Wavebreaking and mixing in the Northern Hemisphere summer stratosphere

Abstract

The cause of zonal ozone variations observed by the Polar Ozone and Aerosol Measurement II (POAM II) instrument in the Northern Hemisphere summer stratosphere from ∼55° to 65°N and ∼20 to 30 km is investigated using United Kingdom Meteorological Office (UKMO) stratospheric data. Eliassen-Palm (E-P) flux vectors calculated from the UKMO data show that wave activity propagates vertically from the troposphere into the stratospheric easterlies during the Northern Hemisphere summer. In the layer between 20 and 30 km the E-P flux divergence is small but nonzero. Space-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 following the Charney-Drazin theorem. Air parcel trajectories and effective diffusivity calculations show substantial mixing due to breaking of westward propagating waves in the summer easterly wind regime. Changes in mixing properties during the summer season are consistent with changes in the mean state and the wave forcing. We conclude that breaking of large-scale westward propagating waves in the summer easterlies causes meridional transport and the observed local maximum in ozone variability. Copyright 2000 by the American Geophysical Union.