QUASI 16-DAY OSCILLATION IN THE MESOSPHERE AND LOWER THERMOSPHERE

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 degrees N, 38 degrees E) and Saskatoon, Canada (52 degrees N, 107 degrees 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.