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Summary and Conclusions

Equatorial oscillations in the middle atmosphere have been studied using the Doppler spread gravity wave drag parameterization of Hines [1997]. Results from a simple mechanistic model using a prescribed mean upwelling from the Canadian Middle Atmosphere Model demonstrated Dunkerton's [1997] conjecture that an additional source of momentum flux from small-scale gravity waves is required to drive the QBO in the presence of realistic Brewer-Dobson circulation in the equatorial lower stratosphere. Results from several simulations of the CMAM were analyzed. The main conclusions from that part of the study are:

• The simulated ``QBO'' in the CMAM exhibited a period which was synchronized with the seasonal cycle unlike the observed QBO. For the experiment using enhanced (parameterized) gravity wave momentum fluxes in the tropics a period of approximately 18 months was found. Other simulations using different source settings, which were not discussed, further revealed the tendency of the period of the oscillation to be quantized in integral multiples of 6 months. This is most likely due to the large gravity wave momentum fluxes which are needed to generate the oscillation in this GCM.

• The contribution from the resolved waves to the total wave driving of the QBO in the CMAM was shown to be relatively small. This was attributed to the cumulus convection parameterization of Zhang and McFarlane [1995] which is used in the CMAM and possibly to the relatively course horizontal and vertical resolution of the model.

• The manner in which the parameterized momentum flux divergence is computed was shown to have a large impact on the simulated oscillation in the CMAM. Therefore intercomparion of QBO's in different GCMs using different finite-difference schemes may be difficult.