Climate 2013: La Niña, La Nada

From Bob Tisdale's Climate Observations:

Mid-December 2012 Sea Surface Temperature Anomaly Update

NINO3.4 sea surface temperature anomalies are a commonly used metric for the frequency, strength and duration of El Niño and La Niña events. For the week centered on Wednesday December 12, 2012, they’re close to zero—at about 0.02 deg C. As has been apparent for a few weeks, it looks like this year’s ENSO event will be a La Nada.

Weekly NINO3.4

Global sea surface temperature anomalies appear to have responded quite quickly to the stronger El Niño conditions earlier this year. Then they cooled as abruptly in response to its decay. Now they’re around 0.2 deg C. Will global sea surface temperatures make a secondary rebound as they had during the 2009/10 El Niño, or will they hover, varying with seasonal and weather noise, waiting for the next El Niño or La Niña?  And what will it be, another La Nada–or an El Niño or La Niña?...MUCH MORE

As El Niño is defined as three rolling three month periods (i.e. five consecutive months) the 2012 event barely qualified and more accurately is referred to as "El Niño conditions" which is based on the temperature surpassing the 0.5 deg C threshold without regard to duration.

La Nada is interesting because, as NASA said in 2011:
What's to Blame for Wild Weather? "La Nada" 

Such sheets of fire, such bursts of horrid thunder,
Such groans of roaring wind and rain, I never
Remember to have heard; man's nature cannot carry
The affliction nor the fear … 

from Shakespeare's Tragedy of King Lear

June 21, 2011: 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 Niña," 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 Niña 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 Niña and El Niño are opposite extremes of a great Pacific oscillation. Every 2 to 7 years, surface waters across the equatorial Pacific warm up (El Niño) and then they cool down again (La Niña). Each condition has its own distinct effects on weather.

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 (OSTM)/Jason-2 satellite, Credit: NASA JPL) 

The winter of 2010 began with La Niña conditions taking hold. A "normal" La Niña 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 Niña petered out quickly, and no El Niño rose up to replace it. The jet stream was free to misbehave.

"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." MUCH MORE

The risk of tornado outbreaks is greater during La Niña and La Nada than it is during El Niño.
In 2012 the Great Plains Drought combined with the El Nino conditions resulting in a potential all-time low tornado count.