The impact of the stratospheric quasi-biennial oscillation on Arctic polar stratospheric cloud occurrence

Abstract. Polar stratospheric clouds (PSCs) play a critical role in stratospheric ozone depletion. Previous studies have shown that the quasi-biennial oscillation (QBO) influences the Arctic stratospheric polar vortex and ozone, yet few studies have thoroughly analyzed the impact of the QBO on Arctic PSC occurrence. This study examines this impact using CALIPSO observations from 2006 to 2021 and SLIMCAT simulations from 1979 to 2022. The results show that the winter PSC coverage area is significantly larger during the westerly QBO (WQBO) phase than during the easterly QBO (EQBO) phase, with a zonal asymmetry in PSC occurrence frequency anomalies. The QBO influences the temperature, water vapour (H2O), and nitric acid (HNO3) in the Arctic stratosphere, which are key factors affecting PSC formation. During the WQBO phase, Arctic stratospheric temperatures show negative anomalies, with the centre of this anomaly biased towards North America. In addition, H2O shows positive anomalies in the Arctic lower stratosphere, mainly due to the stronger polar vortex preventing the transport of high-moisture air at high latitudes to mid-latitudes, causing H2O to accumulate inside the polar vortex. HNO3 shows negative anomalies, primarily caused by denitrification through nitric acid trihydrate (NAT) sedimentation. Sensitivity analyses further indicate that QBO-induced temperature anomalies are the dominant driver of PSC variability, while the direct effect of H2O anomalies on PSCs is relatively small. The reduction of HNO3 mainly affects PSCs in February and March. This work implies that future changes in the QBO may influence ozone by affecting PSCs.