EVAPORATION WIND FEEDBACK AND THE ORGANIZING OF TROPICAL CONVECTION ON THE PLANETARY SCALE .2. NONLINEAR EVOLUTION

The nonlinear evolution of the quasi-linear (QL) evaporation-wind feed back (EWFB) instability obtained in Part I of this study is investigat ed in a two-level model of the aqua-planet atmosphere. In this model, the QL-EWFB instability causes tropical convection to organize on the planetary scale and a wavenumber one Kelvin wave-like structure domina tes the east-west circulation in the tropics. An increase of the stati c stability that is in phase with the surface evaporation stabilizes t he EWFB mode. For large surface humidity, a hierarchy of convective st ructures appear as a result of the nonlinear adjustment of the QL mode at large amplitudes. Isolated grid-size individual convective zones m ove randomly, while a wavenumber one envelope of this convection propa gates eastward at a constant speed. In the conditionally unstable para meter regime, the model atmosphere is found to be stable on the planet ary scale, but it can be conditionally unstable on the scale of indivi dual convection events. The EWFB and conditional instabilities are not mutually exclusive as in the QL model but cooperate in organizing con vection. The development of the fast-growing conditional instability a cts to stabilize the large-scale atmosphere, allowing the EWFB mechani sm to organize convection into a wavenumber one structure.