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

Historic winter tornadic activity (January, February, and March) between 1950 and 2003 was analyzed to determine the possible effect of seasonally averaged sea surface temperatures in the equatorial Pacific Ocean on the location and strength of tornado outbreaks in the United States. Tornadic activity was gauged through analyses of tornadoes occurring on tornado days (a calendar day featuring by 6 or more tornadoes in within the contiguous United States) and strong and violent tornado days (a calendar day featuring 5 or more tornadoes rated F-2 and greater within the contiguous United States). The tornado days were then stratified according to warm (37 tornado days, 14 violent), cold (43 tornado days, 28 violent), and neutral (61 tornado days, 44 violent) phases of winter average sea surface temperature anomalies.

It is seen that during winter periods of neutral tropical sea surface temperatures in the Pacific, there is a tendency for United States tornado outbreaks to be generally stronger and more frequent than they are during winter periods of anomalously warm tropical Pacific sea surface temperatures (El Nino). During winter periods with anomalously cool sea surface temperatures (La Nina), the frequency and strength of United States tornado activity lies between that of the neutral and El Nino phase. Shifts in tornadic activity according to ENSO phase are also observed. During the neutral phase, tornado outbreaks typically occur from the Central Oklahoma and Kansas eastward through the Carolinas. During cold Phases, historic tornado outbreaks typically occur in a zone stretching from Southeastern Texas northeastward into Illinois, Indiana, and Michigan. During anomalously warm phases activity is mainly limited to the Gulf Coast States including Central Florida.

These findings are examined statistically to indicate the strength of the relationship, and synoptically to show that they are meteorologically reasonable.