The interaction between convectively excited waves and the mean zonal wind in the equatorial lower stratosphere is investigated with a simplified general circulation model (GCM). The model has T42 truncation, and the vertical resolution is about 700 m in the stratosphere. Although it is an ``aqua-planet'' model with uniform sea surface temperature, cumulus convection in low latitudes has realistic hierarchical structures with reasonable space-time spectral distributions. The model produced an oscillation having quite similar features to the equatorial quasi-biennial oscillation (QBO), although the period is 400 days. It is expected that the period will be approximately doubled to have a comparable value to that of the QBO if realistic upwelling of the Brewer-Dobson circulation is incorporated.

Waves in the equatorial lower stratosphere of the model are excited mainly by the cumulus convection in low latitudes. The energy of these waves is a little larger than that observed in the real atmosphere. The dominant waves are gravity waves having an equivalent depth of about 200 m and those of 40-100 m. About half of the transport and deposition of zonal momentum contributing to the oscillation is accounted for by the gravest symmetric gravity modes: westerly momentum by Kelvin waves and easterly momentum by $n$=1 gravity waves. The momentum deposition is done over wide range of zonal wavenumber 2-30, while about half of it is done over period 1-3 days. The deposition has rather continuous phase-speed distributions and a considerable portion of it is provided by waves having critical levels. Since gravity waves with small intrinsic phase speeds have small vertical wavelengths, vertical grid spacings of 700 m or less appear to be required in the lower stratosphere for GCMs in order to simulate the QBO.