Extreme tornado outbreaks have become more frequent: Study

The average number of tornadoes per outbreak -- large-scale weather events that can last one to three days and span huge regions -- is rising in North America from about 10 to about 15 since the 1950s, says a new study.

Most death and destruction inflicted by tornadoes in North America occur during outbreaks, and the largest ever recorded happened in 2011. It spawned 363 tornadoes across the US and Canada, killing more than 350 people and causing damages worth $11 billion, the study pointed out.

"When it comes to tornadoes, almost everything terrible that happens, happens in outbreaks. If outbreaks contain more tornadoes on average, then the likelihood they'll cause damage somewhere increases," said lead study author Michael Tippett, climate and weather researcher at Columbia University in New York, US.

The findings appeared in the journal Nature Communications.

For this study, the authors calculated the mean number of tornadoes per outbreak for each year of the study period 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 study was coauthored by Joel Cohen, director of the Laboratory of Populations, which is based jointly at Rockefeller University and Columbia's Earth Institute. Cohen called the results "truly remarkable."

"The analysis showed that as the mean number of tornadoes per outbreak rose, the variance around that mean rose four times faster," study coauthor Joel Cohen, director of the Laboratory of Populations, which is based jointly at Rockefeller University and Columbia's Earth Institute, noted.

Extreme outbreaks have become more frequent because of two factors, Tippett said.

First, the average number of tornadoes per outbreak has gone up; second, the rapidly increasing variance, or variability, means that numbers well above the average are more common.

The study's authors said they do not know what is driving the changes.

"One big question raised by this work, and one we're working on now, is what in the climate system has been behind this increase in outbreak severity," Tippett said.