El Niño

Annual cycle together with El Niño sea surface temperature anomalies generate combination mode winds

Malte Stuecker and Fei-Fei Jin from the UH Mānoa Department of Meteorology and Axel Timmermann from the International Pacific Research Center have discovered a rhythm behind El Niño’s sporadic behavior. According to their findings, reported in the May 26, 2013, online issue of Nature Geoscience, El Niño’s peaking time around Christmas and rapid decrease by February to April is due to an unusual wind pattern. This wind pattern straddles the equatorial Pacific during strong El Niño events and swings back and forth with a period of 15 months, explaining El Niño’s close ties to the annual cycle.

“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 Stuecker.

When anomalous trade winds shift south, they can terminate an El Niño by generating eastward propagating equatorial Kelvin waves that eventually resume upwelling of cold water in the eastern equatorial Pacific. This wind shift is part of the larger, unusual atmospheric pattern accompanying El Niño events, in which a high-pressure system hovers over the Philippines and the major rain band of the South Pacific rapidly shifts equatorward.

With the help of numerical atmospheric models, the scientists discovered that this unusual pattern originates from an interaction between El Niño and the seasonal evolution of temperatures in the western tropical Pacific warm pool.

A study of the evolution of the anomalous wind pattern in the model reveals a rhythm of about 15 months accompanying strong El Niño events, which is considerably 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 Jin, professor of meteorology and co-author of the study.

“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 Timmermann, climate scientist at the International Pacific Research Center and professor at the UH Mānoa Department of Oceanography. He adds, “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. Moreover, they say the latest climate model projections suggest that El Niño events will be accompanied more often by this combination tone wind pattern, which will also change the characteristics of future El Niño rainfall patterns.

For more information, read the UH Mānoa press release.

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