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1. Uncoupled El Niño Warming

   https://doi.org/10.1029/2020GL087621  

   Hu, Zeng‐Zhen; McPhaden, Michael J.; Kumar, Arun; Yu, Jin‐Yi; Johnson, Nathaniel C. (April 2020, Geophysical Research Letters)

   Abstract In light of a warming climate, the complexity of the El Niño/Southern Oscillation (ENSO) makes its prediction a challenge. In addition to various flavors of ENSO, oceanic warming in the central and eastern tropical Pacific is not always accompanied by corresponding atmospheric anomalies; that is, the atmosphere and ocean remain uncoupled. Such uncoupled warm events as happened in 1979, 2004, 2014, and 2018 are rare and represent an unusual form of ENSO diversity. A weaker zonal sea surface temperature anomaly gradient across the tropical Pacific compared to a conventional El Niño may partially account for the decoupling. Also, the uncoupled warm events typically start late in the calendar year, which raises the possible influence of seasonality in background conditions for the lack of coupling. Without coupling, the impact of the warming in the central and eastern tropical Pacific on extratropical climate is different from that of its coupled counterpart. 

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2. The El Niño–Southern Oscillation Pattern Effect

   https://doi.org/10.1029/2021GL095261  

   Ceppi, Paulo; Fueglistaler, Stephan (October 2021, Geophysical Research Letters)

   Abstract El Niño–Southern Oscillation (ENSO) variability is accompanied by out‐of‐phase anomalies in the top‐of‐atmosphere tropical radiation budget, with anomalous downward flux (i.e., net radiative heating) before El Niño and anomalous upward flux thereafter (and vice versa for La Niña). Here, we show that these radiative anomalies result mainly from a sea surface temperature (SST) “pattern effect,” mediated by changes in tropical‐mean tropospheric stability. These stability changes are caused by SST anomalies migrating from climatologically cool to warm regions over the ENSO cycle. Our results are suggestive of a two‐way coupling between SST variability and radiation, where ENSO‐induced radiative changes may in turn feed back onto SST during ENSO. 

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3. El Niño–Southern Oscillation complexity

   https://doi.org/10.1038/s41586-018-0252-6  

   Timmermann, Axel; An, Soon-Il; Kug, Jong-Seong; Jin, Fei-Fei; Cai, Wenju; Capotondi, Antonietta; Cobb, Kim M.; Lengaigne, Matthieu; McPhaden, Michael J.; Stuecker, Malte F.; et al (July 2018, Nature)
4. Historical change of El Niño properties sheds light on future changes of extreme El Niño

   https://doi.org/10.1073/pnas.1911130116  

   Wang, Bin; Luo, Xiao; Yang, Young-Min; Sun, Weiyi; Cane, Mark A.; Cai, Wenju; Yeh, Sang-Wook; Liu, Jian (November 2019, Proceedings of the National Academy of Sciences)

   null (Ed.)

   El Niño’s intensity change under anthropogenic warming is of great importance to society, yet current climate models’ projections remain largely uncertain. The current classification of El Niño does not distinguish the strong from the moderate El Niño events, making it difficult to project future change of El Niño’s intensity. Here we classify 33 El Niño events from 1901 to 2017 by cluster analysis of the onset and amplification processes, and the resultant 4 types of El Niño distinguish the strong from the moderate events and the onset from successive events. The 3 categories of El Niño onset exhibit distinct development mechanisms. We find El Niño onset regime has changed from eastern Pacific origin to western Pacific origin with more frequent occurrence of extreme events since the 1970s. This regime change is hypothesized to arise from a background warming in the western Pacific and the associated increased zonal and vertical sea-surface temperature (SST) gradients in the equatorial central Pacific, which reveals a controlling factor that could lead to increased extreme El Niño events in the future. The Coupled Model Intercomparison Project phase 5 (CMIP5) models’ projections demonstrate that both the frequency and intensity of the strong El Niño events will increase significantly if the projected central Pacific zonal SST gradients become enhanced. If the currently observed background changes continue under future anthropogenic forcing, more frequent strong El Niño events are anticipated. The models’ uncertainty in the projected equatorial zonal SST gradients, however, remains a major roadblock for faithful prediction of El Niño’s future changes. 

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5. Diverse NPMM conditions deviate the 2023/24 El Niño from the 1997/1998 and 2015/2016 extreme El Niño events

   https://doi.org/10.1038/s41612-025-01013-z  

   Lin, Yong-Fu; Chen, Mengyan; Liu, Lingling; Zheng, Fei; Ding, Ruiqiang; Wang, Xin; Wu, Chau-Ron; Lo, Min-Hui; Hsu, Huang-Hsiung; Chen, Jiepeng; et al (December 2025, npj Climate and Atmospheric Science)

   Abstract The 2023/24 El Niño commenced with an exceptionally large warm water volume in the equatorial western Pacific, comparable to the extreme 1997/98 and 2015/16 events, but did not develop into a super El Niño. This study highlights the critical role of contrasting Northern Pacific Meridional Mode (NPMM) conditions in this divergence. Warm NPMM conditions during the 1997/98 and 2015/16 events created a positive zonal sea surface temperature (SST) gradient in the equatorial western-central Pacific and enhanced Madden-Julian Oscillation (MJO) propagation, driving sustained westerly wind bursts (WWBs) and downwelling Kelvin waves that intensified both events. In contrast, the cold NPMM during 2023/24 induced a negative SST gradient and suppressed MJO activity, resulting in weaker WWBs and limited eastward wave activity, preventing the event from reaching super El Niño intensity. A 2,200-year CESM1 pre-industrial simulation corroborates these observational findings, underscoring the importance of NPMM interference in improving El Niño intensity predictions. 

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