The stratopause semiannual oscillation in the Berlin troposphere-stratosphere-mesosphere GCM

The tropical stratopause semiannual oscillation (SAO) in the Berlin troposphere-stratosphere-mesosphere general circulation model (TSM GCM) is investigated. The model is able to produce a semiannual oscillation that is properly located in time and space in comparison with observations; however, the westerly phase is 10-15 m s(-1) too weak while the second easterly phase of the year is about the same amount too strong. A case study is performed to examine the westerly forcing of the SAO in detail. At 1 hPa in one particular March, when the strongest westerly acceleration of the year takes place, only 18% of the forcing is due to the dissipation of Kelvin waves, whereas 72% of the forcing is due to the dissipation of other slow eastward propagating planetary-scale waves and 10% is due to the dissipation of eastward propagating inertio-gravity waves. Throughout the year the Kelvin wave contribution to the total body force is below 50%. The modeled Kelvin waves are dissipated at higher altitudes than observed Kelvin waves with the same phase speeds. To examine the effect of the QBO on the wave spectrum a relaxation of the zonal-mean wind to an idealized QBO has been implemented into the model. During the westerly phase of the QBO, slow eastward propagating waves with wavenumbers greater than 1 can no longer reach the upper stratosphere. This leads to a reduction of the westerly momentum available to generate the westerly phase of the SAO, and the westerly winds do not descend below 1 hPa as in the control experiment. This reduction in the descent is also present in observations, but there it is less pronounced. As the upper-stratospheric Kelvin waves in the model are partly too slow, they are too strongly affected by the changes in the propagation properties due to the QBO.