Seasonal, interannual and short-term variability of planetary waves in Met Office stratospheric assimilated fields

Met Office stratospheric assimilated data have been used to study the variability of travelling planetary waves in the upper stratosphere during the last decade (1992-2001). To distinguish the various travelling and quasi-standing waves, a new space-time filtering method has been developed. The results of this analysis show that longer-period planetary waves (10 < T < 20 days) have significant amplitudes mainly during winter in both hemispheres. Westward-propagating waves are dominant in the northern hemisphere, but in the southern hemisphere eastward- and westward-propagating waves have comparable amplitudes and sometimes eastward-propagating waves are dominant. It is suggested that the main forcing of longer-period travelling waves is an internal variability of the quasi-stationary planetary waves in the tropo/stratosphere, and that the substantial activity of eastward-propagating waves in the southern hemisphere is associated with seasonal changes of the general circulation of the middle atmosphere. In the shorter-period part of the wavelet spectra (T < 10 days), westward-propagating waves are dominant and the observed maxima in amplitude spectra can be identified with the first symmetric normal atmospheric modes, the so-called 5- and 4-day waves. These waves, present in the stratosphere during most of the year, increase in amplitude during seasonal transitions. Analysis of the behaviour of wavelet amplitudes at different pressure levels shows that these normal-mode waves propagate upward from the lower atmosphere and their amplitudes increase with altitude. There exists also a substantial long-period modulation of the 5-day wave amplitude with quasi-period of about 30 days.