Transient flow patterns of an annular-like stratospheric polar vortex

A two-component study is undertaken of sub-planetary scale flow features associated with the wintertime polar-night vortex at upper-stratospheric levels. First, case studies based upon reanalysis data are presented to illustrate the scale, structure, evolution and dynamics of the features potential vorticity in three disparate flow settings along with signatures of some accompanying flow, thermal, and constituent variables. The features are shown to be present near the periphery of the vortex and are associated with the vortex's predilection to develop a structure akin to an annular-like band of enhanced potential vorticity. Second, theoretical considerations and numerical model simulations of perturbations of an annular band of enhanced absolute vorticity using a non-divergent barotropic model on a polar β-plane yield results consistent both with the occurrence and character of the observed sub-planetary scale features, and with the rapid reconfiguration of the band when subject to a planetary scale wave perturbation. The latter result suggests that that the break-up of the polar vortex at stratopause-levels can be facilitated by the synergetic combination of strong planetary-scale Rossby-wave forcing from below acting upon the vortex's annular potential vorticity band.