Editing IPCC:AR6/SROCC/Chapter-6 (section)

==== 6.5.1.1 Extreme El Niño, La Niña ====

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AR5 (Christensen et al., 2013 <sup>[[#fn:r481|481]]</sup> ) and SREX do not provide a definition for an extreme El Niño but mention such events, especially in the context of the 1997–1998 El Niño and its impacts. AR5 and SREX concluded that confidence in any specific change in ENSO variability in the 21st century is low. However, they did note that due to increased moisture availability, precipitation variability associated with ENSO is likely to intensify. Since AR5 and SREX, there is now a limited body of literature that examines the impact of climate change on ENSO over the historical period.

Palaeo-ENSO studies suggest that ENSO was highly variable throughout the Holocene, with no evidence for a systematic trend in ENSO variance (Cobb et al., 2013 <sup>[[#fn:r482|482]]</sup> ) but with some indication that the ENSO variance over 1979–2009 has been much larger than that over 1590–1880 (McGregor et al., 2013 <sup>[[#fn:r483|483]]</sup> ). Palaeo-ENSO reconstruction for the past eight centuries suggests that central Pacific ENSO activity has increased between the last two decades (1980-2015; Liu et al., 2017b <sup>[[#fn:r484|484]]</sup> ), with an increasing number of central Pacific El Niño events compared to east Pacific El Niño events (Freund et al., 2019 <sup>[[#fn:r485|485]]</sup> ). Further proxy evidence exists for changes in the mean state of the equatorial Pacific in the last 2000 years (Rustic et al., 2015 <sup>[[#fn:r486|486]]</sup> ; Henke et al., 2017 <sup>[[#fn:r487|487]]</sup> ). Simulations using an Earth System Model indicate significantly higher ENSO variance during 1645–1715 than during the 21st century warm period, though it is unclear whether these simulated changes are realistic (Keller et al., 2015 <sup>[[#fn:r488|488]]</sup> ). For the 20th century, the frequency and intensity of El Niño events were high during 1951–2000, in comparison with the 1901–1950 period (Lee and McPhaden, 2010 <sup>[[#fn:r489|489]]</sup> ; Kim et al., 2014b <sup>[[#fn:r490|490]]</sup> ; Roxy et al., 2014 <sup>[[#fn:r491|491]]</sup> ). Current instrumental observational records are not long enough and the quality of data before 1950 is limited, to assert these changes with ''high confidence'' (Wittenberg, 2009 <sup>[[#fn:r492|492]]</sup> ; Stevenson et al., 2010 <sup>[[#fn:r493|493]]</sup> ) though the palaeo records mentioned here signal the emergence of a statistically significant increase in ENSO variance in recent decades.

Since SREX and AR5, an extreme El Niño event occurred in 2015–2016. This has resulted in significant new literature regarding physical processes and impacts but there are no firm conclusions regarding the impact of climate change on the event. The SST anomaly peaked toward the central equatorial Pacific causing floods in many regions of the world such as those in the west coasts of the USA and other parts of North America, some parts of South America close to Argentina and Uruguay, the UK and China (Ward et al., 2014 <sup>[[#fn:r494|494]]</sup> ; Ward et al., 2016 <sup>[[#fn:r495|495]]</sup> ; Zhai et al., 2016 <sup>[[#fn:r496|496]]</sup> ; Scaife et al., 2017 <sup>[[#fn:r497|497]]</sup> ; Whan and Zwiers, 2017 <sup>[[#fn:r498|498]]</sup> ; Sun and Miao, 2018 <sup>[[#fn:r499|499]]</sup> ; Yuan et al., 2018 <sup>[[#fn:r500|500]]</sup> ).

The main new body of literature concerns future projections of the frequency of occurrence and variability of extreme ENSO events with improved confidence (Cai et al., 2014a <sup>[[#fn:r501|501]]</sup> ; Cai et al., 2018 <sup>[[#fn:r502|502]]</sup> ). These studies define extreme El Niño events as those El Niño events which are characterised by a pronounced eastward extension of the west Pacific warm pool and development of atmospheric convection, and hence a rainfall increase of greater than 5 mm day -1 during December to February (above 90th percentile), in the usually cold and dry equatorial eastern Pacific (Niño 3 region, 150°W–90°W, 5°S–5°N; Cai et al., 2014a <sup>[[#fn:r503|503]]</sup> ), such as the 1982–1983, 1997–1998 and 2015–2016 El Niños (Santoso et al., 2017 <sup>[[#fn:r504|504]]</sup> ; Figure 6.5).

The background long-term warming puts the 2015–2016 El Niño among the three warmest in the instrumental records (24 El Niño events occurred during 1900–2018; Huang et al., 2016 <sup>[[#fn:r505|505]]</sup> ; Santoso et al., 2017 <sup>[[#fn:r506|506]]</sup> ). The 2015–2016 event can be viewed as the first emergence of an extreme El Niño in the 21st century – one which satisfies the rainfall threshold definition, but not characterised by the eastward extension of the west Pacific warm pool (L’Heureux et al., 2017 <sup>[[#fn:r507|507]]</sup> ; Santoso et al., 2017 <sup>[[#fn:r508|508]]</sup> ).

Based on the precipitation threshold, extreme El Niño frequency is projected to increase with the global mean temperatures ( ''medium confidence'' ) with a doubling in the 21st century under 1.5°C of global warming, from about one event every 20 years during 1891–1990, to one every 10 years (Cai et al., 2014a <sup>[[#fn:r509|509]]</sup> ; Figure 6.5). The increase in frequency continues for up to a century even after global mean temperature has stabilised at 1.5°C, thereby challenging the limits to adaptation, and hence indicates high risk even at the 1.5°C threshold (Wang et al., 2017 <sup>[[#fn:r510|510]]</sup> ; Hoegh-Guldberg et al., 2018 <sup>[[#fn:r511|511]]</sup> ). Meanwhile, the La Niña events also tend to increase in frequency and double under RCP8.5 (Cai et al., 2015 <sup>[[#fn:r512|512]]</sup> ), but indicate no further significant changes after global mean temperatures have stabilised (Wang et al., 2017 <sup>[[#fn:r513|513]]</sup> ). Particularly concerning is that swings from extreme El Niño to extreme La Niña (opposite of extreme El Niño) have been projected to occur more frequently under greenhouse warming (Cai et al., 2015 <sup>[[#fn:r514|514]]</sup> ). The increasing ratio of Central Pacific El Niño events to East Pacific El Niño events is projected to continue, under increasing emissions (Freund et al., 2019 <sup>[[#fn:r515|515]]</sup> ). Further, CMIP5 models indicate that the risk of major rainfall disruptions has already increased for countries where the rainfall variability is linked to ENSO variability. This risk will remain elevated for the entire 21st century, even if substantial reductions in global GHG emissions are made ( ''medium confidence'' ). The increase in disruption risk is caused by anthropogenic warming that drives an increase in the frequency and magnitude of ENSO events and also by changes in background SST patterns (Power et al., 2013 <sup>[[#fn:r516|516]]</sup> ; Chung et al., 2014 <sup>[[#fn:r517|517]]</sup> ; Huang and Xie, 2015 <sup>[[#fn:r518|518]]</sup> ). While many of these studies have adopted the precipitation view of an extreme El Nino, studies also indicate an increase in SST variability for events with their main SST anomalies in the east Pacific (Cai et al., 2018 <sup>[[#fn:r519|519]]</sup> ). Also, a role of cross-equatorial winds has been identified (Hu and Fedorov, 2018 <sup>[[#fn:r520|520]]</sup> )

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'''Figure 6.5 | Frequency of extreme El Niño Southern Oscillation (ENSO) events, adapted from Cai et al. (2014a). (a) December to February mean meridional sea surface temperature (SST) gradient (x-axis: 5oN–10oN, 210oE–270oE minus 2.5oS–2.5oN, 210oE–270oE) versus equatorial Pacific anomalous rainfall (y-axis: 5oS–5oN, 210oE–270oE). Data from only those Coupled Model Intercomparison Project Phase 5 (CMIP5) models […]'''
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Figure 6.5 | Frequency of extreme El Niño Southern Oscillation (ENSO) events, adapted from Cai et al. (2014a). (a) December to February mean meridional sea surface temperature (SST) gradient (x-axis: 5oN–10oN, 210oE–270oE minus 2.5oS–2.5oN, 210oE–270oE) versus equatorial Pacific anomalous rainfall (y-axis: 5oS–5oN, 210oE–270oE). Data from only those Coupled Model Intercomparison Project Phase 5 (CMIP5) models that capture the observed relationship between Pacific SST and rainfall are shown. Black dots are from observations with extreme El Niño and extreme La Niña years indicated. The horizontal line denotes the threshold of 5 mm day–1 for an extreme event. (b)Histogram showing the relative frequency of rainfall rates. The vertical line denotes the 5 mm day–1 threshold. Higher counts of extreme events under the Representative Concentration Pathway (RCP)8.5 scenario suggest an increase in the frequency of extreme El Niño under global warming.

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