ELLIPTIC DIAGNOSTICS OF STRATOSPHERIC POLAR VORTICES

Diagnostics based on the spatial moments of isopleths of a long-lived tracer, or of potential vorticity, are presented that enable the struc ture and evolution of stratospheric polar vortices to be concisely sum marized and quantified. The area, centre, aspect ratio and orientation of the 'equivalent ellipse' of the vortex, on an isentropic surface, are defined using the second-and lower-order spatial moments of contou rs within the vortex-edge region. By examining the variations of these 'elliptical' diagnostics with time and altitude, the temporal evoluti on and vertical structure of the polar vortices can be quantified. The usefulness of the diagnostics is assessed by examining nitrous oxide data from the Geophysical Fluid Dynamics Laboratory 'SKYHI' general-ci rculation model. The diagnostics show, and quantify, several differenc es between the Arctic and Antarctic vortices in the SKYHI model. The A rctic vortex moves further off the pole, is generally more elongated, and has a more complicated vertical structure than the Antarctic vorte x (with larger variations of both the vortex centre and elongation wit h height). The elliptical diagnostics also identify the occurrence of large-scale Rossby-wave breaking events, both at the vortex edge and i n the subtropics, in the model.