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

The polar night jet (PNJ) is a strong stratospheric westerly circumpolar wind at around 65°N in winter, and the strength of the climatological PNJ is widely recognized to increase from October through late December. Remarkably, the climatological PNJ temporarily stops increasing during late November. A detailed understanding of the short break in late November is important in terms of the dynamic meteorology of intraseasonal variations in the stratosphere. No previous studies explicitly showed this climatological short break, nor have yet been addressed in terms of dynamic meteorology. We thus examined this "short break" in terms of the atmospheric dynamical balance and the climatological seasonal march. Examination of the atmospheric dynamical balance showed that an increase in upward propagation of planetary waves from the troposphere to the stratosphere in late November is accompanied by convergence of the Eliassen-Palm (EP) flux in the lower stratosphere, which brings about this short break in the PNJ. The upward propagation of Rossby (planetary) waves over Siberia from the troposphere to the stratosphere is a dominant cause of the short break. This upward propagation of planetary-scale Rossby waves at high latitudes (see Figure) is associated with amplification of eddy geopotential height and air temperature, that is, with a strengthening of the trough over Siberia. Further, we inferred that this strengthening of the trough is forced by the high-latitude land-sea thermal contrast caused by their different heat capacities.

The part of this presentation was published as the article, Ando et al. (2018).

Reference

Ando, Y., K. Yamazaki, Y. Tachibana, M. Ogi, and J. Ukita, 2018: Detection of a climatological short break in the polar night jet in early winter and its relation to cooling over Siberia. Atmospheric Chemistry and Physics, 18, 12639-12661, doi:10.5194/acp-18-12639-2018.