[1] Coupling between the troposphere and lower thermosphere due to upward propagating tides is investigated using temperatures measured from the SABER instrument on the TIMED satellite. The data analyzed here are confined to 20-120 km altitude and ±40°l atitude during 20 July to 20 September 2002. Apart from the migrating (Sun-synchronous) tidal components, the predominant feature seen (from the satellite frame) during this period is a wave-4 structure in longitude with extrema of up to ±40-50 K at 110 km. Amplitudes and longitudes of maxima of this structure evolve as the satellite precesses in local time and as the wave(s) responsible for this structure vary with time. The primary wave responsible for the wave-4 pattern is the eastward propagating diurnal tide with zonal wave number s = 3 (DE3). Its average amplitude distribution over the interval is quasi-symmetric about the equator, similar to that of a Kelvin wave, with maximum of about 20 K at 5°S and 110 km. DE3 is primarily excited by latent heating due to deep tropical convection in the troposphere. It is demonstrated that existence of DE3 is intimately connected with the predominant wave-4 longitude distribution of topography and land-sea difference at low latitudes, and an analogy is drawn with the strong presence of DE1 in Mars atmosphere, the predominant wave-2 topography on Mars, and the wave-2 patterns that dominate density measurements from the Mars Global Surveyor (MGS) spacecraft near 130 km. Additional diurnal, semidiurnal, and terdiurnal nonmigrating tides are also revealed in the present study. These tidal components are most likely excited by nonlinear interactions between their migrating counterparts and the stationary planetary wave with s = 1 known to exist in the Southern Hemisphere during this period just prior to the austral midwinter stratospheric warming of 2002.

A quasi‐16‐day wave in the mesosphere and lower thermosphere is investigated through analyses of radar data during January/February 1979 and through numerical simulations for various background wind conditions. Previous workers have examined about 19 days of tropospheric and stratospheric data during January 10–28, 1979, and present conflicting evidence as to whether a large westward propagating wavenumber 1 oscillation observed during this period can be identified in terms of the second symmetric Rossby normal mode of zonal wavenumber 1, commonly referred to as the “16‐day wave.” In the present work we have applied spectral analysis techniques to meridional and zonal winds near 95 km altitude obtained from radar measurements over Obninsk, Russia (54°N, 38°E) and Saskatoon, Canada (52°N, 107°W). These data reveal oscillations of the order of ±10 m s−1 with a period near 16 days as well as waves with periods near 5 and 10 days. These periodicities all correspond to expected resonant frequencies of atmospheric disturbances associated with westward propagating free Rossby modes of zonal wavenumber 1. Numerical simulations are performed which demonstrate that the 95‐km measurements of the 16‐day wave are consistent with upward extension of the oscillation determined from the tropospheric and stratospheric data. Noteworthy features of the model in terms of its applicability in the mesosphere/lower thermosphere regime are explicit inclusion of eddy and molecular diffusion of heat and momentum and realistic distributions of mean winds, especially between 80 and 100 km. The latter include a westerly wind regime above the summer easterly mesospheric jet, thus providing a ducting channel enabling interhemispheric penetration of the winter planetary wave disturbance. This serves to explain the appearance of a quasi‐16‐day wave recently reported in the high‐latitude summer mesopause (Williams and Avery, 1992). However, the efficiency of this interhemispheric coupling may be reduced by gravity wave stress. No significant penetration of the 16‐day oscillation above about 100 km is predicted by the model. Reported signatures of a 16‐day periodicity in ionospheric data therefore require modulation of tidal or gravity wave accessibility to the thermosphere, or perhaps in situ excitation.

The roles of equatorial trapped waves (EQWs) and internal inertia-gravity waves in driving the quasibiennial oscillation (QBO) are investigated using a high-resolution atmospheric general circulation model with T213L256 resolution (60-km horizontal and 300-m vertical resolution) integrated for three years. The model, which does not use a gravity wave drag parameterization, simulates a QBO. Although the simulated QBO has a shorter period than that of the real atmosphere, its amplitudes and structure in the lower stratosphere are fairly realistic. The zonal wavenumber/frequency spectra of simulated outgoing longwave radiation represent realistic signals of convectively coupled EQWs. Clear signals of EQWs are also seen in the stratospheric wind components. In the eastward wind shear of the QBO, eastward EQWs including Kelvin waves contribute up to ;25%-50% to the driving of the QBO. The peaks of eastward wave forcing associated with EQWs and internal inertia-gravity waves occur at nearly the same time at the same altitude. On the other hand, westward EQWs contribute up to ;10% to driving the QBO during the weak westward wind phase but make almost zero contribution during the relatively strong westward wind phase. Extratropical Rossby waves propagating into the equatorial region contribute ;10%-25%, whereas internal inertia-gravity waves with zonal wavelength &1000 km are the main contributors to the westward wind shear phase of the simulated QBO.

Interactions between breaking internal gravity waves with phase speeds of 0, ±10, ±20, ±30 ms-1, and the diurnal tide in the mesosphere and lower thermosphere (70-120 km) are investigated using a time dependent numerical model of the tide. The gravity wave breaking and stress are calculated using a modified Lindzen's parameterization making allowance for interactions with a diurnally varying zonal wind superimposed on a zonal mean wind field. In order to avoid difficulties with the WKB approximation used in the parameterization, the interactions are only considered in the extratropics. However, the gravity wave stress is shown to suppress the amplitude of the diurnal tide at all latitudes in the upper mesosphere and lower thermosphere. It is also shown that gravity wave stresses modified by the diurnal tide induce significant semidiurnal and terdiurnal tides in the mesosphere and thermosphere.

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