The Stratopause Semiannual Oscillation in the Berlin Troposphere–Stratosphere–Mesosphere GCM

Abstract

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.