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

本文发展了一个新的大气环流模式(GCM)，该模式包含了从地面到外基面的区域，并用来研究春分条件下日潮的迁移行为。我们研究了中层和热层中迁移的日、半日潮的逐日变化及其与低层大气产生的迁移日潮的关系。例如，在20~300公里高度范围内发现了17-18天和25天周期的移动日振幅波动，表明了中间层和热层与低层大气之间的动力耦合。波-均流与超快开尔文波和日潮的相互作用是驱动季节内振荡的重要因素。无论是迁徙的日潮，还是非迁徙的日潮，对季节内振荡的激发都起着重要的作用。在20~90公里高度范围内，超快开尔文波和日潮汐的振幅存在季节内变化。这些波幅的季节内变化与湿对流活动的变化密切相关。与准两年振荡相关的平流层低层纬向风的年际变化调制了MLT中向上传播的重力波。这种准BO调制的MLT重力波活动产生了半年振荡的年际变化。

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.