Orographic Effects of the Maritime Mountains on the Eastward Propagation and the Rainfall of the 2007-08 Madden-Julian Oscillation

This study investigates the orographic effects of the Maritime Continent (MC) on the eastward propagation of the Madden-Julian Oscillation (MJO) and the rainfall associated with it, during the strong 2007-08 MJO event (MJO07-08). The eastward propagation of the MJO has been observed in the Indian Ocean and the western Pacific. The corresponding deep convection and near-surface wind anomalies appears to move around the mountainous islands such as, Sumatra, Java, and New Guinea; instead of naturally moving across the islands. To study the orographic effects, the event simulated by the Advanced Research Weather Research and Forecasting (WRF) with three domains of 10, 5 and 1 km grid resolutions. The simulated flow and moisture fields resemble the observed fields reasonably well. The simulated results are then analyzed to investigate the orographic effects on the dominant mesoscale convective system (MCS) and the embedded cloud clusters when the MJO07-08 event passes over the mountains of MC. The WRF model demonstrated a strong orographic blocking on the MCS by the mountainous islands of the MC, such as the weakening of the deep convection, prohibiting the MJO from propagating over the mountain continuously, and the regeneration of the MCS on the lee side of the mountains.

The simulation also shows that the MJO goes through three different stages during its propagation across the mountainous islands of the MC. During each of these stages the convection associated with the MJO is weakened by the orographic effects of the mountains until the eastward propagation is not continuous and there is a decrease of the heavy rainfall that is associated with the MJO. Common ingredients and major mechanisms responsible for producing heavy rainfall are identified and proposed, respectively, to improve the understanding of the fundamental dynamics of the rainfall associated with the MJO07-08 during its passage over the MC.