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This paper documents changes in the relationship between warm water volume (WWV), which is an index for upper ocean heat content, and El Niño/Southern Oscillation (ENSO) SST anomalies during the period 1980–2010. Upper ocean heat content represents a major source of predictability for ENSO, with WWV integrated along the equator leading ENSO SST anomalies by 2–3 seasons during the 1980s and 1990s. For the first decade of the 21st century however, WWV variations decreased and lead time was reduced to only one season, mainly due to the diminished persistence of WWV anomalies early in the calendar year. These changes are linked to a shift towards more central Pacific (CP) versus eastern Pacific (EP) El Niños in the past decade. The results are consistent with a reduced impact of thermocline feedbacks on ENSO SST development and potentially imply reduced seasonal time scale predictability during periods dominated by CP El Niños. Key Points The relationship between ENSO SST and upper ocean heat content is changing These changes are related to the increase in central Pacific El Ninos There are potential implications for the predictability of ENSO

Satellite observations suggest that the intensity of El Niño events in the central equatorial Pacific (CP) has almost doubled in the past three decades, with the strongest warming occurring in 2009–10. This is related to the increasing intensity as well as occurrence frequency of the so‐called CP El Niño events since the 1990s. While sea surface temperature (SST) in the CP region during El Niño years has been increasing, those during neutral and La Niña years have not. Therefore, the well‐documented warming trend of the warm pool in the CP region is primarily a result of more intense El Niño events rather than a general rise of background SST.

The Madden–Julian Oscillation (MJO) is the most dominant mode of subseasonal variability in the tropics, characterized by an eastward-moving band of rain clouds. The MJO modulates the El Niño Southern Oscillation 1 , tropical cyclones 2 , 3 and the monsoons 4 – 10 , and contributes to severe weather events over Asia, Australia, Africa, Europe and the Americas. MJO events travel a distance of 12,000–20,000 km across the tropical oceans, covering a region that has been warming during the twentieth and early twenty-first centuries in response to increased anthropogenic emissions of greenhouse gases 11 , and is projected to warm further. However, the impact of this warming on the MJO life cycle is largely unknown. Here we show that rapid warming over the tropical oceans during 1981–2018 has warped the MJO life cycle, with its residence time decreasing over the Indian Ocean by 3–4 days, and increasing over the Indo-Pacific Maritime Continent by 5–6 days. We find that these changes in the MJO life cycle are associated with a twofold expansion of the Indo-Pacific warm pool, the largest expanse of the warmest ocean temperatures on Earth. The warm pool has been expanding on average by 2.3 × 10 5  km 2 (the size of Washington State) per year during 1900–2018 and at an accelerated average rate of 4 × 10 5  km 2 (the size of California) per year during 1981–2018. The changes in the Indo-Pacific warm pool and the MJO are related to increased rainfall over southeast Asia, northern Australia, Southwest Africa and the Amazon, and drying over the west coast of the United States and Ecuador. Since the 1980s, rapid warming of the Indo-Pacific warm pool has altered global rainfall pattern by changing the residence time of the Madden–Julian Oscillation, decreasing it by 3–4 days over the Indian Ocean and increasing it over the Indo-Pacific by 5–6 days.

The El Niño–Southern Oscillation (ENSO) is the dominant and most consequential climate variation on Earth, and is characterized by warming of equatorial Pacific sea surface temperatures (SSTs) during the El Niño phase and cooling during the La Niña phase. ENSO events tend to have a centre—corresponding to the location of the maximum SST anomaly—in either the central equatorial Pacific (5° S–5° N, 160° E–150° W) or the eastern equatorial Pacific (5° S–5° N, 150°–90° W); these two distinct types of ENSO event are referred to as the CP-ENSO and EP-ENSO regimes, respectively. How the ENSO may change under future greenhouse warming is unknown, owing to a lack of inter-model agreement over the response of SSTs in the eastern equatorial Pacific to such warming. Here we find a robust increase in future EP-ENSO SST variability among CMIP5 climate models that simulate the two distinct ENSO regimes. We show that the EP-ENSO SST anomaly pattern and its centre differ greatly from one model to another, and therefore cannot be well represented by a single SST ‘index’ at the observed centre. However, although the locations of the anomaly centres differ in each model, we find a robust increase in SST variability at each anomaly centre across the majority of models considered. This increase in variability is largely due to greenhouse-warming-induced intensification of upper-ocean stratification in the equatorial Pacific, which enhances ocean–atmosphere coupling. An increase in SST variance implies an increase in the number of ‘strong’ EP-El Niño events (corresponding to large SST anomalies) and associated extreme weather events. Despite inter-model differences in predicting the details of the eastern Pacific El Niño, a robust increase in the corresponding sea surface temperature variability under greenhouse warming is found across models.

Both the Indian and Pacific Oceans exhibit prominent decadal time scale variations in sea surface temperature (SST), linked dynamically via atmospheric and oceanic processes. However, the relationship between SST in these two basins underwent a dramatic transformation beginning around 1985. Prior to that, SST variations associated with the Indian Ocean basin mode (IOB) and the interdecadal Pacific oscillation (IPO) were positively correlated, whereas afterward they were much less clearly synchronized. Evidence is presented from both observations and coupled state-of-the-art climate models that enhanced external forcing, particularly from increased anthropogenic greenhouse gases, was the principal cause of this changed relationship. Using coupled climate model experiments, it is shown that without external forcing, the evolution of the IOB would be strongly forced by variations in the IPO. However, with strong external forcing, the dynamical linkage between the IOB and the IPO weakens so that the negative phase IPO after 2000 is unable to force a negative phase IOB-induced cooling of the Indian Ocean. This changed relationship in the IOB and IPO led to unique SST patterns in the Indo-Pacific region after 2000, which favored exceptionally strong easterly trade winds over the tropical Pacific Ocean and a pronounced global warming hiatus in the first decade of the twenty-first century.

Understanding the interaction between the tropical Pacific and Atlantic Oceans has challenged the climate community for decades. Typically, boreal summer Atlantic Niño events are followed by vigorous Pacific events of opposite sign around two seasons later. However, incorporating the equatorial Atlantic information to variabilities internal to the Pacific lends no significant additional predictive skill for the subsequent El Niño‐Southern Oscillation (ENSO). Here we resolve this conundrum in a physically consistent frame, in which the nascent onset of a Pacific event rapidly induces an opposite‐signed summer equatorial Atlantic event and the lead correlation of Atlantic over Pacific is a statistical artifact of ENSO's autocorrelation. This Pacific‐to‐Atlantic impact is limited to a short window around late spring due to seasonally‐amplified Atlantic atmosphere‐ocean coupling. This new frame reconciles the discrepancies between the observed and multi‐model simulated inter‐basin relationship, providing a major advance in understanding seasonally‐modulated inter‐basin climate connections as well as their predictability. Plain Language Summary Previous studies interpreted the observed lead/lag relationship between Atlantic Niño/Niña and Pacific Niño/Niña sea surface temperature anomalies as evidence for a precursory role of the equatorial Atlantic on the development of El Niño–Southern Oscillation (ENSO) events. This study clearly demonstrates that this statistical relationship is not related to Atlantic‐to‐Pacific causality, but can rather be explained by seasonally modulated equatorial Atlantic's response to ENSO. We find that Pacific ENSO events drive equatorial Atlantic events rather than vice versa, and reconcile the apparent discrepancies between the observed and multi‐model simulated tropical Pacific/Atlantic relationship. Key Points The lead correlation of the equatorial Atlantic over the Pacific is not related to Atlantic‐to‐Pacific causality The tropical Pacific/Atlantic interaction is consistent with the nascent onset of Pacific events driving the equatorial Atlantic events The discrepancies between the observed and multi‐model simulated tropical Pacific/Atlantic relationship can be reconciled in this new frame

El Niño and La Niña, collectively referred to as the El Niño–Southern Oscillation (ENSO), are not only highly consequential 1 – 6 but also strongly nonlinear 7 – 14 . For example, the maximum warm anomalies of El Niño, which occur in the equatorial eastern Pacific Ocean, are larger than the maximum cold anomalies of La Niña, which are centred in the equatorial central Pacific Ocean 7 – 9 . The associated atmospheric nonlinear thermal damping cools the equatorial Pacific during El Niño but warms it during La Niña 15 , 16 . Under greenhouse warming, climate models project an increase in the frequency of strong El Niño and La Niña events, but the change differs vastly across models 17 , which is partially attributed to internal variability 18 – 23 . Here we show that like a butterfly effect 24 , an infinitesimal random perturbation to identical initial conditions induces vastly different initial ENSO variability, which systematically affects its response to greenhouse warming a century later. In experiments with higher initial variability, a greater cumulative oceanic heat loss from ENSO thermal damping reduces stratification of the upper equatorial Pacific Ocean, leading to a smaller increase in ENSO variability under subsequent greenhouse warming. This self-modulating mechanism operates in two large ensembles generated using two different models, each commencing from identical initial conditions but with a butterfly perturbation 24 , 25 ; it also operates in a large ensemble generated with another model commencing from different initial conditions 25 , 26 and across climate models participating in the Coupled Model Intercomparison Project 27 , 28 . Thus, if the greenhouse-warming-induced increase in ENSO variability 29 is initially suppressed by internal variability, future ENSO variability is likely to be enhanced, and vice versa. This self-modulation linking ENSO variability across time presents a different perspective for understanding the dynamics of ENSO variability on multiple timescales in a changing climate. Modelling experiments show that the El Niño response to global warming is self-modulating and depends on its historical variability; if current variability is high, future variability will be low.

A major open question about El Niño–Southern Oscillation (ENSO) is what causes ENSO amplitude asymmetry, with strong El Niños generally larger than strong La Niñas. The authors examine a leading hypothesis— that the ENSO state modifies the fetch and/or wind speed of westerly wind bursts (WWBs) that create asymmetric forcing and an asymmetric ENSO response. Further, in El Niño forecasts, the number of WWBs expected increases in the month following a strong WWB when compared with the month preceding it. Using a conceptual model, a relationship is derived between the magnitude of the westerly wind burst state dependence on ENSO and ENSO asymmetry. It is found that this relationship between the magnitude of the state dependence and ENSO asymmetry holds in both the observations and 21 coupled climate models. Finally, it is found that because of state-dependent westerly wind burst forcing, extreme El Niño events tend to be of the eastern Pacific variety.

CMIP5 simulations reveal that the frequency of extreme El Niño events doubles under the 1.5 °C Paris target, and continues to increase long after global temperatures stabilize due to emission reductions. Extreme La Niña events, however, see little change at either 1.5 °C or 2 °C warming. The Paris Agreement aims to constrain global mean temperature (GMT) increases to 2 °C above pre-industrial levels, with an aspirational target of 1.5 °C. However, the pathway to these targets 1 , 2 , 3 , 4 , 5 , 6 and the impacts of a 1.5 °C and 2 °C warming on extreme El Niño and La Niña events—which severely influence weather patterns, agriculture, ecosystems, public health and economies 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 —is little known. Here, by analysing climate models participating in the Climate Model Intercomparison Project’s Phase 5 (CMIP5; ref.  17 ) under a most likely emission scenario 1 , 2 , we demonstrate that extreme El Niño frequency increases linearly with the GMT towards a doubling at 1.5 °C warming. This increasing frequency of extreme El Niño events continues for up to a century after GMT has stabilized, underpinned by an oceanic thermocline deepening that sustains faster warming in the eastern equatorial Pacific than the off-equatorial region. Ultimately, this implies a higher risk of extreme El Niño to future generations after GMT rise has halted. On the other hand, whereas previous research suggests extreme La Niña events may double in frequency under the 4.5 °C warming scenario 8 , the results presented here indicate little to no change under 1.5 °C or 2 °C warming.

El Niño-Southern Oscillation (ENSO) features strong warm events in the eastern equatorial Pacific (EP), or mild warm and strong cold events in the central Pacific (CP), with distinct impacts on global climates. Under transient greenhouse warming, models project increased sea surface temperature (SST) variability of both ENSO regimes, but the timing of emergence out of internal variability remains unknown for either regime. Here we find increased EP-ENSO SST variability emerging by around 2030 ± 6, more than a decade earlier than that of CP-ENSO, and approximately four decades earlier than that previously suggested without separating the two regimes. The earlier EP-ENSO emergence results from a stronger increase in EP-ENSO rainfall response, which boosts the signal of increased SST variability, and is enhanced by ENSO non-linear atmospheric feedback. Thus, increased ENSO SST variability under greenhouse warming is likely to emerge first in the eastern than central Pacific, and decades earlier than previously anticipated. Under global warming, increased variability in El Niño sea surface temperature was projected to be detectable by about 2070. Here the authors show that the increased variability of a type of more impactful El Niño events is likely detectable by 2030.