Researchers: (alphabetic order) - Doglioli, Andrea1 - Griffa, Annalisa2,3 - Magaldi, Marcello3 - 1UNIGE, Italy - 2CNR, Italy - 3RSMAS, Miami Sponsors: - CNR - INFM Publications: - Doglioli A. M., A. Griffa, M.G. Magaldi, 2004: Numerical study of a coastal current on a steep slope in presence of a cape: the case of the Promontorio di Portofino. J. Geophys. Res., 109, c12033, doi:10.1029/2004JC002422 (PDF) - Aliani S., A. Griffa, A. Molcard, 2003: Floating debris in the Ligurian Sea, North-Western Mediterranean. Marine Pollution Bulletin, 46, (9), 1142-1149.

The response of a narrow shelf current in presence of a cape has been investigated. The results have been applied to the winter circulation in the area of the Promontorio of Portofino (North-Western Mediterranean) (Fig. 1, Fig. 2). Historical current measurements in the area suggest the presence of a recirculating eddy in the lee of the Promontorio (cape), with intensity of 10% of the incoming shelf current and with extension of 15 km. A sensitivity study is first performed considering the response of a steady incoming current which propagates on an idealized shelf topography (maintaining the shallower depths on the right) and interacts with the cape. Numerical experiments are performed using both 2D (vertically integrated) and 3D versions of the POM model. From the 2D results, the main controlling parameter appears to be the equivalent Reynolds number, while the system appears quite insensitive to the values of the Rossby number, i.e. to the intensity of the incoming current. In the 3D case, the dependence on the vertical Ekman number is investigated and a significant intensification of the attached eddy with respect to the 2D solutions is obtained, while the eddy size is basically unchanged. The main difference between the 2D and 3D dynamics is the presence of a resolved bottom Ekman layer in 3D, introducing a vertical shear in the incoming current. Downstream the cape in the surface layers, a current directed inshore and associated with an upwelling vertical velocity at the shelf break is observed. This current could be explained by the formation of a ``secondary circulation'' in the case of dominant Coriolis effect and appears responsible for the observed eddy intensification. Experiments with realistic bathymetry are then performed, The 3D results, obtained for realistic values of the parameters, show a good qualitative agreement with the measurement results (Fig. 3). The present results are expected to have significant consequences from the point of view of biological transport, pointing out to a mechanism for eddy intensification in the lee of a cape, connected with surface inshore transport and upwelling. Examples of idealized Lagrangian particles advected by the flow are shown in (Fig. 4). The eddy retention and the effects of upwelling/downwelling on particle transport are clearly shown. Figures: Figure 1 Location of the area of interest, situated along the western Italian coast in the North-Western Mediterranean Sea. Figure 2 Coastline and bottom topography in the area surrounding the Promontorio di Portofino. Red squares indicate the positions of historical current meter measurements. Figure 3 Results for vertically integrated velocity from 2D (upper panel) and 3D (lower panel) simulations. The numbers in the small boxes indicate average velocity from observations (blue) and model (red) in 3 locations. In the 2D simulations the gyre appears significantly weaker than observations, while the 3D simulations appear in good qualitative agreement with observations. Figure 4 Examples of idealized Lagrangian particles advected by the 3-D flow in Fig.3. The animation refers to a 20 day simulation with 12 hour interval. Different colors indicate different release regions, while the symbol size indicate particle depth (surface particles are characterized by maximum size).