| Summary: | The quasi-biennial oscillation (QBO) dominates variability of the equatorial stratosphere and shows a weakening trend in the lower levels in recent studies. This study focuses on the vertical structure of QBO amplitude changes in observation, the dynamic reasons involved, and possible changes under future scenarios. The QBO amplitude has an increased trend in the upper levels of stratosphere and a decreased one in the lower during 1960–2022. Results indicate that the QBO amplitude changes are mainly controlled by the tropical upwelling and wave forcing, where the former dampens the lower QBO but the latter primarily maintains the whole QBO amplitude. In addition, the entire QBO could be weakened in the future, responding to the high emission scenario in CMIP6. Under the high scenario, the enhanced tropical upwelling would weaken the amplitude of QBO at lower levels of stratosphere, and meanwhile, the reduced wave forcing, mainly caused by the increased tropospheric stability and weakened convection activity, would make the QBO amplitude maintenance hard.
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