Classification of Stratosphere Winter Evolutions Into Four Different Scenarios in the Northern Hemisphere
Résumé
Among the 70 northern hemisphere winters from 1950 to 2020, 61 have been classified into
four independent scenarios with a new technique based on analyzing empirical orthogonal functions of
stratospheric zonal wind fluctuation patterns at the edge of the polar vortex. These four scenarios represent a
typical zonal wind evolution in the stratosphere modulated by timings of stratospheric warmings occurring in
winter. First, there are three scenarios (January mode, February mode, and Double mode) for which the polar
vortex generally breaks down with a reversing of zonal winds in midwinter, known as sudden stratospheric
warmings (SSWs) events, at different times. The last one is a non-perturbed scenario with two under-modes for
which only the timing of the polar vortex transition to its summer state differs, either radiative or dynamical.
Consistently with these wind patterns, the wave-1 and wave-2 anomaly evolutions confirm that these scenarios
are associated with independent dynamics behaviors and significant differences between wave-1 and wave-2
activities. We found that the wave-1 anomaly evolutions drop systematically for each scenario when the
stratospheric winds weaken, while this correlation with the wave-2 anomaly evolutions is not observed for all
scenarios. These different dynamical behaviors confirm that the scenarios are either mainly dynamically driven
by wave-1 or both driven by wave-1 and wave-2. Until early December, the modes possess similar increasing
evolutions of the wave-1 activity due to the seasonal cycle. After, the scenarios separate, suggesting that the
mechanisms responsible for the winter unfolding act in the previous months.
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