The role of chemical processes in the quasi-biennial oscillation (QBO) signal in stratospheric ozone

Abstract

The stratospheric quasi-biennial oscillation (QBO) is known to cause variations in the transport of chemical tracers but the role of chemical processes in modulating their QBO signals remains unclear. Using ozone observations (Stratospheric Water and OzOne Satellite Homogenized, SWOOSH) and a chemical transport model (SLIMCAT), we analyze the role of chemical processes in the QBO impact on stratospheric ozone in the tropics and Northern Hemisphere during winter and early spring over the period 1979–2015. During easterly QBO (EQBO) phases tropical ozone concentrations decrease in the lower stratosphere (40–100 hPa, ~16–23 km) and middle stratosphere (above 15 hPa, ~30 km) but increase in the transition region between 15 and 40 hPa (~23–30 km) compared to westerly (WQBO) phases. Although the contributions of chemical processes to the ozone QBO signal are less than those of dynamical processes, this study highlights their non-negligible role. The negative chemical ozone tendencies in the tropical lower stratosphere, that are attributed to less downward ultraviolet radiation and weaker oxygen photolysis, partially contribute to the negative ozone anomalies at this altitude during EQBO phases, particularly in February and March. In contrast, the negative chemical ozone tendencies related to increased NO2 and stronger NO2-catalyzed ozone loss, which are important in the middle stratosphere, cannot explain the ozone increases in the transition region during EQBO phases, which is mainly caused by the QBO-induced transport of ozone-rich air from above. The positive chemical ozone tendencies in the extratropics during EQBO phases are mainly related to lower temperatures and less active catalytic ozone loss cycles. In addition, reduction in polar stratospheric cloud occurrence associated with the warmer Arctic stratosphere during EQBO phases weakens the heterogeneous chemical processes in the lower stratosphere and decreases chlorine activation, partially contributing to the positive chemical ozone tendency and ozone increases in this region.