Evaporation-wind feedback and the organizing of tropical convection on the planetary scale. Part I: quasi-linear instability

Abstract

This paper reports that a planetary-scale mode can be generated in a very simple reduced gravity model that is linear except for two nonlinearities in its cumulus parameterization: conditional heating and wind speed-dependent surface evaporation. The behavior of the model solution is shown to be independent of the perturbation amplitude so that a constant growth rate can be defined. This amplitude-independent nonlinear system is here called the quasi-linear (QL) system. An instability is found in a moist stable atmosphere at rest, which is stable in existing theories. A global integral theorem confirms the existence of the QL instability. The instability has an equatorially trapped, zonal wavenumber one structure, growing exponentially and propagating eastward at a speed close to that of the neutral, linear moist Kelvin wave. A new type of evaporation-wind feedback (EWFB) is responsible for the instability, which does not require the existence of mean easterlies and arises from an in-phase relaiton between temperature perturbation and condensational heating directly due to surface evaporation. -from Authors