The Effect of Orbital Forcing on the MJO’s Amplitude and Phase Speed

Abstract

The Madden-Julian Oscillation (MJO) has a pronounced seasonal cycle, drifting from the Southern Hemisphere in boreal winter to the Northern Hemisphere in boreal summer, with a maximum intensity during boreal winter in the Southern Hemisphere. The reason for this maximum intensity during boreal winter is not fully understood. In this study we perturbed the seasonal cycle in the Superparameterized Community Atmosphere Model, which reasonably represents the MJO, by changing Earth's orbital parameters. In response to the orbital forcing, boreal winter mean precipitation increases, consistent with a stronger Hadley circulation, and the mean lower-tropospheric westerly winds shift southward over the Indo-Pacific warm pool. The increase in the tropical precipitation variance are explained using the linearized approximation of the non-linear moisture-precipitation relationship. The increase in the tropical precipitation variance is mostly due to a decrease in the convective moisture adjustment timescale, which is mainly due to the increase in mean precipitation. The seasonal cycle of the MJO is amplified, with the largest increases in the MJO's precipitation variance occuring during boreal winter. The changes in the convective moisture adjustment timescale and MJO scale moisture predict a larger increase in the MJO precipitation variance than in the tropical precipitation variance. Additionally, a weaker damping effect from the latent heat flux is found due to the southward shift in the mean lower tropospheric westerlies.