Spontaneous Stratospheric QBO-like Oscillations Simulated by the GFDL SKYHI General Circulation Model

Abstract

The tropical stratospheric mean flow behavior in a series of integrations with high vertical resolution versions of the Geophysical Fluid Dynamics Laboratory (GFDL) ?SKYHI? model is examined. At sufficiently high vertical and horizontal model resolution, the simulated stratospheric zonal winds exhibit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. This appears to be a spontaneous internally generated oscillation and closely resembles the observed quasi-biennial oscillation (QBO) in many respects, although the simulated oscillation has a period less than half that of the real QBO. The same basic mean flow oscillation appears in both seasonally varying and perpetual equinox versions of the model, and most of the analysis in this paper is focused on the perpetual equinox cases. The mean flow oscillation is shown to be largely driven by eddy momentum fluxes associated with a broad spectrum of vertically propagating waves generated spontaneously in the tropical troposphere of the model. Several experiments are performed with the model parameters perturbed in various ways. The period of the simulated tropical stratospheric mean flow oscillation is found to change in response to large alterations in the sea surface temperatures (SSTs) employed. This is a fairly direct demonstration of the link between the stratospheric mean flow behavior and tropical convection that is inherent in current theories of the QBO. It is also shown in another series of experiments that the oscillation is affected by the coefficients used for the subgrid-scale diffusion parameterization. These experiments demonstrate that at least one key reason why reasonably fine horizontal resolution is needed for the model to simulate a mean flow oscillation is the smaller horizontal diffusion that can be used at high resolution.