Revisit: Impacts of ENSO on United States tornadic activity

Studies of the El Niņo Southern Oscillation (ENSO) interannual fluctuations have usually focused on the synoptic scale environment. However in the past decade the spotlight has shifted to its influence on meso and micro-scale variability. This study aims to create an accurate link between ENSO phases and tornado frequency in the United States with additional data, 1950-2007, and revised methods including the use of a monthly ENSO index. The investigation is centered on the eastern two-thirds of the United States: 25°N to 50°N and 75°W to 110°W. A higher resolution grid is utilized to examine tornado frequency variability. This region is partitioned into 1° by 1° bins for statistical analysis to provide confidence levels for observed changes in tornado occurrences. In addition to statistical significance tests, large-scale climate variables associated with ideal conditions for tornado development are analyzed to explain regional shifts in tornado variability. A threshold, which takes into account the upward trend in tornado observations, is used to identify the top active days in the period of record. Tornado active day frequency and spatial patterns are examined as a function of ENSO phase.

Regions that exhibit higher tornado frequency will be identified as they relate to the ENSO phase and season. Preliminary results indicate that 80% of the top 15 active days occurred during the neutral phase of ENSO. During El Niņo active days, spatial patterns reveal higher occurrences of tornadoes within Tornado Alley. La Niņa active days produced a small peak of tornado frequency along the Ohio River Valley. ENSO's neutral phase leads to higher concentrations of active day tornadoes observed in the Central United States. When compared to the neutral phase of ENSO, there is a decrease in tornado occurrences in Tornado Alley during spring El Niņo episodes and there is an increase of events in the belt stretching from Mississippi to Indiana during La Niņa, late winter to spring.