Solar-related and terrestrial drivers modulating the northern polar vortex

The wintertime stratosphere is dominated by the polar vortex, a strong westerly wind, which surrounds the cold polar region. In the northern hemisphere the polar vortex can vary a lot during the winter and these variations affect the surface weather, e.g., in Europe and North America. Earlier studies have shown that the northern polar vortex is modulated by different terrestrial drivers and two solar-related drivers: electromagnetic radiation and energetic particle precipitation. Solar radiation varies in concert with the sunspot cycle by affecting the upper atmosphere at lower latitudes. Energetic electron precipitation (EEP) is driven by the solar wind and affects the polar stratosphere and mesosphere by forming ozone depleting NOx and HOx compounds. However, it is unclear how the effects of these solar-related and other, terrestrial drivers compare to each other. In this study we examine the effects of two solar-related drivers (solar radiation and EEP) and three terrestrial drivers (Quasi-Biennial Oscillation (QBO), El-Nino Southern Oscillation (ENSO) and volcanic aerosols) on the northern polar vortex. We use a new composite dataset including ERA-40 and ERA-Interim reanalysis of atmospheric variables and the multilinear regression analysis to estimate atmospheric responses to these five drivers in years 1957 – 2017. We confirm the findings of earlier studies that westerly QBO wind, cold ENSO, volcanic aerosols and increased EEP are associated with a stronger polar vortex. Furthermore, we find that EEP produces the strongest and most significant effect on the northern polar vortex among the studied variables. Only in December the effect of QBO is comparable to the EEP effect. We also find that EEP effect is strong and significant in the easterly QBO phase, while in the westerly phase it does not stand out from the effects of other drivers.