Climatic controls on severe weather in the U.S.: Modulation by the Madden-Julian Oscillation

The MJO is the leading mode of intraseasonal variability of the tropical atmosphere, and it has been shown to modulate U.S. atmospheric circulation, temperature, rainfall, and even cloud-to-ground lightning. However, very little is known about the relationship between the MJO and severe weather events. The primary hypothesis of this study is that deep convection associated with the MJO generates Rossby waves that propagate eastward and northward, and, upon reaching the U.S., drive quantifiable intraseasonal variability in the synoptic and storm-scale conditions that support severe convective storms. By examining metrics of severe activity, created from the tornado, hail, and wind reports in the NOAA storm event database from 1990-2010, the this study will (1) quantify the variability of U.S. severe weather events by phase of the MJO; (2) quantify the intraseasonal variability in tropospheric variables known to be physically associated with severe weather; and (3) quantify the intraseasonal variability by season, geographic region, and phase of other climate oscillations. Preliminary results show a statistically significant modulation in both tornado and hail frequency that is supported by composites of convective available potential energy and storm-relative helicity.