Frequency of tornadoes hail linked to el nino

http://www.sciencedaily.com/releases/2015/03/150316135146.htm

Climate scientists can spot El Niño and La Niña conditions developing months ahead of time, and they use this knowledge to make more accurate forecasts of droughts, flooding and even hurricane activity around the world. Now, a new study shows that El Niño and La Niña conditions can also help predict the frequency of tornadoes and hail storms in some of the most susceptible regions of the United States. The study appears in the current issue of the journal Nature Geoscience.

While the information can't pinpoint when and where storms will wreak havoc, it will nevertheless be useful for governments and insurance companies to prepare for the coming season, Allen said. In recent weeks, researchers from IRI and other institutions have detected El Niño conditions over the Pacific, which implies that this spring will be a relatively quiet one for severe storms in the southern United States.

Last year, 47 people died in tornadoes. But in 2011--a La Niña year-- tornadoes killed more than 550 people, higher than in the previous 10 years combined. Hail storms and tornadoes cause an average estimated $1.6 billion in insured losses each year in the United States, according to the insurer Munich RE. Powerful, isolated events such as the 2011 Joplin, Missouri, tornado can smash that average. That storm alone caused several billion dollars in damage and killed 158 people.

The idea that ENSO can affect the frequency and locations of tornadoes and other severe storm systems isn't new. It is already known to exert a strong influence on temperatures and rainfall in the United States, and affect the position of the jet stream. Yet scientists have had difficulty quantifying ENSO's role in tornadoes, for two reasons. First, a variety of other factors can make them seemingly random: one year can see hundreds of twisters, while another sees few. Also, historical weather records are not reliable for long enough to make strong statistical connections. This is true especially for tornadoes, which often flare up and die quickly.

They note caveats, however. First, ENSO is not the only driver of severe storms. "Any kind of extreme weather is at most only loosely controlled by coherent, predictable climate phenomena like ENSO, and tornadoes are no exception," said coauthor Adam Sobel, who also is at Columbia's engineering school, as well as its Lamont-Doherty Earth Observatory. Second, the current study shows robust correlation only in the southern states, where the ENSO signal is especially clear. "A lot of the year-to-year variability is for all practical purposes random and unpredictable," said Sobel, who also directs a new Columbia University Initiative on Extreme Weather and Climate.