The Science Behind This Terrible Tornado Season

So far, 2011 has proved a year destined for the tornado record books.

Nearly 1,200 tornadoes have swarmed the United States this year, according to preliminary numbers from the National Oceanic and Atmospheric Administration (NOAA). Four of these storms have been rated at the highest tornado strength, an EF-5. The death toll from these tornadoes has likely topped 500, a number not seen since 1953.

But why has this year seen so many and such devastating twisters? Scientists point to several large-scale climate factors, some of which have been at work behind the scenes since winter. And at least some of the mind-boggling tornado numbers, believe it or not, can be chalked up to humans — there are more of us around to see them.

La Niña’s exit

Some of the blame for the wild tornado streak lies with La Niña, a cyclical system of trade winds that cools the waters of the equatorial Pacific Ocean. (El Niño is La Niña’s warm-water counterpart.)

Although we were in the grip of one of the most powerful La Niñas on record this last year, La Niña made a sudden exit about three months ago, said Bill Patzert, a climatologist at NASA’s Jet Propulsion Laboratory in California.

“La Niña would have been beneficial for all these people that have been so clobbered,” Patzert said. “If La Niña had maintained its strength, perhaps we wouldn’t have seen so many tornadoes.”

How do trade winds in the Pacific relate to deadly storms in the southern and central United States? It has to do with the jet stream, a high-speed air current that is essentially an atmospheric fence where cool, dry air meets up with warm, moist air — two of the main ingredients for severe storms.

La Niña has a stabilizing effect on the jet stream, and pushes it to higher latitudes.

Without La Niña around, the jet stream has gone rogue, Patzert told OurAmazingPlanet. “This time of the year it should be farther north,” he said.

Instead, the jet stream has spent April and May draped across the middle of the country, where it has the chance to violently mix cool, dry northern air with warm, moist southern air.

And in 2011, those two air masses have been on the extreme ends of the temperature scale.

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