Features of Large-Scale Dynamics of the Arctic Atmosphere in the 2020–2021 Winter Season that Influenced the Formation of Two Sudden Stratospheric Warmings

This study examines the preconditions for the formation of a sudden stratospheric warming during the winter season of 2020–2021, which led to the destabilization of the stratospheric polar vortex. Particular attention is given to external forcings, including anomalies in sea ice cover area (SICA) in the Arctic Ocean and long-period tropical oscillations. Although the phases of the tropical oscillations did not favor weakening of the polar vortex or amplification of planetary wave activity, the vortex gradually lost stability throughout December, while upward wave activity fluxes from the troposphere intensified. This can be linked to the record anomalies in SICA observed in the Arctic Ocean during autumn 2020. Following the loss of polar vortex stability, the Northern Annular Mode index in the lower stratosphere–troposphere remained negative until early March—that is, for approximately two months after the first sudden stratospheric warming. This persistence allowed the event to be classified as a downward-propagating sudden stratospheric warming, which exerts a prolonged influence on tropospheric dynamics. In the lead-up to the sudden stratospheric warming in early January, a meridional redistribution of air masses occurred in the stratosphere, with transport from the Eastern Hemisphere to the Western Hemisphere. This redistribution contributed to the strengthening of upward planetary wave activity fluxes. The enhanced planetary wave activity led to further deformation and displacement of the polar vortex from the pole. Using the example of pressure differences between 60° N and the polar cap average (60°–90° N) on the 850 K isentropic surface, it is demonstrated that, in isentropic coordinates, sudden stratospheric warmings are detected earlier than in conventional isobaric coordinate analysis.