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

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 with a limited domain to represent a convective column in the tropics. The large-scale circulation is parameterized using the weak temperature gradient (WTG) approximation. Temperature variations typical of those seen in easterly and westerly phases are imposed in the lower stratosphere and tropical tropopause layer, and the responses are studied at different sea surface temperatures, corresponding to different equilibrium precipitation rates in the control simulation.

The equilibrium precipitation rate shows a slight increase in response to an QBO easterly phase temperature perturbation over small SST anomaly, and a strong decrease over a large SST anomaly, and vice versa for the QBO westerly phase perturbation. 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, and suggests an interpretation of the geographic dependence of the observed QBO influence on precipitation.