Solar-related and internal drivers of the northern polar vortex

During the winter, a polar vortex, a strong westerly thermal wind, forms in the polar stratosphere. In the northern hemisphere the polar vortex varies significantly during and between winters. The Sun and the solar wind affect the polar vortex via two separate factors: electromagnetic radiation and energetic particle precipitation. Earlier studies have shown that increased energetic electron precipitation (EEP) decreases ozone in the polar upper atmosphere and strengthens the northern polar vortex, while solar irradiance affects temperature and ozone in the stratosphere directly at low latitudes and indirectly at high latitudes. In addition to the solar-related drivers, the northern polar vortex is also affected by different atmospheric internal factors such as Quasi-Biennial Oscillation (QBO), El-Nino Southern Oscillation (ENSO) and volcanic aerosols. Several studies have shown that the QBO modulates the effects that the solar-related drivers and ENSO cause to the polar vortex. In this study we examine and compare effects of the two solar-related drivers (solar radiation and EEP) and three atmospheric internal factors (QBO, ENSO and volcanic aerosols) on the polar vortex. We use multiple linear regression analysis to estimate the effects of each factor on temperature and zonal wind. We concentrate on the northern wintertime stratosphere and troposphere and examine the period of 1957-2017 using a combination of ERA-40 and ERA-Interim re-analysis data. We also study these effects separately in the two QBO phases. While we confirm that increased EEP is associated with a strengthened polar vortex, in accordance with the earlier studies, we further show that EEP is the largest and most significant factor among those studied affecting  the northern polar vortex variability. We also find that the EEP effect on polar vortex is particularly strong in the easterly phase of QBO while in the westerly phase the EEP effect is weakened and does not stand out from other effects.