Behavior of Planetary Waves before and after Stratospheric Sudden Warming Events in Several Phases of the Equatorial QBO

Yoko Naito Department of Geophysics, Kyoto University, Kyoto, Japan

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Shigeo Yoden Department of Geophysics, Kyoto University, Kyoto, Japan

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Abstract

Almost a thousand stratospheric sudden warming (SSW) events are obtained through long time integrations with a simple global circulation model, and a statistical analysis based on such a large number of samples is made to investigate behavior of planetary waves before and after SSW events depending on the phase of the equatorial quasi-biennial oscillation (QBO). An idealized zonal momentum forcing to mimic a phase of the QBO is imposed under a perpetual winter condition, and eight phases of the QBO-wind forcing are examined for 8 × 10 800-day datasets.

Some systematic dependence on the phase of the QBO-wind forcing is seen in the anomaly of the Eliassen–Palm (EP) flux in the winter hemisphere, both in the 10 800-day average and in the composites for SSW events. The composite analysis shows that before SSW events, the upward EP flux in the troposphere and midlatitude lower stratosphere as well as the equatorward flux above the tropopause is larger in the westerly forcing runs than in the easterly forcing runs. After SSW events, the upward EP flux in the troposphere is still larger in the westerly forcing runs. Correlation associated with the differences among SSW events that occurred in each run is significantly positive between the magnitude of the warming and the planetary wave activity flux before all the events in QBO-wind forcing in the stratosphere, but only in the easterly forcing runs in the troposphere.

Corresponding author address: Yoko Naito, Dept. of Geophysics, Kyoto University, Kyoto 606-8502, Japan. Email: naito@kugi.kyoto-u.ac.jp

Abstract

Almost a thousand stratospheric sudden warming (SSW) events are obtained through long time integrations with a simple global circulation model, and a statistical analysis based on such a large number of samples is made to investigate behavior of planetary waves before and after SSW events depending on the phase of the equatorial quasi-biennial oscillation (QBO). An idealized zonal momentum forcing to mimic a phase of the QBO is imposed under a perpetual winter condition, and eight phases of the QBO-wind forcing are examined for 8 × 10 800-day datasets.

Some systematic dependence on the phase of the QBO-wind forcing is seen in the anomaly of the Eliassen–Palm (EP) flux in the winter hemisphere, both in the 10 800-day average and in the composites for SSW events. The composite analysis shows that before SSW events, the upward EP flux in the troposphere and midlatitude lower stratosphere as well as the equatorward flux above the tropopause is larger in the westerly forcing runs than in the easterly forcing runs. After SSW events, the upward EP flux in the troposphere is still larger in the westerly forcing runs. Correlation associated with the differences among SSW events that occurred in each run is significantly positive between the magnitude of the warming and the planetary wave activity flux before all the events in QBO-wind forcing in the stratosphere, but only in the easterly forcing runs in the troposphere.

Corresponding author address: Yoko Naito, Dept. of Geophysics, Kyoto University, Kyoto 606-8502, Japan. Email: naito@kugi.kyoto-u.ac.jp

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