Dynamics of tropical low-frequency waves: an analysis of the moist Kelvin wave

Stability of the equatorial atmosphere to a quasi-zonal, low frequency (order of 10^-6s^-1) disturbance is investigated, using a model that consists of a two-layer free atmosphere and well-mixed boundary layer. The inclusion of boundary layer convergence leads to a circulation-dependent heating more nearly in accord with the behavior of numerically simulated low-frequency waves. Slowly eastward moving unstable waves were found in a parameter regime to inviscid wave-CISK. A derived instability criterion suggests that amplification occurs when the condensational heating supported jointly by interior wave convergence and frictional convergence dominates over the dissipations due to longwave radiation and boundary layer viscosity. Behaviors of model unstable waves appear to resemble the observed 40-50 day mode in many aspects, such as vertical structure, energy source, zonal phase propagations, characteristic zonal and meridional scales, and its relation to SST. -from Author