Time for El Ni帽o's half brother to take a bow?
For those of us living on the US East and Gulf Coasts, the 2009 Atlantic hurricane season has been pretty quiet -- thanks in no small part to , forecasters note. (Wipe hand across forehead here.)
But over the past few years, researchers have uncovered an odd form of El Ni帽o. Think of it as El Ni帽o's half brother. Now, a team of atmospheric scientists from Korea, Germany, and the US suggests that this form of El Ni帽o may become more common than the El Ni帽os we've experienced up to now.
The likely cause of the shift? Global warming, the team says -- although other researchers caution that this half brother may be yet another form of natural variation that appears and disappears on longer time scales than El Ni帽o does.
The team, whose results appear in the current issue of the journal , posits that global warming's effect on ocean temperatures in the Pacific have changed how those temperatures shift with depth -- prompting the emergence of El Ni帽o's half brother.
For its part, El Ni帽o shows up as a vast pool of unusually warm ocean water that has migrated from the far western Pacific east along the equator until it bumps into South and Central America.
When such a large source of heat to the atmosphere shifts locations so dramatically, atmospheric circulation patterns shift too. So El Ni帽o's arrival can alter seasonal rainfall, drought, and other weather patterns far from where the warm pool has docked. Indeed, a quiet North Atlantic hurricane season represents one of these "teleconnections" during an El Ni帽o year.
El Ni帽os crop up once every 3 to 8 years. During the "off" years, that warm pool of ocean water usually migrates back to the far western Pacific, accompanied by another set of long-range changes to atmospheric circulation patterns. This has become known as La Ni帽a.
Two years ago, however, a team of Japanese scientists published a study in the Journal of Geophysical Research showing that a kind of half El Ni帽o had been lurking undiscovered in climate records for a period that runs from 1979 to 2005.
Traveling half-way
In this case, instead of migrating all the way across the tropical Pacific, the warmest portion of the warm pool hunkers down half-way across, leaving cooler water to the east and west. The Japanese dubbed this El Ni帽o Modoki -- essentially pseudo-El Ni帽o.
The team's curiosity was piqued by weather patterns in 2004 around the Pacific rim. It was supposed to be an El Ni帽o year, but the resulting weather patterns didn't match expectations. Among them: The Atlantic hurricane season was far more active than seasonal forecasts had indicated it should be. As the Japanese scientists took a closer look at historical weather data, they uncovered this pseudo-El Ni帽o.
What kind of effect can it have? Last July, a trio of tropical-cyclone scientists at Georgia Tech in Atlanta published a paper in the journal Science showing that during these half-brother El Ni帽os, North Atlantic hurricane activity reached normal or above-normal levels, rather than the expected below-normal pace.
Better seasonal hurricane forecasts?
That sounds grim if you combine the frequency of these pseudo events with the return rate for La Ni帽a, which also leads to active hurricane seasons. The good news: Apparently these pseudo events are easier to forecast ahead of time than El Ni帽o itself, which holds the promise of improving seasonal hurricane forecasts, says Peter Webster, an atmospheric scientist at Georgia Tech and one of the members of the team reporting the results.
The team received a pat on the back from Chris Landsea, a scientist at NOAA's hurricane forecast hub in Miami. He writes in an email that this work "may provide an advance in the field of seasonal hurricane forecasting."
At the least, he adds, it helps explain why the 2004 hurricane season was far more intense than seasonal forecasts suggested.
Global warming's potential role
The the prospect that global warming will drive an increase in this pseudo El Ni帽o comes from a modeling exercise, in which the team reporting the results first identified the models that did the best job of capturing El Ni帽o activity. Then they ran projections assuming business-as-usual greenhouse gas emissions through the rest of the century. These pseudo El Ni帽os became more common than the traditional El Ni帽os -- as much as five times more common.
Researchers still need to learn more about this phenomenon, note Karumuri Ashok and Toshio Yamagata, two of the Japanese scientists who uncovered this phenomenon in the first place. Not the least of which is testing the notion that this pseudo El Ni帽o is itself a cyclical feature that may appear and disappear on timescales longer than El Ni帽o's -- decades to centuries, rather than every few years.
If all of this seems a little too much, here's comedian Chris Farely's take on El Ni帽o:
