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

==== 4.5.3.2 El Niño–Southern Oscillation ====

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The AR5 assessed that it is ''very likely'' that the El Niño–Southern Oscillation (ENSO) will remain the dominant mode of interannual variability in the future. Moreover, due to increased moisture availability, the associated precipitation variability on regional scales was assessed to ''likely'' intensify. An eastward shift in the patterns of temperature and precipitation variations in the North Pacific and North America related to El Niño and La Niña teleconnections was projected with ''medium confidence'' . The stability of teleconnections to other regional implications including those in Central and South America, the Caribbean, parts of Africa, most of Asia, Australia and most Pacific Islands were assessed to be uncertain ( [[#Christensen--2013|Christensen et al., 2013]] ).

There is no consensus on changes in amplitude of ENSO SST variability across CMIP iterations. The main factors driving the diversity of ENSO SST amplitude change in climate models are internal variability, SST-mean warming pattern, and model systematic biases. First, pronounced low-frequency modulations of ENSO exist even in unforced control simulations due to internal variability, which leads a large uncertainty in quantifying future ENSO changes ( [[#Wittenberg--2009|Wittenberg, 2009]] ; [[#Vega-Westhoff--2017|Vega-Westhoff and Sriver, 2017]] ; [[#Zheng--2018|Zheng et al., 2018]] ). Second, ENSO characteristics depend on the climate mean state of the tropical Pacific; however, ENSO can also influence the mean state through non-linear processes ( [[#Cai--2015|Cai et al., 2015]] ; [[#Timmermann--2018|Timmermann et al., 2018]] ). The response of the tropical Pacific mean state to anthropogenic forcing is characterized by a faster warming on the equator compared to the off-equatorial region, a faster warming of the eastern equatorial Pacific compared to the central tropical Pacific (e.g., El Niño-like mean SST warming, see Section 7.4.4.2), and a weakening of the Walker circulation in most models. Those models with a El Niño-like warming tend to project a strengthening of ENSO SST variability whereas models with a La Niña-like warming tend to project a weakening of variability ( [[#Zheng--2016|Zheng et al., 2016]] ; [[#Kohyama--2017|Kohyama and Hartmann, 2017]] ; J. [[#Wang--2017b|]] [[#Wang--2017|Wang et al., 2017]] b ; [[#Cai--2018a|Cai et al., 2018a]] ; [[#Fredriksen--2020|Fredriksen et al., 2020]] ). Third, how to take model biases into account leads to different ENSO changes. [[#Kim--2014|Kim et al. (2014)]] suggested that a subset of CMIP5 models that simulate linear ENSO stability realistically exhibit a decrease in ENSO amplitude by the second half of the 21st century. However, an increase of ENSO SST variability has been projected when considering biases in ENSO pattern simulation by different models ( [[#Zheng--2016|Zheng et al., 2016]] ; [[#Cai--2018a|Cai et al., 2018a]] ). This highlights the importance of constraining tropical Pacific mean state changes in order to enhance confidence in the projected response of ENSO.

There is also no robust consensus on changes in ENSO diversity. Several studies suggest that an increase in Eastern Pacific (EP)-ENSO events tends to be projected particularly in the models with an El Niño-like warming ( [[#Zheng--2016|Zheng et al., 2016]] ; [[#Cai--2018a|Cai et al., 2018a]] ; [[#Fredriksen--2020|Fredriksen et al., 2020]] ). However, [[#Freund--2020|Freund et al. (2020)]] suggested that models with a El Niño-like mean warming show a tendency toward more Central Pacific (CP) events but fewer EP events compared to models with an La Niña-like warming in both CMIP5 and CMIP6 models.

Even though there is ''low agreement'' in simulated changes in ENSO SST variability, the majority of models project an increase in amplitude of ENSO rainfall variability attributable to the increase in mean SST and moisture in CMIP5 ( [[#Power--2013|Power et al., 2013]] ; [[#Watanabe--2014|Watanabe et al., 2014]] ; [[#Huang--2015|Huang and Xie, 2015]] ) and CMIP6 ( [[#Yun--2021|Yun et al., 2021]] ). It is ''likely'' that extreme El Niño events, accompanied by the eastern equatorial Pacific rainfall exceeding the 5 mm day <sup>–1</sup> rainfall threshold, will increase in intensity ( [[#Cai--2014a|Cai et al., 2014a]] , 2017). However, it has also been suggested that historical model biases over the equatorial Pacific cold tongue in CMIP5 may lead to the greater precipitation mean change and amplification of extreme ENSO-associated rainfall in CMIP5 ( [[#Stevenson--2021|Stevenson et al., 2021]] ).

There is ''limited'' intermodel ''agreement'' on future changes in ENSO teleconnections largely depending on changes in the mean state and changes in ENSO properties ( [[#Yeh--2018|Yeh et al., 2018]] ). Many CMIP5 and CMIP6 models project that the centres of the extratropical teleconnection over North Pacific and North America will shift eastward in association with an eastward shift in tropical convective anomalies ( [[#Yeh--2018|Yeh et al., 2018]] ; [[#Fredriksen--2020|Fredriksen et al., 2020]] ). There is an indication that tropical cyclones will become more frequent during future El Niño events (and less frequent during future La Niña events) by the end of the 21st century ( [[#Chand--2017|Chand et al., 2017]] ), thus contributing to the projected increase in ENSO-associated hydro-climate impacts.

While CMIP6 models show no robust change in ENSO SST amplitude in the mid- and long-term period across all four SSPs, a robust increase in ENSO rainfall amplitude is found particularly in SSP2-4.5, SSP3-7.0, and SSP5-8.5 (Figure 4.10). The changes in ENSO rainfall amplitude in the long-term future (2081–2100) relative to the recent past (1995–2014) are statistically significant at the 95% confidence.

To conclude, the forced change in ENSO SST variability is highly uncertain in CMIP5 and CMIP6 models ( ''medium confidence'' ). However, it is ''very likely'' that ENSO-related rainfall variability will increase significantly regardless of ENSO amplitude changes in the mid- and long-term future. It is ''likely'' that the pattern of ENSO teleconnection over the North Pacific and North America will shift eastward.

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