Role of topography on the MJO in the maritime continent: a numerical case study

The role of topography on a Madden–Julian Oscillation (MJO) event in the Maritime Continent (MC) is explored using a regional model. Four simulations are conducted: lower-resolution (12 km) simulations using cumulus parameterization in the presence (LR) and absence (LR-Flat) of topography, and higher-resolution (4 km) simulations without cumulus parameterization in the presence (HR) and absence (HR-Flat) of topography. In the LR simulation, the MJO remains unorganized with no clear eastward propagation, while the LR-Flat simulation captures the MJO and its eastward propagation across the MC. In the absence of cumulus parameterization, both HR and HR-Flat capture the MJO and show several similarities and differences compared to the LR and LR-Flat simulations. To better understand these differences, a moisture budget analysis is conducted during the passage of the MJO. In the LR-Flat simulation, vertical advection of moisture is increased to the east of the islands, leading to continuity in MJO-associated convection, continuity that was not present in the LR simulation. The increase in vertical advection in the absence of topography is due to an increase in the mean moisture advection by the anomalous vertical winds. In the middle of the MC, horizontal advection seems to be the most important for an uninterrupted eastward propagation of the MJO. The increase in the horizontal advection in the absence of topography is primarily due to an increase in the anomalous moisture advection by the mean zonal winds. To what extent the MJO was influenced by the upstream effect from the New Guinea topography was also explored. These results indicate that the important physical processes for MJO-associated convection may be different in different parts of the MC. Further implications of these results in the context of other recent studies on MJO propagation across the MC are discussed.