Properties of QBO and SAO generated by gravity waves

In this paper we present an extension for the 2D (zonal mean) version of ou r numerical spectral mode (NSM) that incorporates Hines' Doppler spread par ameterization (DSP) for small-scale gravity waves (GW). This model is appli ed to describe the seasonal variations and the semi-annual and quasi-bienni al oscillations (SAO and QBO). Our earlier model reproduced the salient fea tures of the mean zonal circulation in the middle atmosphere, including the QBO extension into the upper mesosphere inferred from UARS measurements. T he model is extended to reproduce the upwelling at equatorial latitudes tha t is associated with the Brewer-Dobson circulation - which affects signific antly the dynamics of the stratosphere as Dunkerton had pointed out. In the presence of GW, this upwelling is produced in our model with tropospheric heating, which generates also zonal jets outside the tropics similar to tho se observed. The resulting upward vertical winds increase the period of the QBO. To compensate for that, one needs to increase the eddy diffusivity an d the GW momentum flux, bringing the latter closer to values recommended in the DSP. The QBO period in the model is 30 months (mo), which is conducive to synchronize this oscillation with the seasonal cycle of solar forcing. Associated with this QBO are interannual and interseasonal variations that become increasingly more important at higher altitudes - and this variabili ty is interpreted in terms of GW filtering that effectively couples the dyn amical components of the mesosphere. The computed temperature amplitudes fo r the SAO and QBO are in substantial agreement with observations at equator ial and extra-tropical latitudes. At high latitudes, however, the observed QBO amplitudes are significantly larger, which may be a signature of propag ating planetary waves not included in the present model. The assumption of hydrostatic equilibrium not being imposed, we find that the effects from th e vertical Coriolis force associated with the equatorial oscillations are l arge for the vertical winds and significant for the temperature variations even outside the tropics, but the effects are small for the zonal winds. (C ) 2000 Elsevier Science Ltd. All rights reserved.