Due to the shortcomings of contemporary global climate models (GCMs), the impacts of climate change on the frequency and seasonality of tornadoes has not been well studied. Prevailing research on the topic favors an ingredients-based methodology which uses instability, moisture and wind shear estimations derived from GCM output data in order to project the change in the number of favorable environments for severe weather. However, by utilizing synoptic climatological methods, this thesis takes an entirely different approach to directly projecting future tornado occurrence.
Using a six-step process, that includes principal components analysis, cluster analysis, and discriminant function analysis, this thesis first creates a continental-scale map pattern classification at three levels of the atmosphere, from geopotential height (at 500mb and 700mb) and temperature (850mb) data fields. These patterns are then associated to F2 and stronger United States (US) tornado days from 1957-2002 using binary logistic regression. Then, using output data from two GCMs, spanning five different model emissions scenarios, this synoptic climatology of tornadoes is utilized in order to project the changes in the frequency and seasonality of US tornadoes due to a changing climate.
Results indicate that US tornado days will increase anywhere from 3.8% to 12.7% by the 2090s. The majority of this increase is likely to be manifested in the earlier part of the tornado season – from February through April, while a slight decrease is expected over the US as a whole from May through August. In addition to the shift in seasonality, a broadening of the peak tornado season is also noticed under some scenarios – from a one month peak in May in the 20th Century, to a two or three month peak by the end of the 21st Century. Geographically, portions of the Northern and Central Plains, the High Plains, the Lower Great Lakes, the Mid-Atlantic states, and the Southeast are projected to experience an increase in tornado days under some future scenarios. The Upper Great Lakes states and the Southern Plains are projected to experience a decrease in tornado days.