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

=== Large-scale Circulation and Modes of Variability ===

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'''In the near term, the forced change in Southern Annular Mode in austral summer is''' ''likely'' '''to be weaker than observed during the late 20th century under all five SSPs assessed.''' This is because of the opposing influence in the near- to mid-term from stratospheric ozone recovery and increases in other greenhouse gases on the Southern Hemisphere summertime mid-latitude circulation ( ''high confidence'' ). In the near term, forced changes in the Southern Annular Mode in austral summer are therefore ''likely'' to be smaller than changes due to natural internal variability. {4.3.3, 4.4.3}

'''In the long term, the Southern Hemisphere mid-latitude jet is''' ''likely'' '''to shift poleward and strengthen under SSP5-8.5 relative to 1995–2014.''' This is ''likely'' to be accompanied by an increase in the Southern Annular Mode index in all seasons relative to 1995–2014. For SSP1-2.6, CMIP6 models project no robust change in the Southern Annular Mode index in the long term. It is ''likely'' that wind speeds associated with extratropical cyclones will strengthen in the Southern Hemisphere storm track for SSP5-8.5. {4.5.1, 4.5.3}

'''The CMIP6 multi-model ensemble projects a long-term increase in the boreal wintertime Northern Annular Mode index under the high-emissions scenarios of SSP3-7.0 and''' '''SSP5-8.5''' ''', but regional changes may deviate from a simple shift in the m''' '''id-latit''' '''ude circulation.''' Substantial uncertainty and thus ''low confidence'' remain in projecting regional changes in Northern Hemisphere jet streams and storm tracks, especially for the North Atlantic basin in winter; this is due to large natural internal variability, the competing effects of projected upper- and lower-tropospheric temperature gradient changes, and new evidence of weaknesses in simulating past variations in North Atlantic atmospheric circulation on seasonal-to-decadal time scales. One exception is the expected decrease in frequency of atmospheric blocking events over Greenland and the North Pacific in boreal winter in SSP3-7.0 and SSP5-8.5 scenarios ( ''medium confidence'' ). {4.5.1}

'''Near-term predictions and projections of the sub-polar branch of the Atlantic Multi-decadal Variability (AMV) on the decadal time scale have improved in CMP6 models compared to CMIP5''' ( ''high confidence'' ''').''' This is ''likely'' to be related to a more accurate response to natural forcing in CMIP6 models. Initialization contributes to the reduction of uncertainty and to predicting subpolar sea surface temperature. AMV influences on the nearby regions can be predicted over lead times of 5–8 years ( ''medium'' ''confidence'' ). {4.4.3}

'''It is''' ''virtually certain'' '''that the El Niño–Southern Oscillation (ENSO) will remain the dominant mode of interannual variability in a warmer world.''' There is no model consensus for a systematic change in intensity of ENSO sea surface temperature variability over the 21st century in any of the SSP scenarios assessed ( ''medium confidence'' ). However, it is ''very'' ''likely'' that ENSO rainfall variability, used for defining extreme El Niños and La Niñas, will increase significantly, regardless of amplitude changes in ENSO SST variability, by the second half of the 21st century in scenarios SSP2-4.5, SSP3-7.0, and SSP5-8.5. {4.3.3, 4.5.3, 8.4.2}

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