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

==== 4.5.3.1 Northern and Southern Annular Modes ====

<div id="h3-30-siblings" class="h3-siblings"></div>

<div id="4.5.3.1.1" class="h4-container"></div>

<span id="the-northern-annular-mode"></span>
===== 4.5.3.1.1 The Northern Annular Mode =====

<div id="h4-12-siblings" class="h4-siblings"></div>

The AR5 assessed from CMIP5 simulations that the future boreal wintertime NAM is ''very likely'' to exhibit natural variability and forced trends of similar magnitude to that observed in the historical period and is ''likely'' to become slightly more positive in the future. Considerable uncertainty is related to physical mechanisms to explain the observed and projected changes in the NAM, but NAM trends are clearly closely connected to projected shifts in the mid-latitude jets and storm tracks.

NAM projections from climate models analysed since AR5 reveal broadly similar results to the late 21st century. CMIP6 models show a positive ensemble-mean trend in most seasons and the higher emissions scenarios that is comparable to between-model or between-realization variability (Figure 4.30a). The NAM generally becomes more positive by the end of the century except in boreal summer (JJA) when there is no change in the NAM in these simulations. In boreal winter (DJF) under SSP5-8.5, the central estimate is an increase in the NAM by almost 3 hPa in the long-term compared to 1995–2014. This can be compared to a multi-model mean interannual standard deviation in the winter NAM index of 3.4 hPa during the period 1850–1900. We conclude with ''high confidence'' that in the mid- to long-term, the boreal wintertime surface NAM is more positive under SSP3-7.0 and SSP5-8.5, while under SSP1-1.9 and SSP1-2.6, the NAM does not show any robust change.

<div id="_idContainer078" class="Basic-Text-Frame"></div>

[[File:53baf3a96361d152ad7389c95f52a07d IPCC_AR6_WGI_Figure_4_30.png]]

'''Figure 4.''' '''30 |''' '''CMIP6 Annular Mode index change from 1995–2014 to 2081–2100. (a)''' Northern Annular Mode (NAM) and '''(b)''' Southern Annular Mode (SAM). The NAM is defined as the difference in zonal mean SLP at 35°N and 65°N ( [[#Li--2003|Li and Wang, 2003]] ) and the SAM as the difference in zonal mean SLP at 40°S and 65°S ( [[#Gong--1999|Gong and Wang, 1999]] ). The shadings are the 5–95% ranges across the simulations. The numbers near the top are the numbers of model simulations in each SSP ensemble. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

<div id="4.5.3.1.2" class="h4-container"></div>

<span id="the-southern-annular-mode-1"></span>
===== 4.5.3.1.2 The Southern Annular Mode =====

<div id="h4-13-siblings" class="h4-siblings"></div>

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.

<div id="4.5.3.2" class="h3-container"></div>

<span id="el-niñosouthern-oscillation-2"></span>