Propagation of the Madden–Julian Oscillation through the Maritime Continent and scale interaction with the diurnal cycle of precipitation

<jats:title>Abstract</jats:title><jats:p>The convectively active part of the Madden–Julian Oscillation (<jats:styled-content style="fixed-case">MJO</jats:styled-content>) propagates eastward through the warm pool, from the Indian Ocean through the Maritime Continent (the Indonesian archipelago) to the western Pacific. The Maritime Continent's complex topography means the exact nature of the <jats:styled-content style="fixed-case">MJO</jats:styled-content> propagation through this region is unclear. Model simulations of the <jats:styled-content style="fixed-case">MJO</jats:styled-content> are often poor over the region, leading to local errors in latent heat release and global errors in medium‐range weather prediction and climate simulation.</jats:p><jats:p>Using 14 northern winters of <jats:styled-content style="fixed-case">TRMM</jats:styled-content> satellite data it is shown that, where the mean diurnal cycle of precipitation is strong, 80% of the <jats:styled-content style="fixed-case">MJO</jats:styled-content> precipitation signal in the Maritime Continent is accounted for by changes in the amplitude of the diurnal cycle. Additionally, the relationship between outgoing long‐wave radiation (<jats:styled-content style="fixed-case">OLR</jats:styled-content>) and precipitation is weakened here, such that <jats:styled-content style="fixed-case">OLR</jats:styled-content> is no longer a reliable proxy for precipitation. The canonical view of the <jats:styled-content style="fixed-case">MJO</jats:styled-content> as the smooth eastward propagation of a large‐scale precipitation envelope also breaks down over the islands of the Maritime Continent. Instead, a vanguard of precipitation (anomalies of 2.5 mm day<jats:sup>−1</jats:sup> over 10<jats:sup>6</jats:sup> km<jats:sup>2</jats:sup>) jumps ahead of the main body by approximately 6 days or 2000 km. Hence, there can be enhanced precipitation over Sumatra, Borneo or New Guinea when the large‐scale <jats:styled-content style="fixed-case">MJO</jats:styled-content> envelope over the surrounding ocean is one of suppressed precipitation.</jats:p><jats:p>This behaviour can be accommodated into existing <jats:styled-content style="fixed-case">MJO</jats:styled-content> theories. Frictional and topographic moisture convergence and relatively clear skies ahead of the main convective envelope combine with the low thermal inertia of the islands, to allow a rapid response in the diurnal cycle which rectifies onto the lower‐frequency <jats:styled-content style="fixed-case">MJO</jats:styled-content>. Hence, accurate representations of the diurnal cycle and its scale interaction appear to be necessary for models to simulate the <jats:styled-content style="fixed-case">MJO</jats:styled-content> successfully.</jats:p>