August 16, 2016

Center of U.S. tornado activity shifting east and south, possibly due to climate change

WEST LAFAYETTE, Ind. – A Purdue University research team has found that the center of tornado activity in the United States has shifted in recent decades, and this shift is possibly influenced by climate change.

“This completely redefines annual tornado activity in the United States,” said Ernest Agee, a professor in the Department of Earth, Atmospheric, and Planetary Sciences.

Findings detailed in a paper that appeared Aug. 4 in the Journal of Applied Meteorology and Climatology, published by the American Meteorological Society, show evidence that the central area of annual tornado activity has moved from Oklahoma to Alabama.

Agee’s team studied data from the past 60 years to look for a shift in annual tornado activity. The team divided the 60 years into two groups: 1954-1983, which was a time of cooler temperatures compared to an increasingly warmer second period, from 1983-2013.

Data showed a notable decrease in both annual counts and tornado days in the traditional “tornado alley” of the central plains, aided by declines in summer and autumn. However, annual values were sustained in the southeast with some increase in “Dixie alley” due in part to substantial autumn seasons increases from Mississippi to Indiana, Agee said.

One (250km x 250km) region in Oklahoma, for example, had the greatest annual number of tornado days in the first period. However, in the latter period, a similar Oklahoma region recorded the largest decrease in tornado days, while central Tennessee had the greatest increase.

These data support the research being performed as a part of the National Oceanic and Atmospheric Administration’s VORTEX-SE project, said Michael Baldwin, an associate professor in Purdue’s Department of Earth, Atmospheric, and Planetary Sciences. Baldwin leads a team that was selected as one of nine to be a part of the Verification of the Origins of Rotation in Tornadoes Experiment Southeast, or VORTEX-SE. The research will continue into 2017.

“As compared to the Great Plains, the southeastern United States experiences more unexpected tornadoes from small storms, tornadoes at night and tornadoes outside of the traditional spring tornado season,” he said.

While more research is needed, climate change may be influencing these results.

 “The geographical shift in tornado activity has been established through powerful statistical methods and is shown to occur during two successive 30-year periods moving from a colder weather pattern to warmer conditions,” Agee said. “More research is needed to search for changing climate trends responsible for tornado formation and this geographical shift, but climate change is a distinct possibility.”

The paper was co-authored by Agee and research assistant Jennifer Larson; and undergraduate students Alexandra Marmo and Samuel Childs in Purdue’s Department of Earth, Atmospheric and Planetary Science. Childs is now a graduate student in the Department of Atmospheric Science at Colorado State University. 

Sources: Ernest Agee, 765-494-3282, eagee@purdue.edu

Michael Baldwin, baldwin@purdue.edu


ABSTRACT

Spatial Redistribution of U.S. Tornado Activity between 1954 and 2013

Ernest Agee and Jennifer Larson

Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana

Samuel Childs

Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

Alexandra Marmo

Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana

Climate change over the past several decades prompted this preliminary investigation into the possible effects of global warming on the climatological behavior of U.S. tornadoes for the domain bounded by 30°–50°N and 80°–105°W. On the basis of a warming trend over the past 30 years, the modern tornado record can be divided into a cold “Period I” from 1954 to 1983 and a subsequent 30-year warm “Period II” from 1984 to 2013. Tornado counts and days for (E)F1–(E)F5, significant, and the most violent tornadoes across a 2.5° × 2.5° gridded domain indicate a general decrease in tornado activity from Period I to Period II concentrated in Texas/Oklahoma and increases concentrated in Tennessee/Alabama. These changes show a new geographical distribution of tornado activity for Period II when compared with Period I. Statistical analysis that is based on field significance testing and the bootstrapping method provides proof for the observed decrease in annual tornado activity in the traditional “Tornado Alley” and the emergence of a new maximum center of tornado activity. Seasonal analyses of both counts and days for tornadoes and significant tornadoes show similar results in the spring, summer, and winter seasons, with a substantial decrease in the central plains during summer. The autumn season displays substantial increases in both tornado counts and significant-tornado counts in the region stretching from Mississippi into Indiana. Similar results are found from the seasonal analysis of both tornado days and significant-tornado days. This temporal change of spatial patterns in tornado activity for successive cold and warm periods may be suggestive of climate change effects yet warrants the climatological study of meteorological parameters responsible for tornado formation. 


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