June 26, 2011
What’s To Blame For Wild Weather? ‘La Nada’
By Dauna Coulter - Science@NASA
Record snowfall, killer tornadoes, devastating floods: There's no doubt about it. Since Dec. 2010, the weather in the USA has been positively wild. But why?Some recent news reports have attributed the phenomenon to an extreme "La NiÃ±a," 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 NiÃ±a 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 NiÃ±a and El NiÃ±o are opposite extremes of a great Pacific oscillation. Every 2 to 7 years, surface waters across the equatorial Pacific warm up (El NiÃ±o) and then they cool down again (La NiÃ±a). Each condition has its own distinct effects on weather.
The winter of 2010 began with La NiÃ±a conditions taking hold. A "normal" La NiÃ±a 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 NiÃ±a petered out quickly, and no El NiÃ±o rose up to replace it. The jet stream was free to misbehave.
"By mid-January 2011, La NiÃ±a weakened rapidly and by mid-February it was 'adios La NiÃ±a,' 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."1
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 NiÃ±a?
"La NiÃ±a and El NiÃ±o 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 NiÃ±a 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."
(1) Other atmospheric factors also contributed to the inflow of frigid polar air, says Patzert. One of the most significant was a weakening in the whirlpool motion of the air around the North Pole. As a result of this weakening, more cold air flowed away from the pole and down toward the states. Climatologists call this an "arctic oscillation."
(2) Imagine a paddle wheel oriented like a Ferris wheel and placed in winds that that are much stronger at the top than at the bottom. The wheel will spin in the direction of the strong winds above. This spring, these strong, turning winds led to ongoing rotation of the supercells themselves. So we ended up with intense rotation and updraft close to Earth's surface -- conditions ripe for strong tornadoes.
(3) On May 26, 2011, Patzert posted a comment about this topic on Andrew Revkin's The New York Times' DOT EARTH Blog, "Demography, Design, Atom Bombs and Tornado Deaths." See comment 6 at this URL.
Image 1: The blue and purple band in this satellite image of the Pacific Ocean traces the cool waters of the La NiÃ±a phenomenon in December 2010. (from Ocean Surface Topography Mission (OSTM)/Jason-2 satellite, Credit: NASA JPL)
Image 2: This satellite image, taken in April 2011, reveals La NiÃ±a's rapid exit from the equator near the US coast. The cool (false-color blue) water was gone by early spring. (from Ocean Surface Topography Mission (OSTM)/Jason-2 satellite, Credit: NASA JPL)