by Paulina Ćwik
“I saw devastation. The town was just destroyed. This was the end of Brandenburg (Kentucky) as I knew it” recalls Jane Willis after a violent tornado ravaged her hometown during a Super Tornado Outbreak of 1974 (“When Weather Changed History – Super Outbreak”). In the United States, stories like Jane’s echo almost every year, specifically across communities in the regions east of the Rocky Mountains. Major tornado outbreaks, defined as seven or more tornadoes in a group rated F2 or higher on the Fujita damage-based scale, are particularly dangerous, because they comprise multiple, violent, long-track tornadoes that can affect more places and therefore more people. With the ever-growing population and expansion of urban areas, exposure of people to future tornado outbreak impacts is increasing.
Recently, the increase in global mean temperature trends provoked questions on the potential impacts of climate change on tornado outbreak characteristics. Specifically, as the earth continues to warm, more heat, energy, and moisture are available in the atmosphere to influence the conditions that favor or disfavor tornadic storms. For example, some studies documented an increase in tornado frequency in the eastern part of the United States and a decrease in the central Great Plains, historically known as Tornado Alley. This is a challenge, because the increasing trend occurs in the region that is already particularly vulnerable to tornadic storms because of a large population density and complex, forested terrain with limited visibility. Some changes have been found in the annual timing of tornadoes, with a trend towards earlier tornado season. Additionally, an increase in the annual number of extreme tornado outbreaks and in the mean number of tornadoes per tornado outbreak has been recently reported.
These changes in the tornado outbreak spatiotemporal characteristics bring us to a question: What will future tornado outbreaks look like? That’s where my research comes in. In order to anticipate when and where these extreme events may be occurring in the future, we first must look at the present and the past.
Research found that tornadic storms occur in specific environmental settings that are supportive to tornado formation. Specifically, major tornado outbreaks, such as the one in 1974 or more recently in April of 2011, require at least a few conditions, such as (1) high atmospheric instability, (2) strong vertical wind shear (the change in wind speed and/or direction with height), (3) an abundant amount of moisture in the lower atmosphere (between the surface and 3 km), and (4) an initiating mechanism, for example, an orographic feature (mountain chain), or a frontal zone. These atmospheric components, or tornado building blocks, are often combined into metrics, or parameters, that can detect when atmospheric conditions are ripe to produce violent tornadoes.
Using reanalysis data, which are datasets describing recent historic state of the atmosphere, land surface, and oceans, we can search for the environmental conditions that occurred during the times of identified historic tornado outbreaks. Then, by means of various statistical methods, such as maximum covariance analysis (MCA), we can search for atmospheric patterns of various environmental variables or parameters, that are most common among historic tornado outbreaks.
Finally, we can examine future climate projections for those atmospheric patterns most commonly associated with historic tornado outbreaks. If they are well represented, simulations of future environmental conditions under different climate scenarios can reveal how frequency and location of the patterns may change.
Although assessing possible connections between tornado outbreaks and anthropogenic climate change is a difficult and time-consuming task, it is an important one! In the process, we can gain insights as to the future of tornado outbreak characteristics, which can be translated to meaningful actions that protect people who find themselves in harm’s way.
Additional readings:
Ćwik, P., R. A. McPherson, and H. E. Brooks, 2021a: What is a Tornado Outbreak? Perspectives through Time. Bull. Amer. Meteor. Soc., 102, E817–E835, https://doi.org/10.1175/BAMS-D-20-0076.1.
——, ——, M. B. Richman, and A. E. Mercer, 2021b: Identification of mid-tropospheric patterns associated with tornado outbreaks in the United States. Mon. Wea. Rev., 1–48.
Elsner, J. B., S. C. Elsner, and T. H. Jagger, 2015: Elsner, J.B., Elsner, S.C. and Jagger, T.H., 2015. The increasing efficiency of tornado days in the United States. Climate Dynamics, 45(3), pp.651-659. Clim. Dyn., 45, 651–659.
Gensini, V. A., and H. E. Brooks, 2018: Spatial trends in United States tornado frequency. npj Clim. Atmos. Sci., 1, 1–5, https://doi.org/10.1038/s41612-018-0048-2.
Moore, T. W., 2018: Annual and seasonal tornado trends in the contiguous United States and its regions. Int. J. Climatol., 38, 1582–1594, https://doi.org/10.1002/joc.5285.
Tippett, M. K., C. Lepore, and J. E. Cohen, 2016: More tornadoes in the most extreme U.S. tornado outbreaks. Science, 354, 1419–1423, https://doi.org/10.1126/science.aah7393.
Paulina Ćwik is a PhD student in the Department of Geography and Environmental Sustainability at the University of Oklahoma. Her dissertation examines atmospheric patterns that cause violent tornado outbreaks. For any questions or additional information, contact Paulina at paulinacwik@ou.edu. Follow her on Twitter at @paulinacwik
Early Career Climate Network 