2008 has seen a record outbreak of tornadoes in the United States from California to the Midwest, from the South through the central plains, to the Appalachian states.
"The assessment for 2008 is that there will be a strong La Niña event in the Pacific, which will limit the warming trend for the year (whilst still being one of the warmest years)."
The La Niña phenomenon is an upwelling of colder waters resulting in a change in ocean temperature that causes a shift in the jet stream, reducing corresponding climate temperature. A NOAA study from October, 1999, still referenced on their site, which uses data from 1950 through 1996, concluded there was no tornadic connection to the El Niño/La Niña event. Since then, however, Joseph Schaefer, Director of NOAA's National Weather Service's Storm Prediction Center, according to this February MSNBC report, has revised his position:
Tornadoes do happen in February, but a study by Schaefer two years ago found that winter tornadoes in parts of the South occur more frequently and are stronger when there is a La Niña.
The La Niña connection was also cited in a Purdue University study reported in Science Daily (also from 1999), this one with data from 1916 to 1996, which offered the intriguing possibility that the La Niña event could be traced to a geographical shift in tornadic activity:
"Though the study provides little reason to expect more or fewer tornadoes overall, the findings show clear evidence of geographical shifts in tornado activity within the United States when comparing strong El Niño years to La Niña years," says [Ernest] Agee, professor of atmospheric sciences at Purdue who has studied tornadoes for more than 30 years.
The La Niña event, the opposite of the El Niño phenomenon, has long been associated with intensified hurricane seasons (a prediction for the 2008 season), colder winters (i.e. China) and, as evidenced by recent events, with increased tornadic activity:
La Niña, which means "the little girl," is characterized by unusually cold ocean temperatures in the eastern tropical Pacific, as compared to El Nino, which is characterized by unusually warm temperatures in the same waters. A La Niña event, the subtropical jet -- the jet stream that brings warm moist air from the south -- shifts to the far north, bringing an influx of warmth and moisture to these regions, and increasing the odds for tornadoes.
The geographical shift theory supports this University of Oklahoma assessment from February, 2008, after large scale tornadoes touched down in Kentucky and Tennessee:
February tornadoes usually pop up near the Gulf Coast, not in Kentucky or Tennessee, said University of Oklahoma meteorology professor Howard Bluestein.
Bluestein went on to cite that one factor in the storms' intensity was the record warmth in Oklahoma (84 degrees) which the storm moved through on its "path of destruction."
In February, 97 weather stations broke or tied temperature records in Arkansas, Alabama, Tennessee and Kentucky -- the hardest-hit states.
Tornadoes are a natural phenomenon that have existed long prior to climate change. They are the product of specific conditions, upper level lows, strong winds, geography, the earth's rotation, the confluence of warm and cold fronts, etc.
The La Niña event is also a natural phenomenon with a recorded history prior to any reports of climate change. What begs further investigation, given the record February temperatures in the effected areas, is whether a change in sea level due to melting ice may have had or will have an impact on the phenomena and/or if the warmer air produced by climate change has intensified the process by which dangerous storms that may be related to La Niña are created.
The La Niña event is expected to stay strong through the end of the year.
More on this topic at The Environmentalist.