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

The manifestation of the diurnal cycle (DC) over the Maritime Continent is influenced by the large-scale environment, including the state of the Madden–Julian Oscillation (MJO). It is widely recognised in existing literature that the amplitude of the DC of precipitation is greatest at around the same time as, or perhaps slightly ahead of, greatest mean precipitation. However, there is weaker consensus on the impact of the MJO (if any) on the phase of the DC.

Here, the boreal winter (DJF) composite DC of precipitation is calculated for each of the 8 MJO phases (P1–8) defined by Wheeler and Hendon (2004), using 20 years of IMERG — the state-of-the-art gridded satellite-derived precipitation data product with 30-minute resolution. With such high temporal resolution, subtle changes in the phase of the DC can be, and are, identified. The western sides of Sumatra, Borneo and Java typically experience an earlier than usual diurnal precipitation maximum around P5–6, while the eastern sides of these islands experience an earlier than usual diurnal precipitation maximum around P2–3. An analogous west–east divide in DC phase regime is also observed over certain water bodies, such as the Makassar Strait. The magnitude of this phase fluctuation is greatest across eastern Sumatra, parts of eastern Borneo, the western Java Sea and the eastern Makassar Strait, where a range of DC phase of over four hours is frequently observed. The opposing nature of the western and eastern local regimes is a consequence of changes to the direction of phase propagation; westward phase propagation is favoured over the Makassar Strait and the eastern sides of Sumatra and Borneo during P2–3, while eastward phase propagation dominates across entire islands and water bodies during P5–6.

If, by extension, the DC of cloud cover shows strong regional fluctuations in timing, these results will have significant implications for the influence of the MJO on regional radiation budgets.