows smoothly along the coast and the only clear instabilities are localized in the lee of promontory, where isolated eddies can be recognized (lower left panel). At other times, instabilities appear upstream and downstream surrounding the whole promontory (lower right panel). We show that wind direction is the main mechanism controlling this variability.

The following three-dimensional movie is for a simulation without wind. It shows that the curvature due to the promontory is still essential for the formation of the large lee anticyclone.

Dynamical regimes similar to the ones observed in reality are visible in our idealized experiments. In these cases, the obstacle is a triangular prism with sloping sides. It represents an idealized headland extending from the coast. Each flow regime is characterized by different turbulent coherent structures. The flow sometimes does not separate from the coastline and it follows the obstacle. A fully-attached regime (or lee-wave regime) with pronounced internal waves is then established (upper panel). In some cases, smaller tip eddies can be found near the apex of the cape. At other times, large isolated eddies can be observed in the lee of the cape. They can remain attached to the cape (eddy-attached regime) or detach from it (eddy shedding regime, lower panel).

We show that different regimes appear downstream of the cape depending on the complex interplay between rotation, stratification and the slope of the obstacle. We use the greek letter "α" for the slope, while rotation and stratification are combined in one parameter, the Burger number "Bu". We determine flow regime diagrams in the "Bu-α" space. The following picture shows the diagram for layers close to the bottom. Such diagrams provide important information for a correct coastal management. For example, sewage pipes located at depth in the lee of the cape should be avoided when the eddy-attached regime is expected to occur. Due to the recirculating waters, horizontal dispersion is strongly reduced and anoxic conditions are more likely to occur.

The Portofino Promontory is a blunt headland situated in the Ligurian Sea (Northwestern Mediterranean). Historical current measurements suggest the presence of an attached recirculating eddy in the lee of the promontory. We run idealized and realistic simulations to check under which conditions this hypothesis is true.

An attached anticyclone can be observed in both two-dimensional (2D) and three-dimensional (3D) runs (figure below). The 3D cases show a significant intensification of the eddy which agrees more with the measured values. We show that these differences are related to Ekman-layer dynamics which are explicitly resolved in the 3D cases.