This is an excellent explanation
of what hit us this past several months in terms of the weather. What is clear is that it is a low probability
combination of influences that we are unlikely to see again in our lifetimes.
In the meantime, we have the
worst that Mother Nature can throw at us on record to remind us when someone
wants to diddle statistics to show trends proclaiming the end of the world.
It is way more compelling than
reports from the nineteenth century that are long forgotten.
Is La Nada To Blame For Recent Wild US Weather Ride
by Dauna Coulter
June 21, 2011: Record snowfall, killer tornadoes, devastating floods:
There's no doubt about it. Since Dec. 2010, the weather in the USA
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 the newfound freedom--and the results were 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-January 2011, La Nina weakened rapidly and by mid-February
it was 'adios La Nina,' allowing the jet stream to meander wildly around the US US
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.2
In Patzert's words, "The jet stream - on steroids - acted as an
atmospheric mix master, causing tornadoes to explode across Dixie and Tornado
Alleys, and even into Massachusetts
All this because of a flaky La Nina?
"La Nina and El Nino 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 Nina conditions observed over the tropical Pacific."
And of course there's this million dollar question: "Does any
research point to climate change as a cause of this wild weather?"
"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."3
What will happen next? And please don't say, "La Nada."
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