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

Abstract

self-sustained oscillation dynamically analogous to the equatorial quasi-biennial oscillation (QBO) was obtained as a radiative?moist-convective quasi-equilibrium state in a minimal model of the stratosphere?troposphere coupled system, which is a two-dimensional cloud-system-resolving nonhydrostatic model with a periodic lateral boundary condition. The QBO-like oscillation shows downward propagation of the zonal mean signals in the stratosphere. In addition, in the troposphere there are periodic variations associated with the QBO-like oscillation, including organized features of moist-convective systems characterized as squall-line- or back-building-type precipitation patterns. Details of the momentum budget variation are examined to study the stratosphere?troposphere dynamical coupling associated with the QBO-like oscillation. The vertical flux of horizontal momentum is separated into three contributions of convective momentum transport (CMT) and momentum transports by upward- and downward-propagating gravity waves?that is, upward and downward gravity wave momentum transports (GWMTs)?and the time?height variations of each contribution are evaluated quantitatively. The method is based on the linear theory of gravity waves to separate upward- and non-upward-propagating contributions and uses the phase speed spectra of the total cloud mixing ratio to identify the CMT contribution. The upward GWMT predominates in the stratosphere and contributes to the acceleration of the zonal mean zonal wind. The CMT and downward GWMT are confined to the troposphere, and the former predominates. The variations of the mean zonal wind modulate the organization of convective systems, and the squall-line- and back-building-type patterns appear alternately. According to the modulation of convective systems, the spectral features of every momentum transport vary periodically.