Weather types and rainfall variability during the Northeast Monsoon over Malaysia

Malaysia frequently experiences extreme rainfall throughout the Northeast Monsoon season. However, the connection between extreme rainfall and distinct monsoonal synoptic circulations remains to be fully investigated. This study aims to identify the dominant synoptic circulation patterns and the associated extreme precipitation using weather type classification method. K-means algorithm was employed to classify daily weather types (WTs) over Malaysia region (3°S–10°N, 98°–122°E) during the Northeast Monsoon season, which occurs from November to February. The classification was based on 850-hPa wind data obtained from the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) dataset from 1981 to 2020. Four distinct WTs were identified, and further examined their circulation pattern, frequency of occurrence, typical progression and persistence, and associated rainfall characteristics. Among the identified synoptic circulation patterns, the Borneo Vortex, cold surge, and cross-equatorial surge were prominent within the resultant WTs. Over the past 40 years, the co-occurrence of the Borneo Vortex and cold surges has shown a significant decreasing trend, while the other three patterns, including the Borneo Vortex occurring over the South China Sea, weak cold surges, and cross-equatorial surges have exhibited increasing trends. The cold surges contribute to increased rainfall in western Borneo, particularly in the Sarawak region. The occurrence of the Borneo Vortex leads to increased rainfall in eastern Borneo, while cross-equatorial surges are associated with enhanced rainfall in northeastern Borneo. Lastly, this study looks into how El Niño–Southern Oscillation (ENSO) modulates the occurrence of each of the four WTs. Borneo Vortex events occur more frequently during La Niña years compared to El Niño years. As Malaysia continues to face the challenges of climate change, this study helps in developing strategies to manage the risks related to extreme events, helping communities and industries adapt and sustain resilience.