The nature of Arctic polar vortices in chemistry–climate models

Abstract

† The contributions of these authors were written in the course of their employment at the Met Office, UK, and are published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.The structure of the Arctic stratospheric polar vortex in three chemistry-climate models (CCMs) taken from the CCMVal-2 intercomparison is examined using zonal mean and geometric-based methods. The geometric methods are employed by taking 2D moments of potential vorticity fields that are representative of the polar vortices in each of the models. This allows the vortex area, centroid location and ellipticity to be determined, as well as a measure of vortex filamentation. The first part of the study uses these diagnostics to examine how well the mean state, variability and extreme variability of the polar vortices are represented in CCMs compared to ERA-40 reanalysis data, and in particular for the UMUKCA-METO, NIWA-SOCOL and CCSR/NIES models. The second part of the study assesses how the vortices are predicted to change in terms of the frequency of sudden stratospheric warmings and their general structure over the period 1960-2100.In general, it is found that the vortices are climatologically too far poleward in the CCMs and produce too few large-scale filamentation events. Only a small increase is observed in the frequency of sudden stratospheric warming events from the mean of the CCMVal-2 models, but the distribution of extreme variability throughout the winter period is shown to change towards the end of the twentyfirst century.

Stratospheric polar vortex splits and displacements in the high‐top CMIP5 climate models

“…Their analysis is extended here to consider the two-dimensional variability of the polar vortex using vortex moment (or "elliptical") diagnostics [e.g., Waugh, 1997], see further details in section 2.2. Mitchell et al [2012] previously compared vortex moment diagnostics in climate model simulations from the second Chemistry-Climate Model Validation (CCMVal-2) project, but their analysis was limited because only 3 of the 18 models in CCMVal-2 provided the daily potential vorticity (PV) necessary for the calculation of moment diagnostics. They also did not classify vortex splits and displacements per se, instead focusing on the mean state of the vortex.…”

“…Previous studies which have calculated moment diagnostics for the stratospheric polar vortex have used PV on an isentropic surface, a quantity which is conserved for adiabatic processes. However, this is not commonly output by climate models and is computationally expensive to calculate, leading the majority of models being excluded from previous studies [Mitchell et al, 2012]. Motivated by this, Seviour et al [2013] developed a method to calculate the moment diagnostics using geopotential height on isobaric levels, a quantity archived by almost all climate models.…”

Uncertainty in the Response of Sudden Stratospheric Warmings and Stratosphere‐Troposphere Coupling to Quadrupled CO2 Concentrations in CMIP6 Models

“…For instance, one can find in the literature a number of single‐model studies that report a significant increase in the frequency of SSWs in the future (Charlton‐Pérez et al, 2008; Bell et al, ), while other studies report a nonstatistically significant increase (e.g., Ayarzagüena et al, ; Mitchell, Osprey, et al, ) and others no significant change in SSW frequency at all (Karpechko & Manzini, ; McLandress & Shepherd, ; Scaife et al, ). Multimodel intercomparisons of Chemistry Climate Model Validation (CCMVal) and Coupled Model Intercomparison Project 5 (CMIP5) models have reported large discrepancies in the sign of change among models (Kim et al, ; Mitchell, Charlton‐Perez, et al, ).…”

Noise‐induced vortex‐splitting stratospheric sudden warmings

“…There have been a number of studies aimed at assessing SSW frequency under plausible scenarios for both greenhouse gas emissions and ozone recovery, using atmosphere‐only mechanistic models (Butchart et al ), chemistry–climate models (Mitchell et al ; Ayarzagüena et al ) and coupled ocean–atmosphere models (Mitchell et al ). Overall, the results of these studies are indeterminate, with some suggestion of changes in the timing of SSWs, but no statistically significant changes in their frequency.…”