Reconsidering the role of Rossby waves and inertia-gravity waves in the Madden-Julian Oscillation (MJO)

Many studies attempt to gain insight into atmospheric and oceanic phenomena in the tropics using the Gill (1980) model because it simplifies the equatorial wave spectrum by making the “longwave approximation.” How can the equatorial wave spectrum be simplified to reproduce the waves important for the Madden-Julian Oscillation (MJO)? The MJO is the main intraseasonal fluctuation in tropical weather, modulating precipitation, pressure, and winds all year. Global Climate Models (GCMs) do not simulate the MJO well due in part to a lack of physical understanding. In order to improve physical understanding, the dynamical aspects of the MJO were investigated in a simple model framework. Steady state anomalies of winds and geopotential heights, as well as momentum fluxes in two simplified shallow-water models on the equatorial beta-plane, were compared to those in a “complete” shallow-water model on the equatorial beta-plane, which represented all waves. The simplified models consisted of a “filtered” model, which produced all Rossby and Kelvin waves, and a “truncated” model, which produced all Kelvin waves and only long Rossby waves. Three case scenarios of diabatic heating were analyzed. Winds, geopotential heights, and momentum fluxes were generally weaker, and maximum values were less concentrated in the “filtered” and “truncated” models. Therefore, short Rossby waves must be well-represented in idealized models of the MJO, and inertia-gravity waves may play an important role as well. Idealized models such as the one used here can help improve physical understanding of the MJO, and GCMs can be interpreted and revised so that the MJO is simulated with more accuracy.