Abstract. The quasi-biennial oscillation (QBO) dominates the variability of the equatorial stratosphere (---16-50 km) and is easily seen as downward propagating easterly and westerly wind regimes, with a variable period averaging approximately 28 months. From a fluid dynamical perspective, the QBO is a fascinating example of a coherent, oscillating mean flow that is driven by propagating waves with periods unrelated to that of the resulting oscillation. Although the QBO is a tropical phenomenon, it affects the stratospheric flow from pole to pole by modulating the effects of extratropical waves. Indeed, study of the QBO is inseparable from the study of atmospheric wave motions that drive it and are modulated by it. The QBO affects variability in the mesosphere near 85 km by selectively filtering waves that propagate upward through the equatorial stratosphere, and may also affect the strength of Atlantic hurricanes.

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Fluctuations in vertical profiles of atmospheric temperature and horizontal wind in the 20-60 km altitude range have been isolated from meteorological rocket measurements during 1977-87 at 15 widely separated sites. The seasonal, geographical, and vertical variability of the variance of horizontal velocities, + p, and relative-temperature perturbations, F , were studied. The bulk of the variance of both quantities in the 2-10 km and 2-20 km vertical-wavelength bands was associated with gravity-wave motions, although in-depth study of the wave polarization shows that planetary-scale equatorial wave modes contribute to the variance at equatorial sites. Annual mean variances varied widely among -the 15 stations, suggesting appreciable geographical variability in stratospheric wave activity. Whereas u'* + u '~ values generally increased significantly with altitude throughout the stratosphere, F values grew less substantially and often decreased with altitude at upper heights. Rotations of wave-velocity phasors with height were always more frequently clockwise than anticlockwise in the northern hemisphere, consistent with upward-propagating wave energy, yet these percentages (>50%) showed a marked semi-annual variation, with equinoctial maxima and minima at the solstices. At high latitudes (-50°N-80"N) variances exhibited a strong annual variation, with the minimum in summer and a strong peak during winter at both lower (20-40 km) and upper (40-60 km) heights. The annual variance cycle attenuated somewhat at mid-latitudes (-25"N-40"N), and a strong peak in August dominated the d 2 + variations at 4 0 4 0 km. The peak was also evident in p, but was smaller relative to the winter peak. At low latitudes (-15"N-25'") the wave morphology was broadly similar to that at mid-latitudes, apart from an additional upper-level peak in the variance in May. This peak in May occurred in some years but not in others at mid-latitude stations. At the equatorial stations (-10"N-10%) the low-level variance showed little systematic seasonal variability, but exhibited clear modulation over a quasi-two-year period. Much of this variance was consistent with the Kelvin modes thought to drive the eastward phase of the stratospheric quasibiennial oscillation (QBO). However, the uniform east-west alignment of waves was inconsistent with the expected polarization of the mixed Rossby-gravity wave mode which is believed to drive the westward phase of the QBO. At 40-60 km, the variance was strongly attenuated around April-May and November, when both 8 + ? and Fdecreased with height around the 40-45 km range, indicating that wave dissipation occurs here. This produced a semi-annual variation at upper heights, with maxima around January and July, which may contribute significantly to the semi-annual wave driving of the equatorial upper stratosphere. Polarization studies showed that this variance in the 2-10 km band was mostly due to gravity waves, although equatorial modes contributed during December-February .

A statistical study is made of the interannual variability of the northern winter stratospheric circulation in connection with the equatorial quasi-biennial oscillation (QBO) and the solar cycle, by using the 37-year stratospheric dataset of the Freie Universitat Berlin and the 31-year NMC global data.During the period 196263-197778, analyzed first by Holton and Tan (1980, referred to as HT), the polar-night jet is stronger in the W (westerly) than in the E (easterly), as was mentioned by Holton and Tan (1980). However, the difference between the W and the E is barely significant in 'the latter half period ' (197879-199394). When the whole period is classified into two groups defined as 'Min' and 'Max' with respect to the intensity of the 10.7-cm solar flux, it is clearly shown that the late-winter jet in the W is much stronger than in the E (the value of Student's t test exceeds 6) in the Min group, whereas it is no stronger in the Max group. The reason why the result from the HT period resembles that from the Min is probably because the HT period includes two solar minima and one maximum. In early winter, the circulation seems to be correlated with the QBO irrespective of the solar cycle. This difference between early and late winter suggests that the equatorial QBO influences the extratropical circulation in early winter and that the solar cycle modifies it in late winter.An extensive analysis of wave components is also made. The result from the Min is similar to that of the HT period, and the difference between the W and the E is larger than in the HT period. In late winter, the result from the Max is the inverse of the Min result.Finally, the occurrence of major warmings is shown to be related significantly to the QBO and the solar cycle. Such a relationship is clearly illustrated by plotting the occurrence of the major warming onto a 2D phase space of the solar flux and the equatorial wind.

An analysis was made of the structure and behaviour of small-scale motions in the stratosphere and lower mesosphere with the aid of meteorological rocket observations over the period of six years from 1977 to 1982, covering the wide range of latitudes.By applying a filter to observed wind data with respect to height, wind fluctuations with characteristic vertical scales close to 10km are separated from large-scale components such as the mean field, planetary waves and tides.From the hodograph analysis it is found that at northern hemisphere stations most of horizontal wind vectors show the clockwise rotation with increasing height while they rotate anti-clockwise in the southern hemisphere. This strongly suggests that the wind fluctuations are due mainly to upward propagating inertia-gravity waves.On the basis of a simplified theory of inertia-gravity waves, the wave-frequency distribution is estimated statistically from the degree of elliptic polarization of holographs, and it is shown that the most predominant values of f*/* (f; the Coriolis parameter, *; the intrinsic wave frequency) fall into a range of 0.20.4. Namely, the typical time scale of these waves is of the order of several hours in middle and high latitudes and of a day in low latitudes.Further discussions are made of the vertical profile of the wave energy density, and it is suggested , from the uniform decay of the wave amplitude with height that the wide spectra of horizontal phase velocities should be taken into account.