Impact of the Madden–Julian oscillation and equatorial waves on tracked mesoscale convective systems over southeast Asia

Southeast Asia is a region dominated by high-impact weather, but numerical weatherprediction here is a challenge owing to the complex orography and interactions betweensmall- and large-scale phenomena. Localised mesoscale convective systems (MCSs) canproduce intense precipitation. Here, we track MCSs over a 5-year period in Himawarisatellite data, characterise the distribution of MCSsin the region, and investigate howthey are modulated by the Madden–Julian oscillation (MJO) and equatorial waves.Between 10◦S and 10◦N in southeast Asia, MCSs account for 45–70% of the precipitationduring boreal extended winter (November–April). Over most of the region, the frac-tional MCS contribution to rainfall is higher than average on days with extreme rainfall(>55%). Long-lived (>12hr) MCSs contribute disproportionately, providing 85% of therainfall despite comprising only 34% of all MCSs. Variability in MCS rainfall accountsfor>50% of the total rainfall variability during an MJO cycle, mostly due to larger num-bers of MCSs in convectively active MJO phases. Variations in MCS size and mean rainrate due to shifts in the stratiform proportion provide compensating effects. In the westof the region, a shift to faster moving MCSs in active MJO phases and slower movingMCSs in inactive phases resulted in fast-moving MCSs having the greatest impact on theMJO-associated variability. Variability is larger in the west than in the east. EquatorialKelvin waves modulate MCS rainfall, with MCSs accounting for 20–50% of local rainfallanomalies. This variability is again enhanced in the west. By contrast, rainfall anoma-lies due to westward-propagating mixed Rossby–gravity waves and Rossby-1 waves aredominated by tropical-cyclone-related rainfall. Skill at local scales may be extracted fromforecasts of subseasonal drivers such as the MJO and Kelvin waves, by understandinghow these modulate the number and characteristics of MCSs.