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

=== 2.4.6 Atlantic Multi-decadal Variability ===

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The AR5 reported no robust changes in Atlantic Multi-decadal Variability (AMV) reconstructions based on paleo records due to low consistency between different AMV reconstructions prior to 1900. AR5 concluded that there have been no significant trends in the AMV index during the instrumental period and there was difficulty in interpreting the AMV signal because of the long-term underlying SST warming trend.

The AR5 conclusions about large uncertainties in AMV paleo reconstructions ( [[#Hernández--2020|Hernández et al., 2020]] ) have been reinforced by recent studies of tree rings (J. [[#Wang--2017b|]] [[#Wang--2017|Wang et al., 2017]] b ), Greenland ice ( [[#Chylek--2012|Chylek et al., 2012]] ), and corals ( [[#Kilbourne--2014|Kilbourne et al., 2014]] ; [[#Svendsen--2014b|Svendsen et al., 2014b]] ; J. [[#Wang--2017b|]] [[#Wang--2017|Wang et al., 2017]] b ). The AMV exhibited a generally positive state over the first millennium of the CE ( [[#Mann--2009|Mann et al., 2009]] ; [[#Singh--2018|Singh et al., 2018]] ). Paleo reconstructions over the last millennium consistently show a negative AMV phase during 1400–1850 CE and a positive phase during 900–1200 CE ( [[#Mann--2009|Mann et al., 2009]] ; J. [[#Wang--2017b|]] [[#Wang--2017|Wang et al., 2017]] b ; [[#Singh--2018|Singh et al., 2018]] ), consistent with warmer surface temperatures from tropical Atlantic records ( [[#Kilbourne--2014|Kilbourne et al., 2014]] ).

Instrumental observations show that AMV is characterized by basin-wide warm and cool periods with an average variation in SST of about 0.4°C, but with larger variations in the North Atlantic subpolar gyre. Despite small differences in indices used to define the AMV (Annex IV), they all show warm periods occurring approximately between 1880–1900, 1940–1960, and from the mid-1990s to present, with cool periods in between (Figure 2.38) but no overall sustained change during the instrumental period ( [[#Booth--2012|Booth et al., 2012]] ; [[#Gulev--2013|Gulev et al., 2013]] ; [[#Bellomo--2018|Bellomo et al., 2018]] ).

The oceanic changes are seen in salinity and temperature variations over the upper 3000 m of the North Atlantic ( [[#Polyakov--2005|Polyakov et al., 2005]] ; [[#Keenlyside--2015|Keenlyside et al., 2015]] ), and in sea level variations in the western North Atlantic along the Gulf Stream passage ( [[#McCarthy--2015|McCarthy et al., 2015]] ). The pattern and strength of the AMV differs among the periods (e.g., [[#Svendsen--2014b|Svendsen et al., 2014b]] ; [[#Reynolds--2018|Reynolds et al., 2018]] ) and there are indications that there may have been a shift since 2005 toward a negative phase of the AMV ( [[#Robson--2016|Robson et al., 2016]] ).

In summary, no sustained change in AMV indices has been observed over the instrumental period ( ''high confidence'' ). However, instrumental records may not be long enough to distinguish any oscillatory behaviour from trends in the AMV. There is ''low confidence'' in the paleo AMV reconstructions due to a paucity of high-resolution records.

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