Occurrence of tornado outbreaks in the context of solar wind coupling to magnetosphere-ionosphere-atmosphere

The National Oceanic and Atmospheric Administration National Weather Service database of tornadoes provided by the Storm Prediction Center is used to investigate the occurrence of tornado outbreaks in the United States from 1963 to 2021 in the context of solar wind that impacts the Earth’s magnetosphere. A link between the solar wind and large tornado outbreaks is found. Superposed epoch analysis of tornado occurrence reveals a peak in the cumulative number of tornadoes near the interplanetary magnetic field sector boundary (heliospheric current sheet) crossings. The latter often closely precede or coincide with co-rotating interaction regions at the leading edge of high-speed streams from coronal holes. Most of the large tornado outbreaks (20 or more tornadoes per 24 hours) are associated with high-density plasma adjacent to heliospheric current sheets and co-rotating interaction regions. Other large tornado outbreaks followed impacts of interplanetary coronal mass ejections or occurred in a declining phase of major high-speed streams. We consider the role of the solar wind coupling to the magnetosphere-ionosphere-atmosphere system in severe weather development, mediated by globally propagating aurorally excited atmospheric gravity waves. While these gravity waves reach the troposphere with attenuated amplitudes, when over-reflecting in regions of low-level wind shear and opposing winds, they can contribute to conditional symmetric instability release in frontal zones of extratropical cyclones leading to intensification of supercells that spawn tornado outbreaks. The ERA5 meteorological re-analysis is used to evaluate slantwise convective available potential energy (SCAPE) to assess conditional symmetric instability and slantwise convection in cases of large tornado outbreaks.