Relationships between phases of the El Nino-Southern Oscillation and character of the local severe-storms season in the central United States

Warm El Nino-Southern Oscillation (ENSO) events, indicated by large negative values of the Southern Oscillation Index (SOI), are significantly correlated with decreases in frequency of strong and violent tornadoes (Fujita scale F2-F5) during February-July in three of four regions of the central United States from 1955-1994. Cold ENSO events, indicated by large positive values of SOI, are significantly correlated with higher incidences of F2-F5 tornadoes in the same three regions during the warm convective seasons. Thermodynamic-parameter cap strength in the tornado-prone Southern Low Plains (SLP) region appears enhanced during cold ENSO phases and reduced during warm events. Upper air flow patterns suggest a reason for enhanced activity: strong and persistent capping inversions produced by southwesterly flows of continental tropical (cT) air from the Mexican Plateau associated with high pressure within and east of the SLP.