The Impact of the El Nino Southern Oscillation (ENSO) on Winter and Early Spring U.S. Tornado Outbreaks

Abstract

n recent years, the potential of seasonal outlooks for tornadoes has attracted the attention of researchers. Previous studies on this topic have focused mainly on the influence of global circulation patterns (e.g., El Niño Southern Oscillation (ENSO), North Atlantic Oscillation, Pacific Decadal Oscillation) on spring tornadoes. However, these studies have yielded conflicting results of the roles of these climate drivers on tornado intensity and frequency. The present study seeks to establish linkages between ENSO and tornado outbreaks over the U.S. during winter and early spring. These linkages are established in two ways: 1) Statistically, by relating raw counts of tornadoes in outbreaks (defined as six or more tornadoes in a 24-hour period in the U.S. east of the Rocky Mountains), and their destructive potential, to sea surface temperature anomalies observed in the Niño 3.4 region; and 2) Qualitatively, by relating ENSO to shifts in synoptic-scale atmospheric phenomena that contribute to tornado outbreaks. The latter approach is critical for interpreting the statistical relationships, thereby avoiding the deficiencies in a few of the previous studies that did not provide physical explanations relating ENSO to shifts in tornado activity. The results suggest that shifts in tornado occurrence are clearly related to ENSO. In particular, La Niña conditions consistently foster more frequent and intense tornado activity compared with El Niño, particularly at higher latitudes. Furthermore, it is found that tornado activity changes are tied not only to the location and intensity of the subtropical jet during individual outbreaks, but also to the positions of surface cyclones, low-level jet streams, and instability axes.