Hg. Mayr et al., THE GRAVITY-WAVE DOPPLER SPREAD THEORY APPLIED IN A NUMERICAL SPECTRAL MODEL OF THE MIDDLE ATMOSPHERE .2. EQUATORIAL OSCILLATIONS, J GEO RES-A, 102(D22), 1997, pp. 26093-26105
Mayr et al. [this issue] discussed a two-dimensional version of the nu
merical spectral model (NSM) of Chan et al. [1994a, b] that incorporat
es the Doppler spread parameterization (DSP) for momentum deposition b
y small-scale gravity waves (GW) developed by Hines [1997a, b] and pre
sented numerical results describing the global scale seasonal variatio
ns in the temperature and wind fields of the middle atmosphere. Even w
ith the simplest assumptions for the GW flux emanating from the tropos
phere, to be isotropic and independent of latitude and season, this mo
del also produces significant oscillations in the equatorial zonal cir
culation which are discussed here. Our model results lead to the follo
wing conclusions: (1) At altitudes above 40 km, a periodicity of 6 mon
th dominates, resembling the observed semiannual oscillation (SAO). Th
e peak amplitude of this oscillation is close to 18 m/s near 50 km (20
-30 m/s observed). A secondary maximum is excited near 80 km with an a
mplitude of about 11 m/s (15-25 m/s observed), whose phase is opposite
to that at 50 km. In this altitude range, the downward phase progress
ion is about 9 km/month, in agreement with observations. The computed
SAO is confined to equatorial latitudes, as observed. (2) At altitudes
below 40 km, the period of the computed oscillation is almost 21 mont
hs, approaching that of the observed quasi-biennial oscillation (QBO).
The maximum wind amplitudes are close to 8 m/s (20 m/s observed), and
the downward phase progression is about 1.6 km/month (1.3 km/month ob
served). The model also produces a QBO in the upper mesosphere, in qua
litative agreement with recent UARS measurements [Burrage et al., 1996
]. (3) When the eddy diffusivity is reduced by a factor of two, the QB
O period increases to 30 months and the maximum wind amplitude approac
hes 13 m/s. Computer experiments are discussed for constant, equinocti
al solar heating to elucidate the GW excitation mechanism for the equa
torial oscillations in the zonal circulation.