Normal Modes versus Moisture Modes: Fundamental Differences between Matsuno Waves and the Madden–Julian Oscillation in Theory and Models (Invited Presentation)

Dave Raymond has spent much of his career studying what he calls “the subtle interplay of dynamics and thermodynamics” between large scale circulations and tropical convection. Dave and his collaborators are especially well known for developing “toy models” that have been highly successful at describing the fundamental character of tropical disturbances coupled to convection, including tropical cyclones. This talk reviews those aspects of observed equatorial waves that are central to Dave’s ideas of the key processes that lead to the maintenance and propagation of Kelvin waves and the MJO. The dry linear theory of Matsuno can successfully account for the gross dynamical features of observed dry equatorial waves, but the scale and vertical structure of these waves is significantly altered by interaction with convection. In Kelvin waves, convection is coupled to dynamics through the mass circulation, which modifies the wave’s dry dynamical phase speed, presumably through changes in the gross moist stability. The observational diagnostics of Mike Herman, who was Dave’s graduate student at the time, found that the role of convective inhibition appeared to be crucial to the control of convective activity in Kelvin waves, as predicted in the Raymond-Fuchs model of Kelvin waves. In the case of the MJO, which has no corresponding linear theory as a framework for its dynamics, the “moisture mode” concept as developed by Dave and Zeljka Fuchs accounts for the growth and maintenance of the MJO through the feedback between diabatic processes and the circulation, with radiation also playing a key role. Observations from the DYNAMO field campaign by Ji-Eun Kim et al. will be used to examine the radiative and other diabatic terms in the moist static energy budget through the apparent heat sources and moisture sinks, confirming the critical role of radiation in the MJO, as posited by Dave almost 20 years ago.