Editing IPCC:AR6/WGI/Chapter-8 (section)

===== 8.4.2.9.2 Extratropical modes =====

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CMIP6 projections indicate that the Northern Annular Mode (NAM; Annex IV.2.1) is expected to become more positive in winter throughout the 21st century in the SSP3-7.0 and SSP5-8.5 scenarios ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.1|Section 4.5.1]] ). In the near term, the Southern Annular Mode (SAM, Annex IV.2.2) is projected to become less positive than observed during the end of the 20th century during the austral summer in all SSPs scenarios ( [[IPCC:Wg1:Chapter:Chapter-4#4.3.3.1|Section 4.3.3.1]] ).

In the CMIP5 RCP8.5 scenario, increased amplitude and frequency of the North Atlantic Oscillation (NAO, Annex IV.2) during boreal winter (December–January–February, DJF) is associated with higher precipitation in northern Europe and lower precipitation in southern Europe ( [[#Tsanis--2019|Tsanis and Tapoglou, 2019]] ). However, large-ensemble analyses show how the NAO leads to significant uncertainty in future changes of regional climate ( [[#8.5.2|Section 8.5.2]] ). For example, more than a 85% increase in precipitation is projected over northern Europe, western Russia and much of eastern North America, with similar decreasing resulting in drying over north-western Africa and regions adjacent to the Mediterranean Sea ( [[#Deser--2017|Deser et al., 2017]] ).

In the SH, the positive trend projected for the SAM in the CMIP5 RCP8.5 scenario appears to mitigate the wetting in the mid- to high latitudes and the drying over the subtropics, but with strong seasonal dependence ( [[#Lim--2016|Lim et al., 2016]] ). Regional precipitation changes in South America, South Africa, Southern Australia and New Zealand are not well explained by changes in the SAM, but are related to broad-scale changes in north – south temperature gradients associated with enhanced warming of the tropical upper troposphere and strengthening of the stratospheric polar vortex ( [[#Mindlin--2020|Mindlin et al., 2020]] ).

In summary, projected changes in the intensity, frequency and phase of extratropical MoVs (see also Sections 4.3 and 4.5) may amplify regional changes in precipitation and contribute to an increase in their intra-seasonal and interannual variability ( ''medium confidence'' ). Regionally, there are potentially significant precipitation and atmospheric circulation changes associated with changes in extratropical dynamics ( ''low con'' ''fidence'' ).

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