It is assumed that low- and high-frequency tropical waves are generated by the united mechanisms consisting of the evaporation-wind feedback (EWF), saturation-triggering (ST), and lateral-triggering mechanisms. Through the EWF mechanism, some waves become unstable owing to evaporation-wind feedback. Through the ST mechanism, other waves are triggered by the intermittent onset of moist convection, upon saturation, to neutralize any pre-existing conditionally unstable stratification. These mechanisms are theoretically interpreted by partitioning moist convective adjustment into two consecutive processes of diagnostic and prognostic adjustments. The two processes respectively restore and maintain convective equilibrium, and are crucial to the ST and EWF mechanisms.
As a step to toward a unified theory, EWF instability is examined by the use of a theoretical Kelvin-wave model, which incorporates only the prognostic-adjustment process in the linearized perturbation equations, thereby excluding the ST mechanism. The solutions indicate that wave instability results from the EWF mechanism and not from the wave-CISK mechanism. For a plausible choice of adjustable parameters, one strongly unstable mode corresponds to the observed 40-50-day oscillation, while two weakly unstable modes correspond to the observed 25-30-day and 10-20-day oscillations.
These results are compared with those from the numerical experiments conducted in Part I, using a nonlinear model incorporating the original moist convective adjustment scheme. It is then speculated that the 40-50- and 25-30-day modes can strongly grow through the linear and nonlinear EWF mechanisms respectively, while the 10-20-day mode can strongly amplify through the ST mechanism.