The Madden-Julian Oscillation (MJO) has a pronounced seasonal cycle, oscillating from the Southern Hemisphere in boreal winter to the Northern Hemisphere in boreal summer, with a maximum intensity during boreal winter. The mechanism behind the MJO’s seasonal progression in latitudinal position and intensity is not fully understood. In this study we perturb the climatological seasonal cycle of insolation by changing the eccentricity and precession of Earth's orbit in the Superparameterized Community Atmosphere Model.

In response to changes in insolation due to changes in orbital parameters, the boreal winter mean precipitation and precipitation variance increases, consistent with a stronger Hadley circulation, and the mean lower-tropospheric westerly winds shift southward over the Indo-Pacific warm pool. The seasonal cycle of the MJO is amplified when the eccentricity is increased in the model, with the largest increases in the MJO's precipitation variance occurring during boreal winter. The changes in MJO and tropical scale precipitation variance are explained using the linearized approximation of the non-linear moisture-precipitation relationship, which highlights the convective moisture adjustment timescale and anomalous moisture. The increase in the tropical precipitation variance is largely due to a decrease in the convective moisture adjustment timescale, which is due to an increase in the mean precipitation. The tropical moisture anomalies decrease in the high eccentricity simulation, which works to dampen the increase in the tropical precipitation variance. The changes in the MJO-scale moisture anomalies show small changes, which lead to a larger increase in the predicted MJO precipitation variance.

Following the moisture mode framework, the changes in the moisture budget of the MJO driven by the orbital forcing are examined to explain the amplification of the MJO in response to a larger eccentricity. The moisture budget suggests that the changes in the MJO’s maintenance are largely driven by changes in the position and magnitude of anomalous surface latent heat flux associated with the MJO-scale zonal wind. When eccentricity is increased there is a southward shift in the mean lower tropospheric westerlies, which when combined with the MJO-scale westerly winds have a positive contribution to the total wind. The increase in the total winds increases the latent heat flux feedback and reduces the damping effect of evaporation on the MJO’s maintenance. This suggests that the seasonal cycle of the mean winds leads to a positive contribution in the latent heat flux feedback to MJO’s maintenance and increased amplitude in the boreal winter.