An objective determination of the polar vortex using Ertel's potential vorticity

TLDR

The authors determine the polar vortex edge by locating the maximum gradient of Ertel's potential vorticity on equivalent latitudes, constrained by the maximum wind jet along Epv isolines, and define the boundary region at the local maximum convex/concave curvature surrounding the edge, using the 1992–1993 period to show that increases in Epv are linked to diabatic surface air movement. The study establishes objective criteria for polar vortex boundary, onset, and breakup dates, showing that on the 450 K isentropic surface these events occur when the maximum wind speed along Epv isolines crosses ~15.2 m s⁻¹, and that increases in Epv are linked to diabatic surface air movement while decreases are tied to radiatively induced stability changes.

Abstract

We have developed objective criteria for choosing the location of the northern hemisphere polar vortex boundary region and the onset and breakup dates of the vortex. By determining the distribution of Ertel's potential vorticity (Epv) on equivalent latitudes, we define the vortex edge as the location of maximum gradient of Epv constrained by the location of the maximum wind jet calculated along Epv isolines. We define the vortex boundary region to be at the local maximum convex and concave curvature in the Epv distribution surrounding the edge. We have determined that the onset and breakup dates of the vortex on the 450 K isentropic surface occur when the maximum wind speed calculated along Epv isolines rises above and falls below approximately 15.2 m s −1 . We use 1992–1993 as a test case to study the onset and breakup periods, and we find that the increase of polar vortex Epv values is associated with the dominance of the term in the potential vorticity equation involving the movement of air through the surface due to the diabatic circulation. We also find that the decrease is associated with the dominance of the term involving radiatively induced changes in the stability of the atmosphere.