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