Cold surges are synoptic weather systems that occur over the Maritime Continent during the boreal winter. They are characterised by the strengthening of prevailing low-level northerly to north-easterly winds, temperature falls of a few degrees over several days, and in some cases, extreme prolonged rainfall and flooding. We investigate the synoptic structure and development of cold surges through composites of dry, moderate and wet surges. Each surge category is defined by the distribution of precipitation averaged within a specified domain over the equatorial South China Sea.

Over the Maritime Continent, most of the dry (wet) surges occur during the suppressed (active) phases of the Madden-Julian Oscillation (MJO). Dry surges are characterised by cross-equatorial flow and positive mean sea-level pressure anomalies which reach the Southern Hemisphere, and enhanced descent or weaker ascent. Wet surges coincide with a cyclonic circulation over Borneo, a lack of cross-equatorial flow, and enhanced moisture and ascent. We find that diurnal precipitation patterns are consistent with convective onset being controlled by the mid-tropospheric buoyancy of an idealised entraining plume. This buoyancy diagnostic suggests that wet surges are characterised by a moister free troposphere because this reduces the effect of entrainment and allows convection to penetrate the lower troposphere.

Finally, deep (shallow) and relatively strong (weak) westerlies are found over southern Java and northern Australia during the dry (wet) surges. Consequently, Australian summer monsoon bursts are more likely to occur following dry cold surges. The westerlies are also explained as part of the larger-scale MJO circulations.