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

The impact of El Niño-Southern Oscillation (ENSO) on United States tornadoes has been a controversial topic in the last 15 years. Widely varied and contradicting conclusions exist regarding the topic, especially when attempting to correlate shifts in location of tornadoes to ENSO phase. For instance, Hagemeyer (1998) noted a tendency toward stronger, more frequent tornadoes in Florida during El Niño years. Meanwhile, Schaefer and Tatum (1998) could find no statistical basis for correlation between Florida tornadoes and ENSO phase, and Bove (1998) suggested that Florida experiences fewer tornadoes in both El Niño and La Niña phases. Research by Schaefer and Tatom (1998) and Marzban and Schaefer (2000) found some change in tornado frequency for La Niña years in the Mid-eastern and Northeastern United States, respectively. Browning (1998) also saw some change in frequency of tornadoes across northwestern Missouri as a function of ENSO phase. However, Agee and Zurn-Birkhimer (1998) noted ENSO related shifts in tornadic activity throughout the entire two-thirds of the United States. Some controversy also exists in discerning the role of ENSO phase in the strength of tornadoes. Some research (Knowles and Pielke 1993, Bove 1998) suggests that stronger tornadoes occur in La Niña patterns, while others (Agee and Zurn-Birkhimer 1998, Schaefer and Tatom 1998) suggest phase does not have a role in the development of stronger or longer track tornadoes in any phase. In addition, researchers do not agree on the seasonal and monthly variations of tornado activity as a function of ENSO phase. Schaefer and Tatom (1998) address monthly variations in tornado activity as a function of ENSO phase and suggest very little correlation between tornado occurrences and phase throughout the entire United States east of the Rockies. Sankovitch et al. (2004) investigated winter ENSO phase associated with southeast United States tornado patterns and could not identify shifts in tornadic activity due to ENSO phase, although they did find shifts in thermodynamic and kinematic upper air profiles which were associated with ENSO phase. Agee and ZurnBirkhimer (1998) found shifts in tornado maxima across the Central and Southern Plains and Gulf Coast in strong El Niño years, shifting to the Ohio and Tennessee Valleys and mid-Atlantic region during La Niña years. These results contradict those of Schaefer and Tatom (1998) and Schaefer and Marzban (2000), which show very little indication of seasonal and monthly variations of tornado activity as a function of ENSO phase. The present paper discusses shifts in historic winter tornadic activity across the United States according to ENSO phase. The strength and length of tornadoes occurring in individual outbreaks, along with the frequency of tornado outbreaks, will also be discussed. Shifts in occurrence of tornado outbreaks will then be related with shifts in jet stream patterns prescribed by the Climate Prediction Center (CPC). This paper will conclude with a brief discussion on severe weather in relation to ENSO.