Jm. Forbes et al., QUASI 16-DAY OSCILLATION IN THE MESOSPHERE AND LOWER THERMOSPHERE, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 100(D5), 1995, pp. 9149-9163
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.