Impact of the Madden–Julian Oscillation on extreme precipitation over the western Maritime Continent and Southeast Asia

The western Maritime Continent (MC) and Southeast Asia lie at the heart of the largest area of high precipitation on Earth. Extreme precipitation is one of the major high-impact weather events to affect the population of over 500 million in this region. The deep convection associated with this extreme precipitation is difficult to forecast, even with modern high-resolution numerical weather prediction with explicit convection. However, larger-scale organised convective systems, such as the Madden–Julian Oscillation (MJO), can be skilfully predicted to 3–5 weeks lead time. The MJO has a well-known precipitation signal, and it is likely that it also modulates extreme precipitation. Here, the extreme precipitation signal of the MJO is analysed in detail for the western MC and Southeast Asia using 19 years of high-resolution Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (GPM IMERG) data. The probability of experiencing extreme precipitation increases robustly by a factor of two, and decreases by a factor of half, dependent on location and the phase of the MJO. The spatial pattern of these changes in extreme precipitation does not describe a smooth eastward propagation, but shows rapid variation over short distances, tied to the complex distribution of land and sea within the archipelago. There is also a seasonal dependence of this MJO modulation in some locations. A more detailed analysis of the effect of the MJO on extreme precipitation is presented for the major cities in the region. Extreme precipitation days over the MC are generally linked with an amplification of the diurnal cycle. However, although an active MJO increases the frequency of extreme precipitation days and therefore an amplified diurnal cycle, there was no further amplification of the diurnal cycle in the active MJO, compared with extreme precipitation days during non-active MJO periods.