Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.4.1.1 Northern Annular Mode (NAM)/North Atlantic Oscillation (NAO) ====

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The AR5 reported that the shift towards a positive NAO, a mode of variability in the North Atlantic that is closely related to the hemispheric-scale NAM, from the 1950s to the 1990s was largely reversed by more recent changes ( ''high confidence'' ). Moreover, periods of persistent negative or positive NAO states observed during the latter part of the 20th century were not unusual, based on NAO reconstructions spanning the last half millennium ( ''high confidence'' ).

New multi-millennial-timescale NAO reconstructions are derived from marine and lake sediments, speleothems, tree rings and ice cores ( [[#Ortega--2015|Ortega et al., 2015]] ; [[#Faust--2016|Faust et al., 2016]] ; [[#Sjolte--2018|Sjolte et al., 2018]] ). NAO variability over the past 8 kyr suggests the presence of a significant 1.5 kyr periodicity ( [[#Darby--2012|Darby et al., 2012]] ). Positive NAO conditions dominated during the MH, while the prevailing NAO sign during the early and late Holocene was negative according to most reconstructions ( [[#Olsen--2012|Olsen et al., 2012]] ; [[#Ait%20Brahim--2019|Ait Brahim et al., 2019]] ; [[#Røthe--2019|Røthe et al., 2019]] ; [[#Hernández--2020|Hernández et al., 2020]] ). For the CE, reconstructions developed since AR5 indicate no dominant NAO phase during 1000–1300 CE ( [[#Jones--2014|Jones et al., 2014]] ; [[#Baker--2015|Baker et al., 2015]] ; [[#Ortega--2015|Ortega et al., 2015]] ; [[#Lasher--2019|Lasher and Axford, 2019]] ; [[#Hernández--2020|Hernández et al., 2020]] ), with either negative ( [[#Baker--2015|Baker et al., 2015]] ; [[#Faust--2016|Faust et al., 2016]] ; [[#Mellado-Cano--2019|Mellado-Cano et al., 2019]] ) or a more variable phase of the NAO ( [[#Jones--2014|Jones et al., 2014]] ; [[#Ortega--2015|Ortega et al., 2015]] ; [[#Sjolte--2018|Sjolte et al., 2018]] ; [[#Cook--2019|Cook et al., 2019]] ) between 1400 and 1850 CE. Several instrument-based NAO reconstructions extending back to the 17th and 18th centuries highlight the presence of multi-decadal variations in the NAO phases ( [[#Cornes--2013|Cornes et al., 2013]] ; [[#Cropper--2015|Cropper et al., 2015]] ), although these studies have limitations considering the seasonality of the centres of action and the locations of the stations used. Recent reconstructions of the large scale sea level pressure field yield more robust NAO analysis, showing a persistently negative NAO phase from the 1820s to the 1870s, with positive values dominating during the beginning of the 20th century followed by a declining trend over 1920–1970, with a recovery thereafter to a period of consistently high values between 1970 and the early 1990s ( [[#Delaygue--2019|Delaygue et al., 2019]] ; [[#Mellado-Cano--2019|Mellado-Cano et al., 2019]] ). Based on the evaluation of several NAO reconstructions for recent centuries, [[#Hernández--2020|Hernández et al. (2020)]] highlighted that the strong positive NAO phases of the 1990s and early 21st century were not unusual.

The predominantly positive phase during the 1990s was followed by partial reversal and a tendency towards stronger variability in boreal winter NAM and NAO since the late 1990s ( [[#Pinto--2012|Pinto and Raible, 2012]] ; [[#Hanna--2018|Hanna et al., 2018]] ). This is particularly evident in December NAO ( [[#Hanna--2015|Hanna et al., 2015]] ) and NAM ( [[#Overland--2015|Overland and Wang, 2015]] ) indices, and is not unusual on multi-decadal time scales ( [[#Woollings--2018a|Woollings et al., 2018a]] ). Since the 1990s, a statistically significant summer NAO decline was reported, which is, to a lesser extent, also evident in the winter NAO, linked to an enhanced blocking activity over Greenland ( [[#Hanna--2015|Hanna et al., 2015]] , 2016; [[#Wachowicz--2021|Wachowicz et al., 2021]] ). However, this was moderated by persistent positive NAO values since 2015 (Annex IV.2.1). Based on observations and reanalysis datasets, multi-decadal variations were found for the NAM patterns: the Atlantic centre remained unchanged throughout 1920–2010, whereas the Pacific centre was stronger during 1920–1959 and 1986–2010 and weaker during 1960–1985 ( [[#Gong--2018|Gong et al., 2018]] ). Multidecadal changes were also observed in the position of the centres of action of the NAO ( [[#Moore--2013|Moore et al., 2013]] ; [[#Zuo--2016|Zuo et al., 2016]] ).

In summary, positive trends for the NAM/NAO winter indices were observed between the 1960s and the early 1990s, but these indices have become less positive or even negative thereafter ( ''high confidence'' ). The NAO variability in the instrumental record was ''very likely'' not unusual in the millennial and multi-centennial context.

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