Role of Diurnal Cycle of Insolation on the MJO Propagationin the Maritime Continent

The diurnal cycle of convection over the Maritime Continent (MC) has long been hypothesized to act as a major barrier to the eastward propagation of the Madden–Julian Oscillation (MJO), yet the physical mechanisms responsible for this barrier effect remain incompletely understood. To investigate this problem, we use a regional numerical model that realistically reproduces the large-scale structure and evolution of an observed MJO event during boreal spring 2013, a case in which the MJO weakened substantially and eventually stalled over the MC. Two experiments are conducted to isolate the role of the diurnal cycle: a control simulation with the full diurnal cycle of insolation included (CTL), and a sensitivity simulation in which the diurnal cycle of solar forcing is removed (NO_DC). MJO propagation in these simulations is identified and tracked using a large-scale precipitation tracking framework, which is able to distinguish propagating from non-propagating events more effectively than the conventional Real-time Multivariate MJO index. Results show that in the NO_DC experiment, the absence of daytime insolation suppresses the development of strong afternoon convection and reduces precipitation over the major islands of the MC, thereby weakening the local convective barrier and allowing the MJO to propagate eastward more continuously. Further diagnosis based on the moist static energy budget indicates that enhanced MJO maintenance in NO_DC is associated with increased longwave radiative heating and reduced total advection, while the more persistent eastward propagation is linked to enhanced horizontal and vertical advection together with reduced longwave heating and surface latent heat flux ahead of the convective center. These results suggest that the impact of the diurnal cycle on MJO propagation operates through multiple thermodynamic pathways and that these processes differ across various parts of the Maritime Continent, highlighting the strong regional complexity of MJO propagation through this convectively active area.