A PV-approach for dense water formation along fronts: Application to the Northwestern Mediterranean

Abstract : The mechanisms of dense water formation (r > 29:0 kg m 23) at work in the baroclinic cyclonic gyre of the NorthWestern Mediterranean basin are investigated through a PV-budget (PV: Potential Vortici-ty). The PV-budget is diagnosed from an eddy-resolving (1=36 o) ocean simulation driven in surface by hourly air-sea fluxes provided by a nonhydrostatic atmospheric model at 2:5 km resolution. The PV-budget is controlled by the diabatic, frictional, and advective PV-fluxes. Around the gyre the surface diabatic PV-flux dominates the PV-destruction, except along the northern branch of the North Current where the surface frictional PV-flux is strongly negative. In this region, the bathymetry stabilizes the front and maintains the current northerly in the same direction as the dominant northerly wind. This configuration leads to optimal wind-current interactions and explains the preponderance of frictional PV-destruction on diabatic PV-destruction. This mechanical forcing drives a cross-front ageostrophic circulation which subducts surface low-PV waters destroyed by wind on the dense side of the front and obducts high-PV waters from the pycnocline on the light side of the front. The horizontal PV-advections associated with the geostrophic cyclonic gyre and turbulent entrainment at the pycnocline also contribute to the PV-refueling in the frontal region. The surface nonadvective PV-flux involves energy exchanges down to 21400 W m 22 in the frontal zone: this flux is 3.5 times stronger than atmospheric buoyancy flux. These energy exchanges quantify the coupling effects between the surface atmospheric forcing with the oceanic frontal structures at submesoscale.