A Minimal Model of QBO-Like Oscillation in a Stratosphere-Troposphere Coupled System under a State of Radiative-Moist Convective Quasi-Equilibrium

Quasi-Biennial Oscillation (QBO) is a prominent internal variations in the equatorial stratosphere due to the interactions between a mean zonal wind and waves that propagate upward in the　stratosphere. Over two decades ago, Held et al. (1993, hereafter HHR93) investigated radiative-moist convective quasi-equilibrium states with a two-dimensional explicit moist convection model and obtained a QBO-like oscillation. Their model contains the fundamental dynamical processes of the QBO, though it is a highly-idealized two-dimensional model for a periodic domain without Coriolis effects. The periodic boundary condition enables the zonally averaged winds to evolve freely. In this study, we re-examine the QBO-like oscillation found by HHR93 with a long enough time-integration period over two years, by using Advanced Research WRF Modeling System. We also investigate the sensitivity of the QBO-like oscillation in regards of different factors such as domain size, resolution and boundary conditions (e.g., prescribed zonal wind at the top and sea surface temperature). The first report of a series of numerical studies was published as Yoden et al. (2014).

The control experiment has a similar configuration to that of HHR93; 640 km domain width with a resolution of 5 km, 130 vertical levels up to 26 km. Convective parameterization is turned off in all simulations and only a cloud microphysics scheme is used. Other physics options are standard ones for short- and long-wave radiations, surface fluxes, planetary boundary layer, turbulence and diffusion, and Rayleigh damping near the top boundary. After spin up, the mean zonal wind shows a clear QBO-like oscillation with a period of 120.6 days. Unlike the observed QBO, the oscillation has a clear signal in the troposphere, in which moist convections dominate and gravity waves are generated. Such convectively generated gravity waves propagate into the stratosphere to produce the QBO-like oscillation in the stratosphere. On the other hand, intensity and propagation of organized convective systems are modulated in accordance with the oscillation of mean zonal wind in the troposphere. The zonal mean precipitation also varies associated with the oscillation, though its day-to-day fluctuation is very large compared to the long-period oscillation.

In a series of experiments in which SST is increased, the oscillation period becomes longer; 111.9 days, 120.6 days, and 133.2 days for 25°C, 27°C and 30°C, respectively. This relationship is opposed to the expectation of shorter periods due to more convections and wave momentum fluxes. In a case of “warm rain” with Kessler cloud microphysics, we did not obtain any QBO-like oscillations. The vertical profile of the zonal mean temperature is very different from those with QBO-like oscillations. In this quasi-equilibrium state without QBO-like oscillation, moist convections are not very active and much smaller lapse rate of 3.2 K/km is maintained up to the elevated tropopause at the height of 24 km just below the Rayleigh damping layer. The changed lapse rate would be a consequence of the very different spatial distributions of clouds and moisture that give different diabatic heating by the atmospheric radiation and cloud microphysics. In another series of “low top” experiments with top boundary at 20 km or 15 km, we still have QBO-like oscillation but the oscillation is irregular and the mean period is about a half or less of the control case. These experiments suggest the QBO-like oscillation in the stratosphere is important to regulate the tropospheric QBO produced by the modulation of momentum transports by slant-wise convective systems.

Held, I. M., R. S. Hemler, and V. Ramaswamy, 1993: Radiative-convective equilibrium with explicit two-dimensional moist convection. J. Atmos. Sci., 50, 3909-3927.

Yoden, S., H.-H. Bui, and E. Nishimoto, 2014: A minimal model of QBO-like oscillation in a stratosphere-troposphere coupled system under a radiative-moist convective quasi-equilibrium state. SOLA, 10, 112-116, doi:10.2151/sola.2014-023.