Do El Nino and La Nina affect tornado activity in the United States?

The effect of El Nino and La Nina on the North Atlantic hurricane season is relatively easy to analyze compared to the effect of ENSO on tornado activity in the United States. However, Mark C. Bove of the Center for Ocean-Atmospheric Predication Studies has tackeled the very complex analysis of data and reached a conclusion. The following is the abstract of his findings. Use the link at the end to access his full report. (Note: El Viejo is another term for La Nina.)

Impacts Of ENSO On United States Tornadic Activity

Mark C. Bove

Center for Ocean-Atmospheric Prediction Studies (COAPS)
The Florida State University
Tallahassee, FL 32306-3041

Submitted to the
Bulletin of the American Meteorological Society
9 April 1999

Abstract

Changes in annual and seasonal tornadic activity during El Niño/Southern Oscillation (ENSO) events are assessed in 2.5° by 2.5° and 1.25° by 1.25° bins from 25° to 50° N and 75° to 110° W (the eastern two-thirds of the United States). Forty-two years (1950-1992) of observed tornado occurrences are classified, using sea surface temperature anomaly data from the equatorial Pacific Ocean, as occurring during El Niño (anomalously warm tropical Pacific waters), La Niña (anomalously cold tropical waters), or neither (neutral). Statistical distributions of seasonal tornado occurrences are estimated by resampling available data via a 'bootstrap' technique.

The amount of annual and seasonal tornadic activity during an ENSO phase is calculated according to where the tornado touched ground. Tornado occurrences are assumed to be independent random variables with Poisson distributions over an entire year. Mean differences in tornadic activity between extreme ENSO phases and neutral years are examined for statistical significance. This significance is calculated for four different three month spring seasons during an 'ENSO year', defined as running from October through the following September.

The results indicate that El Niño events reduce tornadic activity in the southern plain states, while El Viejo events increase tornadic activity in the Ohio River Valley and Deep South. Results further show that El Niño inhibits the chances of multiple tornado outbreaks, while La Niña facilitates large tornadic outbreaks and produces more devastating tornadoes.

For details, maps, graphs, and diagrams, read the full article.