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

==== 4.3.3.2 El Niño–Southern Oscillation ====

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The El Niño–Southern Oscillation (ENSO) is the most dominant mode of variability on interannual time scales and also the dominant source of seasonal climate predictability ( [[#box-11.3|Box 11.3]] and Annex IV, Section AIV.2.3; [[#Timmermann--2018|Timmermann et al., 2018]] ). The AR5 assessed from CMIP5 simulations that ENSO variability will ''very likely'' remain the dominant mode of interannual climate variability in the future, and that associated ENSO precipitation variability on regional scales is ''likely'' to intensify ( [[#Christensen--2013|Christensen et al., 2013]] ). However, they assessed there was ''low confidence'' in projected changes in ENSO variability in the 21st century due to a strong component of internal variability.

Among a range of indices proposed for representing ENSO, we use the most prominent one, the Niño 3.4 index, defined as the average equatorial SST or precipitation across the central equatorial Pacific (5°S–5°N, 170°W–120°W; Section AIV.2.3). Here, we consider the evolution of the amplitude of Niño 3.4 index for SST and precipitation over the 21st century as projected by CMIP6 models. Analysis of CMIP6 models shows there is no robust model consensus on the forced changes in the amplitude of ENSO SST variability even under the high-emissions scenarios SSP3-7.0 and SSP5-8.5, but a significant increasing trend in the amplitude of ENSO precipitation variability is projected across the 21st century in the four SSPs (Figure 4.10). This is broadly consistent with results from CMIP5 models ( [[#Christensen--2013|Christensen et al., 2013]] ; [[#Power--2013|Power et al., 2013]] ; [[#Cai--2015|Cai et al., 2015]] ; [[#Chen--2017|Chen et al., 2017]] ; [[#Wengel--2018|Wengel et al., 2018]] ), recent studies with CMIP6 models ( [[#Brown--2020|Brown et al., 2020]] ; [[#Fredriksen--2020|Fredriksen et al., 2020]] ; Freund et al., 2020; [[#Yun--2021|Yun et al., 2021]] ), and large initial-condition ensemble experiments ( [[#Maher--2018|Maher et al., 2018]] ; [[#Zheng--2018|Zheng et al., 2018]] ; [[#Haszpra--2020|Haszpra et al., 2020]] ).

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[[File:992f415ee80cdda28cdc960542811e0c IPCC_AR6_WGI_Figure_4_10.png]]

'''Figure 4.10 |''' '''Changes in amplitude of ENSO Variability.''' Variability of '''(a)''' SST and '''(b)''' precipitation anomalies averaged over Niño 3.4 region for 1950–2014 from CMIP6 historical simulations and for 2015–2100 from four SSPs. Thick lines stand for multi-model mean and shading is the 5–95% range across CMIP6 models for historical simulation (grey), SSP1-2.6 (blue) and SSP3-7.0 (pink), respectively. The amplitude of ENSO SST and rainfall variability is defined as the standard deviation of the detrended Niño 3.4-area averaged SST and rainfall index, respectively, over 30-year running windows. The standard deviation in every single model is normalized by each model’s present-day standard deviation averaged from 1995 to 2014. The number of available models is listed in parentheses. This figure is adopted from [[#Yun--2021|Yun et al. (2021)]] . Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

It is therefore ''very likely'' that the amplitude of ENSO rainfall variability will intensify in response to global warming over the 21st century although there is no robust consensus from CMIP6 climate models for a systematic change in amplitude of ENSO SST variability even in the high-emissions scenarios of SSP3-7.0 and SSP5-8.5.

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