QUASI 16-DAY OSCILLATION IN THE MESOSPHERE AND LOWER THERMOSPHERE

A quasi-16-day wave in the mesosphere and lower thermosphere is invest igated 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 stra tospheric data during January 10-28, 1979, and present conflicting evi dence as to whether a large westward propagating wavenumber 1 oscillat ion observed during this period can be identified in terms of the seco nd symmetric Rossby normal mode of zonal wavenumber 1, commonly referr ed to as the ''16-day wave.'' In the present work we have applied spec tral analysis techniques to meridional and zonal winds near 95 km alti tude 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 Ro ssby modes of zonal wavenumber 1. Numerical simulations are performed which demonstrate that the 95-km measurements of the 16-day wave are c onsistent 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 re gime are explicit inclusion of eddy and molecular diffusion of heat an d momentum and realistic distributions of mean winds, especially betwe en 80 and 100 km. The latter include a westerly wind regime above the summer easterly mesospheric jet, thus providing a ducting channel enab ling interhemispheric penetration of the winter planetary wave disturb ance. This serves to explain the appearance of a quasi-16-day wave rec ently reported in the high-latitude summer mesopause (Williams and Ave ry, 1992). However, the efficiency of this interhemispheric coupling m ay be reduced by gravity wave stress. No significant penetration of th e 16-day oscillation above about 100 km is predicted by the model. Rep orted signatures of a 16-day periodicity in ionospheric data therefore require modulation of tidal or gravity wave accessibility to the ther mosphere, or perhaps in situ excitation.