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

===== 4.3.3.1.2 Southern Annular Mode =====

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The Southern Annular Mode (SAM) is the leading mode of large-scale extratropical atmospheric variability in the Southern Hemisphere and influences most of the southern extratropics (Annex IV, Section AIV.2.2). In its positive phase, the SAM characterizes anomalously low pressure over the polar cap and high pressure in southern mid-latitudes ( [[#Marshall--2003|Marshall, 2003]] ). While there are some zonal asymmetries to the structure of the SAM (Section AIV.2.2), it is more symmetric than its NH counterpart ( [[#Fyfe--1999|Fyfe et al., 1999]] ). Throughout this chapter, we use a simple fixed latitude-based SAM index defined as the difference in zonal mean SLP between 40°S and 65°S ( [[#Gong--1999|Gong and Wang, 1999]] ; see Section AIV.2.2 for discussion of other SAM indices). Although the SAM is often used as a proxy for the location of the mid-latitude westerly wind belt, trends in the SAM can reflect a combination of changes in jet position, width, and strength. The changes in the Southern Hemisphere circulation associated with the SAM influence surface wind stress ( [[#Wang--2014|Wang et al., 2014]] ) and hence affect the Southern Ocean.

Over the instrumental period, there has been a robust positive trend in the SAM index, particularly since 1970 ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-2#2.4.1.2|Section 2.4.1.2]] ). There is ''medium confidence'' that the recent trend in the SAM is unprecedented in the past several centuries ( [[IPCC:Wg1:Chapter:Chapter-2#2.4.1.2|Section 2.4.1.2]] ). There is ''high confidence'' that stratospheric ozone depletion and GHG increases have contributed to the positive SAM trend during the late 20th century, with ozone depletion dominating in austral summer, following the peak of the Antarctic ozone hole in September –October, and GHG increases dominating in other seasons ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.2|Section 3.7.2]] ). To capture the effects of stratospheric ozone changes on the SAM, climate models must include a realistic representation of ozone variations ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.2|Section 3.7.2]] ). In models that do not explicitly represent stratospheric ozone chemistry, which includes the majority of the CMIP6 model ensemble, an ozone dataset is prescribed. To properly capture the effects of ozone depletion and recovery on the stratosphere and surface climate, the prescribed ozone dataset must realistically capture observed stratospheric ozone trends with sufficiently high temporal resolution ( [[#Neely--2014|Neely et al., 2014]] ; [[#Young--2014|Young et al., 2014]] ). The CMIP6 experiment protocol recommended the use of a prescribed 4-D monthly mean ozone concentration field for models without stratospheric chemistry ( [[#Eyring--2016|Eyring et al., 2016]] ).

The AR5 assessed that the positive trend in the austral summer/autumn SAM observed since 1970 (see [[IPCC:Wg1:Chapter:Chapter-2#2.4.1.2|Section 2.4.1.2]] ) is ''likely'' to weaken considerably as stratospheric ozone recovers through the mid-21st century, while in other seasons the SAM changes depend on the emissions scenario, with a larger increase in SAM for higher emissions scenarios. In CMIP6 models, the austral summer SAM is more positive by the end of the 21st century under SSP3-7.0 and SSP5-8.5 (Figure 4.9b). On the other hand, under SSP1-1.9 and SSP1-2.6, the SAM is projected to be less positive, especially under SSP1-1.9 where the 5–95% ranges of anomalies relative to 1995–2014 are –3.1 to 0.0 hPa averaged from 2081–2100. In summary, under the highest emissions scenarios in the CMIP6 models, the SAM in the austral summer becomes more positive through the 21st century ( ''high confidence'' ).

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