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

===== 4.5.3.1.2 The Southern Annular Mode =====

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The AR5 assessed it is ''likely'' that the evolution of the SAM over the 21st century will be primarily determined by the interplay between the effects of ozone recovery and changing GHG concentrations and influence the SAM in opposing ways. Owing to the relative effects of these two drivers, CMIP5 model SAM and Southern Hemisphere circulation projections differed markedly across forcing scenarios and across seasons ( [[#Barnes--2013|Barnes and Polvani, 2013]] ; [[#Barnes--2014|Barnes et al., 2014]] ). CMIP5 models simulated a weak negative SAM trend in austral summer for RCP4.5 by the end of the century (F. [[#Zheng--2013|]] [[#Zheng--2013|Zheng et al., 2013]] ), while for RCP8.5 they simulated a weak positive SAM trend in austral summer (F. [[#Zheng--2013|]] [[#Zheng--2013|Zheng et al., 2013]] ). A substantial fraction of the spread in CMIP5 projections of the end of century SH summer jet shift under RCP8.5 may be attributable to differences in the simulated change in break-up of the stratospheric polar vortex, with models that produce a later break-up date showing a larger summertime poleward jet shift ( [[#Ceppi--2019|Ceppi and]] [[#Shepherd--2019|Shepherd, 2019]] ). For RCP2.6, the effect of ozone recovery on the SAM has been found to dominate over that of GHGs in austral summer ( [[#Eyring--2013|Eyring et al., 2013]] ). In austral winter, the poleward shift of the SH circulation in CMIP5 models, and the associated increase in the SAM index, tends to be larger, on average, in higher forcing scenarios though with substantial inter-model spread ( [[#Barnes--2014|Barnes et al., 2014]] ). New research since the AR5 shows that the previous theory for the apparent relationship across models between the annual mean climatological SH jet position and the amplitude of forced SH jet shift ( [[#Kidston--2010|Kidston and Gerber, 2010]] ) does not hold at seasonal time scales ( [[#Simpson--2016|Simpson and Polvani, 2016]] ).

In most seasons, the SAM becomes more positive by the end of the century relative to 1995–2014 under SSP2-4.5, SSP3-7.0, and SSP5-8.5 (Figure 4.30b). Conversely, under SSP1-1.9 and SSP1-2.6, in most seasons the SAM index does not show a robust change compared to 1995–2014 except in austral summer when it becomes significantly more negative. The greatest change in the SAM occurs in austral winter, where CMIP6 models show an ensemble-mean increase in the SAM index of almost 5 hPa in SSP5-8.5. This can be compared to a multi-model mean interannual standard deviation in the austral winter SAM index of 4.0 hPa during 1850–1900. In conclusion, there is ''high confidence'' that in high emissions scenarios (SSP3-7.0 and SSP5-8.5) the SAM becomes more positive in all seasons, while in the lowest scenario (SSP1-1.9) there is a robust decrease in austral summer.

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