Elliptical diagnostics of stratospheric polar vortices

Abstract

Diagnostics based on the spatial moments of isopleths of a long‐lived tracer, or of potential vorticity, are presented that enable the structure and evolution of stratospheric polar vortices to be concisely summarized 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 contours within the vortex‐edge region. By examining the variations of these ‘elliptical’ diagnostics with time and altitude, the temporal evolution 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‐circulation model. The diagnostics show, and quantify, several differences between the Arctic and Antarctic vortices in the SKYHI model. The Arctic vortex moves further off the pole, is generally more elongated, and has a more complicated vertical structure than the Antarctic vortex (with larger variations of both the vortex centre and elongation with height). The elliptical diagnostics also identify the occurrence of large‐scale Rossby‐wave breaking events, both at the vortex edge and in the subtropics, in the model.

Do split and displacement sudden stratospheric warmings have different annular mode signatures?

“…The use of 2-D moment analysis to study the polar vortex was first discussed by Waugh [1997] and Waugh and Randel [1999] and has been recently revisited by Mitchell et al [2011]. The method of S13 describes the geometry of the polar vortex using 10 hPa geopotential height.…”

A sudden stratospheric warming compendium

“…2.3), which may enable classification of split-type SSWs according to the CP07 definition, in which the edges of the vortex are identified by the location of the maximum absolute vorticity gradient. We also provide potential vorticity (PV) interpolated onto isentropic surfaces, and geopotential heights at 10 hPa, both of which can be used to assess vortex moment diagnostics and determine the SSW type (Mitchell et al, 2011;Seviour et al, 2013;Waugh, 1997). We note that the vortex moment diagnostics detect some different dates of SSWs compared to CP07 (and these events are not included in the Compendium), but the provided data would allow classification of the included events.…”

Middle‐atmospheric response to a future increase in humidity arising from increased methane abundance

“…[37] The climatologies of the polar vortices in the various runs are compared using elliptical diagnostics [Waugh, 1997]. This method enables construction of a single ellipse representing the mean state of the polar vortex over an arbitrary averaging period.…”