An Analysis of the Influence of El Nino and La Nina on Tornado Occurrence in the United States

El Niño-Southern Oscillation(ENSO) is a phenomenon that has far reaching implications for global climate. It influences a wide range of meteorological and oceanographic occurrences, but the totality of its effects are not yet fully understood. For example, it is generally accepted that hurricane activity over the Atlantic increases during La Niña years and decreases during El Niño years. It has been suggested that there exists a connection between El Niño and La Niña upon tornado formation in the United States. This paper examines annual tornado frequency data from years with high, moderate and low levels of the Multivariate El Niño Index and suggests evidence of a correlation between ENSO and the number of tornadoes within the United States for the period 1984-1997. Annual tornado frequency was found to increase during El Niño years. This result was especially true in the Midwest, the region often referred to as "Tornado Alley." Further, tornado activity decreased during La Niña, especially over the Southern Plains and, to a lesser extent, over the Upper Midwest. While the length of the data set may be somewhat limited, it does suggest that there is cause to examine a longer period to determine the correlation between tornado activity in the United States and the ENSO cycle.