Do Dry GCMs Generate QBO-like Oscillation?

Abstract

The effect of vertical discretization methods and vertical resolution on Quasi-Biennial Oscillation (QBO)-like oscillations that can occur in mechanistic General Circulation Models (dry GCMs) is investigated. Two models are compared. One model uses the spectral method in the horizontal direction but the finite difference method in the vertical direction (VFD model), while the other is a three-dimensional spectral model that uses the spectral method for discretization in both the horizontal and vertical directions (3DS model). Both models include horizontal hyperdiffusion, simple Newtonian cooling and Rayleigh friction, but as they are dry models, they do not include the effects of moist convection, and no explicit vertical diffusion is used, following a previous study. Long-term numerical integrations of these models show that the 3DS model does not generate QBO-like oscillations at the vertical resolution settings used. On the other hand, the VFD model generates QBO-like oscillations at low vertical resolution, but no QBO-like oscillations at higher vertical resolution. Wavenumber-frequency spectral analyses of wave disturbances show that, in the VFD model, the amplitude of the waves at the sigma-level near the central altitude of the QBO-like oscillations is highly dependent on the vertical resolution of the model. Analyses of the wave contribution to the vertical momentum fluxes and additional numerical experiments show that in the higher vertical resolution setting, steady eastward zonal winds form above the altitude corresponding to the tropopause, and these zonal winds suppress the upward propagation of eastward moving waves. Transformed Eulerian mean analyses are also done for the results of the VFD models to investigate the contribution of the residual circulation and the wave-mean-flow interaction to the QBO-like oscillation.