SEASONAL DEVELOPMENT OF THE EXTRATROPICAL QBO IN A NUMERICAL-MODEL OFTHE MIDDLE ATMOSPHERE

The seasonal (wintertime) development of middle atmosphere circulation in opposite phases of the equatorial quasi-biennial oscillation (QBO) was simulated with a three-dimensional nonlinear numerical model. In the stratosphere, the effect of equatorial QBO was generally consisten t with the extratropical QBO observed by Holton and Tan, namely, a str onger midwinter polar vortex in the westerly phase, and vice versa. Ho wever, the extratropical response to the QBO was sensitive to other fa ctors such as mesospheric gravity wave drag and the amplitude of Rossb y waves specified at the model's lower boundary. The extratropical QBO was realistic only when a drag parameterization was included and Ross by wave amplitudes lay in an intermediate range close to the observed. At somewhat stronger forcing, the model's response was largest in the mesosphere where (in this case) westerlies were stronger in the easte rly phase of equatorial QBO. This was apparently due to a shielding ef fect. The theory of planetary wave-mean flow interaction suggests that the sensitivity to equatorial QBO should be greatest for wave forcing s near a ''bifurcation'' point. Below this threshold the stratosphere approaches radiative equilibrium, shutting off vertical propagation of planetary waves. Supercritical forcing leads to a major warming. The model's sensitivity to forcing, while consistent with this idea, was m ost apparent in perpetual solstice runs without parameterized wave dra g. Seasonal integrations with wave drag produced a more realistic extr atropical QBO, making the bifurcation less conspicuous.