Long-term variability in the mesosphere and lower thermosphere and its relation with the lower atmsophere

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A new general circulation model (GCM) which contains the region from the ground surface to the exobase has been developed, and used to investigate behavior of the migrating diurnal tide for equinox condition. We studied day-to-day variations of migrating diurnal and semidiurnal tides in the mesosphere and thermosphere and their relation with migrating diurnal tides generated in the lower atmosphere. For example, fluctuations of the migrating diurnal amplitude with periods of 17-18 and 25 days are found in the height range from 20 to 300 km, indicating dynamical coupling between the mesosphere and thermosphere and the lower atmosphere.By using the GCM, excitation mechanism of intraseasonal oscillation of the zonal mean zonal wind in the equatorial mesosphere and lower thermosphere (MLT) were investigated. Wave-mean flow interaction with ultra fast Kelvin waves and diurnal tides is important for driving the intraseasonal oscillation. Not only the migrating diurnal tide but also the non-migrating diurnal tide plays an important role in excitation of the intraseasonal oscillation. Intraseasonal variations of amplitudes of the ultra fast Kelvin waves and the diurnal tides are found at height range from 20 to 90 km heights. Intraseasonal variations of amplitudes of these waves are closely related with variations of the moist convective activity.Interannual variation of the semiannual oscillation in the equatorial MLT was investigated. Upward propagating gravity waves in the MLT are modulated by interannual variation of the zonal wind in the lower stratosphere associated with the Quasi-Biennial Oscillation. This QBO modulated gravity wave activity in the MLT produces interannual variation of the semiannual oscillation.