Editing IPCC:AR6/WGI/Chapter-3 (section)

==== 3.3.3.4 Sudden Stratospheric Warming Activity ====

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Sudden stratospheric warmings (SSWs) are stratospheric weather events associated with anomalously high temperatures at high latitudes persisting from days to weeks. [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.5|Section 2.3.1.4.5]] discusses the definition and observational aspects of SSWs. SSWs are often associated with anomalous weather conditions, for example, winter cold spells, in the lower atmosphere (e.g., [[#Butler--2015|Butler et al., 2015]] ; [[#Baldwin--2021|Baldwin et al., 2021]] ).

[[#Seviour--2016|Seviour et al. (2016)]] found that stratosphere-resolving CMIP5 models, on average, reproduce the observed frequency of vortex splits (one form of SSWs) but with a wide range of model-specific biases. Models that produce a better mean state of the polar vortex also tend to produce a more realistic SSW frequency ( [[#Seviour--2016|Seviour et al., 2016]] ). The mean sea level pressure anomalies occurring in CMIP5 model simulations when an SSW is underway, however, differ substantially from those in reanalyses ( [[#Seviour--2016|Seviour et al., 2016]] ). Unlike stratosphere-resolving models, models with limited stratospheric resolution, which make up more than half of the CMIP5 ensemble, underestimate the frequency of SSWs ( [[#Osprey--2013|Osprey et al., 2013]] ; J. [[#Kim--2017|]] [[#Kim--2017|Kim et al., 2017]] ). [[#Taguchi--2017|Taguchi (2017)]] found a general underestimation in CMIP5 models of the frequency of ‘major’ SSWs (which are associated with a break-up of the polar vortex), an aspect of an under-representation in those models of dynamical variability in the stratosphere. [[#Wu--2020|Wu and Reichler (2020)]] found that finer vertical resolution in the stratosphere and a model top above the stratopause tend to be associated with a more realistic SSW frequency in CMIP5 and CMIP6 models.

Some studies find an increase in the frequency of SSWs under increasing greenhouse gases (e.g., [[#Schimanke--2013|Schimanke et al., 2013]] ; [[#Young--2013|Young et al., 2013]] ; J. [[#Kim--2017|]] [[#Kim--2017|Kim et al., 2017]] ). However, this behaviour is not robust across ensembles of chemistry-climate models ( [[#Mitchell--2012|Mitchell et al., 2012]] ; [[#Ayarzagüena--2018|Ayarzagüena et al., 2018]] ; [[#Rao--2021|Rao and Garfinkel, 2021]] ). There is an absence of studies specifically focusing on simulated trends in SSWs during recent decades, and the short record and substantial decadal variability yields ''low confidence'' in any observed trends in the occurrence of SSW events in the Northern Hemisphere winter ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.5|Section 2.3.1.4.5]] ). Such an absence of a trend and large variability would also be consistent with a recent reconstruction of SSWs extending back to 1850, based on sea level pressure observations ( [[#Domeisen--2019|Domeisen, 2019]] ), although this time series has limitations as it is not based on direct observations of SSWs.

In summary, an anthropogenic influence on the frequency or other aspects of SSWs has not yet been robustly detected. There is ''low confidence'' in the ability of models to simulate any such trends over the historical period because of large natural interannual variability and also due to substantial common biases in the simulated mean state affecting the simulated frequency of SSWs.

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