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

=== 2.4.4 Atlantic Meridional (AMM) and Zonal Modes (AZM) ===

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The AR5 reported no changes in the Atlantic Meridional Mode (AMM) during the 20th century or shorter periods thereof. For the Atlantic Zonal Mode (AZM), also referred as the Atlantic Niño, the AR5 reported increases during the 1950–2012 period but neither assessed trends nor provided a confidence statement. The AR5 did not assess paleo evidence for the AZM and AMM.

Paleo-reconstructions of these two modes remain rather limited. Nonetheless, the interhemispheric cross-equatorial SST gradients linked to changes in ITCZ locations characteristic of the AMM has been found during the LGM, Heinrich Stadial 1, and the MH, with the largest shift occurring during HS1 ( [[#McGee--2014|McGee et al., 2014]] ). Similarly, the dipole-like SST pattern in the South Atlantic subtropics, which is related to the AZM ( [[#Venegas--1996|Venegas et al., 1996]] ; [[#Morioka--2011|Morioka et al., 2011]] ; [[#Nnamchi--2016|Nnamchi et al., 2016]] , 2017; [[#Lübbecke--2018|Lübbecke et al., 2018]] ; [[#Rouault--2018|Rouault et al., 2018]] ; [[#Foltz--2019|Foltz et al., 2019]] ), has been reconstructed using SST proxies from marine sediment cores during the past 12 kyr ( [[#Wainer--2014|Wainer et al., 2014]] ). The reconstructed index captures two significant cold events that occurred during the 12.9–11.6 ka and 8.6–8.0 ka periods in the South Atlantic ( [[#Wainer--2014|Wainer et al., 2014]] ).

During the observational period the AZM and AMM (Figure 2.37) are related to the AMV largely controlling the interhemispheric gradient of the SST at decadal to multi-decadal timescales ( [[#Tokinaga--2011|Tokinaga and Xie, 2011]] ; [[#Polo--2013|Polo et al., 2013]] ; [[#Svendsen--2014a|Svendsen et al., 2014a]] ; [[#Li--2015|Li et al., 2015]] ; [[#Lübbecke--2018|Lübbecke et al., 2018]] ). The AZM interannual variability is enhanced ( [[#Lübbecke--2018|Lübbecke et al., 2018]] ; [[#Foltz--2019|Foltz et al., 2019]] ), and is more strongly related to ENSO, during the negative phase of the AMV ( [[#Martín-Rey--2014|Martín-Rey et al., 2014]] , 2018; [[#Polo--2015|Polo et al., 2015]] ; [[#Nnamchi--2020|Nnamchi et al., 2020]] ). The AZM displayed a persistent weakened variability over the 1960–2009 period associated with declined cold tongue upwelling ( [[#Tokinaga--2011|Tokinaga and Xie, 2011]] ), which became pronounced since 2000 ( [[#Prigent--2020a|Prigent et al., 2020a]] , b). Despite these multi-decadal fluctuations, there is ''limited evidence'' for any sustained change in the AMM ( [[#Chang--2011|Chang et al., 2011]] ; [[#Martín-Rey--2018|Martín-Rey et al., 2018]] ) and AZM ( [[#Martín-Rey--2018|Martín-Rey et al., 2018]] ; [[#Nnamchi--2020|Nnamchi et al., 2020]] ) during the instrumental period. The AZM and AMM interact on interannual timescales ( [[#Servain--1999|Servain et al., 1999]] ; [[#Foltz--2010|Foltz and McPhaden, 2010]] ; [[#Pottapinjara--2019|Pottapinjara et al., 2019]] ) leading in 2009 to extremes of both modes in which the negative phase of the AMM ( [[#Foltz--2012|Foltz et al., 2012]] ; [[#Burmeister--2016|Burmeister et al., 2016]] ) preceded an equatorial cold tongue cold event that was unprecedented in the prior 30 years ( [[#Foltz--2010|Foltz and McPhaden, 2010]] ; [[#Burmeister--2016|Burmeister et al., 2016]] ).

In summary, ''confidence'' is ''low'' in any sustained changes to the AZM and AMM variability in instrumental observations. There is ''very'' ''low confidence'' in changes of the paleo AZM and AMM due to extremely limited availability of paleo reconstructions.

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