The deep ocean, a vast thermal reservoir, absorbs excess heat under greenhouse warming, which ultimately regulates the Earth’s surface climate. Even if CO₂ emissions are successfully reduced, the stored heat will gradually be released, resulting in a particular pattern of ocean warming. Here, we show that deep ocean warming will lead to El Niño-like ocean warming and resultant increased precipitation in the tropical eastern Pacific with southward shift of the intertropical convergence zone. Consequently, the El Niño-Southern Oscillation shifts eastward, intensifying Eastern Pacific El Niño events. In particular, the deep ocean warming could increase convective extreme El Niño events by 40 to 80% relative to the current climate. Our findings suggest that anthropogenic greenhouse warming will have a prolonged impact on El Niño variability through delayed deep ocean warming, even if CO₂ stabilization is achieved. https://www.nature.com/articles/s41467-024-50663-9

The global oceans absorb most of the surplus heat from anthropogenic warming, but it is unclear how this heat accumulation will affect the Earth’s climate under climate mitigation scenarios. Here we show that this stored heat will be released at a much slower rate than its accumulation, resulting in a robust pattern of surface ocean warming and consequent regional precipitation. The surface ocean warming is pronounced over subpolar to polar regions and the equatorial eastern Pacific where oceans are weakly stratified to allow vigorous heat release from the deep ocean to the surface layer. We also demonstrate that this ocean warming pattern largely explains changes in the precipitation pattern, including the southward shift of the Intertropical Convergence Zone and more moistening in high latitudes. This study suggests that deep ocean warming may hinder climate recovery in some regions, even if carbon neutrality or net negative emissions are successfully achieved. https://www.nature.com/articles/s41558-024-01928-0

El Niño events are a prominent feature of climate variability with global climatic impacts. The 1997/98 episode, often referred to as ‘the climate event of the twentieth century’, and the 1982/83 extreme El Niño, featured a pronounced eastward extension of the west Pacific warm pool and development of atmospheric convection, and hence a huge rainfall increase, in the usually cold and dry equatorial eastern Pacific. Such a massive reorganization of atmospheric convection, which we define as an extreme El Niño, severely disrupted global weather patterns, affecting ecosystems, agriculture, tropical cyclones, drought, bushfires, floods and other extreme weather events worldwide. Potential future changes in such extreme El Niño occurrences could have profound socio-economic consequences. Here we present climate modelling evidence for a doubling in the occurrences in the future in response to greenhouse warming. We estimate the change by aggregating results from climate models in the Coupled Model Intercomparison Project phases 3 (CMIP3) and 5 (CMIP5) multi-model databases, and a perturbed physics ensemble. The increased frequency arises from a projected surface warming over the eastern equatorial Pacific that occurs faster than in the surrounding ocean waters, facilitating more occurrences of atmospheric convection in the eastern equatorial region. https://www.nature.com/articles/nclimate2100

The latest generation of coupled models, the sixth Coupled Models Intercomparison Project (CMIP6), is used to study the changes in the El Niño–Southern Oscillation (ENSO) in a warming climate. For the four future scenarios studied, the sea surface temperature variability increases in most CMIP6 models, but to varying degrees. This increase is linked to a weakening of the east-west temperature gradient in the tropical Pacific Ocean, which is evident across all models. Just as in previous generations of climate models, we find that many characteristics of future ENSO remain uncertain. This includes changes in dominant time scale, extratropical teleconnection patterns, and amplitude of El Niño and La Niña events. For models with the strongest increase in future variability, the majority of the increase happens in the Eastern Pacific, where the strongest El Niño events usually occur.

Plain Language Summary

The El Niño–Southern Oscillation (ENSO) is a naturally occurring irregular oscillation in the tropical Pacific Ocean alternating between warm (El Niño) and cold (La Niña) phases every 2–7 years. The sea surface temperature anomalies associated with ENSO are linked to variability in key climate quantities, such as temperature, winds, and precipitation over many parts of the globe. Hence, it is of great scientific and societal interest to determine how ENSO may change in a warming climate. We find that the latest generation of climate models shows changes in ENSO in a warmer world. The future variability is increasing, especially in the eastern equatorial Pacific, where the extreme warm events usually occur. This increase appears to be related to a reduced temperature difference between the eastern and western equatorial Pacific. The global weather patterns influenced by both the warm and cold events will also change, but models disagree on how large these changes will be. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL090640