Fewer El Niño and La Niña events in a warmer world
The cycle between warm El Niño and cold La Niña conditions in the eastern Pacific (commonly known as El Niño-Southern Oscillation, ENSO ) has persisted without major interruptions for at least the last 11,000 years. This may change in the future according to a new study published in the journal Nature Climate Change by a team of scientists from the IBS Center for Climate Physics (ICCP) at Pusan National University in South Korea, the Max Planck Institute for Meteorology, Hamburg. Germany and the University of Hawaii at Manoa, USA.
The team conducted a series of global climate model simulations with an unprecedented spatial resolution of 10 km in the ocean and 25 km in the atmosphere. Powered by the power of one of South Korea’s fastest supercomputers (Aleph), new ultra-high-resolution climate model simulations can now realistically simulate tropical cyclones in the atmosphere and waves of tropical instability in the equatorial Pacific Ocean. , which play fundamental roles in the generation and termination of El Niño and La Niña events.
“Our supercomputer ran non-stop for over a year to complete a century-long series of simulations covering the current climate and two different levels of global warming. The model generated 2 quadrillion bytes of data; enough to fill about 2,000 hard drives“says Dr. Sun-Seon Lee, who conducted the experiments.
In analyzing this huge dataset, the team focused on a long-term problem: how ENSO will change in response to increasing greenhouse gas concentrations.
“Two generations of climate scientists have analyzed this issue using climate models of varying complexity. Some models simulated weaker; others predicted larger swings in eastern Pacific temperatures in a warmer future climate. The jury was still out “says Professor Axel Timmermann, co-correspondent author and ICCP director. He adds” What these models have in common is that their simulated temperatures in the equatorial Pacific west of the Galapagos were always too cold. compared to observations. This prevented them from adequately representing the delicate balance between positive and negative feedback processes that are important in the ENSO cycle. “.
By capturing small-scale weather processes with the highest computationally resolution possible, the ICCP team was able to alleviate these ocean temperature biases, leading to substantial improvements in the representations of ENSO and its response to global warming.
“The result of our computer simulations is clear: increasing CO2 concentrations will weaken the intensity of the ENSO temperature cycle.“says Dr. Christian Wengel, first author of the study and former postdoctoral researcher at ICCP, now at the Max Planck Institute for Meteorology in Hamburg in Germany.
By tracking the movement of heat in the coupled atmosphere / ocean system, the scientists identified the main culprit of the collapse of the ENSO system: future events of El Niño will lose heat to the atmosphere more rapidly due to the evaporation of water vapor, which has a tendency to cool the ocean.. Furthermore, reducing the future temperature difference between the eastern and western tropical Pacific will also inhibit the development of extreme temperatures during the ENSO cycle. However, these two factors are partly offset by a projected future weakening of waves of tropical instability.
Typically, these ocean waves, which can encompass up to 30% of the Earth’s total circumference, develop during La Niña conditions. They replace colder equatorial waters with warmer waters outside the equator, thus hastening the disappearance of a La Niña event. New computer simulations, which solve for the detailed structure of these waves, show that the associated negative feedback for ENSO will weaken in the future.
Although it is likely that year-to-year fluctuations in eastern equatorial Pacific temperatures weaken with human-induced warming according to this new study, corresponding changes in rainfall extremes related to El Niño and La Niña will continue to increase due to hydrological cycle intensification in warmer weather and climate, as recent studies by ICCP scientists and their international collaborators show.
“Our research documents that constant warming will likely silence the world’s most powerful natural climate change that has been operating for thousands of years. We do not yet know the ecological consequences of this possible non-analogous situation“, dice Axel Timmermann,”but we are eager to find out “.
Future high-resolution El Niño/Southern Oscillation dynamics. Christian Wengel, Sun-Seon Lee, Malte F. Stuecker, Axel Timmermann, Jung-Eun Chu & Fabian Schloesser. Nature Climate Change (2021). https://www.nature.com/article…