MJO-like coherent structures: Sensitivity simulations using the Cloud-Resolving Convection Parameterization (CRCP)

These coherent structures resemble the Madden-Julian oscillation (MJO) observed in the terrestrial Tropics and are present in simulations applying prescribed and interactive radiation, and increased horizontal resolution of the global model. The MJO-like structures are essential for the development of the mean westerly flow, the superrotation, within the equatorial waveguide. Sensitivity simulations suggest that the coupling among deep convection, free-tropospheric moisture, and the large-scale flow is essential for coherence of the MJO-like structures.

When large-scale fluctuations of convectively generated free-tropospheric moisture are removed on a timescale of a few hours, the coherent structures do not develop and, if already present, these structures disintegrate rapidly. It follows that the moisture-convection feedback, postulated previously to explain the large-scale organization of tropical convection and its coupling with SST fluctuations, operates very efficiently in the CRCP global model.

It is also argued that the feedback plays a role in sensitivity simulations with horizontally uniform surface fluxes, where horizontal variability of the fluxes is required for the development, but not the maintenance, of MJO-like coherent structures. These results are discussed in the context of existing theories and modeling studies, which aim to explain the coupling between convection and the large-scale dynamics in the Tropics on intraseasonal timescales.