Abstract
To better understand the selection of eastward-propagating equatorial wave-CISK modes simulated by Miyahara (1987), and Lau and Peng (1987), a simple linearized wave-CISK model having one or two vertical modes is utilized. Two types of wave-CISK processes (traditional and positive-only) are adopted. Second-order horizontal viscosity/diffusion is included to obtain the preferred modes with horizontal scales comparable to the equatorial radius of deformation. Without this viscosity/diffusion, the horizontal scale of upward motion becomes infinitesimally small. By changing the beta parameter and vertical heating profiles, which makes up the first and second modes, characteristic features of the modes appearing in the model are theoretically and numerically examined.
On a non-rotating plane, excited motions are always symmetric in the zonal direction around the initial impulse. Neither preferential propagation nor selective amplification occurs. Three regimes are obtained: (1)a decaying adjustment mode, (2)a stationary growing mode, and (3)a pair of zonally propagating growing modes having the same properties except for the propagation direction.
On the equatorial beta plane, three regimes are similarly obtained, corresponding to those on the non-rotating plane. However, the deviated westward propagation appears and the symmetric features in the east-west direction are destroyed. This deviated westward propagation is produced by the dual roles of the equatorial beta effect for the equatorially symmetric convective modes-rotation and a side wall effect at the equator.
In the positive-only model on the equatorial beta plane, drastic changes take place between the pair of oppositely propagating modes in regime (3). The westward-propagating modes sequentially create new modes to the east and form a wave packet, resulting in decaying modes. On the contrary, the single eastward-propagating growing (EG) mode is not changed compared with that on the nonrotating plane. As a result, the EG mode is selected as the preferred mode. This EG mode has a westward-tilting baroclinic structure and is similar to those simulated by Miyahara, and Lau and Peng. The selective amplification of the EG mode is due to two oppositely propagating modes, which are isolated by the positive-only wave-CISK, that induce different linear responses on the equatorial beta plane in the east-west direction.
