Extreme tornado outbreaks have become more frequent and the average number of tornadoes in such outbreaks has also risen since 1954, a new study has found.
Most death and destruction inflicted by tornadoes in North America occurs during outbreaks - large-scale weather events that can last one to three days and span huge regions.
The largest ever recorded happened in 2011. It spawned 363 tornadoes across the US and Canada, killing more than 350 people. The researchers said they do not know what is driving the changes.
“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,” said lead author Michael Tippett, from the Columbia University in US.
Many scientists expect the frequency of atmospheric conditions favourable to tornadoes to increase in a warmer climate - but even today, the right conditions do not guarantee a tornado will occur.
Every year, North America sees dozens of tornado outbreaks. Some are small and may give rise to only a few twisters; others, such as the so-called “super outbreaks” of 1974 and 2011, can generate hundreds.
In the simplest terms, the intensity of each tornado is ranked on a zero-to-five scale, with other descriptive terms thrown in.
For the study, researchers calculated the mean number of tornadoes per outbreak for each year as well as the variance around this mean.
They found that while the total number of tornadoes rated F/EF1 and higher each year has not increased, the average number per outbreak has, rising from about 10 to about 15 since the 1950s.
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 scientific community has thought a great deal about how the frequency of future weather and climate extremes may change in a warming climate,” Tippett said.
“The simplest change to understand is a shift of the entire distribution, but increases in variability, or variance, are possible as well. With tornadoes, we’re seeing both of those mechanisms at play,” Tippett added. The study was published in the journal Nature Communications.