The stratospheric final warming (FW) is the final transition of the zonal winds from wintertime westerlies to summertime easterlies as solar heating of the high latitude stratosphere increases in the springtime. Recent observational studies have suggested that the stratospheric final warming makes a significant contribution to the spring transitions in the lower troposphere. However, it is still not clear whether those transitions in the troposphere are totally due to stratospheric downward influence.
Here we explore the hypothesis that much of the observed tropospheric signal of the final warming is initialized from the stratosphere. Final warmings are simulated in a dynamical core of general circulation model (GCM) by imposing a transition in the radiative equilibrium temperature from winter to summer only in the stratosphere. As in the observations, the simulated FWs also occur at different ``dates'' in different realizations. Following previously published analyses of observed final warmings, we form a climatological springtime transition and calculate composite anomalies centered on the final warmings.
The results show that along with the increasing of topographic forcing, the mean breakdown dates of the polar vortex become earlier, and the resulting zonal wind anomaly magnitude also increases, consistent with the NH and SH observations. These stratospheric zonal wind anomalies could extend downward into the surface with large statistical significance, especially for the high topographic forcing FW cases. The zonally symmetric model indicates that those tropospheric anomalies are caused by the zonal wind deceleration prior to the FW, which is due to the eruption of long wave numbers, especially wave number one.
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