Regime behavior in the upper stratosphere as a precursor of stratosphere-troposphere coupling in the northern winter

A new regime index is constructed to capture the seasonal development of the stratospheric polar vortex in the northern winter, based on the standard deviation of Ertel’s potential vorticity in the upper stratosphere in November-December. The narrow-jet flow regime is characterized by a polar vortex that is more confined to high latitudes in the early winter upper stratosphere. This upper-level vortex configuration is more susceptible to the disturbances of upward propagating planetary-scale Rossby waves; the stratospheric polar vortex thus weakens earlier and is vertically shallower. The wide-jet flow regime is characterized by a broader-than-average polar vortex that extends further into the subtropics in the early-winter upper stratosphere. The polar night jet then gradually strengthens, moves poleward and penetrates deep into the lowermost stratosphere, with a sharply-defined polar vortex edge accompanied due to more frequent Rossby wave breaking. Composite differences analyses show that the wide and narrow jet regimes, defined in the uppermost stratosphere in November-December, lead to different circulation anomalies of the lower-stratosphere and the troposphere in January-February, offering the potential for improved predictability of sub-seasonal to seasonal forecasts up to two months ahead. The lower tropospheric responses in January-February are zonally asymmetric. The narrow-jet regime projects most strongly over the North Atlantic, with an equatorward-shifted, and/or broader mid-latitude westerly jet. The wide-jet-regime response is characterized by a weakened mid-latitude westerly jet over the North Pacific. The two flow regimes also differ in their impacts on the storm track over Western Europe and the east coast of America, which may have implications for extreme weather events in those regions.

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