Previous work has pointed to the physical mechanisms behind the nocturnal offshore propagation of convection south-west of Sumatra. Low-level moisture flux convergence due to the land breeze front controls the progression of a squall line away from the coast overnight. However, the diurnal convection over the mountains occurs on only 57% of days in December-February (DJF) and propagates offshore on only 49% of those days. We investigate day-to-day variability in dynamical and thermodynamical conditions to explain the variability in diurnal convection and offshore propagation, using a convection-permitting simulation run for 900 DJF days. A convolutional neural network is used to identify regimes of diurnal cycle and offshore propagation behaviour. The diurnal cycle and offshore propagation are most likely to occur ahead of an active Madden-Julian Oscillation, or during El Niño or positive Indian Ocean Dipole; however, any regime can occur in any phase of these large-scale drivers, since the major control arises from the local scale. When the diurnal cycle of convection occurs, low-level wind is generally onshore, providing convergence over the mountains; and low-level humidity over the mountains is high enough to make the air column unstable for moist convection. When this convection propagates offshore, mid-level offshore winds provide a steering flow, combined with stronger convergence offshore due to the land breeze or convection-triggered cold pools. Low-level moisture around the coast also means that, as the convection propagates, the storm-relative inflow of air into the system adds greater instability than would be the case on other days.

The Madden-Julian Oscillation (MJO) is the leading mode of sub-seasonal variability in the tropical atmosphere and is a source of predictability for extratropical weather through its teleconnections. MJO teleconnection patterns can be modulated by the El Niño-Southern Oscillation (ENSO) on seasonal to interannual time scales. However, changes over decadal time scales are less well understood. ERA5 reanalysis data are used to show that the boreal winter MJO teleconnection pattern in the Northern Hemisphere has changed in recent decades in line with changes in the Pacific Decadal Oscillation and Atlantic Multidecadal Variability. Changes are seen in the circulation, temperature and precipitation responses. In particular, from 1997, intraseasonal cold anomalies appear over Europe and the eastern United States due to MJO convection over the western Pacific; these were not present 20 years previously. The decadal variability observed is not the product of aliasing of ENSO modulation of the teleconnection.

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.