Extenarl Mode Induced by Tropical Heating in the Basic Flow with Vertical Shear and its Propagation

[摘要] The generation of the external-mode ("barotropic" mode) motions from the internal-mode ("baroclinic„ mode) motions which are directly forced by stationary tropical heating is investigated with a two-level global model. The equations are linearized with respect to a solid-body rotation basic flow with vertical shear. We examine in detail the effect of this vertical shear which permits the coupling of the external mode with the internal mode. First, we determine the internal-mode motions forced by internal heating; second, from the vertical shear of the basic flow and the internal-mode motions, we determine the external-mode motions. In this treatment the equations governing internal and external modes can be reduced to the shallow water equations with different values of equivalent depth.The results show that the amplitude of the external mode induced in this way is smaller than that of the internal mode in the tropics, but becomes predominant in the extra-tropics. The external-mode wavetrains emanating northeastward and southeastward from the localized heating region are formed. These wavetrains extend to the antipodes of the emanating region. We examine respective effects of the several terms which express the excitation of the external mode from the coupling between the internal mode and the vertical shear of the basic flow. It is found that the term which expresses an advection of internal-mode vorticity by the basic flow with vertical shear is the most important. By a speculation concerning this forcing term and an inspection of the result of the numerical calculation, we examine in detail the sign of vorticity of the external-mode motions in the region where the internal mode has a large amplitude.Further, we examine the effects of the differential damping and the Ekman pumping on the excitation of the external mode.The external-mode wavetrains are formed by these effects also. The sign of the vorticity of the external-mode motions in the heating region is discussed.

[学科分类] 大气科学