Instability Theory of Large-Scale Disturbances in the Tropics

Abstract

A new model is constructed for the explanation of the several types of large-scale wave motions detected in the tropical troposphere and lower stratosphere through observational investigations. The model also attributes the origin of these disturbances to the CISK mechanism, but it operates with a period-dependent self-regulating process. This process is based on the consideration that the deep cumulus convection, organized by the convergence field of the wave disturbances in the conditionally unstable tropical atmosphere, works as an overturning process and hence requires a continuous supply of moisture through evaporation from the surface for their continued existence as a wave train. This suggests that, for waves with periods, τ shorter than a limiting value τ0 obtainable from the average evaporation rate, the available moisture for the particular wave is equal to the normal value multiplied by τ/τ0. When this influence is taken into consideration, it is found that most of the large-scale disturbances observed in the tropical atmosphere can he identified with the most unstable waves. It is also found that, when the influence of the vertical suction of the planetary boundary layer is included, especially when the normal stable stratification is taken into consideration, the symmetric mixed Rossby waves and Kelvin waves with periods of about 4?5 days become more prominent, while the antisymmetric waves with periods around 9 days grow faster than others. The wavelengths of these most favored disturbances are all in the vicinity of 10?103 km. In addition, Kelvin waves of periods about 15 days and wavelengths about 10?103 km also become prominent under the influence of the vertical suction of a neutral boundary layer.