Hydrogen chloride (HCl) is the main reservoir species of chlorine and chemical decomposition of nitrous oxide (N₂O) is the primary source of NO_x (=NO+NO₂) in the stratosphere. Changes in stratospheric HCl and N₂O play a critical role in modulating variations in stratospheric ozone. Thus, long-term trends in stratospheric HCl and N₂O have been investigated in many studies, whereas short-term changes have not received enough attention. Here, using satellite observations and a chemical transport model, we found that two extreme change events for HCl and N₂O in the Northern Hemisphere mid-latitude middle and lower stratosphere have occurred over past decades, which are characterized by a sharp increase in HCl and a decrease in N₂O over several months; for example, HCl increased (and N₂O decreased) by 0.135 ppbv (−33.352 ppbv) in 1987/1988 and by 0.196 ppbv (−28.553 ppbv) in 2010/2011. Further analysis shows that the extreme change events of stratospheric HCl and N₂O in these two periods are closely related to anomalous residual circulation caused by the joint effects of the strong easterly phase of the semi-annual oscillation and the strong polar vortex.

stratospheric hydrogen chloride; nitrous oxide; extreme change events; residual circulation; semi-annual oscillation; polar vortex