The Seasonal Variation of the Propagating Diurnal Tide in the Mesosphere and Lower Thermosphere. Part I: The Role of Gravity Waves and Planetary Waves

Abstract

The seasonal variation of the propagating diurnal tide in the mesosphere and lower thermosphere is examined using results from a 2-yr simulation of the extended version of the Canadian Middle Atmosphere Model (CMAM). The CMAM is shown to be able to reproduce not only the observed semiannual amplitude variation of the tide in the lower thermosphere but also more subtle features such as amplitude maxima that are stronger in March/April than in September/October, a 4- to 6-h shift in phase between winter and summer in the Northern Hemisphere, and a weaker seasonal variation of phase in the Southern Hemisphere. Part I of this two-part series of papers investigates the importance of two of the mechanisms that have been proposed to explain the observed variation of tidal amplitude, namely, 1) interactions with small-scale gravity waves and 2) interactions with planetary-scale waves like the quasi?2-day wave. Analysis of the tidal momentum and thermodynamic budgets shows that the direct effects of parameterized gravity waves are not important, since the associated drag and vertical diffusion are found to be substantially weaker in magnitude than the wave?wave and wave?mean flow interaction terms. Nonlinear interactions with large-scale disturbances (possibly nonmigrating tides) are found to be an important mechanism to damp the diurnal tide in the lower thermosphere; however, the seasonal variation of these terms is of the wrong sign to explain the seasonal variation of the tidal amplitude. Although the CMAM exhibits a quasi?2-day wave at solstice, its overall impact on the tide is not found to be strong. The budget analysis points to the linear advection terms as being of particular importance in the seasonal variation of the tide.