Maritime Continent Influences on the Madden-Julian Oscillation in a Global Model

This study examines the influences of several features of Maritime Continent (MC) islands on the Madden-Julian Oscillation (MJO): their presence, topography, and diurnal cycle (DC) of rainfall. Sensitivity tests that 1) removed the islands (NOLAND), 2) flattened the islands (NOTOPO), and 3) suppressed the diurnal cycle of solar heating over the islands (NODC) were performed using two versions of the NCAR CAM4. In the first version, convection was parameterized using the model’s standard parameterization; in the second, convection was parameterized using “super-parameterization (SP)” in which convective tendencies are obtained from a two-dimensional, 4-km cloud-permitting model that runs in each grid column of the host model.

For both models (CAM4 and SPCAM4), any change to the islands reduced the mean state eastward and Equatorward moisture gradients over the Warm Pool compared to the control (CTRL) simulation. In CAM4, the meager eastward-propagating intraseasonal rainfall in CTRL became even weaker in the experiments. For SPCAM4, despite the reduced mean moisture gradients in the experiments, eastward propagation improved. Analysis of the rainfall DC suggests that the DC helps maintain the eastward moisture gradient in CAM4, and its absence may account for the reduced eastward propagation (via moisture gradient feedbacks) in CAM4-NOLAND and CAM4-NODC. In contrast, the improved eastward propagation in all experiments in SPCAM4 may be consistent with topographic disruption of MJO circulation anomalies and “diurnal siphoning” of energy from intraseasonal to diurnal timescales in SPCAM4-CTRL.

MC effects on MJO propagation in these experiments will be further explored by analyzing the moist static energy budget, regional spectra of convectively coupled equatorial waves, precipitation feature tracking, and DC variability as a function of MJO phase.