El Niño-Southern Oscillation
Climate Fact: ENSO and Winter Tornadoes
The El Niño-Southern Oscillation (ENSO), or the cyclical movement of heat in the tropical Pacific Ocean, affects atmospheric phenomena throughout the world. Its effects are the most pronounced during the winter, when the temperature difference between the atmosphere and the oceans is the greatest.
The cycle affects the strength and position of the Pacific Jet Stream, an upper atmosphere wind current that flows from the Pacific over North America. During La Niña phases of the cycle, the Jet Stream is weaker than average and flows in an arcing pattern over the northern U.S. During El Niño phases, the Jet Stream strengthens and flows over the southern United States. The position and strength of this air current affect the location and frequency of wintertime (January through March) tornado occurrences. Winter tornado outbreaks are concentrated in the southeastern U.S. For the United States as a whole, winter tornadoes are most frequent during neutral ENSO phases, second most frequent during La Niña phases, and the least frequent during El Niño phases.
(Source: Cook, AR and Schaefer, JT et al. “The Relation of the El Niño-Southern Oscillation (ENSO) to Winter Tornado Outbreaks.” Monthly Weather Review 136 (2008): 3121-3137.)
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