Modeling the downward influence of stratospheric final warming events

The stratospheric final warming (SFW) is the final transition of the zonal winds from wintertime westerlies to summertime easterlies as solar heating of the high latitude stratosphere increases. Previous investigations of the SFW often focused more on the mixing of ozone and other gases rather than its dynamics. Composite analysis of observations indicates that zonal wind anomalies during the SFW may influence the tropospheric circulation: In the northern hemisphere (NH), a coherent pattern of significant zonal wind anomaly is observed to extend downward to the surface, while in the southern hemisphere (SH), only marginally significant anomalies are found in the troposphere. Understanding the physical mechanisms for this downward influence is essential if this effect is to be exploited to enhance atmospheric predictability during spring onset.

Here an idealized atmospheric circulation model is used to simulate SFW events, focusing on the mechanisms for stratosphere-troposphere coupling during spring onset. When the radiative equilibrium temperature in the stratosphere is gradually changed from a winter to a spring state, while the radiative forcing of the troposphere is held fixed, the model generates realistic SFW events. Composites of the model SFWs are generated, as well as anomalies from these composites. The simulated spring onset date and zonal wind transition pattern of our preliminary results without topography are consistent with the SH observations. The zonal wind anomaly transition, however, is somewhat similar to NH observations in that a relatively weak but statistically significant zonal wind anomaly extends downward into the lower troposphere, even the surface. Eliassen-Palm flux analysis of model SFWs shows the presence of strong wave activity near the onset date, similar to the NH and SH observations. Additional dynamical analyses of these model SFWs will be presented, and their implications for understanding the observed downward influence of SFWs will be discussed.