The role of moisture-convection feedbacks in simulating the MJO

The sensitivity of a simulated Madden-Julian Oscillation (MJO) in the National Center for Atmospheric Research's (NCAR) Community Atmosphere Model (CAM) version 3.1 with Relaxed Arakawa-Schubert (RAS) convection modified with the moisture trigger of Tokioka et al. (1988) is analyzed with respect to changes to the specified minimum entrainment rate parameter. Implementation of a non-zero minimum entrainment rate in RAS results in a drier and cooler troposphere due to suppressed convective activity. Higher values of the minimum entrainment rate enhance the sensitivity of convection to tropospheric humidity which appears to enhance the MJO signal in the model. The simulated MJO which emerges resembles a moisture mode. Analysis shows that moisture convection feedbacks are enhanced as the minimum entrainment rate is increased. A reduction in the column integrated moist static energy export by divergent motions indicates that the simulations with non-zero minimum entrainment thresholds are able to achieve negative effective gross moist stability which has been suggested as a necessary condition to produce a moisture mode. Additional simulations were analyzed to investigate the impacts of varying the specified rain re-evaporation fraction. Although both parameters can help produce a stronger MJO in the model, the effect on the mean climate is very different for each. This discrepancy suggests that tropical intraseasonal variability has no unique dependence on the basic state, in contrast to the findings of previous studies.