12MJO North Pacific Jet Regimes Preceding Cool Season Tornado Events in the Lower Mississippi Valley

This study examines the North Pacific jet (NPJ) regimes that precede cool season (October–March) tornado events in the southeast United States for 1979–2019. The NPJ regime and phase diagrams are derived from the two leading modes of variability of the NPJ exit region in the cool season. These two modes are retraction/extension and poleward/equatorward shift of the NPJ exit region. A fifth regime, named “origin”, defines the climatological position of the NPJ exit region. Tornado events are identified using practically perfect hindcasts (PPHs) produced from tornado reports and are defined as days where PPH exceeds 5%, which approximates a slight risk categorical outlook from the Storm Prediction Center. The subset of tornado events that occurred in the lower Mississippi valley (defined by the region 30–37°N and 94–85.5°W) are then stratified by the NPJ regime that precedes the events in the 3–7-day period. Results show that tornado events in the lower Mississippi valley are 18% more frequent than what climatology would predict following poleward shifts and 9% more frequent following jet retractions. While tornado events occur near the climatological rate following jet extensions, they are 15% less frequent following equatorward shifts and 7% less frequent following origin regimes.

Recent studies have also established a potential relationship between tornado events in the United States and the phase and amplitude of the Madden-Julian Oscillation (MJO). In this study, an active MJO is defined as > 1.0 sigma using the outgoing longwave radiation (OLR) MJO Index (OMI) and lower Mississippi valley tornado events are stratified by the MJO regime that precedes the events in the 5–10-day period. Results show that tornado events are 16% more frequent than what climatology would predict following phases 6 and 7 (active MJO located in the West Pacific). Tornado events occur near the climatological rate following phases 2–5 (Indian Ocean and Maritime Continent) and are 9% less frequent following phases 1 and 8 (Western Hemisphere and Africa) and 4% less frequent in weak (OMI < 1) MJO regimes. Interestingly, a poleward shift of the NPJ is the most frequent regime associated with active MJOs in the West Pacific region and both are associated with increased cool season tornado events. The stratospheric Quasi-Biennial Oscillation (QBO), known to influence the amplitude of MJOs, appears to enhance the relationship between the poleward shift NPJ regime and MJO. Statistical and composite analyses will be presented to help diagnose the potential linkage between the QBO, MJO, NPJ regime, and cool season tornado events in the lower Mississippi valley.