Momentum Transports associated with Moist Convection and Gravity Waves in a Minimal Model of QBO-like Oscillation

Held et al. (1993) obtained a QBO-like oscillation in radiative-convective quasi-equilibrium state in a highly-idealized two-dimensional non-hydrostatic model with explicit moist convections under a periodic boundary condition without Coriolis effects. Yoden et al. (2014) re-examined the QBO-like oscillations in such an idealized dynamical framework with a state-of-the-art regional cloud resolving model and showed robustness of the oscillation insensitive to the choice of model configurations. The obtained zonal mean zonal wind shows a clear QBO-like oscillation, together with associated variations of temperature, clouds, and precipitation. The mean period of the oscillation is around 133 days in an experimental run with the top boundary located at 40km. Unlike the observed equatorial QBO, the oscillation has a clear signal in the troposphere, where moist convections dominate and gravity waves are generated.

In this study, we examine periodic variations of the moist convections, gravity waves, and associated momentum transports. The QBO-like oscillation modulates the intensity and propagation directions of each convection and organized convective systems. The organized convective systems change the direction of propagation periodically depending on the phase of the oscillation of the mean zonal wind in the troposphere, and the organization has two types, “squall line” and “back building”, depending on the strength of vertical shear of the mean zonal wind in the lower troposphere; squall-line type for limited periods of stronger vertical shear, whereas back-building type for other periods.

The eddy momentum transports are objectively separated into the upward-propagating and nonupward-propagating contributions using a linear group velocity criterion introduced by Shaw and Lane (2013); the former is associated with vertically propagating gravity waves while the latter is associated with convective updrafts and downdrafts, and coherent tilted structures. The gravity waves are generated near the top of liquid-water content clouds in the lower troposphere and that of ice content clouds in the upper troposphere, and then propagate into the stratosphere with almost constant phase speeds relatively close to the mean zonal wind at the generated levels. These waves deposit their momentum at the corresponding critical level in the stratosphere and accelerate or decelerate the zonal mean zonal wind to produce the QBO-like oscillation. Convective momentum transport dominates the momentum fluxes in the troposphere due to slantwise convection in accordance with the vertical shear of the mean zonal wind in the troposphere.