Reviewed by Alex SmithAug 27 2021
In the eastern pacific, the cycling between cold La Niña and warm El Niño phenomena (commonly referred to as the El Niño-Southern Oscillation, ENSO) has constantly occurred without any strong interruption for the past 11,000 years.
Surface ocean temperatures simulated at unprecedented resolution using a coupled atmosphere-ocean model. The extensive wavy cold structure in the equatorial Pacific corresponds to a tropical instability wave. Simulations were conducted on the IBS/ICCP supercomputer Aleph. Image Credit: IBS Center for Climate Physics.
A team of researchers from the IBS Center for Climate Physics (ICCP) at Pusan National University in South Korea, the Max Planck Institute of Meteorology, Hamburg, Germany and the University of Hawaiʻi at Mānoa, USA, are predicting that this condition may change in the future.
The team undertook a series of experiments with global climate model simulation with an unmatched special resolution of 10 km in the ocean and 25 km in the atmosphere. Aleph, one of the fastest supercomputers in South Korea, was employed to strengthen the research.
The new ultra-high-resolution climate model is capable of making realistic simulations of tropical cyclones in the atmosphere and tropical instability waves in the equatorial Pacific Ocean. These processes play basic roles in the generation and termination of El Niño and La Niña events.
Our supercomputer ran non-stop for over one year to complete a series of century-long simulations covering present-day climate and two different global warming levels. The model generated 2 quadrillion bytes of data; enough to fill up about 2,000 hard disks.
Dr Sun-Seon Lee, Center for Climate Physics, Institute for Basic Science
With the analysis of this bulk dataset, the researchers concentrated on a long-standing issue, how ENSO would respond to the increasing greenhouse gas concentrations.
Two generations of climate scientists have looked at this issue using climate models of varying complexity. Some models simulated weaker; others predicted larger eastern Pacific temperature swings in a future warmer climate. The jury was still out.
Axel Timmermann, Study Co-corresponding Author, Professor, and Director, Center for Climate Physics, Institute for Basic Science
“What is common to these models is that their simulated temperatures in the equatorial Pacific, west of Galapagos, were always too cold compared to the observations. This prevented them from properly representing the delicate balance between positive and negative feedback processes that are important in the ENSO cycle,” added Timmermann.
By recording small-scale climatic processes at the maximum computationally possible resolution, the ICCP researchers could relieve these ocean temperature biases. This resulted in substantial enhancements in the representations of ENSO and its response to Global Warming.
“The result from our computer simulations is clear: Increasing CO2 concentrations will weaken the intensity of the ENSO temperature cycle,” said Dr Christian Wengel, first author of the study and former postdoctoral researcher at the ICCP, currently at the Max Planck Institute of Meteorology in Hamburg, Germany.
By observing the movement of heat in the coupled atmosphere/ocean system, the scientists noted the key reason for the collapse of the ENSO system: Future El Niño occurrences will lose heat to the atmosphere much faster due to water evaporation, which holds the ability to cool down the ocean.
Furthermore, the decreased future temperature difference between the eastern and western tropical Pacific will also prevent the build-up of temperature extremes during the ENSO cycle. These two aspects are somewhat offset by a predicted future weakening of tropical instability waves.
Usually, the oceanic waves that constitute up to 30% of the total circumference of the Earth form during the occurrence of La Niña conditions. They are capable of replacing the equatorial waters with warmer off-equatorial water, thus speeding up the collapse of a La Niña event.
The new computer simulations determine the detailed structure of these waves and show that the correlated negative feedback for ENSO will weaken in the future.
According to ICCP alumni Prof. Malte Stuecker, a co-author of the study and now assistant professor at the Department of Oceanography and the International Pacific Research Center at the University of Hawaiʻi at Mānoa, “There is a tug-of-war between positive and negative feedbacks in the ENSO system, which tips over to the negative side in a warmer climate. This means future El Niño and La Niña events cannot develop their full amplitude anymore.”
Although the year-to-year fluctuations in eastern equatorial Pacific temperatures could possibly fade with human-induced warming, the corresponding alterations in El Niño- and La Niña-associated rainfall extremes will increase constantly due to an intensified hydrological cycle in a warmer climate. This is proposed in the study and presented by the scientists from the ICCP and their international collaborators.
Our research documents that unabated warming is likely to silence the world’s most powerful natural climate swing which has been operating for thousands of years. We don’t yet know the ecological consequences of this potential no-analog situation.
Axel Timmermann, Study Co-corresponding Author, Professor, and Director, Center for Climate Physics, Institute for Basic Science
Journal Reference:
Wengel, C., et al. (2021) Future high-resolution El Niño/Southern Oscillation dynamics. Nature Climate Change. doi.org/10.1038/s41558-021-01132-4.