El Niño events are characterized by surface warming of the tropical Pacific Ocean and weakening of equatorial trade winds that occur every few years. Such conditions are accompanied by changes in atmospheric and oceanic circulation, affecting global climate, marine and terrestrial ecosystems, fisheries and human activities. The alternation of warm El Niño and cold La Niña conditions, referred to as the El Niño-Southern Oscillation (ENSO), represents the strongest year-to-year fluctuation of the global climate system. Here we provide a synopsis of our current understanding of the spatio-temporal complexity of this important climate mode and its influence on the Earth system.

Given the importance of the recent wind-induced trends in Pacific sea level and surface temperature, it is vital to determine the underlying causes.

The El Niño–Southern Oscillation (ENSO), which originates in the Pacific, is the strongest and most well-known mode of tropical climate variability. Its reach is global, and it can force climate variations of the tropical Atlantic and Indian Oceans by perturbing the global atmospheric circulation. Less appreciated is how the tropical Atlantic and Indian Oceans affect the Pacific. Especially noteworthy is the multidecadal Atlantic warming that began in the late 1990s, because recent research suggests that it has influenced Indo-Pacific climate, the character of the ENSO cycle, and the hiatus in global surface warming. Discovery of these pantropical interactions provides a pathway forward for improving predictions of climate variability in the current climate and for refining projections of future climate under different anthropogenic forcing scenarios.

Atmospheric circulation anomalies associated with the interannual El Niño-Southern Oscillation (ENSO) phenomenon 1 exert global impacts on the climate system 2. El Niño events are characterised by positive sea surface temperature anomalies in the eastern equatorial Pacific, whereas La Niña events exhibit an anomalously cold sea surface. ENSO is considered an oscillatory instability of the tropical Pacific coupled ocean-atmosphere system 1, 3-6. The boreal winter peak of El Niño events and the seasonal variance modulation of associated eastern equatorial sea surface temperature anomalies 2 , often referred to as phase-locking 7 , document ENSO's tight interaction with the seasonal cycle. To date there exists no established theory for ENSO's synchronisation with the annual cycle. In the present study, we show that seasonal changes in the western tropical Pacific warm pool region interact with El Niño, giving rise to a near-annual combination climate mode with periods of 10 and 15 months. Associated wind changes trigger the termination of large El Niño events 8 , thereby controlling ENSO's seasonal synchronisation and predictability. This combination mode is shown to cause massive shifts of Earth's largest rainbands, impacting human livelihoods across the Asia-Pacific region and beyond. Current ENSO theories, such as the Recharge Oscillator Paradigm 9 , while very successful in explaining some observational features of ENSO, do not account for the interaction between interannual and seasonal timescales. In particular, they do not provide any insight into why El Niño events peak toward the end of the calendar year (in December-January-February: DJF) and terminate in the subsequent months. Previous extensions to these theories that rely on nonlinear concepts 10-13 , such as subharmonic frequency locking and frequency entrainment, nonlinear resonance, the quasi-periodic transition to chaos or parametric excitation 7, 14, 15 , capture some aspects of ENSO / annual cycle interactions. None of these extended dynamical systems' concepts, however, describe the observational finding 16 that a weakening and southward shift of westerly wind anomalies on the equator accompanies the termination of strong El Niño events. Numerous modelling studies 8, 17-20 have confirmed this observational evidence, thus supporting the notion of strong annual cycle / ENSO interactions originating in the tropical western Pacific. Here, we set out to provide a simple unifying dynamical framework to understand various aspects of ENSO, such as the physics of seasonallypaced El Niño transitions, spectral characteristics and ENSO's hydroclimatic impacts. The seasonal weakening and southward shift of westerly wind anomalies that contributes to the transition between El Niño and La Niña can be readily described in terms of an Empirical

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Nonlinear interactions between ENSO and the western Pacific warm pool annual cycle generate an atmospheric combination mode (C-mode) of wind variability. The authors demonstrate that C-mode dynamics are responsible for the development of an anomalous low-level northwest Pacific anticyclone (NWP-AC) during El Niño events. The NWP-AC is embedded in a large-scale meridionally antisymmetric Indo-Pacific atmospheric circulation response and has been shown to exhibit large impacts on precipitation in Asia. In contrast to previous studies, the authors find the role of air-sea coupling in the Indian Ocean and northwestern Pacific only of secondary importance for the NWP-AC genesis. Moreover, the NWP-AC is clearly marked in the frequency domain with near-annual combination tones, which have been overlooked in previous Indo-Pacific climate studies. Furthermore, the authors hypothesize a positive feedback loop involving the anomalous low-level NWP-AC through El Niño and C-mode interactions: the development of the NWP-AC as a result of the C-mode acts to rapidly terminate El Niño events. The subsequent phase shift from retreating El Niño conditions toward a developing La Niña phase terminates the low-level cyclonic circulation response in the central Pacific and thus indirectly enhances the NWP-AC and allows it to persist until boreal summer. Anomalous local circulation features in the Indo-Pacific (e.g., the NWP-AC) can be considered a superposition of the quasi-symmetric linear ENSO response and the meridionally antisymmetric annual cycle modulated ENSO response (C-mode). The authors emphasize that it is not adequate to assess ENSO impacts by considering only interannual time scales. C-mode dynamics are an essential (extended) part of ENSO and result in a wide range of deterministic high-frequency variability.

Originating in the equatorial Pacific, the El Niño-Southern Oscillation (ENSO) has highly consequential global impacts, motivating the need to understand its responses to anthropogenic warming. In this Review, we synthesize advances in observed and projected changes of multiple aspects of ENSO, including the processes behind such changes. As in previous syntheses, there is an inter-model consensus of an increase in future ENSO rainfall variability. Now, however, it is apparent that models that best capture key ENSO dynamics also tend to project an increase in future ENSO sea surface temperature variability and, thereby, ENSO magnitude under greenhouse warming, as well as an eastward shift and intensification of ENSO-related atmospheric teleconnections -the Pacific-North American and Pacific-South American patterns. Such projected changes are consistent with palaeoclimate evidence of stronger ENSO variability since the 1950s compared with past centuries. The increase in ENSO variability, though underpinned by increased equatorial Pacific upper-ocean stratification, is strongly influenced by internal variability, raising issues about its quantifiability and detectability. Yet, ongoing coordinated community efforts and computational advances are enabling long-simulation, large-ensemble experiments and high-resolution modelling, offering encouraging prospects for alleviating model biases, incorporating fundamental dynamical processes and reducing uncertainties in projections.

Here we show that the characteristics of the Indian Ocean Dipole (IOD), such as its power spectrum and phase relationship with the El Niño–Southern Oscillation (ENSO), can be succinctly explained by ENSO combination mode (C‐mode) wind and heat flux forcing together with a seasonal modulation of the air/sea coupled Indian Ocean (IO) Bjerknes feedback. This model explains the observed high‐frequency near‐annual IOD variability in terms of deterministic ENSO/annual cycle interactions. ENSO‐independent IOD events can be understood as a seasonally modulated ocean response to white noise atmospheric forcing. Under this new physical null hypothesis framework, IOD predictability is determined by both ENSO predictability and the ENSO signal‐to‐noise ratio. We further emphasize that lead/lag correlations between different climate variables are easily misinterpreted when not accounting properly for the seasonal modulation of the underlying climate phenomena.