In the recent multi-model intercomparison studies, the background moisture gradient has been highlighted as a key aspect of the mean state that affects Madden-Julian Oscillation (MJO) propagation. However, it was difficult to isolate the role of the mean state because the models use different physical components such as parameterization schemes that affect both the mean state and the MJO. To assess the effect of the mean state that is unaffected by that of model physics, we investigate the relationship between the mean state and the MJO in a ten-member ensemble of the Historical simulations made with one model. We use the Community Earth System Model version 2 (CESM2), which simulates the observed MJO characteristics and its eastward propagation realistically. Each ensemble member has identical parameterizations and resolution except for the different initial conditions.

Results show that one ensemble member with an exceptionally stronger MJO propagation also exhibits a much steeper background meridional moisture gradient (MMG) over the southern MC region than the other ensemble members. The simulated mean state affects MJO via its impacts on moisture dynamics - a column water vapor budget reveals a greater advection of mean moisture by MJO wind in the southern MC is responsible for the anomalous MJO activity.