UNITED MECHANISMS FOR THE GENERATION OF LOW-FREQUENCY AND HIGH-FREQUENCY TROPICAL WAVES .2. THEORETICAL INTERPRETATIONS

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 evapora tion-wind feedback. Through the ST mechanism, other waves are triggere d by the intermittent onset of moist convection, upon saturation, to n eutralize any pre-existing conditionally unstable stratification. Thes e mechanisms are theoretically interpreted by partitioning moist conve ctive adjustment into two consecutive processes of diagnostic and prog nostic adjustments. The two processes respectively restore and maintai n 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 t he prognostic-adjustment process in the linearized perturbation equati ons, thereby excluding the ST mechanism. The solutions indicate that w ave instability results from the EWF mechanism and not from the wave-C ISK mechanism. For a plausible choice of adjustable parameters, one st rongly 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 fro m the numerical experiments conducted in Part I, using a nonlinear mod el incorporating the original moist convective adjustment scheme. It i s then speculated that the 40-50- and 25-30-day modes can strongly gro w through the linear and nonlinear EWF mechanisms respectively, while the 10-20-day mode can strongly amplify through the ST mechanism.