WAVE-MEAN FLOW INTERACTION ASSOCIATED WITH A QBO-LIKE OSCILLATION SIMULATED IN A SIMPLIFIED GCM

The interaction between convectively excited waves and the mean zonal wind in the equatorial lower stratosphere is investigated with a simpl ified general circulation model (GCM). The model has T42 truncation, a nd the vertical resolution is about 700 m in the stratosphere. Althoug h it is an ''aquaplanet'' model with uniform sea surface temperature, cumulus convection in low latitudes has realistic hierarchical structu res with reasonable space-time spectral distributions. The model produ ced an oscillation having quite similar features to the equatorial qua si-biennial oscillation (QBO), although the period is 400 days. Waves in the equatorial lower stratosphere of the model are excited mainly b y the cumulus convection in low latitudes. The energy of these waves i s a little larger than that observed in the real atmosphere. The domin ant 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 z onal momentum contributing to the oscillation is accounted for by the gravest symmetric gravity modes: eastward momentum by Kelvin waves and westward momentum by n = 1 gravity waves. The momentum deposition is done over a wide range of zonal wavenumber (2-30), while about half of it is done over a period of 1-3 days. The deposition has rather conti nuous phase speed distributions and a considerable portion of it is pr ovided 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 stratosp here for GCMs in order to simulate the QBO.