THE GRAVITY-WAVE DOPPLER SPREAD THEORY APPLIED IN A NUMERICAL SPECTRAL MODEL OF THE MIDDLE ATMOSPHERE .2. EQUATORIAL OSCILLATIONS

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.