J. Atmos. Sci., 72, 3625-3638.
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY
Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.
Observational studies suggest that the stratospheric quasi-biennial oscillation (QBO) can modulate tropical deep convection. We use a cloud-resolving model with a limited domain, representing a convective column in the tropics, to study the mechanisms of this modulation. The large-scale circulation is parameterized using the weak temperature gradient (WTG) approximation, under which the parameterized large-scale vertical motion acts to relax the horizontal mean temperature towards a specied reference prole. Temperature variations typically seen in easterly and westerly phases are imposed in the upper troposphere and lower stratosphere of this reference prole. The responses of convection are studied over different sea surface temperatures, holding the reference temperature prole xed. This can be thought of as studying the response of convection to the QBO over different "relative SST", and also corresponds to dierent equilibrium precipitation rates in the control simulation. The equilibrium precipitation rate shows slight increases in response to an QBO easterly phase temperature perturbation over small SST anomalies, and strong decreases over large SST anomalies, 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 the large-scale ow. The non-monotonicity of the QBO influence on precipitation with respect to SST results from the competition of these two eects.