Rain storms in the Maritime Continent region, which includes the islands, peninsulas and shallow seas of Southeast Asia, tends to form over land during the day but move offshore with heavy rainfall at night. This diurnal cycle of convection during the day and offshore migration at night is attributed to two physical mechanisms: the land breeze and gravity waves. Researchers from Texas A&M and Indonesia’s Geophysical Agency and funded by the Climate Program Office (CPO) Climate Variability & Predictability (CVP) program, investigated which driver propels the nighttime rainfall events offshore from the west coast of Sumatra. The authors used radar observations and reanalysis wind and temperature data to analyze individual offshore rainfall characteristics and demonstrate event frequency in terms of the Madden–Julian oscillation (MJO) and season. Based on their analysis, the authors found that land breezes were the main driver of offshore rainfall while gravity waves played a secondary role. Their work is published in Monthly Weather Review.
The research team defined 117 nighttime offshore (within 180km) rainfall events off the west coast of Sumatra in 2018. The rainstorms interacted strongly with the passage of the MJO, as most events occurred when the MJO was weak or active over the Indian Ocean, as opposed to when the MJO is active over the Maritime Continent and Western Pacific. Rainfall occurrences also varied based on Asian-Australian monsoon seasons, with more events occurring during the rainier transition seasons (March-May, Sept-Nov) and fewer events when the monsoons are active to the north and south of Sumatra (Dec-Feb, June-Aug).