Adam H. Sobel

The authors propose a new analytic approximation to aid understanding of the “moisture modes” that arise in idealized models of tropical intraseasonal oscillations. The approximation makes use of the separation between the time scale of the large scale flow and that of convective adjustment, but does not neglect the moisture tendency. The approximation filters out equatorial Kelvin waves, so that the only remaining mode is the intraseasonal moisture mode. Some physical insights are more transparent under the approximation: (a) vertical advection of moisture or temperature lead to a diffusion‐type operator that damps short waves, and instability is strongest at planetary scale. (b) A positive effective gross moist stability appears in a wave operator, while a negative effective gross moist stability leads to instabilities whose growth rates increase with zonal wavenumber. (c) Zonal moisture advection and surface flux feedbacks are crucial for both planetary instability and eastward propagation.