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

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 (MJO), yet climate models often fail to capture these relationships. This study examines the QBO’s impact on high clouds in CMIP6 historical simulations and MODIS observations, given that cloud feedback remains a major source of uncertainty in climate sensitivity estimates.

The QBO modulates dynamic and thermodynamic properties near the tropical tropopause layer, such as temperature, static stability, and vertical wind shear, all linked to cloud formation. Building on recent findings that highlight the major 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.

Results confirm that the QBO westerly (QBOW) phase is associated with reduced tropical mean high-cloud cover, with strong zonal asymmetry in observations. CMIP6 models successfully capture the reduction in tropical high clouds associated with QBOW, but with a strong inter-model spread. Among the analysed CCFs, upper-tropospheric temperature and relative humidity contribute most to this reduction, followed by static stability. Inter-model differences primarily arise from uncertainty in the high-cloud sensitivity to upper troposphere temperature. The strong inter-model spread highlights that improved constraints on high‑cloud sensitivity to upper‑tropospheric thermodynamics could help enhance how models capture QBO‑related cloud responses.