Fidelity of El Nin~o simulation matters for predicting future climate


Date:
August 28, 2020
Source:
University of Hawaii at Manoa
Summary:
A new study revealed that correctly simulating ocean current
variations hundreds of feet below the ocean surface - the so-called
Pacific Equatorial Undercurrent - during El Nin~o events is key
in reducing the uncertainty of predictions of future warming in
the eastern tropical Pacific.



FULL STORY ==========================================================================
A new study led by University of Hawai'i at Manoa researchers, published
in the journal Nature Communications this week, revealed that correctly simulating ocean current variations hundreds of feet below the ocean
surface -- the so- called Pacific Equatorial Undercurrent -- during El
Nin~o events is key in reducing the uncertainty of predictions of future warming in the eastern tropical Pacific.


========================================================================== Trade winds and the temperatures in the tropical Pacific Ocean experience
large changes from year to year due to the El Nin~o-Southern Oscillation (ENSO), affecting weather patterns across the globe. For instance,
if the tropical Pacific is warmer and trade winds are weaker than
usual -- an El Nin~o event - flooding in California typically occurs
and monsoon failures in India and East Asia are detrimental to local
rice production. In contrast, during a La Nin~a the global weather
patterns reverse with cooler temperatures and stronger trade winds in
the tropical Pacific. These natural climate swings affect ecosystems, fisheries, agriculture, and many other aspects of human society.

Computer models that are used for projecting future climate correctly
predict global warming due to increasing greenhouse gas emissions as
well as short-term year-to-year natural climate variations associated
with El Nin~o and La Nin~a.

"There is, however, some model discrepancy on how much the tropical
Pacific will warm," said Malte Stuecker, co-author and assistant
professor in the Department of Oceanography and International Pacific
Research Center at UH Manoa. "The largest differences are seen in the
eastern part of the tropical Pacific, a region that is home to sensitive ecosystems such as the Galapagos Islands. How much the eastern tropical
Pacific warms in the future will not only affect fish and wildlife
locally but also future weather patterns in other parts of the world." Researchers have been working for decades to reduce the persistent model uncertainties in tropical Pacific warming projections.

Many climate models simulate El Nin~o and La Nin~a events of similar
intensity.

In nature, however, the warming associated with El Nin~o events tends to
be stronger than the cooling associated with La Nin~a. In other words,
while in most models El Nin~o and La Nin~a are symmetric, they are
asymmetric in nature.

In this new study, the scientists analyzed observational data and numerous climate model simulations and found that when the models simulate the subsurface ocean current variations more accurately, the simulated
asymmetry between El Nin~o and La Nin~a increases -- becoming more like
what is seen in nature.

"Identifying the models that simulate these processes associated with El
Nin~o and La Nin~a correctly in the current climate can help us reduce
the uncertainty of future climate projections," said corresponding lead
author Michiya Hayashi, a research associate at the National Institute for Environmental Studies, Japan, and a former postdoctoral researcher at UH
Manoa supported by the Japan Society for the Promotion of Science (JSPS) Overseas Research Fellowships. "Only one-third of all climate models can reproduce the strength of the subsurface current and associated ocean temperature variations realistically." "Remarkably, in these models
we see a very close relationship between the change of future El Nin~o
and La Nin~a intensity and the projected tropical warming pattern due
to greenhouse warming," noted Stuecker.

That is, the models within the group that simulate a future increase of
El Nin~o and La Nin~a intensity show also an enhanced warming trend in
the eastern tropical Pacific due to greenhouse warming. In contrast,
the models that simulate a future decrease of El Nin~o and La Nin~a
intensity show less greenhouse gas-induced warming in the eastern part
of the basin. The presence of that relationship indicates that those
models are capturing a mechanism known to impact climate -- signifying
that those models are more reliable. This relationship totally disappears
in the two-thirds of climate models that cannot simulate the subsurface
ocean current variations correctly.

"Correctly simulating El Nin~o and La Nin~a is crucial for projecting
climate change in the tropics and beyond. More research needs to
be conducted to reduce the biases in the interactions between wind
and ocean so that climate models can generate El Nin~o -- La Nin~a
asymmetry realistically," added Fei-Fei Jin, co-author and professor in
the Department of Atmospheric Sciences at UH Manoa.

"The high uncertainty in the intensity change of El Nin~o and La Nin~a
in response to greenhouse warming is another remaining issue," said
Stuecker. "A better understanding of Earth's natural climate swings such
as El Nin~o and La Nin~a will result in reducing uncertainty in future
climate change in the tropics and beyond."

========================================================================== Story Source: Materials provided
by University_of_Hawaii_at_Manoa. Original written by Marcie
Grabowski. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Michiya Hayashi, Fei-Fei Jin, Malte F. Stuecker. Dynamics for
El Nin~o-La
Nin~a asymmetry constrain equatorial-Pacific warming pattern. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-17983-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200828081046.htm

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