The downward influence of stratospheric final warming events in an idealized model

The stratospheric final warming is the final transition of the zonal winds from wintertime westerlies to summer summertime easterlies as the solar radiation in the high latitudes increases in springtime. Recently observations have shown that the final warming events may affect the tropospheric circulation and the magnitude of the influence in the Northern Hemisphere (NH) is more substantial and stronger than the Southern Hemisphere (SH).

Here we use a dynamic core of GCM to explore the dynamics of final warming events by running tens of seasonal transition cases with equilibrium temperature (Teq) changing sinusoidally between the winter and summer states only in the stratosphere. It is seen that without topography or with low height topographic forcing (h0<1000m), the interannual variability in the timing of the final warmings is not significant, however, when the forcing is continuously increased, the final warming zonal wind transition becomes much more abrupt and considerable interannual variability in the timing of the final warming events is observed. The composite analysis demonstrates that the final warming average onset dates decrease with the height of the topography, consistent with the fact that NH final warming onset time is far earlier than SH. The zonal wind anomaly evolution with respect to the final warming onset, which is often used to characterize the deviation between the composite and climatology, suggests that only when the topographic forcing is strong enough, the statistically significant positive anomalies prior to the onset and negative ones after that may extend downward into the lower troposphere, similar with the NH observations. These zonal wind anomalies are closely associated with the strong wave activities in the stratosphere. The different topographic final warming cases seem to indicate the important roles of planetary waves in the downward influence of the final warming events.