EQUATORIAL WAVES IN A GENERAL-CIRCULATION MODEL SIMULATING A QUASI-BIENNIAL-OSCILLATION

The quasi-biennial oscillation (QBO) in the equatorial lower stratosph ere has been well simulated in the first version of tile Center for Cl imate System Research / National Institute for Environmental Studies ( CCSR/NIES) atmospheric general circulation model (Takahashi, 1996). Th e model horizontal resolution is T21 with 60 layers of vertical resolu tion of about 500 m in the upper troposphere and lower stratosphere, w ith a moist convective adjustment scheme. The period of the simulated oscillation is 1.5 years, which is slightly shorter than that observed . The equatorial wave behavior in the model is examined using the spec tral analysis method. The westerly acceleration phase of the QBO-like oscillation in the model is due to Kelvin waves and eastward-propagati ng gravity waves. The easterly acceleration phase of the oscillation i s due to westward-propagating n = 1 equatorial gravity waves, random w estward-propagating gravity waves and Rossby waves propagating from th e mid-latitudes in the northern winter hemisphere to the equatorial re gion. The Rossby-gravity wave has a role in the easterly acceleration of the QBO-like oscillation in the present general circulation model.