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The structure of the intraseasonal oscillations in the tropics of an idealized general circulation model with a zonally symmetric climate is described. Space-time spectra show a peak in zonal winds and velocity potential at the equator in zonal wavenumbers 1 and 2, corresponding to eastward-propagating power at phase speeds of ≈18 m s−1. This speed is significantly greater than that of the observed oscillation but comparable to that obtained in similar models by Hayashi and Sumi and Swinbank et al. The corresponding eastward-propagating power in the precipitation spectrum is concentrated in wavenumbers 2–5. A composite procedure is used to describe the three-dimensional structure of the model's oscillation. The oscillation is characterized by circulation cells oriented along the equatorial zonal plane, with enhanced precipitation in the region of rising motion. Zonal wind changes tend to be positively correlated with geopotential height changes at the same level. Positive perturbations in the water vapor mixing ratio, evaporation, and lower tropospheric horizontal moisture convergence all exhibit distinct eastward displacements from the center of convection. Two different linear models are used to interpret the GCM results. The response to the GCM's composited diabatic heating field is first computed using a linear primitive equation model on the sphere. This linear model requires strong damping above the heated region, as well as near the surface, to produce a pattern in rough agreement with the GCM. A simple Kelvin wave-CISK model, in which the vertical structure of the heating is taken from the composite, is then shown to be capable of reproducing the phase speed simulated in the GCM.
Lau et al. (Thu,) studied this question.