Convectively Coupled Kelvin Waves in a Simplified Moist General Circulation Model

We present results concerning the determination of the speed and structure of equatorial Kelvin waves in an idealized moist general circulation model. The model consists of the primitive equations on the sphere, a zonally symmetric aquaplanet lower boundary, and various idealized physical parameterizations including gray radiative transfer and a simplified Betts-Miller convection scheme. This simple model framework allows us to study the dependence of Kelvin waves on properties of the convection scheme in a clean manner.

A control simulation with the model produces convectively coupled Kelvin waves which are remarkably persistent and dominate the variability within the tropics. These waves propagate with an equivalent depth of approximately 40 m, but have more of a first-baroclinic mode structure than observations. By varying a convection scheme parameter that increases the fraction of large scale versus convective precipitation, we show that the waves increase in strength, propagate more slowly, and move to larger scales. However, when mostly large scale precipitation occurs, the Kelvin wave disappears, and the tropics are dominated by tropical storm-like variability. We relate the decrease in speed to the gross moist stability of the atmosphere, which is reduced with increased large scale precipitation. We additionally present comparisons with full general circulation model simulations, which indicate that similar behavior is present in comprehensive GCM's as well.