Responses of tropical deep convection to the QBO: cloud-resolving simulations and Observations

Previous observational studies suggest that the stratospheric quasi-biennial oscillation (QBO) can modulate tropical deep convection. To investigate how tropical deep convection responds to the QBO, we use a cloud-resolving model to represent a convective column in the tropics. The effects of large-scale circulations are parameterized using the weak temperature gradient (WTG) approximation. Temperature anomalies near tropopause typically seen in the QBO easterly and westerly phases are imposed on the WTG reference temperature profile. The responses of convection are studied at varying positive relative sea surface temperatures (SST), corresponding to different equilibrium precipitation rates in the tropics. The equilibrium precipitation rate shows slight increases in response to QBO easterly phase temperature anomalies over small relative SST, and strongly decreases over large relative SST, and vice versa for the QBO westerly phases. A column moist static energy budget analysis reveals that the QBO modulates the convective precipitation through two pathways: it changes the high cloud properties and thus the column radiative cooling; and it alters the shape of the large-scale vertical motion and thus the efficiency of energy transport by large-scale flow. The non-monotonic response of the convective precipitation to the QBO over varying SST anomalies is the result of the competition of these two effects. Inspired by the results of the cloud-resolving simulations, using observational data, we compute the composites of tropical precipitation anomalies based on the QBO phases and relate the precipitation anomalies with the local climatological mean precipitation. The observational results qualitatively agree with the results of cloud-resolving simulations over tropical oceanic convective regions. Over tropical continental convective regions, where the orographic effects play important roles in precipitation, the observed responses of precipitation to the QBO are different and need further investigations.