Numerical study of a coastal current on a steep slope in presence of a cape: The case of the Promontorio di Portofino

A process study aimed at investigating the winter circulation in the area of the Promontorio of Portofino (northwestern Mediterranean) is presented, motivated by historical current measurements suggesting the presence of an attached recirculating eddy in the lee of the Promontorio (cape). A sensitivity study is first performed in an idealized setting, considering the interaction of a steady incoming current with the cape. Numerical experiments are performed using both two‐dimensional (2‐D) (vertically integrated) and 3‐D versions of the POM model. From the 2‐D results, the main controlling parameter appears to be the equivalent Reynolds number Ref, in agreement with previous historical results. In the 3‐D case, the dependence on the vertical Ekman number Ekv is investigated at fixed Ref, and a significant intensification of the attached eddy with respect to the 2‐D solutions is obtained. The main difference between the 2‐D and 3‐D dynamics is the presence of a resolved bottom Ekman layer in 3‐D, introducing a vertical shear in the incoming current, particularly noticeable in the region of the shelf break. This shear is likely to be responsible for a “secondary circulation” (dominated by Coriolis effect), inducing an inshore surface current and associated upwelling in the lee of the cape, and causing the observed eddy intensification. Experiments with realistic bathymetry are performed, and the 3‐D results are found to be in good qualitative agreement with the measurements. The results are expected to be relevant for biological transport, pointing out to a mechanism for eddy intensification connected with surface inshore transport and upwelling.