Active Tornado Seasons, Big Outbreaks and Stronger Tornadoes Have Been Shown to Be Associated With La Niñas and Natural Variability in the Pacific
Knowles, J.B. and Pielke Sr., R.A. 2005. The Southern Oscillation and its effect on tornadic activity in the United States. Atmospheric Science Paper No. 755, Colorado State University, Fort Collins, CO 80523, 15 pp. (Originally prepared in 1993, published as a Atmospheric Science Paper in March 2005).
|"The Southern Oscillation has been shown in previous research to cause changes in the weather patterns over the continental United States. These changes, caused by either the warm El Niño or cold La Niña, could potentially affect numbers, locations, and strengths of tornadoes in the United States.
Using a variation of the Southern Oscillation Index, the seven strongest El Niño and five strongest La Niña events during the period 1953-1989 were examined to see what effect, if any, that they would have on: 1) Total tornado numbers, 2) Violent tornado track length, 3) Violent tornado numbers, and 4) >40 tornado outbreaks.
Little difference was found in total tornado numbers between El Niño and La Niña events. However, significant differences were found in the number of violent tornadoes, and in large number tornado outbreaks. La Niña event years were found to have longer than average track lengths, more violent tornadoes, and a good probability of having an outbreak of 40 or more tornadoes. El Niño event years were found to have shorter than average track lengths, less violent tornadoes, and only a slim possibility of having an outbreak.
Possible reasons for the above conclusions include: 1) Warmer than normal temperatures in the western U.S./Canada along with cooler than normal temperatures in the southern U.S. during El Niño years; and 2) Colder than normal temperatures in the western U.S./Canada along with warmer than normal temperatures in the southern U.S. during La Niña years. This would act to weaken/strengthen the interactions between warm and cold air in the Midwest U.S. during El Niño/La Niña event years and decrease/increase the numbers and lengths of violent tornadoes."
Tom Grazulis (the Tornado Project) had likewise found the greatest frequency of large tornado outbreaks and strong F4-F5 tornadoes during cold to neutral ENSO springs.
Several studies in the past 10 years have looked at the influence of ENSO on tornado activity in the U.S. Bove (1999), Marzban and Schaefer (2001), Schaefer and Tatom (1999), and Wikle and Anderson (2003) looked broadly at the influence of ENSO on tornado activity in the U.S. All four of these studies have found a negative relationship between SST in the central and eastern Pacific and tornado activity in the U.S.
It can be seen that despite increasing number of tornado reports due to population growth, better public awareness, Doppler radar, and trained spotter networks, the frequency of violent tornadoes had declined from the 1950s to 1970s period when the Pacific was last cold and La Niñas dominated, supporting the findings of Knowles and Pielke (2005) and Bove (1999).
Forecasters know that La Niñas are characterized by cold and deeper snowpacks in the winter and early spring across the northern and western United States and in Canada with warmer than normal conditions in the Gulf States and offshore waters. They also know La Niñas usually mean drought and early spring heat for the southern plains. The enhanced contrast strengthens the jet stream and storms that ride along the jet. Warm, moist Gulf air collides with cold polar air with frequent dry intrusions from Mexico and dry Texas lifted into mid levels creating convective instability. These are all the ingredients identified by Fawbush and Miller (1954) for severe weather and used by severe weather forecasters at the Storm Prediction Center.
Forecasters have been able to predict an active severe weather and flood season months in advance (which often coincides with severe weather) based on the strong La Niña, cold PDO and antecedent deep northern snowpack, warm Gulf and dry southern plains. Since we have reentered a cold Pacific mode, we can expect more La Niñas, with more frequent severe weather in the Spring in upcoming years.
In summation, the active tornado seasons in 2008 and 2011 can be explained by natural variations in the Pacific and strong resulting La Niñas. Both have been shown by numerous studies to be associated with cooling not global warming. The warming mode during the 1980s and the 1990s actually reduced tornado frequency.
Bove, M. C. 1999. Impacts Of ENSO On United States Tornadic Activity. Preprints, 19th Conference on Severe Local Storms, AMS, 313-316. [Available online at http://www.coaps.fsu.edu/papers/impacts_enso_tornadic_activity/.]
Fawbush, E.G. and Miller, R.C. 1954. The types of air masses in which North American tornadoes form. Bulletin of the American Meteorological Society 35: 154-165.
Marzban, C. and Schaefer, J.T. 2001. The Correlation between U.S. Tornadoes and Pacific Sea Surface Temperatures. Monthly Weather Review 129. 884-895.
Schaefer, J.T. and Tatom, B.F. 1999. The Relationship between El Niño, La Niña, and United States Tornado Activity. Preprints, 19th Conference of Severe Local Storms, Minneapolis MN [Available online at http://www.spc.noaa.gov/publications/schaefer/el_nino.htm.]
Wikle, C.K. and Anderson, C.J. 2003. Climatological Analysis of Tornado Report Counts Using a Hierarchical Bayesian Spatio-Temporal Model. [Available online at http://www.stat.missouri.edu/~wikle/TORNADOpap_v3.pdf.]