Break up of the Arctic vortices: timing and mixing

The timing of, and the mixing following, the breakup of the Arctic lower stratospheric vortex both play an important role in ozone depletion. Late vortex breakups are associated with strong and cold spring vortices, and hence favorable conditions for chemical ozone loss. While the time scale of mixing following the breakup is an important factor controlling the impact of polar processed air on mid-latitude ozone. We will present results from an examination of the interannual variability in (i) the timing of and (ii) the mixing following the Arctic vortex breakup.

This analysis shows that there is a large interannual, and smaller decadal and longer time scale, variations in the timing of the breakup of the Arctic vortex. In particular there is a significant increase in the persistence of the vortices since the mid-1980s. These variations are correlated with the strength and coldness of the spring vortex as well as with the upper tropospheric/lower stratospheric eddy heat flux prior to the breakup. The latter correlation indicates that the variability in the wave activity entering the stratosphere over late-winter to early-spring plays a key role in the variability of the vortex persistence on both interannual and decadal time scales.

There is also a large variation in the characteristics of the mixing following the vortex breakup, as diagnosed by both the stretching rates from contour advection calculations and the persistence of coherent vortex fragments. However this variability is not strongly correlated with the timing of the breakup of the vortex, i.e., whether there is rapid or weak mixing following the breakup does not depend strongly on whether it is an early or late breakup.