El Nino Has A New Rhythm
May 27, 2013

New Research Could Lead To Improved El Nino Forecasting

redOrbit Staff & Wire Reports - Your Universe Online

Climate experts from the University of Hawai'i at Manoa Meteorology Department and International Pacific Research Center have reportedly discovered why the weather phenomenon known as El Nino typically peaks around Christmas and ends sometime between the months of February and April.

As it turns out according to researchers, "An unusual wind pattern that straddles the equatorial Pacific during strong El Nino events and swings back and forth with a period of 15 months explains El Nino's close ties to the annual cycle."

“This atmospheric pattern peaks in February and triggers some of the well-known El Nino impacts, such as droughts in the Philippines and across Micronesia and heavy rainfall over French Polynesia,” lead author Malte Stuecker, a research assistant in the university´s Department of Meteorology, said in a statement.

When trade winds unexpectedly shift to the south, they can halt an El Nino by creating eastward propagating equatorial Kelvin waves, the researchers explain. Those waves ultimately resume the upwelling of cold water in the eastern equatorial Pacific Ocean, and the shift is part of an overarching atmospheric pattern that accompanies El Nino events. As part of that pattern, a high-pressure system tends to hover over the Philippines, and the major rain band found in the South Pacific rapidly begins to shift towards the equator.

Thanks to assistance from numerical atmospheric models, Stuecker and his colleagues were able to discover that this odd pattern has its origins in an interaction between El Nino and the seasonal evolution of temperatures in the western tropical Pacific warm pool.

Stuecker said that not every El Nino is accompanied by this unorthodox wind pattern, but once the phenomenon´s conditions reach a specific “threshold amplitude” at the right time of the year, “it is like a jack-in-the-box whose lid pops open.”

According to the researchers, by studying the wind pattern´s evolution, they were able to discover a 15-month rhythm accompanying strong El Nino events, meaning that it is “considerably faster than the three- to five-year timetable for El Nino events, but slower than the annual cycle.”

They believe that using an improved representation of this 15-month tropical Pacific wind pattern in existing climate models will lead to better El Nino forecasts. In addition, they say that recent climate model projections suggest that future El Nino events will be accompanied by this combination tone wind pattern on a more frequent basis — and that this discovery will likely alter the characteristics of future rainfall patterns caused by the weather event.

Their findings are detailed in Sunday´s online edition of the journal Nature Geoscience.

Image 2 (below): This is a schematic figure for the suggested generation mechanism of the combination tone: The annual cycle (Tone 1), together with the El Niño sea surface temperature anomalies (Tone 2) produce the combination tone. Credit: Malte Stuecker