Modulation of Tornado Activity By Year-Round North American Weather Regimes

Severe convective storms, including tornadoes, can result in large socioeconomic losses. U.S. tornado activity is related to predictable climate signals, such as El Niño-Southern Oscillation (ENSO). In this study, we investigate the extent to which U.S. tornado activity is modulated by weather regimes (WRs), which are recurrent patterns that describe low-frequency atmospheric variability. We also explore whether ENSO influences this WR-tornado relationship. We use a year-round WR classification technique to define four North American regimes — Pacific Trough, Pacific Ridge, Alaskan Ridge, and Greenland High — plus a No Regime state. We find clear WR signals in U.S. tornado activity during all months except June-August. Overall, activity is above average during Pacific Ridge days and below average during Alaskan Ridge and Greenland High days. During winter and spring, La Niña (cool ENSO state) is associated with enhanced Pacific Ridge-related increases in tornado activity.