Mystery shrouds increasing frequency of tornado outbreaks in the United States. According to a recently published research study in Science, the frequency of large-scale tornado outbreaks is increasing in the United States, particularly when it comes to the most extreme events. Findings of the study show that the increase in tornado outbreaks does not appear to be the result of a warming climate as earlier models suggested. Instead, the findings tie the growth in frequency to trends in the vertical wind shear found in certain supercells–a change not so far associated with a warmer climate.
Reporting that tornado outbreaks are large-scale weather events that last one to three days, featuring several thunderstorms and six or more tornadoes in close succession, the study adds: “What’s pushing this rise in extreme outbreaks, during which the vast majority of tornado-related fatalities occur, is far from obvious in the present state of climate science.” The researchers estimated that the number of tornadoes in the most extreme outbreak in a five-year interval doubled over the last half-century. This means that in 1965 the worst outbreak expected over five years would have had about 40 tornadoes, while in 2015 the worst outbreak expected over five years would have had about 80 tornadoes.
To understand the increased frequency in tornado outbreaks, the researchers looked at two factors: convective available potential energy, or CAPE, and storm relative helicity, which is a measure of vertical wind shear. It is reported that past studies had projected a warming climate would increase CAPE, creating conditions favorable to a rise in severe thunderstorms–and potentially tornado outbreaks. But researchers of present study found the increases in outbreaks were driven instead by storm relative helicity, which has not been projected to increase under a warming climate.
Co-author of this study, Michael K. Tippett, says: “Our study raises new questions about what climate change will do to severe thunderstorms and what is responsible for recent trends. The fact that we didn’t see the presently understood meteorological signature of global warming in changing outbreak statistics for tornadoes leaves two possibilities: Either the recent increases are not due to a warming climate, or a warming climate has implications for tornado activity that we don’t understand.”