The emergence of an observed distinctive, meridionally confined surface cooling trend in the tropical Pacific cold tongue over recent decades contrasts sharply with the rapid warming simulated by most climate models, representing a key unresolved feature of climate change. Ocean-only simulations, even when forced with observed atmospheric conditions, likewise fail to reproduce the observed long-term absence of surface-layer warming over the eastern Pacific cold tongue. Here, we examine the long-term heat budget of the surface layer in the equatorial Pacific using atmospheric and oceanic reanalysis data, quantifying contributions from surface heat fluxes, ocean advection, and vertical diffusion inferred through Richardson number-based diffusivity estimates. In the Ocean Reanalysis System 5 (ORAS5), the observed cold tongue cooling cannot be reproduced by the model's intrinsic dynamics alone; instead, it depends on a surface heat flux adjustment imposed during data assimilation. A reduction in the warming effect associated with this adjustment in the eastern Pacific cold tongue over time emerges as the dominant contributor to the long-term cooling in the reanalysis data, a deus ex machina obscuring the actual physical drivers of change in the real ocean. We discuss potential origins of this deus ex machina cooling effect, including the influence of analysis increments, uncertainties in surface forcing, and possible problems in the representation of oceanic processes, particularly subsurface turbulent heat flux induced by oceanic mixing processes. This work emphasizes that reanalysis-based assessments remain subject to inherent biases from the ocean models on which they are built. Resolving the discrepancy between observed and simulated Pacific trends will require confronting the structural limitations of models.

Regional modes of climate variability, modes with consequential impacts on temperature and rainfall over land, have historically been understood as natural, internal fluctuations in the ocean-atmosphere system. There is, however, growing evidence of the influence of external radiative forcing on these modes. Here we use large ensembles of simulations of the historical period with over 50 climate models to estimate the magnitude of the radiatively forced response, here termed ‘signal,’ compared with internal fluctuations, termed ‘noise,’ in both models and observations. We examine three major modes of decadal variability collectively: Atlantic Multidecadal Variability (AMV), the North Atlantic Oscillation (NAO), and the Pacific Decadal Oscillation (PDO). We find strong agreement in recent decades between the observed time history of all these modes and their response to radiative forcing as simulated in climate models, suggesting that the signal of radiative forcing is an important part of their observed behavior. However, the amplitude of the radiatively forced signal is smaller in models than in observations for all three regional climate modes. This erroneously small, forced signal is overwhelmed by internally generated noise in any single run of a climate model. This is the signal-to-noise problem in climate models. This error leads to the mistaken impression that these decadal climate modes vary unpredictably, despite the radiatively forced trends in recent decades. Without this error in climate models, our understanding of the nature of regional decadal climate variability, both historically and into the future, would be very different.

The interhemispheric thermal contrast, defined as the mean sea surface temperature difference between the northern and southern hemispheres, crucially influences tropical climate. Climate models show a positive interhemispheric thermal contrast trend since 1950, with more warming in the northern hemisphere compared to the southern hemisphere, contradicting the observed negative trend. Here we show this discrepancy stems from models overestimating greenhouse gas responses via wind-evaporation-sea surface temperature feedback, while anthropogenic and natural aerosols combine to produce the negative trend in observations. Consequently, models with high equilibrium climate sensitivity exhibit larger discrepancies with observations. Despite model failure to reproduce the trend, the modeled multidecadal interhemispheric thermal contrast variability aligns with observations, enabling a constrained estimate of effective radiative forcing due to aerosol-cloud interactions of - 0.6 ± 0.3 W / m 2 , with a "likely" range 57% narrower than the latest IPCC report. Our study further suggests that future northward shifts of the tropical rain belt are likely to be less pronounced than predicted by climate models with high equilibrium climate sensitivity.

Abstract The lack of warming in the central-to-eastern equatorial Pacific under anthropogenic forcing is a well-documented feature of the observed sea surface temperature (SST) trend. Less recognized is its strong seasonal dependence, characterized by pronounced cooling during boreal winter and mild warming from spring to summer across the central-to-eastern equatorial Pacific, which amplifies the local seasonal cycle and delays the seasonal maximum by one month. In the far eastern Pacific, the SST warming trend likewise peaks in spring–summer and is less in winter–early spring, leading to a reduced amplitude of the seasonal cycle with little change in phase. Consequently, the zonal SST gradient across the equatorial Pacific strengthens most in winter and less in other seasons. This seasonal structure of the long-term trend is consistently identified across observational SST datasets. We further show that the variation in SST is largely associated with wind-driven seasonally varying upper-ocean dynamics, particularly the weakening of geostrophic zonal currents in all seasons except winter and strong thermocline shoaling during winter, while the influence of surface heat fluxes is not well constrained. The lack of long-term warming in the central-to-eastern equatorial Pacific is a net effect of these opposing seasonal dynamical processes rather than a uniform cooling trend.

Substantial interannual-to-decadal variations in the sources and pathways of the Equatorial Undercurrent (EUC) water have been found in observational and modeling studies, yet long-term trends remain underexplored due to limited record lengths and insufficient evaluation of climate model skill in simulating EUC water sources and pathways. As a necessary first step for model skill evaluation, here we investigate EUC water sources and pathways by implementing an offline particle tracer scheme applied to two high-resolution ocean reanalysis datasets, Global Ocean Reanalysis and Simulation (GLORYS) and Ocean Reanalysis System, version 5 (ORAS5). In agreement with observational studies and prior model results, we find that the primary off-equatorial source of the EUC is the southeastern subtropical Pacific (SESP) (15% of the total from the tropical Pacific). During 1979-2023, we identify an increasing trend in the contribution of this source to the western-central Pacific portion of the EUC, alongside a decreasing trend in its contribution to the eastern Pacific portion. Furthermore, we find a deepening trend in the water pathway from SESP into the EUC during 1979-2023, along with a westward shift in the pathway toward the eastern Pacific portion of the EUC during 2003-22. We attribute the trends in the SESP source to increased mixed layer depth and vertical downward pumping. The westward shift trend in the pathway is due to accelerated southwestward currents in SESP, driven by increased southwest trades over the region. The identified trends may contribute to the strengthening of the equatorial Pacific zonal sea surface temperature gradient by allowing for enhanced water mixing along the preferred pathway toward the EUC and cooling of the EUC core.

The Atlantic meridional overturning circulation (AMOC) plays a crucial role in past, present, and future climate, and there is substantial interest in using sea surface temperature (SST) as a fingerprint of past AMOC strength. Using a hierarchy of climate model ensembles, we find that the decline in AMOC, and its SST fingerprint within the North Atlantic warming hole region, are sensitive to external forcing level and wind driven ocean forcing. Once external forcing reaches a level at which sea ice melt increases the Labrador Sea vertical salinity gradient, localized cooling and resulting expansion of the sea ice edge decrease vertical mechanical stirring. Under greenhouse gas only forcing, this mechanism plays a large role and under SSP3.70 forcing, it plays a relatively minor role due to larger buoyancy forcing. This implies that an AMOC fingerprint developed from one simulation or external forcing level cannot be applied to other scenarios.

Extreme rainfall during the Indian summer monsoon can be destructive and deadly to the world's third-largest economy and most populous country. Although El Niño events in the equatorial Pacific are known to suppress total summer rainfall throughout India, we show using observational data spanning 1901 to 2020 that, counterintuitively, they simultaneously intensify extreme daily rainfall. This is partly driven by increases in extreme daily values of convective buoyancy, provided that both the undilute instability of near-surface air and the dilution by mixing with drier air above are considered. El Niño could plausibly drive similar changes in other tropical regions, and our framework could be further applied to changes in hourly extremes, to other internal variability modes, and to forced trends under climate change.

The equatorial cold tongue region has not warmed up in response to historical radiative forcing in the real world, contrary to the strong warming often simulated by climate models. Here we demonstrate that climate models fail to represent one or both of the key processes driving observed sea surface temperature (SST) pattern formation: a realistic surface wind stress pattern shaping subsurface cooling through wind-driven circulation changes, and effective connectivity between subsurface and surface temperatures via upwelling and mixing. Consequently, none of the models approximate the observed lack of cold tongue SST warming and strengthening of zonal SST gradient across the equatorial Pacific. Furthermore, those that come closest achieve this due to interhemispheric warming differences rather than equatorial dynamics as observed. Addressing different origins of subsurface cooling in observations and simulations, and how they connect to SST, will lead to improved understanding of tropical Pacific SST changes to date and how they will evolve in the future.

The Pacific decadal oscillation (PDO)-the leading pattern of climate variability driving changes over the North Pacific and surrounding continents-is now thought to be generated by processes internal to the climate system1,2. According to this paradigm, the characteristic, irregular oscillations of the PDO arise from a collection of mechanisms involving ocean and atmosphere interactions in the North and tropical Pacific3-5. Recent variations in the coupled ocean-atmosphere system, such as the 2015 El Niño, ought to have shifted the PDO into its positive phase6. Yet, the PDO has been locked in a consistent downward trend for more than three decades, remanding nearby regions to a steady set of climate impacts. Here we show that the main multidecadal variations in the PDO index during the twentieth century, including the ongoing, decades-long negative trend, were largely driven by human emissions of aerosols and greenhouse gases rather than internal processes. This anthropogenic influence was previously undetected because the current generation of climate models systematically underestimate the amplitude of forced climate variability. A new attribution technique that statistically corrects for this error suggests that observed PDO impacts-including the ongoing multidecadal drought in the western United States-can be largely attributed to human activity through externally forced changes in the PDO. These results indicate that we need to rethink the attribution and projection of multidecadal changes in regional climate.

AbstractWe present new climate field reconstructions (CFR) of tropical Pacific ENSO sea surface temperatures (HadISST) for the boreal winter season using a circum‐Pacific tree‐ring network from known El Niño rainfall impact regions. We use two different CFR methods: Point‐by‐Point Regression (PPR) and reduced‐space Orthogonal Spatial Regression (OSR). Both methods produce reconstructions with high validation skill, but OSR is preferred because it has less spatial noise and is more efficient. Only the leading EOF of the SST field (EOF1) can be skillfully reconstructed by either method; EOF2 does not validate. The success of EOF1 reflects its importance for ENSO rainfall impacts over land; the failure with EOF2 is from the lack of these impacts. EOF1 allows for the reconstruction of many ENSO indices, including the ENSO Longitudinal Index (ELI). We also find evidence in our reconstructions for a recent increase in ENSO activity.

The El Niño-Southern Oscillation (ENSO) provides most of the global seasonal climate forecast skill1-3, yet, quantifying the sources of skilful predictions is a long-standing challenge4-7. Different sources of predictability affect ENSO evolution, leading to distinct global effects. Artificial intelligence forecasts offer promising advancements but linking their skill to specific physical processes is not yet possible8-10, limiting our understanding of the dynamics underpinning the advancements. Here we show that an extended nonlinear recharge oscillator (XRO) model shows skilful ENSO forecasts at lead times up to 16-18 months, better than global climate models and comparable to the most skilful artificial intelligence forecasts. The XRO parsimoniously incorporates the core ENSO dynamics and ENSO's seasonally modulated interactions with other modes of variability in the global oceans. The intrinsic enhancement of ENSO's long-range forecast skill is traceable to the initial conditions of other climate modes by means of their memory and interactions with ENSO and is quantifiable in terms of these modes' contributions to ENSO amplitude. Reforecasts using the XRO trained on climate model output show that reduced biases in both model ENSO dynamics and in climate mode interactions can lead to more skilful ENSO forecasts. The XRO framework's holistic treatment of ENSO's global multi-timescale interactions highlights promising targets for improving ENSO simulations and forecasts.

The eastern tropical Pacific has defied the global warming trend. There has been a debate about whether this observed trend is forced or natural (i.e., the Interdecadal Pacific Oscillation; IPO) and this study shows that there are two patterns, one that oscillates along with the IPO, and one that is emerging since the mid-1950s, herein called the Pacific Climate Change (PCC) pattern. Here we show these have distinctive and distinguishable atmosphere-ocean signatures. While the IPO features a meridionally broad wedge-shaped SST pattern, the PCC pattern is marked by a narrow equatorial cooling band. These different SST patterns are related to distinct wind-driven ocean dynamical processes. We further show that the recent trends during the satellite era are a combination of IPO and PCC. Our findings set a path to distinguish climate change signals from internal variability through the underlying dynamics of each.

Understanding how the tropical Pacific responds to rising greenhouse gases in recent decades is of paramount importance given its central role in global climate systems. Extensive research has explored the long-term trends of tropical Pacific sea surface temperatures (SST) and the overlying atmosphere, yet the historical change of the upper ocean has received far less attention. Here we present compelling evidence of a prominent subsurface cooling pattern along the thermocline in the central-to-eastern tropical Pacific since 1958. This subsurface cooling has been argued to be contributing to the observed cooling or lack of warming of the equatorial cold tongue SST. We further demonstrate that different mechanisms are responsible for different parts of the subsurface cooling. In the central-to-eastern equatorial Pacific and the southeastern off-equatorial Pacific, where zonal wind stress strengthens, a pronounced subsurface cooling trend emerges just above the thermocline that is closely tied to increased Ekman pumping. In the eastern equatorial Pacific where zonal wind stress weakens, the westward surface current and eastward equatorial undercurrent weaken as well, resulting in reduced vertical current shear and increased ocean stability, which suppresses vertical mixing and leads to local cooling. We conclude that the historical subsurface cooling is primarily linked to dynamical adjustments of ocean currents to tropical surface wind stress changes.

Extreme rainfall in the Indian summer monsoon can be destructive and deadly. Although El Niño/ events in the equatorial Pacific make dry days and whole summers more likely throughout India, their influence on daily extremes is not well established. Despite this summer-mean drying effect, we show using observational data spanning 1901-2020 that El Niño increases extreme rainfall likelihoods within monsoonal India, especially in the the summer's core rainy areas of central-eastern India and the narrow southwestern coastal band. Conversely, extremes are broadly suppressed in the drier southeast and far northwest, and more moderate accumulations are inhibited throughout the domain. These rainfall signals appear driven by corresponding ones in convective buoyancy, provided both the undilute instability of near-surface air and its dilution by mixing with drier air above are accounted for. When the summer ENSO state is predicted from a seasonal forecast ensemble initialized in May, the extreme rainfall patterns broadly persist, suggesting the potential for skillful seasonal forecasts. The framework of analyzing the full distributions of rainfall and convective buoyancy could be usefully applied to hourly extremes, other tropical regions under ENSO, other variability modes, and to trends in extreme rainfall under climate change.

Tropical rainfall variations are of direct societal relevance , drive climate variations worldwide via tele-connections. The convective rainfall tends to occur when sea surface temperature (SST) exceeds a threshold, SSTthr, usu-ally taken to be constant in time and space. We analyze 40-yr monthly observations and find that SSTthr varies by up to 4 & DEG;C in space and with season. Based on local convective instability, we develop a quantitative theory that largely explains the SSTthr variations using the climatological state of the tropical atmosphere. Although it is often assumed that spatial varia-tions of tropical upper-tropospheric temperature are small and can be neglected, it is shown that lower climatological val-ues favor a lower SSTthr. Similarly, a small increase in climatological surface relative humidity also leads to a decrease in SSTthr, as does a lower climatological air-sea temperature difference. Consequently, efforts to understand and predict natural or forced variations in tropical rainfall must account for, in addition to SST, the temperatures aloft and the near-surface humid-ity and temperature and requires improved understanding of what controls their distribution in space and time.

Five out of six La Niña events since 1998 have lasted two to three years. Why so many long-lasting multiyear La Niña events have emerged recently and whether they will become more common remains unknown. Here we show that ten multiyear La Niña events over the past century had an accelerated trend, with eight of these occurring after 1970. The two types of multiyear La Niña events over this time period followed either a super El Niño or a central Pacific El Niño. We find that multiyear La Niña events differ from single-year La Niñas by a prominent onset rate, which is rooted in the western Pacific warming-enhanced zonal advective feedback for the central Pacific multiyear La Niña events type and thermocline feedback for the super El Niño multiyear La Niña events type. The results from large ensemble climate simulations support the observed multiyear La Niña events–western Pacific warming link. More multiyear La Niña events will exacerbate adverse socioeconomic impacts if the western Pacific continues to warm relative to the central Pacific.

The tropical Atlantic climate is characterized by prominent and correlated multidecadal variability in Atlantic sea surface temperatures (SSTs), Sahel rainfall and hurricane activity1-4. Owing to uncertainties in both the models and the observations, the origin of the physical relationships among these systems has remained controversial3-7. Here we show that the cross-equatorial gradient in tropical Atlantic SSTs-largely driven by radiative perturbations associated with anthropogenic emissions and volcanic aerosols since 19503,7-is a key determinant of Atlantic hurricane formation and Sahel rainfall. The relationship is obscured in a large ensemble of CMIP6 Earth system models, because the models overestimate long-term trends for warming in the Northern Hemisphere relative to the Southern Hemisphere from around 1950 as well as associated changes in atmospheric circulation and rainfall. When the overestimated trends are removed, correlations between SSTs and Atlantic hurricane formation and Sahel rainfall emerge as a response to radiative forcing, especially since 1950 when anthropogenic aerosol forcing has been high. Our findings establish that the tropical Atlantic SST gradient is a stronger determinant of tropical impacts than SSTs across the entire North Atlantic, because the gradient is more physically connected to tropical impacts via local atmospheric circulations8. Our findings highlight that Atlantic hurricane activity and Sahel rainfall variations can be predicted from radiative forcing driven by anthropogenic emissions and volcanism, but firmer predictions are limited by the signal-to-noise paradox9-11 and uncertainty in future climate forcings.

Abstract The Atlantic multidecadal variability (AMV) – a basin-scale sea surface temperature (SST) fluctuation in the Atlantic – has dramatic influences on climate1–8. Understanding its causes has important social-ecological implications. However, the driver of AMV and its impacts remains debated because of limitations of current climate models9–13. Here we identify a systematic interhemispheric SST bias in the CMIP6 models that causes biased trends in large-scale atmospheric circulation and rainfall simulations. After removing the bias, we find the simulated AMV and associated impacts agree with observations. We show statistically and physically that AMV and its tropical impacts since 1950 have been driven primarily by anthropogenic aerosols and natural forcings, while internal dynamics likely drives them before 1950. We suggest using the tropical Atlantic SST contrast (dSST) as an AMV index because it naturally eliminates the global warming trend and can explain tropical impacts via a local Gill-type response. In the positive phase, this response features low-level westerly winds that enhance Sahel rainfall, high28 level easterly winds that affect vertical wind shear and hence North Atlantic Hurricanes, and a circum-global teleconnection that affects climate in other regions. Our results highlight that external forcing is responsible for the recent AMV, its widespread impacts, and the connection between the two.

Prediction of the seasonal monsoon rainfall over India relies largely on the well-known relationship with El Niño and Southern Oscillation (ENSO) and is possible because reasonably reliable seasonal predictions of ENSO are now available. Usually, the cold phase of ENSO is associated with above-normal monsoon rainfall and the warm phase of ENSO with below-normal rainfall. There are, however, exceptions: years in the cold phase of ENSO with below-normal monsoon rainfall and even drought conditions. We term these exceptional events ‘rogue La Niñas’. Clearly, an explanation of these exceptional cases will improve the predictive skill. Here we show that for the part of the Arabian Sea, east of the upwelling region and north of the equatorial belt (60°–70°E, 10°–23°N), the correlation of outgoing longwave radiation with Indian summer monsoon rainfall is even higher than that with the equatorial central Pacific associated with ENSO. Convection over this region is triggered by ENSO, but is modulated by the underlying sea surface temperature (SST). There is a minimum of SST of about 28.1°C above which the convection over the Arabian Sea is high enough and there are no rogue La Niñas. Furthermore, we show that, in this region, the SST of June–September is related to the SST of April–May. When April–May SST is >29.6°C, June–September mean SST is always >28.1°C and there are no rogue La Niñas; the monsoon rainfall is always normal or above normal as expected with a La Niña. Thus the chance of a rogue La Niña can be predicted from the April–May SST of the Arabian Sea.

Most current climate models predict that the equatorial Pacific will evolve under greenhouse gas-induced warming to a more El Niño-like state over the next several decades, with a reduced zonal sea surface temperature gradient and weakened atmospheric Walker circulation. Yet, observations over the last 50 y show the opposite trend, toward a more La Niña-like state. Recent research provides evidence that the discrepancy cannot be dismissed as due to internal variability but rather that the models are incorrectly simulating the equatorial Pacific response to greenhouse gas warming. This implies that projections of regional tropical cyclone activity may be incorrect as well, perhaps even in the direction of change, in ways that can be understood by analogy to historical El Niño and La Niña events: North Pacific tropical cyclone projections will be too active, North Atlantic ones not active enough, for example. Other perils, including severe convective storms and droughts, will also be projected erroneously. While it can be argued that these errors are transient, such that the models' responses to greenhouse gases may be correct in equilibrium, the transient response is relevant for climate adaptation in the next several decades. Given the urgency of understanding regional patterns of climate risk in the near term, it would be desirable to develop projections that represent a broader range of possible future tropical Pacific warming scenarios-including some in which recent historical trends continue-even if such projections cannot currently be produced using existing coupled earth system models.