Impact of the Extreme Indian Ocean Boreal Summer Intraseasonal Oscillation on the Weather Pattern Over the Western Maritime Continent

The impact of the extreme Boreal Summer Intraseasonal Oscillation (BSISO1) phases 2 and 3 passing through Indian Ocean-East Asia on the weather pattern anomaly over the Western Maritime Continent was investigated using ECMWF Reanalysis 5 (ERA5) and Global Satellite Mapping of Precipitation (GSMaP) during May to September (MJJAS) with record spanning from 2011 to 2022. The extreme BSISO1 was identified by its intensity and statistic applied with > 1.4 and above the 95th percentile value, respectively. The results indicated that the distribution of strong BSISO1 events (33 cases) in phase 2 (24 cases) was more robust than in phase 3 (9 cases), with the extreme values reaching 2.687 and 2.153, respectively. We also found that the trend of strong BSISO1 in phase 2 decreased; conversely, in phase 3, it tended to increase during the last decade. This study suggested that the impact of BSISO1 phase 3 was more vigorous in modifying weather patterns over the Western Maritime Continent by concentrating extreme precipitation over south of Sumatra and southwest of Java. Of note, the BSISO1 phase 3 was strengthened by a cyclonic vortex marked by the straight line of southwesterly vertical moisture transport from the center of the vortex to Sumatra. This study provided early warning information about vulnerable regions in weather-related hazards associated with extreme precipitation.