Wave breaking along the stratospheric polar vortex as seen in ERA-40 data

Planetary wave breaking through the depth of the Northern Hemispheric stratosphere is observed in daily ERA-40 reanalysis data. Strong wave breaking along the vortex edge is objectively diagnosed by noting the large-scale overturning of potential vorticity contours on isentropic surfaces spanning the stratospheric vortex. Breaking events show distinctly different climatologies depending on whether they occur along the upper or lower portion of the stratospheric vortex. During early winter there is a strong negative correlation between the frequency of breaking events in these two regions. Frequent wave breaking in the lower stratosphere in early winter appears to both shield the upper portion of the vortex from wave disturbances and reduce the net upward wave activity flux into the troposphere, thereby allowing the vortex to strengthen into mid-winter.