Downward impact of the QBO on the tropical troposphere under different ENSO conditions

The Quasi-Biennial Oscillation (QBO) of descending zonal winds is the leading mode of variability in the tropical stratosphere, influencing tropospheric processes, including the El Niño-Southern Oscillation (ENSO). While an upward influence of ENSO on the QBO is accepted, the potential downward impact of the QBO on ENSO remains less explored. Recent studies indicate that the QBO can modulate the divergent circulation over the tropical Indian-Pacific sector, particularly during summer and autumn, with effects that vary with the ENSO phase. However, separating QBO teleconnections in the tropical troposphere from the dominant ENSO influence remains challenging, and the mechanisms involved in the QBO-ENSO interaction are not yet fully understood.  This study investigates the potential tropospheric impacts of the QBO under different ENSO conditions using three atmosphere-only experiments performed with the EC-EARTH model as part of the APARC-QBOi framework. These simulations represent El Niño, La Niña, and neutral ENSO conditions, allowing to isolate QBO-related signals. The analysis focuses on the QBO modulation of temperature and zonal wind in the upper troposphere-lower stratosphere and the associated impacts on static stability, vertical wind shear and relative vorticity.  Our findings show that the QBO affects deep tropical convection over the Maritime Continent in all three experiments. For La Niña and neutral conditions, the westerly QBO phase is associated with reduced upper-level divergence, ascending motion and lower-level convergence over this region, with the strongest anomalies occurring in summer and autumn. These changes in the divergent circulation alter the zonal Walker circulation and, more prominently, the meridional Hadley circulation. For El Niño conditions, the QBO impact is largely opposite due to modifications in both the location of mean precipitation and the QBO period. Overall, results indicate that these tropospheric impacts are primarily driven by QBO-induced changes in upper-tropospheric static stability, which exhibit a strong correlation with convective anomalies over the Maritime Continent and occur simultaneously.