Dynamics and Predictability of Troposphere-Stratosphere Dynamical Coupling during Stratospheric Sudden Warming Events

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In this study, we obtained the following results on the dynamics and the predictability of the stratosphere-troposphere dynamical coupling during stratospheric sudden warming (SSW) events by examining 1-month ensemble forecast dataset provided from the Japan Meteorological Agency (JMA) and long-term global analysis datasets as well as performing a series of numerical experiments based on atmospheric general circulation models and the operational numerical weather forecast model of the JMA :1.The occurrence of SSWs with rather simple time evolution, such as a SSW in December 2001, is predictable at least 2 weeks in advance. The predictable period of such SSWs is at least two times longer than that of the tropospheric circulation.2.Sensitivity to the initial condition for the forecast of SSWs becomes very high during the onset period of the SSWs.3.The importance of the tropospheric blocking for the precursory event of the SSW is statistically confirmed. Prediction experiments also indicate that the tropospheric circulation anomaly associated with the blocking specified for the initial condition actually causes the subsequent SSW. The stratospheric circulation nonlinearly responds to the magnitude of the specified anomaly.4.The downward migration of the zonal-mean zonal wind anomaly after the SSW is predictable at most 1 week in advance. The downward migration is caused by the E-P flux convergence associated with zonal wavenumber (WN) 2 planetary waves propagating upward from the troposphere. The propagating property of WN 2 components strongly depends on the profile of the zonal-mean zonal wind around the tropopause region in high latitudes.5.Predictable period of SSWs with relatively complicated time evolution such as a SSW in January 2004 is at most 9 days.6.Forecast skill of the stratospheric annular mode tends to be improved for winter seasons when the variability of the stratospheric annular mode becomes large.