Abstract
Major stratospheric sudden warmings (SSWs) are the largest instance of wintertime variability in the Arctic stratosphere. Due to their relevance for the troposphere-stratosphere system, several previous studies have focused on their potential response to anthropogenic forcings. However, a wide range of results have been reported, from a future increase in the frequency of SSWs to a 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. Here we revisit the question of future SSWs changes, using an identical set of metrics applied consistently across 12 different models participating in the Chemistry Climate Model Initiative. From analyzing future integrations we find no statistically significant change in the frequency of SSWs over the 21st century, irrespective of the metric used for the identification of SSWs. Changes in other SSWs characteristics, such as their duration and the tropospheric forcing, are also assessed: again, we find no evidence of future changes over the 21st century.
Highlights
Stratospheric sudden warmings (SSWs) are the largest manifestation of the internal variability of the wintertime polar stratosphere in the Northern Hemisphere, consisting of a very rapid temperature increase accompanied by a reversal of the westerly wintertime circulation
One question of particular relevance is whether SSWs will change in the future, as a consequence of increasing greenhouse gas (GHG) concentrations and ozone recovery
We have revisited the question of whether SSWs will change in the future, analysing 12 state-of-the-art stratosphere resolving models that participated in CCMI
Summary
Stratospheric sudden warmings (SSWs) are the largest manifestation of the internal variability of the wintertime polar stratosphere in the Northern Hemisphere, consisting of a very rapid temperature increase accompanied by a reversal of the westerly wintertime circulation (the polar vortex). Because of its simplicity and its dynamical insight, the WMO criterion (and its recent simplified version) is the most commonly used criterion in modelling studies as well Such an absolute metric might not always be the best choice to measure the polar stratospheric variability in these studies, as it does not account for potential model biases in the polar vortex climatology or possible changes in this climatology in the future projections (McLandress and Shepherd, 2009; Mitchell et al, 2012a; Butler et al, 2015; Kim et al, 2017). Unlike other previous studies such as Kim et al (2017), our analysis is restricted to the mean frequency of SSWs; we examine the possible future changes in other characteristics, such as the duration of events, the related deceleration of the polar night jet, or the wave activity preceding their occurrence To our knowledge this is the first time that a multi-model assessment of these different SSW features is performed.
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