Modulation of US Tornado Activity by Year-Round North American Weather Regimes

Here, we examine the relation between U.S. tornado activity and a new year-round classification fi cation of North American weather regimes. The regime classification fi cation is based on 500-hPa geopotential height anomalies and classifies fi es each day as Pacific fi c Trough, Pacific fi c Ridge, Alaskan Ridge, Greenland High, or No regime. During the period 1979-2022, we fi nd statistically significant fi cant relations between average tornado report numbers and weather regimes in all months except June- August. Tornado activity is enhanced on Pacific fi c Ridge days during late winter and spring, reduced on Pacific fi c Trough days in spring, and reduced on Alaskan Ridge and Greenland High days during fall and early winter. During active regimes, the probability of many tornadoes occurring also increases, and there is greater variability in the number of tornadoes reported each day. A reanalysis-based tornado index reproduces the regional features of the modulation of tornado activity by the weather regimes and attributes them to changes in storm relative helicity, convective available potential energy, and convective precipitation. The phase of El Ni & ntilde;o-Southern Oscillation (ENSO) also plays a role. In winter and spring, Pacific fi c Ridge days occur more often and average more reports per day during cool ENSO conditions. During warm ENSO conditions, Pacific fi c Trough days occur more often and are associated with widespread reduced tornado activity.