Extreme tornado outbreaks may be far more common now than they once were. It turns out that something is causing more tornadoes to spawn together, which could be a major problem in areas of the United States.
"The science is still open," said Michael Tippet, a climate and weather researcher at Columbia University's School of Applied Science and Engineering and Columbia's Data Science Institute. "It could be global warming, but our usual tools, the observational record and computer models, are not up to the task of answering this question yet."
Each year, the United States experiences dozens of tornado outbreaks. Some of these are small and may only give rise to a few twisters. Others, though, can generate hundreds of tornadoes and cause severe damages. For example, in 2011 a severe outbreak spawned 363 tornadoes across the U.S. and Canada. This outbreak killed more than 350 people and caused $11 billion in damages.
The intensity of each tornado is ranked on a scale from zero to five with other descriptive terms thrown in. The lower gradations cause only light damage, while top ones can rip houses from their foundations.
In this latest study, the researchers calculated the mean number of tornadoes per outbreak for each year as well as the variance, or scatter, around this mean. They found that while the total number of tornadoes rated F/EF1 and higher each year hasn't increased, the average number per outbreak has, rising from about 10 to about 15 since the 1950s.
"The analysis showed that as the mean number of tornadoes per outbreak rose, the variance around that mean rose four times faster," said Joel Cohen, co-author of the new study and director of the Laboratory of Populations. "While the mean rose by a factor of 1.5 over the last 60 years, the variance rose by a factor of more than 5, or 1.5 x 1.5 x 1.5 x 1.5. This kind of relationship between variance and means has a name in statistics: Taylor's power law of scaling. We have seen [Taylor's power law] in the distribution of stars in a galaxy, in death rates in countries, the population density of Norway, securities trading, oak trees in New York and many other cases. But this is the first time anyone has shown that it applies to scaling in tornado statistics."
The findings show that there may be more tornadoes per outbreak. However, what's exactly causing this remains a mystery. Now, researchers are focusing on what in the climate system has increased outbreak severity.
