The Response of Equatorial Waves to Thermal Forcing

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Abstract
The response of the stratospheric equatorial waves to thermal forcing is theoretically discussed, taking into account various factors such as the meridional distribution of heating and the vertical shear of the mean zonal wind. First we discuss analytically these effects based on normal mode models and then we treat a realistic three-dimensional model by numerical method. The horizontal structure of waves obtained in the models including the above effects indicates that the dominant wave modes are identified as Kelvin waves, mixed Rossby-gravity waves and n=1 Rossby waves, and that gravity waves are not prominent. The coexistence of gravity waves with the dominant modes, however, in the realistic model with maximum heating near the ITCZ makes the heating and vertical motion to be in phase, in agreement with observations, while in the single-component normal mode model they are in quadrature. It is concluded that mixed Rossby-gravity waves have the maximum response at wavenumbers 4-5 as observed, in view of the effect of vertical shear of the mean zonal wind on the selectivity of wavenumbers. In normal mode models hitherto treated the vertical wavelength of the dominant waves (resonant waves) was somewhat longer and hence the period was shorter, compared with observations. The defect still remains in the present model, though improved to some extent by introducing the realistic distribution of heating. Vertical structure of wind and temperature for predominant waves also shows different features from observed ones in the troposphere. Although some characteristics of observed equatorial waves are accounted for in light of the present model, these results suggest that the observed waves cannot be interpreted merely as the resonant waves by thermal forcing.

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