Climatological features of mesosphere and lower thermosphere stationary planetary waves within ±40° latitude

[1] Zonal and meridional wind measurements from the High Resolution Doppler Imager (HRDI) and Wind Imaging Interferometer (WINDII) instruments on the Upper Atmosphere Research Satellite (UARS) are used to construct monthly average stationary planetary wave (SPW) structures from 15 km to 110 km over the period December 1991 to September 1994. Because of viewing constraints of the instruments, intercomparisons between Northern Hemisphere (NH) and Southern Hemisphere (SH) structures and seasonal evolutions are mainly confined to ±40° latitude. The largest zonal wave number s = 1 amplitudes (10–25 m s−1) in the stratosphere and mesosphere at ±40° exist in the zonal wind fields and occur during September–January in the NH and July–October in the SH. SPW s = 2 structures are much less prevalent, especially above 80 km because of more restrictive filtering effects due to the mean winds. Above about 70 km, s = 1 wave amplitudes (∼5–10 m s−1) are more uniform, extending over all nonsummer months in both hemispheres up to 80–90 km, with similar amplitudes existing throughout the year at higher altitudes. Our interpretation is that the SPW above about 80–90 km during summer owe their existence to ducting from the winter hemisphere, in agreement with numerical simulations by Pogoreltsev and Sukhanova [1993]. The origin of SPW above about 80 km during winter at midlatitudes is not possible to determine unambiguously. Phase structures undergo a transition from vertical propagation with long vertical wavelengths (hundreds of kilometers) below ∼80 km to either evanescent behavior or an extreme shortening of vertical scale at higher altitudes. This change in phase behavior may be connected with negative values in refractive index Q for quasi-geostrophic wave propagation, which occur over a narrow height region near 80 km during almost every month. This interruption of normal phase progression makes it impossible to distinguish the origin of the planetary wave oscillations from (1) those that “tunnel” through the Q < 0 region and (2) those that might be excited in situ by zonally asymmetric momentum deposition from gravity waves which have undergone filtering by the stratospheric planetary wave wind field. In addition, quantitative estimates of aliasing due to the diurnal tide indicate that it may be of sufficient magnitude to contaminate retrieved SPW structures between 80–90 km and 100–110 km, where local time coverages are incomplete. Comparisons are made between the observed SPW structure and those from the horizontal wind model [Hedin et al., 1993, 1996]. This model is found to provide a reasonable approximation to NH winter SPW s = 1 structures below 80 km, but severely underestimates the amplitudes of SPW in the SH.

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