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

===== 8.3.2.9.2 Extratropical modes =====

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A positive trend has been observed in the Northern Annular Mode (NAM; Section AIV.2.1) in the second half of the 20th century, which partially reversed since the 1990s ( [[IPCC:Wg1:Chapter:Chapter-2#2.4.5.1|Section 2.4.5.1]] ), but the detection and attribution of these changes remain difficult ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.1|Section 3.7.1]] ). The linkages of the NAM with weather and climate extremes in the northern extratropics are still unclear in models and observations ( [[#Vihma--2014|Vihma, 2014]] ; [[#Overland--2016|Overland et al., 2016]] ; [[#Screen--2018|Screen et al., 2018]] ). However, robust links are identified between precipitation trends and variability in Europe and the phases of the Atlantic component of the NAM, that is, the NAO ( [[#Moore--2013|Moore et al., 2013]] ; [[#Comas-Bru--2014|Comas-Bru and McDermott, 2014]] ). Reduced winter precipitation is well correlated with the NAO over Southern Europe and Mediterranean countries (Kalimeris et al. , 2017; Corona et al. , 2018; [[#Vazifehkhah--2018|Vazifehkhah and Kahya, 2018]] ; Neves et al. , 2019). NAO teleconnections in those regions include influences on groundwater and streamflow ( [[#Zamrane--2016|Zamrane et al., 2016]] ; [[#Massei--2017|Massei et al., 2017]] ; [[#Jemai--2018|Jemai et al., 2018]] ). Remote teleconnections of the NAO have been identified over Northern China, the Yangtze River valley and India ( [[#Jin--2017|Jin and Guan, 2017]] ; [[#Di%20Capua--2020|Di Capua et al., 2020]] ). The summer phase of the NAO is significantly correlated with variations in summer rainfall in East China, with the thermal forcing of the Tibetan Plateau providing a link to this Eurasian teleconnection (Z. [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|Wang et al., 2018]] ).

In the Southern Hemisphere (SH), an observed positive trend is identified in the strength of the Southern Annular Mode (SAM, Section AIV.2.2) since 1950, especially in austral summer ( ''high confidence'' , [[IPCC:Wg1:Chapter:Chapter-2#2.4.1.2|Section 2.4.1.2]] ). While stratospheric ozone depletion and GHG increases largely contributed to this change, climate models still have trouble simulating the SAM and its response to ozone and GHGs ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.2|Section 3.7.2]] ). Shifts in the south-westerly winds ( [[#Fletcher--2018|Fletcher et al., 2018]] ) and the expansion of the SH Hadley cell ( [[#Kang--2011|Kang and Polvani, 2011]] ; H. [[#Nguyen--2018|]] [[#Nguyen--2018|Nguyen et al., 2018]] ) influence SAM-related rainfall anomalies in in southern South America and southern Australia during the austral spring–summer. Over New Zealand, large-scale SLP and zonal wind patterns associated with SAM phases modulate regional river flow ( [[#Li--2017|Li and McGregor, 2017]] ). The SAM also influences precipitation and water vapour changes over Antarctica via moisture fluxes ( [[#Marshall--2017|Marshall et al., 2017]] ; [[#Oshima--2017|Oshima and Yamazaki, 2017]] ; [[#Grieger--2018|Grieger et al., 2018]] ) but CMIP5 models are limited in their ability to simulate these regional teleconnections ( [[#Marshall--2015|Marshall and Bracegirdle, 2015]] ; [[#Palerme--2017|Palerme et al., 2017]] ). SAM and its interaction with other large-scale modes of climate variability, like ENSO ( [[#Fogt--2011|Fogt et al., 2011]] ) and the Indian Ocean Dipole ( [[#Hoell--2017a|Hoell et al., 2017a]] ), are responsible for fluctuations in southern African rainfall ( [[#Nash--2017|Nash, 2017]] ) and southern South America ( [[#Gergis--2017|Gergis and Henley, 2017]] ). In May, the SAM can trigger a southern Indian Ocean Dipole SSTA favoring more or less precipitation over the Indian sub-continent and adjacent areas ( [[#Dou--2017|Dou et al., 2017]] ), also affecting subsequent summer monsoon in the South China Sea (T. [[#Liu--2018|]] [[#Liu--2018|]] [[#Liu--2018|]] [[#Liu--2018|Liu et al., 2018]] ). Over South America, a positive SAM is associated with dry conditions ( [[#Holz--2017|Holz et al., 2017]] ) due to reduced frontal and orographic precipitation and weakening of moisture convergence. Regions particularly affected include Chile ( [[#Boisier--2018|Boisier et al., 2018]] ) and the rivers of central Patagonia ( [[#Rivera--2018|Rivera et al., 2018]] ).

In summary, while the attribution of 20th century variations of the NAM/NAO is still unclear, there is a strong relationship with precipitation changes over Europe and in the Mediterranean region ( ''high confidence'' ). SAM teleconnections are associated with changes in moisture transport and extend to South America, Australia and Antarctica ( ''high confidence'' ) with documented drying occurring as a result of the ''very likely'' human-induced SAM trend toward its positive phase observed from the 1970s until the 1990s ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.2|Section 3.7.2]] ).

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