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. The polar night jet (PNJ) is a strong stratospheric westerly circumpolar windat around 65 ∘  N in winter, and the strength of the climatologicalPNJ is widely recognized to increase from October through late December.Remarkably, the climatological PNJ temporarily stops increasing during lateNovember. We examined this “short break” in terms of the atmospheric dynamicalbalance and the climatological seasonal march. We found that it results froman increase in the upward propagation of climatological planetary waves fromthe troposphere to the stratosphere in late November, which coincides with amaximum of the climatological Eliassen–Palm (EP) flux convergence in the lowerstratosphere. The upward propagation of planetary waves at 100 hPa, which isstrongest over Siberia, is related to the climatological strengthening of thetropospheric trough over Siberia. We suggest that longitudinally asymmetricforcing by land–sea heating contrasts caused by their different heatcapacities can account for the strengthening of the trough.

Detection of a climatological short break in the polar night jet in early winter and its relation to cooling over Siberia