Destruction of potential vorticity by winds

The destruction of potential vorticity (PV) at ocean fronts by wind-stress driven frictional forces is examined using PV flux formalism and numerical simulations. When a front is forced by ``down-front'' winds, i.e. winds blowing in the direction of the frontal jet, a nonadvective frictional PV flux that is upward at the sea surface is induced. The flux extracts PV out of the ocean, leading to the formation of a boundary layer thicker than the Ekman layer, with nearly zero PV and non-zero stratification. The PV reduction is not only active in the Ekman layer but is transmitted through the boundary layer via secondary circulations that exchange low PV from the Ekman layer with high PV from the pycnocline. Extraction of PV from the pycnocline by the secondary circulations results in an upward advective PV flux at the base of the boundary layer that leads to the deepening of the layer. The advective PV flux and hence efficacy in boundary layer deepening is scaled by the parameter $f S^2 \tau_o /(\rho_o \delta_e)$, where $f$ is the Coriolis parameter, $S^2$ the magnitude of the lateral buoyancy gradient of the front, $\tau_o$ the ``down-front'' component of the wind-stress, $\rho_o$ a reference density, and $\delta_e$ the Ekman layer depth. Scaling arguments suggest that at fronts forced by ``down-front'' winds, in addition to PV destruction by heat loss, PV destruction by winds may play an important role in the formation of mode water.