Earlier this year, a Nature Communications paper reported that although the number of tornadoes per year in the United States remains consistent, the average number of tornadoes per outbreak has increased steadily from 10 per outbreak to 15 since 1954. They had no idea why.
The researchers also found that outbreaks with extraordinarily large numbers of tornadoes—like an outbreak in 2011 that included a tornado that razed neighborhoods and killed more than 100 people in Joplin, Mo., alone—have become more common as well.
The same research team, plus another colleague, set out to investigate possible causes of these expanding clusters of tornadoes. They found something unexpected: Effects from global warming may not be the cause of this increasing number. Their findings appeared last week in Science.
Every year, about 1200 tornadoes hit the United States, concentrated disproportionately in a region called “Tornado Alley,” which spans the south central United States (although meteorologists note that tornadoes can strike anywhere the environmental conditions are right). Tornadoes form out of thunderstorms because humid, warm, buoyant air meets relatively dry, cold air and winds moving at different speeds interact to form a vortex.
A tornado “outbreak” is defined many ways but generally refers to six or more tornadoes rated 1 or greater on the Fujita or enhanced Fujita intensity scale forming from the same weather system.
Led by climate and weather expert Michael Tippett (who was also the lead author on the Nature Communications paper) of Columbia University in New York City, the team homed in on two key meteorological ingredients for tornado formation to explore what may be causing the extreme outbreaks.
One of these ingredients is vertical wind shear, which refers to differences in wind speed and direction as you change altitude, Tippett told Eos. Another key ingredient in tornado formation is convective available potential energy (CAPE). This refers to the potential for the upward rush of warm air that invigorates a thunderstorm. Higher CAPE values mean higher chances that tornadoes will spawn.
Chasing the Cause
The researchers wanted to know to what degree the increasing number of tornadoes per outbreak was connected to climate change. Models of climate change show that “in a warmer climate, there tends to be generally more moisture in the atmosphere, which would allow values of CAPE to increase,” Tippett said. Prior studies have indicated that increases in CAPE are a signature effect of climate change on severe thunderstorms, he added.
The researchers dug into two data sets: one that compiled observations of actual tornadoes between 1951 and 2015 and one that compiled meteorological observations surrounding the tornadoes starting in 1979. The researchers specifically looked at the changes in the daily averages of CAPE and vertical wind shear and how they changed over the years compared with tornado occurrence.
The results surprised them: Instead of an increase in CAPE, which would tie the rise in the number of tornadoes per outbreak to climate change, they found an increase in vertical wind shear, Tippett said. This increase in wind shear may be what’s driving the increasing number of tornadoes per outbreak, the authors speculate.
“The study is important because it addresses one of the hypotheses that has been raised to explain the observed change in number of tornadoes in outbreaks,” said Harold Brooks, senior scientist at the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory, in a statement. “Changes in CAPE can’t explain the change. It seems that changes in shear are more important.”
However, the causes of the greater wind shear—and the accompanying weather patterns—remain unknown and an area for future research, Tippett said.
Although no signature of global warming emerged from the new analysis, the new findings don’t rule warming out as a cause. If climate change is contributing to the greater number of tornadoes in each outbreak, “then we might expect to see either continued increases in the number of tornadoes per outbreak or at least no return to earlier levels,” the 1 December Science paper notes.
Another possibility is that the outbreak trend results from shorter-term fluctuations in climate on timescales of several years or decades. In that case, the researchers write, “a return toward earlier levels [of the number of tornadoes per outbreak] might be possible in the future.”
—JoAnna Wendel (@JoAnnaScience), Staff Writer