Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.5.3.6 Atlantic Multi-decadal Variability ====

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Based on paleoclimate reconstructions and model simulations, AR5 assessed that AMV is ''unlikely'' to change its behaviour in the future. However, AMV fluctuations over the coming decades are ''likely'' to influence regional climate, enhancing or offsetting some of the effects of global warming.

Recent proxy-derived reconstructions of AMV-related signals show persistent multi-decadal variability over the last three centuries ( [[#Kilbourne--2014|Kilbourne et al., 2014]] ; [[#Svendsen--2014|Svendsen et al., 2014]] ; [[#Moore--2017|Moore et al., 2017]] ), up to the last millennium ( [[#Chylek--2011|Chylek et al., 2011]] ; [[#Zhou--2016|Zhou et al., 2016]] ; J. [[#Wang--2017b|]] [[#Wang--2017|Wang et al., 2017]] b ) and beyond ( [[#Knudsen--2011|Knudsen et al., 2011]] ). This implies that in the past AMV properties were little affected by large climatic excursions.

AMV long-term changes under future warming scenarios have so far scarcely been investigated. A study on the CMIP5 multi-model simulations under RCP8.5 scenario by ( [[#Villamayor--2018|Villamayor et al., 2018]] ) found no substantial differences in the simulated SST patterns (and in the related tropical rainfall response) when RCP8.5, historical and piControl simulations are compared. Such results suggest that the AMV is not expected to change under global warming. A more recent single-model large ensemble study ( [[#Hand--2020|Hand et al., 2020]] ) shows a pronounced change in the AMV pattern under global warming linked to a strong reduction of the mean AMOC and its variability. However, since a superposition of multiple processes controls the AMV, as extensively discussed in [[IPCC:Wg1:Chapter:Annex-iv|Annex IV]] (Section AIV.2.7), in [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.7|Section 3.7.7]] ), and in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.2.3.1), the length of the RCP8.5 simulations might be not sufficient to properly evaluate the respective weight and interplay of internal components and influences from external forcing on AMV projections.

In conclusion, on the basis of paleoclimate reconstructions and CMIP5 model simulations, there is ''low confidence'' that the AMV is not expected to change in the future.

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