The Relationship between the Madden–Julian Oscillation and U.S. Violent Tornado Outbreaks in the Spring

The Madden–Julian oscillation (MJO) has been linked to weather variability in the midlatitudes via its associated overturning circulations and Rossby wave trains that redistribute the thermal and mass fields at higher latitudes. This work examines the relationship between the MJO and violent tornado outbreaks in the United States. A census of events shows that violent tornado outbreaks during March–April–May (MAM) are more than twice as frequent during phase 2 of the Real-time Multivariate MJO (RMM) index as during other phases or when the MJO was deemed inactive. Composite analyses show the global circulation patterns simultaneously associated with the MJO and the tornado outbreaks and also indicate the most favored low-frequency circulation pattern that precedes tornado outbreaks in RMM phase 2. An index of 300-hPa geopotential height data is generated by projecting 60-day mean values onto the composite low-frequency pattern. When that index exceeds one standard deviation and the MJO is in RMM phase 2 with an amplitude exceeding one standard deviation during MAM, violent tornado outbreaks occur 50% of the time, relative to the average frequency of less than 4%. Results demonstrate that the anomalous large-scale midlatitude circulation modulated by the MJO and lower-frequency signals can make conditions more or less favorable for tornado outbreaks.