Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.6.1.3 Atmospheric Circulation ====

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The AR5 reported that the application of pattern scaling to extract information on variables other than surface temperature and precipitation was relativelyunexplored. Since AR5, new studies have examined the relationship between projections of mid-latitude atmospheric circulation and GSAT both in terms of interpreting spread in responses across the CMIP5 multi-model ensemble ( [[#Grise--2014a|Grise and Polvani, 2014a]] , 2016) and to investigate variations in the circulation response as a function of GSAT change over time within a given forcing experiment ( [[#Grise--2017|Grise and Polvani, 2017]] ; [[#Ceppi--2018|Ceppi et al., 2018]] ).

At a fixed time horizon, the CMIP5 multi-model spread in GSAT explains only a small fraction of the spread in the shift of the Northern Hemisphere mid-latitude circulation due to an abrupt quadrupling in CO <sub>2</sub> ( [[#Grise--2016|Grise and Polvani, 2016]] ). The fraction of model spread explained by GSAT in the shift of the Southern Hemisphere circulation is larger, but still fairly small ( [[#Grise--2014a|Grise and Polvani, 2014a]] , 2016). At a fixed time horizon and for a given emissions scenario, CMIP5 multi-model spread in storm track shifts, and the closely related mid-latitude jets, can be better explained by multi-model spread in lower and upper level meridional temperature gradients than by GSAT ( [[#Harvey--2014|Harvey et al., 2014]] ; [[#Grise--2016|Grise and Polvani, 2016]] ).

In the North Atlantic, North Pacific, and Southern Hemisphere, the transient response of the mid-latitude jets to forcing behaves non-linearly with GSAT ( [[#Grise--2017|Grise and Polvani, 2017]] ; [[#Ceppi--2018|Ceppi et al., 2018]] ). This is a consequence of the time-dependence of the relationship between radiative forcing and GSAT and the temporal evolution of SST patterns ( [[#Ceppi--2018|Ceppi et al., 2018]] ), with a potential seasonal component in the SH associated with polar stratospheric temperature changes ( [[#Grise--2017|Grise and Polvani, 2017]] ). Consequently, the epoch approach applied to a transient simulation of the 21st century will overestimate the mid-latitude circulation response in a stabilized climate. Dedicated time slice experiments simulating stabilized climates are therefore required to assess differences in mid-latitude circulation at given levels of global warming ( [[#Li--2018|Li et al., 2018]] ). A further complication in the SH is the competing influences of ozone recovery and increasing GHG concentrations on the austral-summer mid-latitude circulation during the first half the 21st century ( [[#Barnes--2013|Barnes and Polvani, 2013]] ; [[#Barnes--2014|Barnes et al., 2014]] ). Using transient 21st century experiments to diagnose changes in SH mid-latitude circulation at different levels of warming therefore confounds the effects of ozone recovery and GHG increases ( [[#Ceppi--2018|Ceppi et al., 2018]] ). Given these various limitations, we do not apply epoch analysis to assess mid-latitude atmospheric circulation changes and related annular modes of variability.

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