Potential Vorticity Sources and Sinks in a Quasi-geostrophic Ocean: beyond Western Boundary Currents

Abstract

Potential vorticity dynamics for a quasi-geostrophic eddy-resolving general circulation model (EGCM) are studied in order to determine the effects of mesoscale variability on the potential vorticity distribution of a wind-driven ocean. The study employs both Eulerian and Lagrangian analyses in the effort to describe the potential vorticity gain/loss cycle along the path of a particle. While the mean wind stress curl is the dominant potential vorticity source for the interior of the upper layer, a redistribution of eddy potential vorticity creates sources of potential vorticity for the multiple gyres in the lower layers. This redistribution is a result of the local generation of eddies via baroclinic instabilities. These eddies are advected by the western boundary current into the midlatitude jet where they are responsible for a cross-gyre potential vorticity exchange. This exchange is concentrated at the entrance to the eastward jets where northward and southward boundary currents converge. From a Lagrangian viewpoint the vorticity exchange is accomplished via dissipative meandering rather than particle exchange across gyre fronts.