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Summary

From the Northern Hemisphere polar temperature anomaly data we find that the stratosphere behaves sometimes like a "conductor" which allows a warm anomaly to propagate from the upper stratosphere to the troposphere, and sometimes like a "resistor" which prohibits downward propagation. The "conductivity" of polar atmosphere is determined by the strength of wave activity and the structure of zonal mean flow. If the initial wave forcing is so strong and persistent that could reverse the polar westerly wind, then the warm anomaly would descend with the "critical line" because waves cannot propagate into easterly wind. Further downward propagation of the warm anomaly requires continuous energy supply by tropospheric waves, which can be induced by the positive feedback due to changes in zonal mean flow in the mid-high latitudes. In the propagating case, the zonal mean flow in the post-warming period changed to a state favorable for poleward transport of tropospheric waves, which induced a second pulse of wave energy flux to reinforce the warm anomaly in the lower stratosphere and extend its downward propagation. We may call this condition of polar atmosphere as "conductive," compared with the "resistive" atmosphere which lacks descending "critical line" and continuous supply of wave energy. The impact of downward propagating warm anomaly could be felt in lower tropospheric weather systems through the link of upper tropospheric jet streams. Changes in the strength and position of subtropical jet are often indicative of changes in weather patterns. In addition, the south shift of the North America and Atlantic jet stream by the downward propagating stratospheric polar warm anomaly is consistent with the negative phase of AO or NAO (Thompson and Wallace, 1999).