Dynamical Features of the Polar Vortex Intensification in the Northern Hemisphere Winter of 2019/2020

Abstract:

A strong polar vortex was continuously maintained in the Northern Hemisphere winter of 2019/2020 except in early February when a minor sudden stratospheric warming (SSW) occurred. In this study, we make a statistical evaluation of the strength of the polar vortex in this winter and investigate a plausible mechanism for the intensification of the polar vortex, by using MERRA-2 reanalysis data. On the basis of four indices evaluating the strength of the polar vortex with reference to previous studies (e.g., Wallace and Chang, 1982), it is found that the polar vortex of this winter began to be strengthen around mid-February after the minor SSW occurrence and remained strong until the end of March; as a result, the average of the strength of the polar vortex from February to March was the strongest in the analysis period from 1980 to 2020. Furthermore, we make dynamical analyses based on the transformed Eulerian mean equations to elucidate the maintenance of the strong polar vortex and obtain the following two important factors: The first is the intensification mechanism for the polar vortex after the minor SSW. A strong divergence of the Eliassen-Palm (EP) flux appeared in mid-latitudes of the upper stratosphere, leading to intensify the polar vortex in the upper stratosphere so strongly that the upward propagation of planetary waves was restricted. Our linear stability analysis indicates a possibility that the EP flux convergence might be caused by barotropic instability of a modified polar night jet structure after the minor SSW. The second is that the planetary wave activity was continuously very weak in the stratosphere after mid-February. Instead, the downward propagation of the zonal wavenumber 2 was dominant in the lower stratosphere. Consequently, the upward propagation of planetary waves is effectively restricted to keep the westerly polar night jet strong in the stratosphere.