Responses of tropical deep convection to the QBO : 1 cloud - resolving simulations

4 Observational studies suggest that the stratospheric quasi-biennial oscillation (QBO) can 5 modulate tropical deep convection. We use a cloud-resolving model with a limited domain, 6 representing a convective column in the tropics, to study the mechanisms of this modulation. 7 The large-scale circulation is parameterized using the weak temperature gradient (WTG) 8 approximation, under which the parameterized large-scale vertical motion acts to relax the 9 horizontal mean temperature towards a specified reference profile. Temperature variations 10 typically seen in easterly and westerly phases are imposed in the upper troposphere and 11 lower stratosphere of this reference profile. The responses of convection are studied over dif12 ferent sea surface temperatures, holding the reference temperature profile fixed. This can be 13 thought of as studying the response of convection to the QBO over different ”relative SST”, 14 and also corresponds to different equilibrium precipitation rates in the control simulation. 15 The equilibrium precipitation rate shows slight increases in response to an QBO easterly 16 phase temperature perturbation over small SST anomalies, and strong decreases over large 17 SST anomalies, and vice versa for the QBO westerly phase perturbation. A column moist 18 static energy budget analysis reveals that the QBO modulates the convective precipitation 19 through two pathways: it changes the high cloud properties and thus the column radiative 20 cooling, and it alters the shape of the large-scale vertical motion and thus the efficiency of 21 energy transport by the large-scale flow. The non-monotonicity of the QBO influence on 22 precipitation with respect to SST results from the competition of these two effects. 23