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

Yoden et al. (2014) re-examined radiative-convective quasi-equilibrium states in a highly-idealized two-dimensional non-hydrostatic regional model with explicit moist convections under a periodic boundary condition without Coriolis effects, as a similar configuration as Held et al. (1993). The obtained zonal mean zonal wind shows a clear QBO-like oscillation, and the mean period of the oscillation is around 133 days when the top boundary is located at 40km. Unlike the observed equatorial QBO, the oscillation has associated signals even in temperature, clouds, and precipitation in the troposphere, where moist convections dominate and gravity waves are generated. In this study, the momentum budget of the QBO-like oscillation is examined by taking account of these organized motions.

Two spatial patterns of precipitation appear in accordance with the oscillation of zonal mean zonal wind in the troposphere. One precipitation pattern is a squall-line (SL) type, which appears during a limited period when the near-surface wind is strong, and the other is a back-building (BB) type during the rest of the period. For the SL type, a new precipitation system emerges at the front side of a propagating system. On the other hand, a new system emerges at the rear side for the BB type. These precipitation patterns have good coincidence with space-time variation of the vertical flux of horizontal momentum (Fz); Fz becomes large in the stratosphere for the BB type whereas it becomes large in the troposphere for the SL type.

Fz and its convergence are objectively separated into the upward-propagating, stationary, and downward-propagating contributions using a linear group velocity criterion (c.f., Shaw and Lane, 2013). In the stratosphere, the oscillation in the zonal-mean zonal wind is almost maintained by the Fz convergence near the critical level and the convergence is induced by gravity waves propagating from the troposphere. In the troposphere, strong signals of stationary and downward-propagating contributions coexist. It is difficult to perfectly separate them, because slantwise moist convections and gravity waves coexist in the domain at the same time. However, downward-propagating contribution has a dominant role in the acceleration in the troposphere. The method introduced here, therefore, is useful to evaluate the time variations of the relative importance of each contribution. The results also show the importance of the dynamical coupling between the stratosphere and the troposphere through the modulation of precipitation patterns associated with the oscillation of the zonal mean zonal wind.