Record snowfall, killer tornadoes, devastating floods: There's no doubt about
it. Since December 2010, the weather in the USA has been positively wild. But
why?
Some recent news reports have attributed the phenomenon to an extreme "La Nina,"
a band of cold water stretching across the Pacific Ocean with global
repercussions for climate and weather.
But NASA climatologist Bill Patzert names a different suspect: "La Nada." "La
Nina was strong in December," he says. "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 its newfound freedom,
and the results were even more disastrous."
La Nina and El Nino are opposite extremes of a great Pacific oscillation. Every
2 to 7 years, surface waters across the equatorial Pacific warm up (El Nino) and
then they cool down again (La Nina).
Each condition has its own distinct effects on weather. The winter of 2010 began
with La Nina conditions taking hold.
A "normal" La Nina would have pushed the jet stream northward, pushing cold
arctic air, one of the ingredients of severe weather, away from the lower US.
But this La Nina petered out quickly, and no El Nino rose up to replace it. The
jet stream was free to misbehave. "By mid-February 2011 the jet stream was
meandering wildly around the U.S. The weather pattern became dominated by strong
outbreaks of frigid polar air, producing blizzards across the West, Upper
Midwest, and northeast U.S." The situation lingered into spring - and things got
ugly.
Russell Schneider, Director of the NOAA-NWS Storm Prediction Center, explains:
"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.
In Patzert's words, "The jet stream, on steroids, acted as an atmospheric mix
master, causing tornadoes to explode across the deep south and Tornado Alley,
and even into Massachusetts."
The action's not over. This is weather, after all. What will happen next? Please
don't say "La Nada."
For more information about wild weather, on Earth and other planets, please
visit Science.nasa.gov
Uploaded by ScienceAtNASA on Jun 30, 2011
Visit
http://science.nasa.gov/science-news/science-at-nasa/2011/24jun_wildweather/ for
the full story.
Документальный фильм "Wild Weather - What's to Blame?"
