Planetary wave‐mean flow interaction in the stratosphere: A comparison between northern and southern hemispheres

By the use of stratospheric height and temperature field data obtained from the TIROS satellite, dynamical interaction between planetary waves and mean zonal winds in the stratosphere is investigated. Special attention is paid to differences between northern and southern hemispheres. an analysis is made for pressure levels from 70 to 0.4 mb during the period from June 1981 to May 1982. Eliassen‐Palm (E‐P) flux diagnostics are used as a powerful and useful tool to investigate the wave‐mean flow interaction. From winter to summer the seasonal marches of wave activity (measured by the E‐P flux) and of mean zonal wind are different between the two hemispheres. In the NH, wave activity varies intermittently with a characteristic timescale of about two weeks. Once the core of the stratospheric westerly jet shifts poleward due to a minor warming, the subsequent wave activity breaks the westerlies dramatically to replace them by the easterlies in association with the sudden warming, whilst in the SH wave activity in mid‐winter is quiet, corresponding to small time variations of the maximum westerly speed. In late winter the core of the stratospheric westerly jet suddenly shifts poleward and downward due to a wavenumber‐2 minor warming. After the shifting of the westerly jet the wave activity of wavenumber 1 is enhanced, and continues until early summer. On the basis of this observational evidence we discuss: (i) the seasonal evolutions of wave activity and the mean zonal wind; (ii) the interannual variability of the stratospheric circulations; and (iii) the time variation of wave activity.