United Mechanisms for the Generation of Low- and High-Frequency Tropical Waves. Part II

Abstract

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 connective 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.

Toward an Understanding of the Madden-Julian Oscillation: With a Mesoscale-Convection-Resolving Model of 0.2 Degree Grid

“…The WISHE mechanism was proposed by Emanuel [71] and Neelin et al [72], as mentioned in Section 1. Hayashi and Golder [85] argued that intraseasonal oscillations are maintained primarily through the evaporation-wind feedback (EWF: similar to WISHE) mechanism. The present author has considered that the dependence of surface heat and moisture fluxes on the wind speed is one of the important components of CISK (including wave-CISK).…”

Indo‐Pacific Ocean response to atmospheric intraseasonal variability: 1. Austral summer and the Madden‐Julian Oscillation

“…In the first, Hendon [2000] coupled a one‐dimensional mixed layer model to each ocean grid point of the R30 GFDL climate model [cf. Hayashi and Golder , 1997] and found that the coupled mixed layer had virtually no impact on the modeled MJO. However, analysis of the model results showed that the simulated MJO‐induced latent heat flux anomalies were relatively incoherent and did not exhibit the proper (i.e., observed) phase relationship relative to the convection, in part because of the model's basic state.…”