Dynamics of the Northwestern Mediterranean during the HyMeX/ASICS Experiment: A PV-Perspective

Abstract:

Mode water formation has long been treated as a buoyancy flux problem, however this approach fails to explain the spatial distribution and variability of dense water in the Northwestern Mediterranean. This paper proposes to adopt a PV-perspective (PV: Potential Vorticity) rather than the usual surface flux approach to identify the processes of dense water formation during the HyMeX/ASICS experiment.

The PV-budget was diagnosed from an ocean simulation performed with the NEMO-WMED36 ocean model (1/36°-resolution), driven in surface by the hourly air-sea fluxes from the AROME-WMED forecasts model (2.5km-resolution) during winter 2012-2013.

If a large part of dense water is produced at the centre of the cyclonic gyre, a significant production of dense water (rho>29 kg/m3) is also found along the rim of the cyclonic gyre where the current (North Current) and gradients of density are strong. The spatial distribution of dense waters is well collocated with the PV-destruction associated with the surface frictional and buoyancy PV-fluxes. This suggests that surface PV destructions by winds are sources of destratification and are the relevant forcings of dense water formation.

The negative PV created around the gyre forces a cross-front ageostrophic circulation which subducts subsurface low-PV into interior and obducts high-PV from the thermocline to the surface. The horizontal and vertical advections associated with the 3D ageostrophic circulation in the frontal region are positive and plays a role of PV-refueling destroyed by surface winds. Finally eddies formed by baroclinic instability are expulsed from the cyclonic gyre and transport the low-PV produced in the frontal region towards the centre of the gyre. This non-local process contributes to destratify the convection area.