United Mechanisms for the Generation of Low- and High-Frequency Tropical Waves. Part I: Control Experiments with Moist Convective Adjustment

To examine several mechanisms for the generation of lowand high-frequency tropical waves, numerical experiments are conducted using an idealized nine-level R21 spectral model with the original scheme of moist convective adjustment (MCA). The model prescribes globally uniform, time-independent distributions of sea surface temperatures and insolation, thereby excluding stationary waves and extratropical baroclinic waves. The idealized model, however, still produces tropical intraseasonal oscillations, superclusters, Kelvin waves, and mixed Rossby–gravity waves. When eliminating the wind fluctuations in the parameterized surface fluxes of latent and sensible heat, the intraseasonal oscillations are profoundly weakened while other waves are not substantially weakened. Subsequently, the MCA scheme is modified to neutralize any conditionally unstable stratification that would otherwise develop during periods of nonsaturation. This modification suppresses the part of the MCA process that neutralizes, upon saturation, any preexisting unstable stratification. In spite of the presence of moisture convergence, all tropical transient waves then disappear, in contrast to the wave-CISK (conditional instability of the second kind) mechanism. The above results are consistent with the united mechanisms proposed as follows. Intraseasonal oscillations are maintained primarily through the evaporation–wind feedback mechanism. Other waves are maintained primarily through the ‘‘saturation-triggering mechanism’’ and/or the lateral-triggering mechanism. The saturationtriggering mechanism hypothesizes that transient waves can be triggered by the intermittent onset of nonequilibrium moist convection, upon saturation, to neutralize any preexisting unstable stratification.