Fewer El Nin~o and La Nin~a events in a warmer world

Date:
August 26, 2021
Source:
Institute for Basic Science
Summary:
The cycling between warm El Nin~o and cold La Nin~a conditions in
the eastern Pacific has persisted without major interruptions for
at least the last 11,000 years. This may change in the future.



FULL STORY ==========================================================================
The cycling between warm El Nin~o and cold La Nin~a conditions in
the eastern Pacific (commonly referred to as the El Nin~o-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 of 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. Boosted by the power of one of South Korea's fastest supercomputers (Aleph), 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, which both play fundamental roles in the generation and termination of
El Nin~o and La Nin~a 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 byte of data; enough to fill up about 2,000 hard disks," says Dr. Sun-Seon Lee who conducted the experiments.

Analyzing this enormous dataset, the team focused on a long-standing
problem: how will ENSO change in response to 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." says Prof. Axel Timmermann, co-corresponding author and Director of the ICCP. He adds "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 important in the ENSO cycle." By capturing small-scale climatic
processes at the highest computationally possible resolution, 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 from 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 the ICCP, now at the Max Planck
Institute of Meteorology in Hamburg in Germany.

By tracing the movement of heat in the coupled atmosphere/ocean system the scientists identified the main culprit of the collapse of the ENSO system: Future El Nin~o events will lose heat to the atmosphere more quickly due
to the evaporation of water vapor, which has the tendency to cool the
ocean. In addition, the reduced future temperature difference between the eastern and western tropical Pacific will also inhibit the development
of temperature extremes during the ENSO cycle. However, these two factors
are partly offset by a projected future weakening of tropical instability waves. Normally these oceanic waves, which can encompass up to 30% of the earth's entire circumference, develop during La Nin~a conditions. They
replace colder equatorial waters with warmer off-equatorial water, thereby accelerating the demise of a La Nin~a event. The new computer simulations, which resolve the detailed structure of these waves, demonstrate 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 Nin~o and La Nin~a events cannot develop
their full amplitude anymore" comments ICCP alumni Prof. Malte Stuecker, co-author of the study and now assistant professor at the Department
of Oceanography and the International Pacific Research Center at the
University of Hawaii at Manoa.

Even though the year-to-year fluctuations in eastern equatorial
Pacific temperatures are likely to weaken with human-induced warming
according to this new study, the corresponding changes in El Nin~o and
La Nin~a-related rainfall extremes will continue to increase due to an intensified hydrological cycle in a warmer climate, as shown in recent
studies by 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" says Axel Timmermann, "but we are eager to find out." ========================================================================== Story Source: Materials provided by Institute_for_Basic_Science. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Wengel, C., Lee, SS., Stuecker, M.F. et al. Future high-resolution
El
Nin~o/Southern Oscillation dynamics. Nat. Clim. Chang., 2021 DOI:
10.1038/s41558-021-01132-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210826111636.htm

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