Representation of the Madden–Julian Oscillation in Model E2.1: Role of the Mean State and Air–Sea Interaction

Recent modeling studies have suggested two factors that are important for global climate model (GCM) to correctly represent the Madden-Julian Oscillation (MJO): the mean state, in particular, the horizontal moisture gradient over the Indo-Pacific warm pool region, and air-sea interaction.

This study investigates the effect of the mean state and air-sea interaction on MJO simulation using a recent version of the NASA Goddard Institute for Space Studies GCM – Model E2.1. A set of three long-term simulations were conducted by varying air-sea coupling strategy: i) a fully coupled simulation, in which the atmosphere model fully interacts with an ocean model, ii) an atmosphere-only simulation, in which the climatological sea surface temperature obtained from the fully coupled simulation is prescribed, and iii) a semi-coupled simulation, in which the atmospheric component model is coupled with a slab-ocean model.

The coupled simulation realistically represents the coherent eastward propagation of the MJO over the Indo-Pacific warm pool. In the atmosphere-only and the semi-coupled simulations, the MJO is weaker with more pronounced Maritime Continent barrier effect than in the coupled simulation. Even though the climatological SST is identical between the fully-coupled and the atmosphere-only simulations, the mean horizontal moisture gradient over the Indian Ocean is much steeper in the fully-coupled simulation. Albeit closer to the observed pattern, the mean horizontal moisture gradient in the semi-coupled simulation is much weaker than those in the other simulations. The column-integrated moist static energy (MSE) budget of the MJO shows that the surface latent heat flux feedback weakly helps the MJO’s eastward propagation in the fully coupled simulation, while it drags the propagation in the atmosphere-only and semi-coupled simulations.