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

Abstract

Winter tornado activity (January–March) between 1950 and 2003 was analyzed to determine the possible effect of seasonally averaged sea surface temperatures in the equatorial Pacific Ocean, the ENSO phase, on the location and strength of tornado outbreaks in the United States. Tornado activity was gauged through analyses of tornadoes occurring on tornado days (a calendar day featuring six or more tornadoes within the contiguous United States) and strong and violent tornado days (a calendar day featuring five or more tornadoes rated F2 and greater within the contiguous United States). The tornado days were then stratified according to warm (37 tornado days, 14 violent days), cold (51 tornado days, 28 violent days), and neutral (74 tornado days, 44 violent days) winter ENSO phase. It is seen that during winter periods of neutral tropical Pacific sea surface temperatures, there is a tendency for U.S. tornado outbreaks to be stronger and more frequent than they are during winter periods of anomalously warm tropical Pacific sea surface temperatures (El Niño). During winter periods with anomalously cool Pacific sea surface temperatures (La Niña), the frequency and strength of U.S. tornado activity lies between that of the neutral and El Niño phase. ENSO-related shifts in the preferred location of tornado activity are also observed. Historically, during the neutral phase, tornado outbreaks typically occurred from central Oklahoma and Kansas eastward through the Carolinas. During cold phases, tornado outbreaks have typically occurred in a zone stretching from southeastern Texas northeastward into Illinois, Indiana, and Michigan. During anomalously warm phases activity was mainly limited to the Gulf Coast states, including central Florida. The data are statistically and synoptically analyzed to show that they are not only statistically significant, but also meteorologically reasonable.

A Bayesian modelling framework for tornado occurrences in North America

“…In winter, VWSH (and to a lesser extent SHEAR) is characterized by the highest-standardized slopes. This result is on parity with existing evidence from the literature that the influence of the subtropical jet to tornado activity is strongest in the winter, and can be a major factor of tornado formation and tornado outbreaks under extratropical storms 12,31 . Moreover, strong vertical wind shear (i) can play a role in organizing quasi-linear convective systems that can include individual or multiple supercells 32 and (ii) can produce a large vertical pressure gradient when horizontal vorticity is tilted and thus facilitates nocturnal convection to overcome large negative buoyancy, thereby producing tornadoes 33 .…”

The boreal spring variability of the Intra-Americas low-level jet and its relation with precipitation and tornadoes in the eastern United States

“…Additionally, Trapp et al (2005) found that tornado activity was strongly associated with squall lines and bow echoes (as opposed to cells) for the US states situated along a curved axis from Louisiana to Pennsylvania and that activity was higher in the first few months of the year (i.e., from January to April). Tornadic activity in a similar region was found by Cook and Schaefer (2008) to be susceptible to the influence of ENSO conditions in boreal winter.…”

Weather Risks in the USA

“…Natural climate variability, such as El Niño and La Niña phases, has an impact on severe thunderstorm activity in North America (Bove, M. C., 1999;Cook & Schaefer, 2008). During strong La Niña phases, the region frequently hit by significant tornadoes (EF2 -EF5) during winter and spring extends markedly eastwards from Tornado Alley, whereas El Niño phases tend to spawn tornadoes more often in southern parts of the USA.…”