Interannual Variability in the Decay of Lower Stratospheric Arctic Vortices

The interannual variability of the decay of lower stratospheric Arctic vortices is examined using NCEP/NCAR re-analyses between 1958 and 2000. There is large interannual variability in the characteristics of the decay of the vortex air, with very different characteristics for early and late vortex breakups. In early breakup years (when the vortex breaks up in February and early March) the remnants of the vortex survive as coherent potential vorticity structures for around two months, whereas in late breakups (late April and May) the potential vorticity remnants quickly disappear. There is a similar contrast in the stirring around the vortex between early and late breakup years, as diagnosed by the lengthening of material contours in contour advection calculations. In years with an early breakup there is a gradual decrease in the stretching rates from large winter to small summer values, whereas in late breakup years stretching rates are roughly constant until late spring when there is a rapid decrease. These differences in the decay of coherent vortex structures and stirring suggest that there are large differences in the mixing of vortex air into the surrounding middle latitudes between years with early