Climatology of wave breaking and mixing in the Northern Hemisphere summer stratosphere

Abstract

The cause of loge zonal ozone variations observed by POAM II (Polar Ozone and Aerosol Measurement II) in the Northern Hemisphere summer stratosphere between 55N̊-65N̊ and ~20-30 km is investigated using the United Kingdom Meteorological Office stratospheric data set with time-mean anomalies removed. This study tests the hypothesis from Hoppel et at. (1999) that breaking of westward-propagating planetary waves in the region of maximum ozone variance (RMV) induces substantial meridional transport which is responsible for the observed ozone variance. EP-flux vectors calculated from the UKMO data set show that wave activity propagates vertically from source regions in the lower midlatitude troposphere into the stratosphere and RMV during the NH summer. In the RMV, the EP-flux divergence is clearly nonzero, which means the zonal-mean zonal flow is forced by waves in this region. Close examination of the individual zonal wavenumber contributions to the climatological monthly-mean El'-flux divergence shows that contributions by zonal wavenumbers 1-5 generally account for over 90% of the forcing of the zonal-mean flow in the lower half of the RMV from June to August. In the upper half of the RMV, planetary waves 1-3 dominate the forcing in June and July, but waves 1-5 appear to be able to penetrate to all RMV levels in August. The forcing of the zonal-mean flow in the RMV is shown to be driven primarily by dissipation of waves that are breaking at critical levels in the region. Power spectra show power in westward-propagating waves 1-5 with phase speeds equal to the zonal-mean flow at all RMV levels. Therefore, breaking waves in the RMV are westward- propagating waves 1-5. EP flux divergence, meridional mixing (transport), and Ozone parlance in the RMV are all shown to decrease from June to July and increase in August in a similar fashion. This trend is also mirrored specifically in the amplitudes of zonal waves 1-5, further connecting the meridional transport (which induces the ozone variance in the RMV) to the breaking of westward-propagating planetary-scale waves 1-3 and medium-scale waves 4-5 in the RMV, lending credibility to the hypothesis in Hoppel et at. [1999].