Q. J. R. Meteorol. Soc. (2004), 130, pp. 2445–2458 doi: 10.1256/qj.02.200 Seasonal, interannual and short-term variability of planetary waves in Met Office stratospheric assimilated fields By I. N. FEDULINA 1 ∗ , A. I. POGORELTSEV 2 and G. VAUGHAN 1 1 University of Wales, Aberystwyth, UK 2 Institute for Meteorology, Leipzig University, Germany (Received 16 October 2002; revised 26 January 2004) SUMMARY 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. KEYWORDS: Middle atmosphere Normal mode Travelling planetary wave 1. I NTRODUCTION A characteristic feature of stratospheric dynamics, particularly in winter, is the substantial nonzonality of the meteorological fields (Andrews et al. 1987). Global-scale disturbances of the zonally averaged state can be represented by the Fourier series of zonal harmonics with small zonal wave numbers, the so-called planetary waves. Quasi-stationary planetary waves (SPWs), generated by orography and diabatic heating in the troposphere, propagate into the upper atmospheric layers and modify the mean flow by depositing heat and momentum. In the middle atmosphere there also exists a broad spectrum of travelling planetary waves, which are forced by irregular thermal or mechanical forcing in the lower atmosphere and/or by a short-period variability of the SPW activity. Behaving like a resonant wave-guide, the atmosphere preferentially amplifies the westward-propagating normal-mode Rossby waves (the free atmospheric oscillations) (Longuet-Higgins 1968; Volland 1988). In the lower atmosphere these transients have smaller intensities than those of the SPWs; nevertheless, propagating into the upper atmosphere, they play an important role in the dynamics of the mesosphere and lower thermosphere (MLT) (Salby 1984). Many observational studies on normal atmospheric modes have been performed using an analysis of tropospheric and lower stratospheric meteorological fields (Delan 1964; Eliasen and Machenhauer 1965; Diky and Golitsyn 1968; Madden 1978; Ahlquist 1982, 1985; Lindzen et al. 1984), and comprehensive reviews of the first experimental results and theoretical properties of Rossby waves were presented by Madden (1979) and Salby (1984). Ten years of global tropospheric data from the European Centre ∗ Corresponding author, current affiliation: Institute of the Ionosphere, Kamenskoe plato, Almaty 480020, Kazakhstan. e-mail: inna@fedul.almaty.kz c  Royal Meteorological Society, 2004. 2445