The role of the QBO for tropical high-cloud variability in CMIP6 models and observations

Abstract. The Quasi-Biennial Oscillation (QBO) is a dominant mode of stratospheric zonal wind variability. Observations indicate that the QBO influences tropical phenomena such as convection, precipitation, and the Madden–Julian Oscillation, yet climate models often fail to capture these relationships. This study examines the QBO’s impact on high clouds, in particular tropical tropopause layer clouds, in CMIP6 historical simulations and CALIPSO observations. Building on recent findings that identified relevant cloud-controlling factors (CCFs) for high clouds, we apply CCF analysis to assess QBO-driven changes in high-cloud amount and interpret these changes in terms of contributions from controlling factors. Our results confirm that the QBO westerly (QBOW) phase is associated with reduced tropical mean high-cloud cover. CMIP6 models successfully capture the reduction in tropical high clouds associated with QBOW, but with a weaker magnitude and strong inter-model spread. Among the analysed CCFs, upper-tropospheric temperature, static stability at 150 hPa and relative humidity contribute most to this reduction in observations, and to the model bias. The substantial model bias and spread suggest that better constraints on high-cloud sensitivity to upper-tropospheric thermodynamics are needed to improve simulated QBO-related cloud responses. This framework may also help assess cloud responses to climate change, including the role of greenhouse gas–driven stratospheric cooling.