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El Niño-Southern Oscillation (ENSO), characterized by anomalous sea surface temperature in the central-eastern equatorial Pacific, is a dominant interannual variability, impacting worldwide weather and socioeconomics. The ENSO cycle contains irregularity, in which La Niña often persists for more than two years, called “multi-year La Niña”. Observational records show that multi-year La Niña tends to accompany strong El Niño in the preceding year, but their physical linkage remains unclear. Here we show using reanalysis data that a strong El Niño excites atmospheric conditions that favor the generation of multi-year La Niña in subsequent years. Easterly wind anomalies along the northern off-equatorial Pacific during the decay phase of the strong El Niño are found crucial as they act to discharge ocean heat content (OHC) via an anomalous northward Ekman transport. The negative OHC anomaly is large enough to be restored by a single La Niña and, therefore, causes another La Niña to occur in the second year. Furthermore, analyses of the Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models support the abovementioned mechanisms and indicate that the occurrence frequencies of multi-year La Niña and strong El Niño are highly correlated.

Future projections of global mean precipitation change (Δ P ) based on Earth-system models have larger uncertainties than projections of global mean temperature changes (Δ T ) 1 . Although many observational constraints on Δ T have been proposed, constraints on Δ P have not been well studied 2 – 5 and are often complicated by the large influence of aerosols on precipitation 4 . Here we show that the upper bound (95th percentile) of Δ P (2051–2100 minus 1851–1900, percentage of the 1980–2014 mean) is lowered from 6.2 per cent to 5.2–5.7 per cent (minimum–maximum range of sensitivity analyses) under a medium greenhouse gas concentration scenario. Our results come from the Coupled Model Intercomparison Project phase 5 and phase 6 ensembles 6 – 8 , in which Δ P for 2051–2100 is well correlated with the global mean temperature trends during recent decades after 1980 when global anthropogenic aerosol emissions were nearly constant. Δ P is also significantly correlated with the recent past trends in precipitation when we exclude the tropical land areas with few rain-gauge observations. On the basis of these significant correlations and observed trends, the variance of Δ P is reduced by 8–30 per cent. The observationally constrained ranges of Δ P should provide further reliable information for impact assessments. Model simulations show that the historical relationship between global temperature and precipitation under a medium greenhouse gas concentration scenario lowers the projected high end of future precipitation change.

El Ni˜no–Southern Oscillation (ENSO) events occasionally recur one after the other in the same polarity, called multiyear ENSO. However, the dynamical processes are not well understood. This study aims to elucidate the unified mechanisms of multiyear ENSO using observations, phase 6 of the Coupled Model Intercomparison Project (CMIP6) models, and the theoretical linear recharge oscillator (RO) model. We found that multiyear El Ni˜no and La Ni˜na events are roughly symmetric except for cases of multiyear La Ni˜na following strong El Niño. The composite multiyear ENSO reveals that anomalous ocean heat content (OHC) in the equatorial Pacific persists beyond the first peak, stimulating another event. This prolonged OHC anomaly is caused by meridional Ekman heat transport counteracting geostrophic transport-induced recharge–discharge process that otherwise acts to change the OHC anomaly. A meridionally wide pattern of sea surface temperature anomalies observed during multiyear ENSO is responsible for the Ekman heat transport and multiple factors such as decadal variability, subtropical processes, and ENSO diversity modulate the ENSO meridional structure. CMIP6 multimodel ensemble shows a significant correlation between the ENSO meridional width and the occurrence ratio of multiyear ENSO, supporting the aforementioned mechanism. A multiyear ENSO-like oscillation was simulated using the linear RO model that incorporates a seasonally varying Bjerknes growth rate and a weak recharge efficiency representing the effect of Ekman transport. When the recharge efficiency parameter was estimated using reanalysis data based on geostrophic transport alone, a multiyear ENSO rarely occurred, confirming the importance of Ekman transport in retarding the recharge–discharge process.

How the tropical Pacific mean‐state sea surface temperature (SST) responds to global warming is an ongoing heated debate, as climate models are flawed and their projections are opposite to the observed zonal gradient strengthening. Here, we build a dynamical tropical Pacific air–sea coupled model that incorporates three key mechanisms of the forced response, the differential evaporative damping, the Pacific Walker circulation (PWC) weakening, and the ocean thermostat, to better understand how the long‐term zonal SST gradient responds to global warming. By choosing suitable parameters, this model can reproduce the long‐term zonal SST gradient weakening seen in climate models, demonstrating its ability to serve as an emulator. We find the sign and magnitude of the zonal SST gradient change depend strongly on the weakening rate of the PWC and the magnitude of the eastern Pacific subsurface warming, providing physical insights into understanding the diversity of the zonal SST gradient change across climate models.

Severe winters have occurred frequently in mid-latitude Eurasia during the past decade. Simulations with a 100-member ensemble of an atmospheric model detect an influence of declining Arctic sea-ice cover. Over the past decade, severe winters occurred frequently in mid-latitude Eurasia 1 , 2 , despite increasing global- and annual-mean surface air temperatures 3 . Observations suggest that these cold Eurasian winters could have been instigated by Arctic sea-ice decline 2 , 4 , through excitation of circulation anomalies similar to the Arctic Oscillation 5 . In climate simulations, however, a robust atmospheric response to sea-ice decline has not been found, perhaps owing to energetic internal fluctuations in the atmospheric circulation 6 . Here we use a 100-member ensemble of simulations with an atmospheric general circulation model driven by observation-based sea-ice concentration anomalies to show that as a result of sea-ice reduction in the Barents–Kara Sea, the probability of severe winters has more than doubled in central Eurasia. In our simulations, the atmospheric response to sea-ice decline is approximately independent of the Arctic Oscillation. Both reanalysis data and our simulations suggest that sea-ice decline leads to more frequent Eurasian blocking situations, which in turn favour cold-air advection to Eurasia and hence severe winters. Based on a further analysis of simulations from 22 climate models we conclude that the sea-ice-driven cold winters are unlikely to dominate in a warming future climate, although uncertainty remains, due in part to an insufficient ensemble size.

The Pacific trade winds have been strengthening over the past two decades, but until now the cause of this has not been known. Now research shows that sulfate aerosols caused the western North Pacific Ocean to warm, leading to the trade-wind intensification. The Pacific trade winds, coupled with the zonal sea surface temperature gradient in the equatorial Pacific Ocean, control regional sea levels 1 , and therefore their trend is a great concern in the Pacific Rim. Over the past two decades, easterly winds have been accelerated in association with eastern tropical Pacific cooling 2 . They may represent natural interdecadal variability in the Pacific 3 and possibly explain the recent global warming hiatus 4 , 5 , 6 , 7 . However, the intensification of the winds has been the strongest ever observed in the past century 2 , 5 , 8 , the reason for which is still unclear. Here we show, using multiple climate simulations for 1921–2014 by a global climate model, that approximately one-third of the trade-wind intensification for 1991–2010 can be attributed to changes in sulfate aerosols. The multidecadal sea surface temperature anomaly induced mostly by volcanic aerosols dominates in the western North Pacific, and its sign changed rapidly from negative to positive in the 1990s, coherently with Atlantic multidecadal variability 9 , 10 , 11 . The western North Pacific warming resulted in intensification of trade winds to the west of the dateline. These trends have not contributed much to the global warming hiatus, but have greatly impacted rainfall over the western Pacific islands.

Northern midlatitudes, over central Eurasia in particular, have experienced frequent severe winters in recent decades1–3. A remote influence of Arctic sea-ice loss has been suggested4–14; however, the importance of this connection remains controversial because of discrepancies among modelling and between modelling and observational studies15–17. Here, using a hybrid analysis of observations and multi-model large ensembles from seven atmospheric general circulation models, we examine the cause of these differences. While all models capture the observed structure of the forced surface temperature response to sea-ice loss in the Barents–Kara Seas—including Eurasian cooling—we show that its magnitude is systematically underestimated. Owing to the varying degrees of this underestimation of sea-ice-forced signal, the signal-to-noise ratio differs markedly. Correcting this underestimation reconciles the discrepancy between models and observations, leading to the conclusion that ~44% of the central Eurasian cooling trend for 1995–2014 is attributable to sea-ice loss in the Barents–Kara Seas. Our results strongly suggest that anthropogenic forcing has significantly amplified the probability of severe winter occurrence in central Eurasia via enhanced melting of the Barents–Kara sea ice. The difference in underestimation of signal-to-noise ratio between models therefore calls for careful experimental design and interpretation for regional climate change attribution.The connections between Arctic sea-ice loss and severe Eurasian winters are complicated by differences among studies. Correcting model underestimates reveals that 44% of the central Eurasian cooling trend is attributable to sea-ice loss in the Barents–Kara Seas.

Changes in the sea surface temperature (SST) pattern in the tropical Pacific modulate radiative feedbacks to greenhouse gas forcing, the pace of global warming and regional climate impacts. Therefore, elucidating the drivers of the pattern is critically important for reducing uncertainties in future projections. However, the causes of observed changes over recent decades, an enhancement of the zonal SST contrast coupled with a strengthening of the Walker circulation, are still debated. Here we focus on the role of external forcing and review existing mechanisms of the forced response categorized as either an energy perspective that adopts global and hemispheric energy budget constraints or a dynamical perspective that examines the atmosphere–ocean coupled processes. We then discuss their collective and relative contributions to the past and future SST pattern changes and propose a narrative that reconciles them. Although definitive evidence is not yet available, our assessment suggests that the zonal SST contrast has been dominated by strengthening mechanisms in the past, but will shift towards being dominated by weakening mechanisms in the future. Finally, we present opportunities to resolve the model–observations discrepancy regarding the recent trends. Focusing on the role of external forcing, an investigation of the causes of observed changes in the tropical Pacific surface warming pattern over recent decades discusses a possible shift in the drivers of this pattern.

Coupled dynamics between westerly wind events (WWEs) and the El Niño–Southern Oscillation (ENSO) is examined using an atmosphere–ocean coupled model with intermediate complexity. The model incorporates state-dependent stochastic noise that mimics observed WWEs, which occur at the edge of the Pacific warm pool when the Niño-4 sea surface temperature (SST) anomaly increases positively. The model parameter that controls the efficiency of the thermocline feedback, γ, is perturbed to elaborate the sensitivity of the results to the system’s stability. Without the noise (experiment NO), the model produces an ENSO-like regular oscillation with a 6-yr period, the variance of which increases with γ. When additive noise is introduced over the western Pacific (experiment AD), the oscillations become irregular with a dominant period of 4–6 years and the increase in the variance relative to the NO experiment depends on γ. When state-dependent noise is included (experiment SD), the oscillatory solution is also irregular, and its variance and asymmetry are increased irrespective of the value of γ. Both the additive and state-dependent noise contribute to the occurrence of two types of variability, corresponding to the eastern Pacific (EP) and central Pacific (CP) El Niños. In SD, the state dependence of the stochastic noise guarantees the existence of CP El Niño regardless of γ since the increased likelihood of WWE occurrence with Niño-4 SSTs results in a positive feedback in the central Pacific. The above results suggest that the state dependence of WWEs plays a crucial role in the asymmetry and diversity of ENSO.

The Pacific Walker circulation—the tropical Pacific zonal overturning circulation of the atmosphere—and the associated sea surface temperature distribution in the tropical Pacific significantly impact global climate. However, climate model historical simulations cannot capture the observed Walker circulation enhancement since around 1980. Although a number of mechanisms have been proposed to explain the observed change, quantitative discussion and clues for reconciling the model-observation discrepancy have not yet been settled. Here we show that the Walker circulation strengthening between 1980 and 2020 can be quantitatively explained by the remote influence of subtropical and extratropical sea surface temperature changes. This conclusion is obtained from climate model pacemaker experiments in which sea surface temperature anomalies outside the tropics are restored towards observations. Influence from the southeastern Pacific, which cools the eastern tropical Pacific, is especially crucial for the Walker circulation strengthening. This equatorward influence occurs mostly through the atmosphere and its thermal coupling with the ocean. We further show that current generation climate models have biases in southeastern Pacific surface temperature changes, which may cause the failure in reproducing the Walker circulation trend. Our results suggest that improved representation of air–sea coupling in this region could enable better projections of future climate. Subtropical and extratropical sea surface temperature changes can explain recent observed Walker circulation strengthening, according to climate model experiments.