Detecting a Climate Signal in the U.S Tornado Record Josh Hatzis, Don Callaway, Colten Presgrove

Department of Geography and Environmental Sustainability, University of Oklahoma

Statement of Purpose • Recent major tornadoes have caused increasing interest in the

climate change implications for tornado frequency and intensity • May 22, 2011: an EF5 tornado hit Joplin, Missouri,

causing 158 direct fatalities and an estimated $2 – 3 billion dollars in damage.

• May 20, 2013: an EF4 tornado hit Moore, Oklahoma, causing 24 direct fatalities (many of them young children) and leveling or severely damage over 300 homes.

• Our project seeks to identify changes in patterns of tornado intensity, frequency and distribution and link these changes to changes in climate.

Results

Introduction • Tornadoes typically form from severe thunderstorms occurring in an

environment of high vertical wind shear and convective available potential energy (CAPE; Brooks et al., 2013)

• Climate change is projected to change severe thunderstorm environments in the following ways

• Increased temperatures may lead to increased evaporation, instability and CAPE (Trapp et al., 2007).

• Reductions in surface albedo may reduce latitudinal temperature gradients and vertical wind shear (IPCC, 2013 ; Trapp et al., 2007)

• Changes in atmospheric circulation patterns may change storm tracks and tornado distributions (Diffenbaugh et al., 2008).

Data and Methods • Tornado reports were collected from the US Storm Prediction Center's

(SPC's) Storm Data and "Storm Events" database (SPC, 2014) for the period of 1954 to 2013

• Data quality is evaluated by Verbout and others (2006). • Temperature and precipitation grids were collected from the PRISM

Climate Group, Oregon State University (http://prism.oregonstate.edu; PRISM , 2014)

• Annual trends were analyzed using a simple linear regression t-test. • Correlation between changes in tornado density, precipitation, and

temperature were analyzed using multiple linear regression

Discussion • Climate change could potentially manifest itself through changes in tornado

frequency, intensity, or distribution. • The general tendency towards a decreasing number of days with tornadoes

and the increase in days with multiple tornadoes mirrored findings by Verbout and others (2006) and Eisner and others (2014) and is in line with expectations for a warmer world.

• We found a significant but weak correlation between Mean Annual Tornado Density and Mean Annual Temperature which indicates that climate change does appear to be impacting tornado distributions but the relationship is far more complex than a simple correlation between two variables.

References • Brooks HE. (2013). Severe thunderstorms and climate change. Atmospheric Research, 67: 73-94 • Diffenbaugh, N.S., Trapp, R.J. and Brooks, H. 2008. Does global warming influence tornado activity? EOS, Transactions, American Geophysical Union. 89:

553-554. • Eisner, JB., Eisner, SC., and Jagger, TH. 2014. The increasing efficiency of tornado days in the United States. Climate Dynamics. DOI 10.1007/s00382-014-

2277-3. • Intergovernmental Panel on Climate Change [IPCC]. (2013). Climate change 2013: The physical science basis. Contribution of Working Group I to the Fifth

Assessment Report of the Intergovernmental Panel on Climate Change. In Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (Eds.). New York: Cambridge University Press.

• PRISM Climate Group, Oregon State University (2014). PRISM Gridded Climate Data. Available from PRISM Climate Group, Oregon State University: http://prism.oregonstate.edu

• Storm Prediction Center [SPC]. (2014) Storm Data and “Storm Events” Database. Available from the National Climatic Data Center’s Web site: http://www.ncdc.noaa.gov/stormevents/ftp.jsp

• Trapp, RJ., Diffenbaugh, NS., Brooks, HE., Baldwin, ME., Robinson, ED., and Pal, JS. (2007). Changes in severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing. Proceedings of the National Academy of Sciences, 104 (50): 19719-19723.

• Verbout, S. M., Brooks, H. E., Leslie, L. M., & Schultz, D. M. (2006). Evolution of the U.S. tornado database: 1954-2003. Weather and Forecasting, 21 (1), 86-93.

Figure 1. Moore, OK Tornado on May 20, 2013

Results

Figure 3. Total number of tornadoes per county during the period of 1954 – 2013. Surrounding figures show the annual tornado density for EF1+ (red) and EF3+ (blue) tornadoes with accompanying trend lines for each climate region in the contiguous US (except for Northwest).

Figure 4. (a) Number of EF1+ (red) and EF3+ (blue) tornadoes per year for the contiguous US with accompanying trend lines. (b) Number of days per year with at least one EF1+ (red) and EF3+ (blue) tornadoes

Figure 5. Number of days with at least 4, 8, 16, and 32 tornadoes for contiguous US, with trend lines

Figure 6. (a) Change in mean annual tornado density per county between the periods of 1954-1983 and 1984-2013. (b) Same as a, only for mean annual temperature. (c) Same as a only for mean annual precipitation. (d) Counties where mean annual tornado density changes in the same direction as mean annual temperature and mean annual precipitation. (e) Same as d only compared to just mean annual temperature. (f) Same as d only compared to just mean annual precipitation.

Figure 2. Lowering wall cloud in Norman, OK on May 19, 2013