Behavior of planetary waves before and after stratospheric sudden warming events in several phases of the equatorial QBO

To investigate how the planetary wave activity before and after a stratospheric sudden warming (SSW) event depends on the phase of the equatorial quasi-biennial oscillation (QBO), a statistical analysis is made based on almost thousand SSW events which were obtained by a numerical experiment. An idealized zonal momentum forcing to mimic a phase of the QBO was imposed in the equatorial stratosphere in a simple global circulation model. Changing the phase of the perpetual "QBO-wind" forcing as an experimental parameter, eight integrations were performed for 12,000 days under perpetual winter condition.

Composite analysis shows that before the SSW events, the upward Eliassen-Palm (EP) flux in the polar stratosphere is stronger in the easterly phase of the QBO, while the upward EP flux in the mid-latitude stratosphere is stronger in the westerly phase. Confidential ellipses are evaluated for each of the eight composites on the phase plane of the temperature at the polar stratosphere and the latitude-averaged upward EP flux at lower levels. In the easterly phase, correlation between the magnitude of the warming events and the intensity of the planetary wave activity at the troposphere before the events is significantly positive. On the other hand, the correlation in the westerly phase is not significant due to the variability in the stronger equatorward EP flux around the tropopause.