Gravity wave and equatorial wave morphology of the stratosphere derived from long-term rocket soundings



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, u′2 + v′2, and relative-temperature perturbations, T′2, 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′2 + v′2 values generally increased significantly with altitude throughout the stratosphere, T′2 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 u′2 + v′2 variations at 40–60 km. The peak was also evident in T′2, but was smaller relative to the winter peak. At low latitudes (∼15°N–25°N) 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°S) 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 quasi-biennial 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 u′2 + v′2 and T′2 decreased 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.