Timing Behind El Niño Revealed

El Niño is famous for the havoc it wreaks across the globe, including shifting weather patterns that in turn give rise to everything from droughts to flash floods.

However, despite the well-known effects of the climate phenomenon, the reasons behind its peak around Christmas and disappearance between February and April have long remained a mystery.

That is, until now.

A new study published in the journal Nature Geoscience uncovers the reasons for El Niño’s rise and fall, citing an unusual wind pattern that straddles the equatorial Pacific during strong El Niño events and swings back and forth over a period of 15 months as the culprit.

"This atmospheric pattern peaks in February and triggers some of the well-known El Niño impacts, such as droughts in the Philippines and across Micronesia and heavy rainfall over French Polynesia," says lead author Malte Stuecker of the University of Hawai’i at Manoa Meteorology Department and International Pacific Research Center.

When unusual trade winds shift south they can end an El Niño by generating eastward waves that eventually resume upwelling of cold water in the eastern equatorial Pacific, according to a press release on the study.

This wind shift is part of the larger, unusual atmospheric pattern accompanying El Niño events, during which a high-pressure system hovers over the Philippines and the major rain band of the South Pacific rapidly shifts toward the equator.

This unusual pattern, the scientists determined via computer models, originates from an interaction between El Niño and the seasonal evolution of temperatures in the western tropical Pacific warm pool.

"Not all El Niño events are accompanied by this unusual wind pattern" notes Malte Stuecker, "but once El Niño conditions reach a certain threshold amplitude during the right time of the year, it is like a jack-in-the-box whose lid pops open."

A study of the evolution of the anomalous wind pattern in the model revealed a rhythm of about 15 months accompanying strong El Niño events, which is faster than the three- to five-year timetable for El Niño events, but slower than the annual cycle.

"This type of variability is known in physics as a combination tone," says Fei-Fei Jin, professor of Meteorology and co-author of the study.

Combination tones, first discovered by violin builder Tartini, refer to the ear’s ability to hear a third tone only though two are played.

"The unusual wind pattern straddling the equator during an El Niño is such a combination tone between El Niño events and the seasonal march of the sun across the equator" says co-author Axel Timmermann, climate scientist at the International Pacific Research Center and professor at the Department of Oceanography, University of Hawai'i. "It turns out that many climate models have difficulties creating the correct combination tone, which is likely to impact their ability to simulate and predict El Niño events and their global impacts."

The scientists are convinced that a better representation of the 15-month tropical Pacific wind pattern in climate models will improve El Niño forecasts and that the latest climate model projections suggest that El Niño events will more often include this combination tone wind pattern, which in turn will change the characteristics of future El Niño rainfall patterns.