A tornado can cause localized destruction, but the most serious problems arise when a storm system spawns multiple tornadoes. This creates what is called a tornado outbreak, which spreads destruction over a wider area. Now, a new study suggests the most violent tornado outbreaks are on the rise. But the researchers behind the study see no indication that the increase in tornado outbreaks is related to our warming climate.
It would make sense for a warming climate to affect tornado activity. After all, higher temperatures mean more energy in the atmosphere, potentially fueling the storms. But previous studies have had mixed results when it comes to tornado activity. There is no significant trend in the number of tornadoes or frequency of outbreaks (defined as six or more tornadoes occurring in rapid succession). At the same time, tornadoes occur in a greater part of the year and the number of tornadoes in outbreaks has become increasingly variable.
A team of researchers from Columbia University (Michael Tippett, Chiarra Lepore and Joel Cohen) decided to take a closer look at this last figure. They collected data on the number of storms in outbreaks during the period between 1965 and 2015. While there was no trend in the number of outbreaks, the number of tornadoes per outbreak has increased over that period. Not only did the average number of tornadoes per outbreak increase, but the more extreme outbreaks — those with the most storms — increased the fastest.
So the authors formally defined an “extreme outbreak” as an event that spawns a dozen or more tornadoes. There were 435 in their study period, but there was no indication of a statistically significant increase over that time. They then divided those 435 events into groups representing different percentages of severity (again measured in the total number of tornadoes per outbreak). With this data, the authors saw a clear trend: the most extreme outbreaks have increased over this period.
While the trend wasn’t extreme, it got bigger over time. “The estimated number of tornadoes in the most extreme 5-year outbreak roughly doubles from 40 in 1965 to nearly 80 in 2015,” the authors note.
The authors then checked whether correlations with known climate cycles, such as the Pacific Decadal Oscillation, could explain the changes. That did not work. It also didn’t match an atmospheric change caused by rising temperatures. What did show a connection was a specific measure of atmospheric wind shear, but that measure had not previously been linked to climate.
The authors argue that this association with wind shear also means that the trend is unlikely to improve meteorologists’ ability to identify tornadoes. But they also note that many of the outbreak measures they’re excited about fell to their lowest levels in a decade last year. So while the trend is significant, we may want to revisit it in a few years.
Science2016. DOI: 10.1126/science.aah7393 (About DOIs).