Wavebreaking and mixing in the Northern Hemisphere summer stratosphere

The cause of zonal ozone variations observed by the Polar Ozone and Aerosol Measurement II (POAM II) instrument in the Northern Hemisphere summer stra tosphere from similar to 55 degrees to 65 degreesN and similar to 20 to 30 km is investigated using United Kingdom Meteorological Office (UKMO) strato spheric data. Eliassen-Palm (E-P) flux vectors calculated from the UKMO dat a show that wave activity propagates vertically from the troposphere into t he stratospheric easterlies during the Northern Hemisphere summer. In the l ayer between 20 and 30 km the E-P flux divergence is small but nonzero. Spa ce-time power spectra show that wave power shifts from eastward propagating waves in the upper troposphere to predominantly westward propagating waves in the middle stratosphere, consistent with phase speed filtering followin g the Charney-Drazin theorem. Air parcel trajectories and effective diffusi vity calculations show substantial mixing due to breaking of westward propa gating waves in the summer easterly wind regime. Changes in mixing properti es during the summer season are consistent with changes in the mean state a nd the wave forcing. We conclude that breaking of large-scale westward prop agating waves in the summer easterlies causes meridional transport and the observed local maximum in ozone variability.