Time variations of the atmospheric state have two periodic components known as a diurnal cycle and an annual cycle, which are periodic responses to the periodic variations of the external forcings due to the earth’s rotation and revolution, respectively. Intraseasonal and interannual variations are defined as deviations from the periodic annual response; generally the intraseasonal variation means lowfrequency variation with week-to-week or month-to-month time scales, while the interannual one means year-to-year variation. Some part of these variations is a response to the time variations of the external forcings or boundary conditions of the atmospheric system, while the rest is generated within the system. Well known forcings with interannual time scales are ll-year solar cycle, irregular and intermittenteruptions of volcanos and interannual variations of sea surface temperature, although the last one should be considered as internal variation of the coupled system of the atmosphere and oceans. Quasi-Biennial Oscillation(QBO) of the mean zonal wind in the equatorial stratosphere, which is basically caused by the interaction between the mean zonal wind and equatorial waves propagated from the troposphere, could be considered as variation of a lateral boundary of the mid-latitude stratosphere. On the other hand, external forcings are not so significant in the intraseasonal time scales; fundamentally intraseasonal variations could be considered as internal one which may exist even under constant external conditions.
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- Numerical Experiments on Intraseasonal and Interannual Variations of the Troposphere-Stratosphere Coupled System
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