Large-Scale Modulation of Mesoscale Convective System Propagation and Diurnal Rainfall over the Western Maritime Continent

<jats:p>The western Maritime Continent exhibits a pronounced regional diurnal cycle of rainfall, to which the propagating Mesoscale Convective Systems (MCSs) make a significant contribution. Using an automated tracking algorithm based on brightness temperature, we quantify the contribution of MCSs to the regional diurnal cycle over Singapore and the South China Sea. MCSs affecting the Singapore sector show two distinct peaks in the diurnal cycle, one in the early morning and the other in the afternoon. These two diurnal peaks reflect the superposition of two opposing MCS propagation pathways: eastward-propagating events originating from Sumatra and westward-propagating events originating from the South China Sea and Borneo, respectively. These propagation regimes occur under systematically different large-scale wind profiles. Eastward and westward propagation are associated with distinct lower- and mid-tropospheric zonal wind conditions, whereas upper-tropospheric winds provide less separation between the two regimes. Seasonal changes in MCS directionality are consistent with changes in the preferred low-level wind direction. The Madden-Julian Oscillation (MJO) modulates the relative frequency of eastward- and westward-propagating events and thereby influences the regional diurnal cycle through its effect on the environmental wind profiles. These results show that the regional diurnal cycle over the western Maritime Continent is shaped not only by local land-sea thermal contrasts but also by distinct populations of propagating MCSs whose preferred pathways are modulated by the large-scale environment.</jats:p>