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

===== 4.4.3.1.2 The Southern Annular Mode =====

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The AR5 assessed that it is ''likely'' that increases in GHGs and the projected recovery of the Antarctic ozone hole will be the principal drivers of future SAM trends. Additionally, the positive trend in austral summer/autumn SAM observed over the past several decades ( [[IPCC:Wg1:Chapter:Chapter-2#2.4.1.2|Section 2.4.1.2]] ; [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] in AR5, [[#Hartmann--2013|Hartmann et al., 2013]] ), is ''likely'' to weaken considerably as stratospheric ozone recovers through to the mid-21st century. The effects of ozone depletion and recovery on the SH circulation primarily occur in austral summer, while GHGs influence the SH circulation year round ( [[#Gillett--2013|Gillett and Fyfe, 2013]] ; [[#Grise--2014b|Grise and Polvani, 2014b]] ). Therefore, they are ''likely'' to be the dominant driver of projected circulation changes outside of austral summer ( [[#Gillett--2013|Gillett and Fyfe, 2013]] ; [[#Barnes--2014|Barnes et al., 2014]] ; [[#Solomon--2016|Solomon and Polvani, 2016]] ). Based on current scenarios specifying future atmospheric decline of ozone depleting substances ( [[#WMO--2011|WMO, 2011]] ), chemistry-climate models project the Antarctic ozone hole in October to recover by around 2060 ( [[#WMO--2014|WMO, 2014]] , 2018; [[#Dhomse--2018|Dhomse et al., 2018]] ). Observational evidence since AR5 shows the onset of Antarctic ozone hole recovery ( [[#Solomon--2016|Solomon et al., 2016]] ; [[#WMO--2018|WMO, 2018]] ) that has been attributed to a pause in the summer SAM trend over the past couple of decades ( [[#Saggioro--2019|Saggioro and]] [[#Shepherd--2019|Shepherd, 2019]] ; [[#Banerjee--2020|Banerjee et al., 2020]] ). In austral summer, ozone recovery and increasing GHGs will have opposing effects on the SAM over the next several decades ( [[#Barnes--2014|Barnes et al., 2014]] ).

Since AR5, there have been advances in understanding the role of internal climate variability for projected near-term SH circulation trends ( [[#Solomon--2016|Solomon and Polvani, 2016]] ). A large initial-condition ensemble following the RCP4.5 emissions scenario shows a monotonic positive SAM trend in austral winter. In austral summer, the SAM trend over the first half of the 21st century is weaker compared to the strongly positive trend observed and simulated over the late 20th century. In that model, the number of realizations required to identify a detectable change in decadal mean austral winter SAM index from a year 2000 reference state decreased to below five by around 2025–2030 ( [[#Solomon--2016|Solomon and Polvani, 2016]] ). However, in December–January–February (DJF) the same criterion is not met until the second half of the 21st century, owing to the near-term opposing effects of ozone recovery and GHGs on the austral-summer SAM. In austral summer, forced changes in the SAM index in the near-term are therefore ''likely'' to be smaller than changes due to internal variability (Figure 4.17b; [[#Barnes--2014|Barnes et al., 2014]] ; [[#Solomon--2016|Solomon and Polvani, 2016]] ).

CMIP6 models show a tendency in the near-term towards a more positive SAM index especially in the austral winter (June–July–August, JJA; Figure 4.17b). In all seasons, the differences between the central estimates of the change in the SAM index for each SSP are much smaller than the inter-model ensemble spread. The number of CMIP6 realizations in Figure 4.17b is larger than the suggested threshold of five realizations needed to detect a significant near-term change in decadal-mean austral winter SAM index for a single CMIP5 model ( [[#Solomon--2016|Solomon and Polvani, 2016]] ), and yet the 5–95% intervals on the CMIP6 ensemble spread encompass zero for all core SSPs. This suggests both internal variability and model uncertainty contribute to the CMIP6 ensemble spread in near-term SAM index changes. Based on these results, it is ''more likely than not'' that in the near-term under all assessed SSP scenarios the SAM index would become more positive than in present-day in austral autumn, winter and spring.

An influence of forcing agents other than stratospheric ozone and GHGs, such as anthropogenic aerosols, on SAM changes over the historical period has been reported in some climate models ( [[#Rotstayn--2013|Rotstayn, 2013]] ), but the response across a larger set of CMIP5 models is not robust ( [[#Steptoe--2016|Steptoe et al., 2016]] ) and depends on how tropospheric temperature responds to aerosols ( [[#Choi--2019|Choi et al., 2019]] ). This gives ''low confidence'' in the potential influence of anthropogenic aerosols on the SAM in the future.

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