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

===== 4.5.3.1.1 The Northern Annular Mode =====

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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.

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'''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).

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