Dynamical constraints on the extreme low values of the potential vorticity in the ocean

High-resolution two and three-dimensional numerical experiments are used to study the manner in which advection of potential vorticity (PV) by frontal instabilities growing out of an initially symmetrically unstable flow or forced by destabilizing wind-forcing constrains the extreme minima in the PV to values near zero. The instabilities act as a PV pump, upwelling high PV water from the pycnocline while subducting low PV surface water. The positive correlation between the velocity and PV fields results in an upward eddy PV flux. When the baroclinic flow is forced by down-front winds, which exert an upward, PV-reducing, frictional PV flux, the eddy and surface frictional PV fluxes were found to scale with one another. This PV pump mechanism tends to increase PV in the surface boundary layer and constrains the extreme low values of the PV from dropping below zero at the expense of reducing the PV in the pycnocline. When the wind alternates orientation between down-front and up-front orientations, PV is both removed from and injected into the fluid. The PV pump mechanism limits the reduction of PV by down-front winds but not the enhancement of PV by up-front winds, causing the skewness of the isopycnal PV distribution to become positive.