Stratospheric Polar Vortex Revisited: New Diagnostic for the Vortex Breakup

Abstract

The stratospheric polar vortex and its breakup are important dynamical phenomena. In previous studies, three diagnostics for vortex breakup have been suggested using the potential vorticity (PV) and zonal wind for the lower stratosphere. These three diagnostics, however, cannot be applied for the upper stratosphere, since the evolution of the polar vortex is more complicated and, therefore, it is more difficult to prescribe key parameters. Here we define the dates of the breakup and formation of the polar vortex by obtaining the maximum peaks in the averaged rates of change in the equivalent latitude, PV, and wind speed at the vortex edge. By applying our new definition to the ERA-Interim reanalysis data, the breakup and formation dates of the Arctic and Antarctic polar vortices for the whole stratosphere were obtained for 1979 ??? 2018. Our newly defined vortex breakup date is compared with the date of the stratospheric final warming, which is defined as the timing of zonal-mean westerlies changing to easterlies without recovering to a westerly exceeding the threshold westerly wind speed. To see if our definition is consistent with atmospheric transport near the vortex edge, the dates of the formation and breakup of the polar vortex are compared with the mixing ratios of long-lived trace species. It turns out that the newly defined dates well match the changes of concentrations of trace gases in the stratosphere for the winter of 1996 - 1997. Considering all the above observations, our definition of the vortex formation and breakup appears to be applicable to the whole stratosphere.