Spontaneous stratospheric QBO-like oscillations simulated by the GFDL SKYHI general circulation model

The tropical stratospheric mean flow behavior in a series of integrations w ith high vertical resolution versions of the Geophysical Fluid Dynamics Lab oratory (GFDL) "SKYHI'' model is examined. At sufficiently high vertical an d horizontal model resolution, the simulated stratospheric zonal winds exhi bit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. Thi s appears to be a spontaneous internally generated oscillation and closely resembles the observed quasi-biennial oscillation (QBO) in many respects, a lthough the simulated oscillation has a period less than half that of the r eal QBO. The same basic mean flow oscillation appears in both seasonally va rying 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 osc illation is shown to be largely driven by eddy momentum fluxes associated w ith a broad spectrum of vertically propagating waves generated spontaneousl y in the tropical troposphere of the model. Several experiments are perform ed with the model parameters perturbed in various ways. The period of the s imulated tropical stratospheric mean flow oscillation is found to change in response to large alterations in the sea surface temperatures (SSTs) emplo yed. This is a fairly direct demonstration of the link between the stratosp heric mean flow behavior and tropical convection that is inherent in curren t theories of the QBO. It is also shown in another series of experiments th at the oscillation is affected by the coefficients used for the subgrid-sca le 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 diffusi on that can be used at high resolution.