A new study published in the journal Nature Communications reveals that the El Niño-Southern Oscillation (ENSO), a key driver of global climate variability, is projected to undergo a dramatic transformation due to greenhouse warming. Using high-resolution climate models (Figure 1), a team of researchers from South Korea, the USA, Germany, and Ireland found that ENSO could intensify rapidly over the coming decades and synchronize with other major climate phenomena, reshaping global temperature and rainfall patterns by the end of the 21^st century.

Figure 1. Snapshot of eastern Pacific sea surface temperatures, showing temperature pattern for a typical La Niña event with equatorial cold temperatures and wave-like structure west of the Galapagos Islands simulated by a high-resolution climate model. Blue to red color shading indicates a transition from colder to warmer surface conditions. The amplitude of La Niña and El Niño conditions can intensify in response to global warming, and the succession of these extremes will also become more regular.

The study projects an abrupt shift within the next 30-40 years from irregular El Niño-La Niña cycles to highly regular oscillations, characterized by amplified sea surface temperature (SST) fluctuations (Figure 2). “In a warmer world, the tropical Pacific can undergo a type of climate tipping point, switching from stable to unstable oscillatory behavior. This is the first time this type of transition has been identified unequivocally in a complex climate model,” says Prof. Malte F. STUECKER, lead author of the study and Director of the International Pacific Research Center at the University of Hawaiʻi at Mānoa, USA. “Enhanced air-sea coupling in a warming climate, combined with more variable weather in the tropics, leads to a transition in amplitude and regularity,” he adds.

According to the high-resolution computer model simulations analyzed in the study, the stronger and more regular ENSO cycles are also expected to synchronize with other climate phenomena, including the North Atlantic Oscillation (NAO), the Indian Ocean Dipole (IOD), and the Tropical North Atlantic (TNA) mode, similar to how multiple weakly connected pendulums adjust to swinging with the same frequency. “This synchronization will lead to stronger rainfall fluctuations in regions such as Southern California and the Iberian Peninsula, increasing the risk of hydroclimate ‘whiplash’ effects,” says Prof. Axel TIMMERMANN, corresponding author of the study and Director of the IBS Center for Climate Physics at Pusan National University, South Korea. “The increased regularity of ENSO could improve seasonal climate forecasts; however, the amplified impacts will necessitate enhanced planning and adaptation strategies,” he adds.

Figure 2. Left: Simulated sea surface temperature anomaly averaged over the eastern equatorial Pacific showing the abrupt transition from weak, irregular to intense, regular El Niño-La Niña cycles around year 2065; Right: Future change between 2080-2100 CE and 2015-2035 CE, in amplitude of year-to-year sea surface temperature variations, showing substantial intensification in the tropical eastern Pacific, eastern Indian Ocean, and tropical North Atlantic due to global warming. The results were obtained using a high-resolution climate model (AWI-CM3), which was subjected to a high-emission greenhouse gas emission scenario.

The research utilized the Alfred Wegener Institute Climate Model (AWI-CM3), with 31 km horizontal resolution in the atmosphere and 4-25 km in the ocean, to simulate climate responses under a high-emission greenhouse gas scenario. Observational data and simulations from other climate models were also analyzed to validate the findings. ​“Our simulation results, which some other climate models support, show that ENSO’s future behavior could become more predictable, but its amplified impacts will pose significant challenges for societies worldwide,” says Dr. Sen ZHAO, co-lead author of the study and researcher at the University of ​Hawaiʻi at Mānoa.

The new study in Nature Communications highlights the potential for anthropogenic climate change to fundamentally alter the characteristics of ENSO, and its interactions with other climate processes, even in regions far away from the equatorial Pacific, such as Europe. ​“Our findings underscore the need for global preparedness to address intensified climate variability and its cascading effects on ecosystems, agriculture, and water resources,” ​says Prof. Axel TIMMERMANN.

In the future, the team will explore the underlying global synchronization processes also in other high-resolution climate model simulations, including those with 9 km and 4 km resolution recently conducted at the IBS Center for Climate Physics on the Aleph supercomputer in South Korea.

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The IBS Center for Climate Physics (ICCP) expands the frontiers of earth system science by conducting cutting-edge research into climate dynamics and utilizing high-performance computer simulations to elucidate how climate has shaped human history and to improve decadal Earth system forecasts and long-term climate, sea-level, and biogeochemical projections. See more on http://ibsclimate.org.