Impacts of the MJO and Equatorial Waves on Tracked Mesoscale Convective Systems Over Southeast Asia

Southeast Asia is a region dominated by intense convection and characterised by the high-impact weather associated with synoptic scale tropical depressions, typhoons, or tropical cyclones (TCs). However, more localised convection such as mesoscale convective systems (MCSs) can also produce intense precipitation which can be a major risk for loss of lives and property for the communities in the region. Due to these high-impact weather features, its complex orography, and the significant impact of large-scale weather features on its meteorological variability, predicting weather in Southeast Asia is of great importance and scientific interest, but is a challenge. We aim to characterise the distribution of MCSs in the region and capture how the systems are modulated by the Madden-Julian Oscillation (MJO) and equatorial waves. 

MCSs in Southeast Asia between 2015 and 2020 were tracked using Himawari satellite data, their associated rainfall estimated using IMERG, and classified by lifetime and propagation speed. TC-related rainfall was also deduced using data from IBTrACS to identify certain cloud clusters as associated with TCs. Between 10S and 10N, MCSs account for 45-70% of the precipitation between November and April, and over most of the region, the fractional MCS contribution to rainfall is higher than average on extreme wet days (>55%). Long-lived  (>12 hours) MCSs contribute disproportionately, providing 84% of the rainfall despite comprising only 34% of all MCSs.

The MJO modulates MCS rainfall in a similar way to total rainfall, contributing >50% of the total rainfall anomaly, with the number of MCSs being greater in convectively active phases. However, in the West part of the region there are more fast-moving MCSs in the active MJO phases and more slow-moving MCSs in the inactive phases, resulting in fast-moving MCSs having a greater impact on the MJO-associated variation in MCS rainfall. This variation in MCS rainfall is larger in the West part of the region than the East. Meanwhile, variation in the area-mean rainfall rate within the storms, and sizes of storms were less well correlated with MCS rainfall in different phases; when areas were large, area-mean rainfall rate was generally low, and vice versa, providing compensating effects.

In the low-level convergence phase of an equatorial Kelvin wave, MCS rainfall and non-organized rainfall both increase, accounting for 20-50% of local rainfall anomalies, a pattern which is again enhanced in the West of the region. By contrast, Westward-propagating Mixed Rossby-Gravity waves, and Rossby-1 waves, do not strongly modulate MCS rainfall, and instead their rainfall anomalies are dominated by TC-related rainfall.

These relationships between MCSs and the MJO and Kelvin waves provide useful insight into forecasting MCSs in Southeast Asia by utilising knowledge of the synoptic weather regimes that are or will be affecting the region.