On the Semiannual Formation of Large Scale Three-Dimensional Vortices at the Stratopause

An examination of the dynamics of the middle atmosphere as reconstructed in the NASA Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reveals the formation of large scale three-dimensional (3D) vortices in the tropical stratosphere following both the vernal and autumnal equinoxes at times when the jet associated with the westerly phase of the semiannual oscillation (SAO) is maximal and extratropical influences from planetary waves are weakest. An empirical orthogonal function (EOF) analysis of the second matrix invariant of the velocity gradient tensor applied to the shear zones about the SAO reveals statistically stationary wave-5 vortex structures that span more than 3,200 km in length and up to 40 km in the vertical. Eliassen-Palm fluxes suggests the vortices are maintained by a combination of local (shear zones) and remote (vertically propagating tropical) sources of momentum. These large-scale coherent features appear to be unique to the stratopause.