Downward Influence of QBO-Like Oscillation on Moist Convection in a Two-Dimensional Minimal Model Framework

The influence of the quasi-biennial oscillation (QBO)-like oscillation on moist convection is examined using the two-dimensional minimal model framework of Yoden et al. with two series of parameter sweep experiments: model-top experiments with varying model height and low-level nudging experiments with reduced zonal-mean zonal wind toward zero from the surface to a certain level. The QBO-like oscillation in the mean zonal wind is a robust feature obtained in all the experiments, including low-top cases in which only the tropospheric portion is retained. The zonal-mean precipitation is modulated with a half period of the mean zonal wind oscillation in the model-top experiments, and a positive correlation exists between the precipitation intensity and the vertical shear of the mean zonal wind near the surface. The precipitation modulation is weakened with a nudging of the low-level mean zonal wind but appears again in the cases with the low-level nudging in the middle and lower troposphere. There is a negative correlation between the precipitation intensity and the vertical shear of the mean zonal wind in the upper troposphere. These results suggest that the QBO-like oscillation modulates the moist convection via two mechanisms related to the vertical shear. Large values of the shear near the surface enhance the longevity and intensity of the moist convective systems by separating the updraft and downdraft. On the other hand, large values of shear near the cloud top tend to disrupt the convective structure and lead to weakening moist convection, although this mechanism seems to be secondary.