Modulation of the observed diurnal cycle of precipitation over the Maritime Continent by the Madden-Julian Oscillation

The Madden–Julian Oscillation (MJO) exerts a downscale influence on the diurnal cycle (DC) of precipitation over the Maritime Continent (MC). We assess the characteristics of this downscale influence in GPM-IMERG data across the western MC, comparing the MJO cycles of daily-mean precipitation, DC amplitude, DC timing and additional diurnal characteristics. During a typical MJO event, islands and surrounding waters experience their greatest DC amplitude 2–4 days ahead of their greatest daily-mean precipitation. The MJO has a greater influence on daily-mean precipitation over water and on DC amplitude over land. Greatest DC amplitude over land leads greatest DC amplitude over surrounding waters by 3–6 days. DC timing varies systematically by MJO phase in most locations, particularly eastern Sumatra, eastern Borneo and the eastern Makassar Strait where the diurnal maximum may systematically vary in timing by over four hours. Over these regions, the diurnal maximum propagates westward before, and eastward after, the active MJO crosses the western MC. As the active MJO crosses, the diurnal maximum gets earlier across western land on large islands, and later across eastern land, creating a west–east regime divide in DC timing variability. Additional diurnal characteristics quantify further changes in the nature of the diurnal oscillation. MJO-induced changes in the diurnal timing of convective cloud cover may influence local radiation budgets. These results provide a detailed benchmark for the modulation of the DC by the MJO against which this scale interaction in models may be assessed.