科学美国人60秒:La Ni?a Conditions Spin Up More Springtime Twisters

It's been a light year for tornadoes in the U.S.—just a few dozen so far. By this time in 2011, a notoriously deadly tornado year, there had already been more than 120 twisters. Why the variability? 

For decades, scientists have hypothesized that tornado frequency may be related to the fluctuation of sea surface temperatures in the equatorial Pacific—known as El Niño and La Niña. But the link wasn't clear in the peak springtime tornado season. Because observations of tornadoes and hail tend to be most prevalent where people can see them—meaning the data isn't that comprehensive.

So researchers took a step back, looking instead at the basic ingredients of tornadoes and hail: factors like wind shear, humidity and temperature. During La Niña, they indeed saw a boost in the conditions that breed springtime twisters and hail in the southeastern U.S. In El Niño years, they saw the opposite. The findings appear in the journal Nature Geoscience. [John T. Allen et al, Influence of the El Niño/Southern Oscillation on tornado and hail frequency in the United States]

study author John Allen, a climatologist at the Earth Institute at Columbia University, says the data might be used to create yearly tornado forecasts, like this one: "So the forecast for this spring is a 60 percent chance of being around about normal, because we've got a relatively weak El Nino pattern, which tends to suppress a little bit. So we'd probably say that it's going to be at or below normal." 

Insurance companies or governments could use that data to gauge risk, he says—but no forecast can guarantee safety. "One tornado will still wreck a house. It will still end somebody's life, or destroy their life. And this is the thing to remember. We're saying there’s a lower chance, but even in a quiet year we’re still seeing about 800 tornadoes in the United States. And you know, if the right conditions match up, a tornado can form anywhere on this planet."

—Christopher Intagliata