Editing IPCC:AR6/WGI/Chapter-3 (section)

=== Human Influence on Modes of Climate Variability ===

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'''It is''' very likely '''that human influence has contributed to the observed trend towards the positive phase of the Southern Annular Mode (SAM) since the 1970s and to the associated''' '''strengthening and southward shift of the Southern''' '''Hemispheric extratropical jet in austral summer.''' The influence of ozone forcing on the SAM trend has been small since the early 2000s compared to earlier decades, contributing to a weaker SAM trend observed over 2000–2019 ( ''medium confidence'' ). Climate models reproduce the summertime SAM trend well, with CMIP6 models outperforming CMIP5 models ( ''medium confidence'' ). By contrast, the cause of the Northern Annular Mode (NAM) trend towards its positive phase since the 1960s and associated northward shifts of the Northern Hemispheric extratropical jet and storm track in boreal winter is not well understood. Models reproduce the observed spatial features and variance of the SAM and NAM very well ( ''high confidence'' ). {3.3.3, 3.7.1, 3.7.2}

'''Human influence has not affected the principal tropical modes''' '''of interannual climate variability or their associated regional''' '''teleconnections beyond the range of internal variability''' ( high confidence ''').''' Further assessment since AR5 confirms that climate and Earth system models are able to reproduce most aspects of the spatial structure and variance of the El Niño–Southern Oscillation and Indian Ocean Basin and Dipole modes ( ''medium confidence'' ). However, despite a slight improvement in CMIP6, some underlying processes are still poorly represented. In the Tropical Atlantic basin, which contains the Atlantic Zonal and Meridional modes, major biases in modelled mean state and variability remain. {3.7.3 to 3.7.5}

'''There is''' medium confidence '''that anthropogenic and volcanic aerosols contributed to observed changes in the Atlantic Multi-decadal Variability (AMV) index and associated regional teleconnections since the 1960s, but there is''' low confidence '''in the magnitude of this influence.''' There is ''high confidence'' that internal variability is the main driver of Pacific Decadal Variability (PDV) observed since pre-industrial times, despite some modelling evidence for potential human influence. Uncertainties remain in quantification of the human influence on AMV and PDV due to brevity of the observational records, limited model performance in reproducing related sea surface temperature (SST) anomalies despite improvements from CMIP5 to CMIP6 ( ''medium confidence'' ), and limited process understanding of their key drivers. {3.7.6, 3.7.7}

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