New Climate Model Captures Key Features of Central Pacific El Nino

Scientists working at NYU Abu Dhabi have developed a sophisticated new theoretical model that may lead to more timely, accurate forecasts of the central Pacific El Niño (CP El Niño), an important global weather-maker.

"You have to say 'the potential for better forecasts'," said Andrew Majda, professor of mathematics and atmosphere/ocean science and principal investigator at NYUAD's Center for Prototype Climate Modeling (CPCM) because achieving practical forecasting will still "require an effort of several years." But the next steps have already started, some of them at the CPCM in Abu Dhabi.

Any El Niño is a period of warmer-than-normal sea-surface temperature in the equatorial Pacific. These anomalies, and the related air pressure fluctuations, can have teleconnections – long-distance effects – on weather far away from the region directly affected. CP El Niño events can last for years, and have been "very frequent in the last 25 years," Majda said, although it's not possible to link the increased frequency to climate change.

CP El Niño, located far from any continental shore, is linked to severe weather around the globe including massive rainfall in the southeastern US, drought in California and Asia, and most notably the yearly Indian monsoon, which can lead to life-giving harvests but also deadly floods.

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It would be great to understand it (CP El Niño) much better, because of its effect on the monsoon.

Andrew Majda, Center for Protoype Climate Modeling

Ultimately, Majda said, reliable long-range forecasting of the monsoon in particular would have "huge societal impacts, in agriculture, disaster relief, drought planning" and other fields.

Majda and Nan Chen, postdoctoral researcher at NYU's Courant Institute of Mathematical Sciences, recently published a scholarly paper explaining what they have learned about the CP El Niño.

Majda explained how Dr. Ajaya Ravindran, senior scientist at the Center for Prototype Climate Modeling, steered him and Chen to the "genuine puzzle" of how a CP El Niño occurs. The CPCM, one of 12 research centers grouped in the NYU Abu Dhabi Institute, brings together about 10 researchers including senior scientists and postdoctoral researchers, Ravindran explained.

New Climate Model Captures Key Features of Central Pacific El Nino

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"I was reminded by Ajaya and his postdoctoral students," Majda said, "that it would be great to understand it much better, because of its effect on the monsoon."

Ravindran noted that a CP El Niño coincides with "severe droughts over India, especially northern India", while the better-known eastern Pacific El Niño is tied to drier weather in parts of Asia and Australia. Knowing which to expect would be hugely useful.

Last winter Majda and Chen, then a doctoral candidate, "just brainstormed for two weeks" at NYUAD, the senior scholar said, studying data from two recent major CP El Niño periods, working to fit different potential factors into a pattern.

"We started with a simple computer model you can run on a laptop," said Majda "and astonishingly we started fitting what was seen in observations of CP El Niño. In those two weeks in Abu Dhabi" — as February 2016 became March — "we had the main ideas."

Their refined theory incorporates how changing ocean currents can move warm water around, and found, Majda explained, "that nonlinear transport of sea-surface temperature was an important new ingredient." Other factors in the new theory are the strengthening of the trade winds that blow east-to-west across the Pacific, and "wind bursts", the incessant, effectively random variations in wind strength during an El Niño.

Assembling these factors, Majda said, produced a model with "a striking resemblance to the CP El Niño events from 1990-1995 and 2001-2006." This "now gives the people who make operational models a way to see what they need to … capture. Unlike the familiar eastern Pacific El Niño, current operational models have difficulties capturing key features of the CP El Niño.”

But practical precise forecasting, Majda cautioned, will demand further theoretical development, and work with "the operational community" of major labs such as the U.S. National Centers for Environmental Prediction, the Geophysical Fluid Dynamics Laboratory at Princeton University, and the Indian Institute of Tropical Meteorology. "We're already working as a group," Majda said.

By Brian Kappler for NYUAD Public Affairs