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Seasonal Frequency and Spatial Distribution of Tornadoes in the United States and Their Relationship to the El Niño/Southern Oscillation

Tornadoes are extremely destructive phenomena that can cause tremendous loss of life and catastrophic damage to the natural and built landscape. Their frequency of occurrence varies notably from season to season and from year to year. Tornado variability in the United States and its covariates have been the focus of numerous recent studies. The El Niño/Southern Oscillation has emerged as a covariate with some predictive skill, but most recent studies have focused on its link to tornado activity in boreal winter and spring. This study provides an analysis of the relationship between tornado activity in the United States and the El Niño/Southern Oscillation in all four seasons and for multiple regions. In doing so, this study illustrates that the relationship between tornado activity and the El Niño/Southern Oscillation is seasonally and regionally dependent. The relationship in winter and spring is confirmed but only in the Southeast and Midwest regions. Furthermore, the relationship is stronger in winter than in spring in the Southeast and Midwest. The ability of the El Niño/Southern Oscillation to predict the odds of below- or above-normal tornado activity is also greatest in winter in these regions. There is little evidence of a relationship between tornado frequency and the El Niño/Southern Oscillation in summer and fall, with the exception of a positive correlation between the El Niño/Southern Oscillation in spring and tornado frequency in summer. Key Words: El Niño/Southern Oscillation, seasonal tornado frequency, spatial distribution of tornadoes, United States.