Abstract
Equatorial waves in a three-dimensional sector (1/5) model with moist convective adjustment are examined. The model is derived from a preliminary version of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) general circulation model. The model produces a QBO-like oscillation in the equatorial upper stratosphere (Takahashi and Shiobara, 1995).
There is a strong signal in the zonal wavenumber s=1 (global zonal wavenumber 5, since the model is a 1/5 sector), meridional mode number n=1 having a period of 2.5 days, corresponding to a westward-propagating equatorial gravity wave. The wave produces the dominant easterly phase of the QBO-like oscillation. The signal is also clearly seen in the precipitation. The eastward-propagating waves which produce the westerlies in the QBO-like oscillation are rather complicated. The signals are difficult to identify with equatorial waves, for example the Kelvin wave or n=1 equatorial gravity wave. The wave structures are like an ensemble of random gravity waves. These gravity waves produce the QBO-like oscillation in the sector model.
A lower horizontal resolution experiment, using the T21 global model, is also performed to compare equatorial wave behavior. Kelvin and Rossby-gravity waves are simulated in the T21 general circulation model. Simulated gravity waves are, however, weak and as a result, the T21 model does not produce a QBO-like oscillation, but rather a steady state of easterlies in the equatorial upper stratosphere with westerlies in the middle stratosphere.
