| Summary: | 碩士 === 國立中央大學 === 太空科學研究所 === 104 === The Quasi-biennial Oscillation (QBO) is a persistent oscillation in the zonal mean zonal winds of the low latitude middle atmosphere that is driven by breaking planetary and gravity waves, with a period near two years. The atmospheric tides that dominate the dynamics of the mesosphere and lower thermosphere region (MLT, between heights of 70 to 120 km) are excited in the troposphere and stratosphere, and propagate through QBO-modulated zonal mean zonal wind fields. This allows the MLT tidal response to also be modulated by the QBO, with implications for ionospheric/thermospheric variability. Meanwhile, interannual oscillation in solar radiation could directly drive the variations in the ionosphere with simultaneous period through the photoionization. Many studies also revealed the connection of the solar activities and QBO signal in ionospheric features, e.g. total electron content (TEC).
In this research, we develop an empirical model to isolate stratospheric QBO-related tidal variability in the MLT diurnal and semidiurnal tides using values from assimilated TIMED satellite data. Tidal fields corresponding to stratospheric QBO eastward and westward phases, as well as the artificial solar forcing with QBO period decomposed by Multi-dimensional Ensemble Empirical Mode Decomposition (MEEMD) analysis from Hilbert-Huang Transform (HHT), are then used to drive the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM).
The numerical experiment results indicate that ionospheric QBO is mainly modulated by the solar QBO by during the solar maximum, since the solar QBO would reach its maximum synchronized with solar cycle. During solar minimum, the ionospheric QBO is modulated from below and above by the stratospheric QBO and solar QBO simultaneously.
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