No robust evidence of future changes in major stratospheric sudden warmings: a multi-model assessment from CCMI

Abstract

Abstract. Major mid-winter stratospheric sudden warmings (SSWs) are the largest instance of wintertime variability in the Arctic stratosphere. Because SSWs are able to cause significant surface weather anomalies on intra-seasonal timescales, several previous studies have focused on their potential future change, as might be induced by anthropogenic forcings. However, a wide range of results have been reported, from a future increase in the frequency of SSWs to an actual decrease. Several factors might explain these contradictory results, notably the use of different metrics for the identification of SSWs and the impact of large climatological biases in single-model studies. To bring some clarity, we here revisit the question of future SSW changes, using an identical set of metrics applied consistently across 12 different modPublished by Copernicus Publications on behalf of the European Geosciences Union. Our analysis reveals that no statistically significant change in the frequency of SSWs will occur over the 21st century, irrespective of the metric used for the identification of the event. Changes in other SSW characteristics -such as their duration, deceleration of the polar night jet, and the tropospheric forcing -are also assessed: again, we find no evidence of future changes over the 21st century.

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

“…Figure 1a shows the average frequency of SSWs during the period 1958-2014 in JRA-55 reanalysis (horizontal dashed line) and the corresponding value for the CMIP6 models (bars; the numerical values are given in supporting information Table S1). In agreement with prior studies (e.g., Ayarzagüena et al, 2018;Charlton-Pérez et al, 2013), we find a large spread across the models in the mean frequency of SSW over that period. This spread is likely due, in part, to the large internal variability of the polar wintertime stratosphere; even with an identical climate model, the frequency of SSWs can vary greatly across different realizations, as demonstrated by Polvani et al (2017).…”

“…We also note that the lack of consensus in the CMIP6 models agrees with the recent study of Ayarzagüena et al (2018), who analyzed the chemistry climate model projections of the CCMI models, which were forced with RCP6.0 scenario. While reporting a general tendency toward an increased frequency of SSWs by the end of the current century, they also emphasized that most changes were not statistically significant.…”

“…There has recently been a considerable discussion in the literature as to which metrics best characterize the variability of the stratospheric polar vortex, in particular, extreme vortex weakening events (Butler et al, 2015;Butler & Gerber, 2018). However, in a recent study, Ayarzagüena et al (2018) found little dependence on the choice of metrics in terms of documenting future changes in SSWs. Thus, we here focus only on a few, widely used and easily implementing metrics of stratospheric variability.…”

response to reviewer 2

“…4c and d. Specifically, ozone anomalies are usually accompanied by anomalies in the Arctic vortex ( Fig. 4a and b), and previous work has shown that spring Arctic vortex anomalies independent of ozone can influence surface conditions (Black and McDaniel, 2007;Ayarzagüena and Serrano, 2009;Hardiman et al, 2011). We now demonstrate that vortex anomalies are associated with polar cap PS anomalies in the CCMI models as well and then try to isolate statistically the relative importance of ASO.…”

Drivers and Surface Signal of Interannual Variability of Boreal Stratospheric Final Warmings

“…The stratospheric polar vortex is also expected to change in the future as a result of long-term anthropogenic changes in atmospheric composition, that is, increasing greenhouse gas (GHG) concentrations and stratospheric ozone layer recovery. For instance, several studies have examined possible trends in the future occurrence of midwinter SSWs (e.g., Ayarzagüena et al, 2018;Bell et al, 2010;Hansen et al, 2014;Karpechko & Manzini, 2017;Kim et al, 2017;Manzini et al, 2014;Mitchell et al, 2012;SPARC CCMVal, 2010), but no consensus has been reached yet. Nevertheless, the effects of projected climate change on SFWs have not been addressed.…”