The Impact of the Diurnal Cycle of Clouds and Precipitation over the Maritime Continent on the Propagation of the MJO into the Western Pacific

The processes that determine the interaction between the islands of the maritime continent (MC) and the eastward propagation of the Madden-Julian Oscillation (MJO) are poorly understood. We are undertaking a series of observational and modeling analyses aimed at understanding how clouds and precipitation over the islands of the MC lead to changes in the intensity of the MJO (inferred by the amplitude of the Real-time Multivariate MJO index [RMM] and other metrics) as it crosses the MC. One component of our analysis uses the long-term measurements from the DOE Atmospheric Radiation Measurement (ARM) sites in the Tropical Western Pacific (TWP) to examine cloud radiative effects as the MJO crosses the MC. Using the multi-year ARM dataset and a cloud resolving model (CRM), we show that the MJO interacts with the diurnal cycle of surface heating, clouds, and precipitation over the islands of the MC in a way that weakens it. Additionally, using a satellite climatology based on the TRMM 3B42 dataset we found that MJO episodes that weaken as they cross the MC are characterized by more frequent precipitation and warmer sea surface temperatures (SSTs) south of the equator and less frequent precipitation north of the equator compared to cases where the MJO intensifies. The north-south polarity in SSTs suggests a seasonal dependence in the ability of the MJO to cross the MC. This seasonality was confirmed by looking the seasonal distribution of changes in MJO amplitude as it crosses the MC. Consistent with the SST result, we found that MJO episodes that intensify as they cross the MC are more likely to occur during the northern hemisphere summer and less likely to occur during the northern hemisphere winter (Fig. 1). A regional CRM and satellite observations are used jointly to explore the processes responsible for this seasonality and to examine the impact of interannual oscillations such as ENSO and monsoons on the ability of the MJO to cross the MC. Fig. 1. The annual distribution of the day of the year when the MJO approaches the MC for cases where the RMM amplitude decreases (black lines) and increases (orange lines) across the MC.