The Relation of El Nino Southern Oscillation (ENSO) to Winter Tornado Environments

Abstract

Previous analyses of the relationship between tornadoes and El Nino Southern Oscillation (ENSO) have been complicated by biases in tornado counts. Here I analyze winter tornado activity across the eastern United States using sounding-based tornado environments rather than tornado counts to determine the possible effect of El Nino Southern Oscillation (ENSO) phase on both the frequency and intensity of tornadoes. A pre-existing tornado environment database spanning 1958-2000 was used as an objective indicator of tornado activity. The database was analyzed in totality and partitioned by environment severity class. Results indicate that during La Nina there is an increase in the frequency of tornado environments over what would be expected by chance. This result holds for both weak and strong environments. This stands in contrast with previous studies using tornado counts, where only strong tornadoes could be linked to ENSO. El Nino results display an apparent southward shift in the frequency of tornado environments relative to neutral conditions. During neutral conditions, the frequency of tornado environments is widespread across a region to the south of the Ohio valley extending from Texas to Florida. Broadly speaking these results confirm conclusions drawn using count data.