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Summary

• Implementing the Warner and McIntyre USSP gravity wave parametrization in the Met. Office Unified Model gives significant improvements in the simulation of the middle atmosphere even with constant, homogeneous and isotropic gravity wave sources.

• Both summer and winter jets show strength and structure close to that in observational analyses, although there is still too little equatorward tilt of the SH winter jet.

• Gravity wave forcing accelerates the flow in most of the stratosphere and decelerates it in the mesosphere wher it also drives a stronger meridional circulation with a single mesospheric cell above the 2-cell Brewer-Dobson circulation in the stratosphere. Without drag, the 2-cell circulation extends up into the mesosphere.

• The model produces a QBO with realistic periods, amplitude and phase asymmetry between the Easterly and Westerly phases.

• Both resolved and parametrized waves provide a significant contribution to the driving of the QBO.

• The westerly phase does not descend quite as far as in observations. This appears to be due to damping by the explicit horizontal diffusion in the model. Although scale selective, the diffusion still affects the large scale QBO due to its long timescale.