Editing IPCC:AR6/WGI/Chapter-2 (section)

=== 2.4.2 El Niño–Southern Oscillation (ENSO) ===

<div id="h2-22-siblings" class="h2-siblings"></div>

The AR5 reported with ''medium confidence'' that ENSO-like variability existed, at least sporadically, during the warm background state of the Pliocene. It was also found ( ''high confidence'' ) that ENSO has remained highly variable during the last 7 kyr with no discernible orbital modulation. The AR5 concluded that large variability on interannual to decadal timescales, and differences between datasets, precluded robust conclusions on any changes in ENSO during the instrumental period. The SROCC reported epochs of strong ENSO variability throughout the Holocene, with no indications of a systematic trend in ENSO amplitude, but with some indication that the ENSO amplitude over 1979–2009 was greater than at any point in the period from 1590–1880 CE. It was also reported that the frequency and intensity of El Niño events in the period from 1951–2000 was high relative to 1901–1950.

[[#Manucharyan--2014|Manucharyan and Fedorov (2014)]] found that ENSO-like variability has been present, at least sporadically, during epochs of millions of years (including the MPWP; Cross-Chapter Box 2.4), with proxy records indicating that this was the case even when cross-Pacific SST gradients were much weaker than present. There is substantial disagreement between proxy records for ENSO activity during the early Holocene ( [[#Zhang--2014|Zhang et al., 2014]] ; [[#White--2018|White et al., 2018]] ), and for ENSO activity and mean state at the LGM ( [[#Leduc--2009|Leduc et al., 2009]] ; [[#Koutavas--2012|Koutavas and Joanides, 2012]] ; [[#Sadekov--2013|Sadekov et al., 2013]] ; [[#Ford--2015|Ford et al., 2015]] , 2018; [[#Zhu--2017|Zhu et al., 2017]] ; [[#Tierney--2020|Tierney et al., 2020]] ). A number of studies ( [[#Cobb--2013|Cobb et al., 2013]] ; H. [[#McGregor--2013|]] [[#McGregor--2013|McGregor et al., 2013]] ; [[#Carré--2014|Carré et al., 2014]] ; [[#Emile-Geay--2016|Emile-Geay et al., 2016]] ; [[#Thompson--2017|Thompson et al., 2017]] ; [[#Tian--2017|Tian et al., 2017]] ; [[#White--2018|White et al., 2018]] ; [[#Grothe--2019|Grothe et al., 2019]] ) have found that ENSO was substantially weaker than at present at various times in the mid-Holocene within the period from 6 to 3 ka, with stronger decreases in variability revealed by remote proxies than by those close to the core region of ENSO activity. However, [[#Karamperidou--2015|Karamperidou et al. (2015)]] find that weakening in ENSO-related variability in eastern Pacific proxies does not necessarily correspond to weakening in central Pacific proxies. [[#Barrett--2018|Barrett et al. (2018)]] concluded that multi-proxy reconstructions are more efficient at identifying eastern Pacific than central Pacific events. This suggests that a weakening of proxy-based signals may indicate an along-equatorial shift in ENSO activity rather than a weakening of ENSO during some periods. Following the period of weak ENSO variability in the mid-Holocene, a number of studies find an increase in ENSO activity which, depending upon the study, commences between 4.4 and 3 ka ( [[#Koutavas--2012|Koutavas and Joanides, 2012]] ; [[#Cobb--2013|Cobb et al., 2013]] ; [[#Zhang--2014|Zhang et al., 2014]] ; S. [[#Chen--2016|]] [[#Chen--2016|Chen et al., 2016]] ; [[#Emile-Geay--2016|Emile-Geay et al., 2016]] ; [[#Thompson--2017|Thompson et al., 2017]] ; [[#Du--2021|Du et al., 2021]] ).

Numerous studies (J. [[#Li--2013|]] [[#Li--2013|Li et al., 2013]] ; S. [[#McGregor--2013|]] [[#McGregor--2013|McGregor et al., 2013]] ; [[#Rustic--2015|Rustic et al., 2015]] ; [[#Hope--2017|Hope et al., 2017]] ; Y. [[#Liu--2017|]] [[#Liu--2017|]] [[#Liu--2017|Liu et al., 2017]] ) find substantial variability in ENSO activity on multi-decadal to centennial timescales over the last 500 to 1 kyr (Figure 2.36). Different proxies show a wide spread in the specific timing and magnitude of events in the pre-instrumental period (e.g., [[#Dätwyler--2019|Dätwyler et al., 2019]] ). Most investigators find that ENSO activity in recent decades was higher than the most recent centuries prior to the instrumental period. [[#Grothe--2019|Grothe et al. (2019)]] also found that ENSO variance of the last 50 years was 25% higher than the average of the last millennium, and was substantially higher than the average of the mid- to late-Holocene. S. McGregor et al., (2010, 2013) looked for common variance changes in pre-existing ENSO proxies, finding stronger ENSO variance for the 30-year period 1979–2009 compared to any 30-year period within the timespan 1590–1880 CE. This finding also holds when adding more recently developed ENSO proxies (Figure 2.36). [[#Koutavas--2012|Koutavas and Joanides (2012)]] , [[#Ledru--2013|Ledru et al. (2013)]] and [[#Thompson--2017|Thompson et al. (2017)]] identify various periods within the range 1000 BCE to 1300 CE when ENSO activity was greater than in the following centuries, and more closely comparable to the mid-20th century onwards behaviour.

<div id="_idContainer089" class="Basic-Text-Frame"></div>

[[File:bb143b9338be6a32e60692bdf649080d IPCC_AR6_WGI_Figure_2_36.png]]

'''Figure 2.3''' '''6 |''' '''Reconstructed and historical variance ratio of El Niño–Southern Oscillation (ENSO). (a)''' 30-year running variance of the reconstructed annual mean Niño 3.4 or related indicators from various published reconstructions. '''(b)''' Variance of June–November Southern Oscillation Index (SOI) and April–March mean Niño 3.4 (1981–2010 base period) along with the mean reconstruction from (a). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Since AR5, updates to datasets used widely in prior ENSO assessments resulted in substantial and important revisions to observed tropical Pacific SST data ( [[#2.3.1.1|Section 2.3.1.1]] ). In particular, ERSSTv4, and then ERSSTv5, addressed known SST biases in ERSSTv3 in the equatorial Pacific which affected the derived mean state and amplitude of indices based on that dataset ( [[#Huang--2015|Huang et al., 2015]] ). During the instrumental period, there is no robust indication of any significant century-scale trend in the east-west SST gradient across the equatorial Pacific Ocean, with periods when gradients have been stronger and weaker than the long-term average on decadal timescales, associated with a predominance of La Niña or El Niño events respectively. The frequency of El Niño and La Niña events is also subject to considerable decadal variability (e.g., [[#Hu--2013|Hu et al., 2013]] ) but with no indication of a long-term signal in the frequency of events. The ENSO amplitude since 1950 has increased relative to the 1910–1950 period, as confirmed by independent proxy records (e.g., [[#Gergis--2009|Gergis and Fowler, 2009]] ), the Southern Oscillation Index (SOI) ( [[#Braganza--2009|Braganza et al., 2009]] ) and SSTs (e.g., [[#Ohba--2013|Ohba, 2013]] ; [[#Yu--2013|Yu and Kim, 2013]] ), although there is a spread between different proxy and instrumental sources as to the magnitude of that increase (Figure 2.36). The El Niño events of 1982–1983, 1997–1998 and 2015–2016 had the strongest anomalies in the Niño 3.4 SST index since 1950. Their predominance was less evident from indices based on de-trended data such as the Oceanic Niño Index (ONI) (which still ranked them as the three strongest events since 1950, but only by a small margin), and in the SOI. [[#Huang--2019a|]] [[#Huang--2019a|B. Huang et al. (2019a)]] also note that analyses based upon buoy and Argo data, which are only available since the 1990s, are more capable of resolving strong events than analyses which do not include such data.

Prior to the 1950s, SST observations in the tropical Pacific were much sparser and hence uncertainties in Niño indices are much larger ( [[#Huang--2020|]] [[#Huang--2020|B. Huang et al., 2020]] ). SOI data and some newer SST-based studies show high ENSO amplitude, comparable to the post-1950 period, in the period from the mid-late 19th century to about 1910, but proxy indicators generally indicate that the late 19th and early 20th century were less active than the late 20th century (Figure 2.36). Yu and Kim’s (2013) implementation of the ONI found a number of events with the ONI above 1.5°C between 1888 and 1905, then no such events until 1972, whilst the SOI indicates comparable or stronger events to the three strongest post-1950 events in 1896 and 1905. [[#Giese--2011|Giese and Ray (2011)]] also found a number of such events between 1890 and 1920 in the SODA ocean reanalysis, corroborated further by [[#Huang--2020|]] [[#Huang--2020|B. Huang et al. (2020)]] and [[#Vaccaro--2021|Vaccaro et al. (2021)]] , who found that the strength of the 1877–1878 event was comparable with that of the 1982–1983, 1997–1998 and 2015–2016 events. There have also been a number of strong La Niña events (e.g., 1973–1974, 1975–1976 and 2010–2011), with few clear analogues in the 1920–1970 period; the proxy-based analysis of [[#McGregor--2010|McGregor et al. (2010)]] indicates that the mid-1970s La Niña period was also extreme in a multi-centennial context. There is no indication that the frequency of high-amplitude events since the 1970s reflects a long-term trend which can be separated from multi-decadal variability, given apparent presence of several high-amplitude events in the late 19th and early 20th centuries, and the relatively large uncertainty in pre-1950 SST data in the tropical Pacific region.

There is a distinction (Annex IV.2.3.1) between El Niño events centred in the eastern Pacific (‘Eastern Pacific’ (EP) or ‘classical’ events) and those centred in the Central Pacific (‘Central Pacific’ (CP) or ‘Modoki’ events), which have different typical teleconnections (e.g., [[#Ashok--2007|Ashok et al., 2007]] ; [[#Ratnam--2014|Ratnam et al., 2014]] ; [[#Capotondi--2015|Capotondi et al., 2015]] ; [[#Timmermann--2018|Timmermann et al., 2018]] ). A number of studies, using a range of indicators, have found an increase in recent decades of the fraction of CP El Niño events, particularly after 2000 ( [[#Yu--2013|Yu and Kim, 2013]] ; [[#Lübbecke--2014|Lübbecke and McPhaden, 2014]] ; [[#Pascolini-Campbell--2015|Pascolini-Campbell et al., 2015]] ; [[#Jiang--2018|Jiang and Zhu, 2018]] ). [[#Johnson--2013|Johnson (2013)]] found that the frequency of CP El Niño events had increased (although not significantly) over the 1950–2011 period, being accompanied by a significant increase in the frequency of La Niña events with a warm (as opposed to cool) western Pacific warm pool. A coral-based reconstruction starting in 1600 CE ( [[#Freund--2019|Freund et al., 2019]] ) found that the ratio of CP to EP events in the last 30 years was substantially higher than at any other time over the last 400 years. Variations in the proportion of CP and EP events have also been found in earlier periods, with [[#Carré--2014|Carré et al. (2014)]] finding a period of high CP activity around 7 ka.

There is no robust indication of any changes in ENSO teleconnections over multi-centennial timescales ( [[#Hernández--2020|Hernández et al., 2020]] ) despite multi-decadal variability. [[#Shi--2018|Shi and Wang (2018)]] found that teleconnections with the broader Asian summer monsoon, including the Indian and the East Asian monsoon, were generally stable since the 17th century during the developing phase of the monsoon, and showed substantial decadal variability, but no clear trend, during the decaying phase. They also found that the weakening of teleconnections between the Indian summer monsoon and ENSO in recent decades had numerous precedents over the last few centuries. [[#Räsänen--2016|Räsänen et al. (2016)]] also found substantial decadal variability, but little trend, in the strength of the relationship between ENSO and monsoon precipitation in South East Asia between 1650 and 2000. [[#Dätwyler--2019|Dätwyler et al. (2019)]] identified a number of multi-decadal periods with apparently changed teleconnections at times over the last 400 years.

In the instrumental period, teleconnections associated with ENSO are well known to vary on decadal to multi-decadal timescales (e.g., [[#He--2013|He et al., 2013]] ; [[#Lee--2015|Lee and Ha, 2015]] ; [[#Ashcroft--2016|Ashcroft et al., 2016]] ; [[#Jin--2016|Jin et al., 2016]] ; [[#Wang--2019|Wang et al., 2019]] ). [[#Yun--2018|Yun and Timmermann (2018)]] found that decadal variations in teleconnections between ENSO and the Indian monsoon did not extend beyond what would be expected from a stochastic process. Many observed decadal changes in teleconnections in the instrumental period are consistent with a shift to more central Pacific El Niño events ( [[#Evtushevsky--2018|Evtushevsky et al., 2018]] ; [[#Yeh--2018|Yeh et al., 2018]] ; [[#Yu--2018|Yu and Sun, 2018]] ; [[#Zhao--2019|Zhao and Wang, 2019]] ). Effects of the PDV ( [[#Kwon--2013|Kwon et al., 2013]] ; S. [[#Wang--2014|]] [[#Wang--2014|Wang et al., 2014]] ; [[#Dong--2018|Dong et al., 2018]] ) and the AMV ( [[#Kayano--2019|Kayano et al., 2019]] ) can also modulate ENSO teleconnections, and affect the frequency of CP versus EP events ( [[#Ashok--2007|Ashok et al., 2007]] ). [[#Chiodi--2015|Chiodi and Harrison (2015)]] found that teleconnections over the most recent decades are broadly consistent with those over the last 100 years. Variability in teleconnections can also occur on timescales longer than characteristic PDV timescales (e.g., [[#Gallant--2013|Gallant et al., 2013]] ).

In summary, there is ''medium confidence'' that both ENSO amplitude and the frequency of high-magnitude events since 1950 are higher than over the period from 1850 and possibly as far back as 1400, but ''low confidence'' that they are outside the range of variability over periods prior to 1400, or higher than the average of the Holocene as a whole. Overall, there is no indication of a recent sustained shift in ENSO or associated features such as the Walker Circulation, or in teleconnections associated with these, being beyond the range of variability on decadal to millennial timescales. A high proportion of El Niño events in the last 20–30 years have been based in the central, rather than eastern Pacific, but there is ''low confidence'' that this represents a long-term change.

<div id="2.4.3" class="h2-container"></div>

<span id="indian-ocean-basin-and-dipole-modes"></span>