Effect of energetic electron precipitation on ozone and the southern polar vortex: The role of chlorine deactivation

The polar vortex is a system of strong westerly winds surrounding the cold polar region which forms in the middle atmosphere every winter. In the southern hemisphere the polar vortex is stronger and lasts longer than its northern counterpart. Consequently, the southern polar vortex provides sufficiently cold circumstances where massive ozone depletion by reactive chlorine oxides (ClO_x) forms a large ozone hole after the polar night. Energetic electron precipitation (EEP) is an external driver which modifies ozone chemistry and, thereby, the thermal and dynamical balance in the wintertime middle atmosphere. Precipitating electrons originate from the near-Earth space and produce nitrogen (NO_x) and hydrogen oxides (HO_x) which catalytically destroy ozone. Earlier studies have shown that EEP-NO_x both decreases ozone and deactivates chlorine oxides in the stratosphere in the southern hemisphere. Moreover, EEP is found to affect the strength of the polar vortex and even surface climate modes like the NAO (North Atlantic Oscillation) and the SAM (Southern Annular Mode), but the mechanisms causing these effects are still unclear. We study here the chemical and dynamical variability related to EEP and its seasonal evolution in the southern mesosphere and stratosphere using the POES and Aura satellite measurements and the ERA5 reanalysis data. We show that EEP increases NO_x and decreases both ozone and ClO in the upper stratosphere in early winter. However, when EEP-NO_x reaches the middle stratosphere during the spring, ClO is still decreased but ozone and temperature are increased, and the polar vortex becomes weaker. Moreover, we found that the correlation between EEP and the southern polar vortex has significantly changed during the last 80 years and is tightly related to the amount of chlorine in the stratosphere. These findings show that EEP weakens the southern springtime vortex and drives negative SAM at least partly via chlorine deactivation.