New findings suggest severe tornado outbreaks are increasingly common

http://www.eurekalert.org/pub_releases/2016-03/ru-nfs030716.php

Public Release: 7-Mar-2016
New findings suggest severe tornado outbreaks are increasingly common
Rockefeller University

One tornado alone can cause intense destruction, but the largest impact on both death rates and economic losses stems from "outbreaks," in which six or more tornadoes occur within a limited time.

New research from Joel Cohen, Abby Rockefeller Mauzé Professor and head of the Laboratory of Populations at The Rockefeller University, and Michael Tippett of Columbia University, shows that the number of tornadoes that occur within each outbreak has increased over the past 60 years. Published recently in Nature Communications, findings supported by mathematical models indicate that the likelihood of extreme events -- outbreaks with considerably more tornadoes than average--is growing over time.

"Over a recent 10 year period, tornadoes in the United States resulted in an average of 110 deaths per year and annual losses ranging from $500 million to $9.6 billion," says Cohen. "Outbreaks are the main offender,

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Using reports from the National Oceanic and Atmospheric Administration, the researchers found that the average number of tornadoes per outbreak increased by about 50 percent from 1954 to 2014. However, the total number of tornadoes has been largely the same from year to year, suggesting that tornadoes are increasingly clustered in outbreaks.

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"Variance is growing faster than I would have guessed, and we don't know why it's so different from what we find elsewhere," says Cohen. "If I were to speculate, I would say that certain physical drivers are accelerating, leading to increased energy in the atmosphere, which affects the forces behind tornadoes. Our results do not directly link climate change to the increasing severity of outbreaks, but we've found an indicator of change that's hard to explain otherwise."

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