A Model of Tidal Rectification by Potential Vorticity Mixing. Part II: Frontal Regime

Abstract

A model of tidal rectification in a homogeneous ocean (Part I) is extended here to include a front that separates shelf and slope waters. The front is approximated as a density discontinuity, the stratification and anchoring position of which are given, but which otherwise is coupled to the flow field. The dynamical closure is formulated through vorticity balance of the two layers and a parameterization of potential vorticity (PV) flux in terms of local tidal amplitude and the mean field. As an example of the frontal effect on the tidally rectified flow, a solution is calculated for the case of negligible interfacial stress and PV flux in the bottom boundary layer and compared with that of a homogeneous ocean. It is found that the mean along-isobath flow outside the frontal zone remains largely unchanged, but is qualitatively altered in the frontal zone. Specifically, the mean flow above the sloping front?being insulated from bottom friction?is greatly intensified by PV mixing, consistent with observed seasonal change over Georges Bank. The mean flow below the frontal interface on the other hand is weakened by enhanced frictional effect?to nearly zero at the foot of the front. The vanishing Ekman transport would strengthen the front as a barrier to offshore transport of a passive tracer.