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Disasters & Extreme WeatherNature

La Nada to Blame for American Wild Weather

Want an explanation for the record snowfalls, killer tornadoes and devastating floods running wild across America? NASA climatologist Bill Patzert believes that “La Nada” is the problem.

“La Niña was strong in December,” he says in response to those who believe that a strong La Niña is responsible for the current wild weather. “But back in January it pulled a disappearing act and left us with nothing – La Nada – to constrain the jet stream. Like an unruly teenager, the jet stream took advantage of the newfound freedom–and the results were disastrous.”

The blue and purple band in this satellite image of the Pacific Ocean traces the cool waters of the La Niña phenomenon in December 2010. (from Ocean Surface Topography Mission

He’s funny, but right on the money.

La Niña and El Niño are opposite extremes of a massive Pacific oscillation. Every 2 to 7 years surface waters across the equatorial Pacific Ocean warm up, which is an El Niño, and then they cool down again, a La Niña.

When winter of 2010 in the Northern Hemisphere kicked into gear, La Niña were taking hold. A stereotypical La Niña event would have pushed the jet stream northward, pushing cold arctic air – one of the main causes for severe weather – away from the lower United States.

However, the most recent La Niña dissipated quickly, and there was no El Niño to rise up and take its place. As a result, the jet stream was free to run amok across the country.

This satellite image, taken in April 2011, reveals La Niña's rapid exit from the equator near the US coast. The cool (false-color blue) water was gone by early spring.

“By mid-January 2011, La Niña weakened rapidly and by mid-February it was ‘adios La Niña,’ allowing the jet stream to meander wildly around the US. Consequently the weather pattern became dominated by strong outbreaks of frigid polar air, producing blizzards across the West, Upper Midwest, and northeast US.”

Russell Schneider, Director of the NOAA-NWS Storm Prediction Center, explains how things got ugly.

“First, very strong winds out of the south carrying warm, moist air from the Gulf of Mexico met cold jet stream winds racing in from the west. Stacking these two air masses on top of each other created the degree of instability that fuels intense thunderstorms.”

Extreme contrasts in wind speeds and directions of the upper and lower atmosphere transformed ordinary thunderstorms into long-lived rotating supercells capable of producing violent tornadoes.

“The jet stream — on steroids — acted as an atmospheric mix master, causing tornadoes to explode across Dixie and Tornado Alleys, and even into Massachusetts,” Patzert clarified.

“La Niña and El Niño affect the atmosphere’s energy balance because they determine the location of warm water in the Pacific, and that in turn determines where huge clusters of tropical thunderstorms form,” explains Schneider. “These storms are the main energy source from the tropics influencing the large scale pattern of the jet stream that flows through the US.”

In agreement with Patzert, he notes that the very strong and active jet stream across the lower US in April “may have been related to the weakening La Niña conditions observed over the tropical Pacific.”

“Global warming is certainly happening,” asserts Patzert, “but we can’t discount global warming or blame it for the 2011 tornado season. We just don’t know … Yet.”

Source: NASA
Image Source: NASA Jet Propulsion Laboratory




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