Equatorial waves in a general circulation model simulating a quasi-biennial-oscillation

The quasi-biennial oscillation (QBO) in the equatorial lower stratosphere has been well simulated in the first version of the Center for Climate System Research / National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (Takahashi, 1996). The model horizontal resolution is T21 with 60 layers of vertical resolution of about 500 m in the upper troposphere and lower stratosphere, with 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 spectral analysis method. The westerly acceleration phase of the QBO-like oscillation in the model is due to Kelvin waves and eastward-propagating gravity waves. The easterly acceleration phase of the oscillation is due to westward-propagating n = 1 equatorial gravity waves, random westward-propagating gravity waves and Rossby waves propagating from the mid-latitudes in the northern winter hemisphere to the equatorial region. The Rossby-gravity wave has a role in the easterly acceleration of the QBO-like oscillation in the present general circulation model.