Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.4.2.9 Modes of Climate Variability and Regional Teleconnections ====

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Following on from the assessment of projected changes in modes of climate variability (MoVs) and regional teleconnections ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.3|Section 4.5.3]] ), here we assess their consequences for projected water cycle changes.

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===== 8.4.2.9.1 Tropical modes =====

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CMIP6 projections indicate that the amplitude of ENSO (Annex IV.2.3) variability will not substantially change during the 21st century ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-4#4.4.3.2|Section 4.4.3.2]] ). However, rainfall variability related to ENSO is projected to increase significantly by the second half of the 21st century, regardless of ENSO amplitude ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.3.2|Section 4.5.3.2]] ). Regional precipitation variability associated with ENSO increases due to increases in atmospheric moisture, regardless of changes in ENSO variability itself ( [[#Pendergrass--2017|Pendergrass et al., 2017]] ). In many regions, the magnitude of the projected changes related to ENSO is small compared with historical interannual variability ( [[#Bonfils--2015|Bonfils et al., 2015]] ; [[#Power--2018|Power and Delage, 2018]] ; [[#Perry--2020|Perry et al., 2020]] ). Uncertainties in precipitation projections related to ENSO depend on internal variability associated with the mode ( [[#8.5.2|Section 8.5.2]] ), hence the need to have relatively large ensembles (about 15 members) to adequately estimate uncertainty (Deser et al. , 2018; N. Maher et al. , 2018; C. Sun et al. , 2018; Zheng et al., 2018) .

Even over regions with statistically significant simulated rainfall teleconnections during the historical period, CMIP5 models do not project clear changes ( [[#Perry--2020|Perry et al., 2020]] ). Nonetheless, CMIP5 models that realistically reproduce Indian summer monsoon rainfall indicate a strengthening of its relationship with ENSO in RCP8.5 projections, though the response is not consistent for different varieties of ENSO events ( [[#Roy--2019|Roy et al., 2019]] ). Inconsistent changes in the ENSO–Indian summer monsoon relationship in response to global warming in CMIP5 and CMIP6 models may be related to statistical issues rather than dynamical changes ( [[#Bódai--2020|Bódai et al., 2020]] ; [[#Haszpra--2020|Haszpra et al., 2020]] ). Over East Africa during the boreal spring and summer, ENSO teleconnections are projected to become stronger in the future ( [[#Endris--2019|Endris et al., 2019]] ). Meteorological drought consequences of each strong El Niño are projected to become more severe in the region ( [[#Rifai--2019|Rifai et al., 2019]] ).

Indian Ocean Dipole (IOD, Annex IV.2.4) and Indian Ocean Basin (IOB, Annex IV.2.4) interactions with ENSO are expected to persist in the future ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.3.3|Section 4.5.3.3]] ) but projected changes in the frequency and intensity of events remain uncertain ( [[#Hui--2018|Hui and Zheng, 2018]] ; [[#Endris--2019|Endris et al., 2019]] ; [[#McKenna--2020|McKenna et al., 2020]] ). Climate extremes such as those associated with the extreme positive IOD event of 2019 are expected to occur more frequently under continued global warming ( [[#Cai--2021|Cai et al., 2021]] ). Projected changes in IOD teleconnections are linked to model performance in representing the IOD and its remote influence in the present climate, apparently dominated by a positive IOD event-like mean state (G. [[#Wang--2017|]] [[#Wang--2017|]] [[#Wang--2017|Wang et al., 2017]] ; [[#Huang--2019|Huang et al., 2019]] ). Interactions between the IOD and the Indian Ocean mean state, via atmosphere–ocean feedbacks, can affect the behaviour of the IOD ( [[#Ng--2018|Ng et al., 2018]] ). In the eastern Horn of Africa, OND rainfall is projected to increase because of IOD-ENSO related SST changes in the Indo-Pacific region and associated Walker circulation changes ( [[#Endris--2019|Endris et al., 2019]] ).

Sensitivity studies generally project increases in Madden Julian Oscillation (MJO, Annex IV.2.8) precipitation amplitude in a warmer climate, with increases of up to 14% °C <sup>–1</sup> of warming ( [[#Arnold--2013|Arnold et al., 2013]] , 2015; [[#Caballero--2013|Caballero and Huber, 2013]] ; [[#Liu--2013|Liu and Allan, 2013]] ; [[#Maloney--2013|Maloney and Xie, 2013]] ; [[#Schubert--2013|Schubert et al., 2013]] ; [[#Subramanian--2014|Subramanian et al., 2014]] ; [[#Carlson--2016|Carlson and Caballero, 2016]] ; [[#Pritchard--2016|Pritchard and Yang, 2016]] ; [[#Adames--2017a|Adames et al., 2017a]] ; [[#Wolding--2017|Wolding et al., 2017]] ; [[#Haertel--2018|Haertel, 2018]] ). However, in CMIP5 models with realistic historical MJO behaviour, the precipitation amplitude over the Indo-Pacific warm pool region changes from – 4% to +8% °C <sup>–1</sup> in the RCP8.5 scenario relative to the end of the 20th century ( [[#Bui--2018|Bui and Maloney, 2018]] ; [[#Maloney--2019|Maloney et al., 2019]] ). When simulated MJO precipitation amplitude increases with warming, the leading factor for such change is the intensification of the lower tropospheric vertical moisture gradient, that supports stronger vertical moisture advection per unit diabatic heating ( Arnold et al. , 2015; Adames et al. , 2017a, b; Wolding et al. , 2017 ). In idealized simulations with constant CO <sub>2</sub> forcing with El Niño-like patterns, the MJO activity penetrates farther east into the central and east Pacific with increased warming ( [[#Subramanian--2014|Subramanian et al., 2014]] ; [[#Adames--2017a|Adames et al., 2017a]] ). Increased MJO convective variability in a warmer climate does not reflect into increased ability of the MJO to force the extratropics ( [[#Wolding--2017|Wolding et al., 2017]] ).

In summary, even though there is ''low confidence'' in how the tropical MoVs will change in the future (Sections 4.3.3.2 and 4.5.3.3), their regional hydrological consequences, in terms of precipitation, are projected to intensify ( ''medium confidence'' ). For example, the ENSO influence on precipitation over the Indo–Pacific sector is projected to strengthen and shift eastward ( ''medium confidence'' ). The MJO is projected to intensify in a warmer climate, with increased associated precipitation ( ''medium co'' ''nfidence'' ).

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===== 8.4.2.9.2 Extratropical modes =====

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CMIP6 projections indicate that the Northern Annular Mode (NAM; Annex IV.2.1) is expected to become more positive in winter throughout the 21st century in the SSP3-7.0 and SSP5-8.5 scenarios ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.1|Section 4.5.1]] ). In the near term, the Southern Annular Mode (SAM, Annex IV.2.2) is projected to become less positive than observed during the end of the 20th century during the austral summer in all SSPs scenarios ( [[IPCC:Wg1:Chapter:Chapter-4#4.3.3.1|Section 4.3.3.1]] ).

In the CMIP5 RCP8.5 scenario, increased amplitude and frequency of the North Atlantic Oscillation (NAO, Annex IV.2) during boreal winter (December–January–February, DJF) is associated with higher precipitation in northern Europe and lower precipitation in southern Europe ( [[#Tsanis--2019|Tsanis and Tapoglou, 2019]] ). However, large-ensemble analyses show how the NAO leads to significant uncertainty in future changes of regional climate ( [[#8.5.2|Section 8.5.2]] ). For example, more than a 85% increase in precipitation is projected over northern Europe, western Russia and much of eastern North America, with similar decreasing resulting in drying over north-western Africa and regions adjacent to the Mediterranean Sea ( [[#Deser--2017|Deser et al., 2017]] ).

In the SH, the positive trend projected for the SAM in the CMIP5 RCP8.5 scenario appears to mitigate the wetting in the mid- to high latitudes and the drying over the subtropics, but with strong seasonal dependence ( [[#Lim--2016|Lim et al., 2016]] ). Regional precipitation changes in South America, South Africa, Southern Australia and New Zealand are not well explained by changes in the SAM, but are related to broad-scale changes in north – south temperature gradients associated with enhanced warming of the tropical upper troposphere and strengthening of the stratospheric polar vortex ( [[#Mindlin--2020|Mindlin et al., 2020]] ).

In summary, projected changes in the intensity, frequency and phase of extratropical MoVs (see also Sections 4.3 and 4.5) may amplify regional changes in precipitation and contribute to an increase in their intra-seasonal and interannual variability ( ''medium confidence'' ). Regionally, there are potentially significant precipitation and atmospheric circulation changes associated with changes in extratropical dynamics ( ''low con'' ''fidence'' ).

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