A PV view of zonal flow vacillation

Abstract

It is suggested here that the zonal flow vacillation of the Southern Hemisphere is caused by a mutual interaction between the barotropic shear of the zonal flow and the evolution of baroclinic eddies during the later stages of their lifecycle. An index of the zonal wind anomaly difference between 40° and 60°S is defined for a 13-yr record of European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. Periods are selected during which the zonal flow is at the extremes of its characteristics vacillation, which are distinguished by a broad westerly jet with minor maxima near 30° and 60°S and a narrower jet peaked near 40°S. Series of synoptic maps of potential vorticity (PV) on isentropic surfaces suggest a very different evolution of the lifecycles of baroclinic waves in the two extreme composites. -from Author