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El Niño and La Niña events expected to wane as Earth heats up
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El Niño and La Niña events expected to wane as Earth heats up

  • Surface ocean temperatures are 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.
    Institute for Basic ScienceSurface ocean temperatures are 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.

The cycling between warm El Niño and cold La Niña conditions in the eastern Pacific has occurred without interruption for the last 11,000 years, but it could be weakened now as climate change ramps up, according to a team of scientists.

To obtain an accurate prediction of what is in store for these oceanic temperature patterns, researchers from the IBS Center for Climate Physics at South Korea's Pusan National University, Germany's Max Planck Institute for Meteorology and the University of Hawaii at Mānoa conducted a series of global climate model simulations using one of South Korea’s fastest computers, known as the Aleph, and with unprecedented spatial resolution.

The new ultra-high resolution climate model simulations can now realistically simulate tropical cyclones in the atmosphere and tropical instability waves in the equatorial Pacific Ocean, both of which play a role in the generation of El Niño and La Niña events, the researchers say.

“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,” study co-author Sun-Seon Lee said in a statement. “The model generated 2 quadrillion bytes of data; enough to fill up about 2,000 hard discs.”

The team analyzed the large dataset to answer the longstanding question of how increasing greenhouse gas concentrations will affect El Niño and La Niña events. Their findings were published Thursday in the journal Nature Climate Change.

“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,” said Axel Timmermann, co-author and director of the IBS Center for Climate Physics, in a statement.

He added, “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.”

The researchers overcame the ocean temperature disparities by capturing small-scale climatic processes at the highest computationally possible resolution, leading to marked improvements in the modeling of the ENSO, or El Niño-Southern Oscillation, and its response to climate change.

“The result from our computer simulations is clear: Increasing CO2 concentrations will weaken the intensity of the ENSO temperature cycle,” said lead author Christian Wengel of the Max Planck Institute for Meteorology.

Scientists traced the movement of heat in the coupled atmosphere/ocean system to identify the key figure in the collapse of the ENSO pattern. They found future El Niño events will lose their heat to the atmosphere more quickly due to water vapor evaporation, which tends to cool the ocean. The reduced temperature difference between the eastern and western tropical Pacific is expected to prevent the development of temperature extremes during the ENSO cycle.

These two factors, however, are partly offset by the projected future weakening of tropical instability waves that develop during La Nina conditions and normally could encompass up to 30% of the earth’s total circumference.

These waves replace colder equatorial waters with warmer off-equatorial water, hastening the end of a La Niña event. The computer simulations show that the associated negative feedback for ENSO will weaken in the future.

“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,” co-author Malte Stuecker of the University of Hawai’i at Mānoa said in a statement.

While fluctuations from year to year in eastern equatorial Pacific temperatures are likely to weaken with climate change, the corresponding changes in El Niño and La Niña-related rainfall extremes will continue to increase due to an intensified hydrological cycle in a warmer climate, as other recent studies have shown.

“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,” Timmermman said. “We don’t yet know the ecological consequences of this potential no-analog situation, but we are eager to find out.”

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