Precipitation characteristics and formation mechanisms of prolonged southwesterly flow around southern Taiwan during the Mei-Yu season

The southwesterly flow (SW) is a synoptic-scale strong wind system that transports warm, moist air toward Taiwan and plays a crucial role in Taiwan's precipitation during the mei-yu season (May 15–June 15). Using ERA5 data (fifth-generation reanalysis from the European Centre for Medium-Range Weather Forecasts) and rainfall observations from surface weather stations in Taiwan, this study examines the flow and rainfall characteristics of prolonged SW (PSW) cases and their relationship with extreme precipitation events around southern Taiwan during the 1979–2022 mei-yu seasons. Statistical analysis reveals a strong co-occurrence between PSW and persistent heavy rainfall (PHR), with 75 % of PHR cases occurring during PSW and 60 % of PSW cases featuring PHR. Furthermore, 83 % of R99.9 events (the 99.9th percentile rainfall intensity) are associated with PSW. Composite analysis demonstrates that the formation and maintenance of PSW involve specific synoptic-scale flow patterns: a slight eastward migration of the western North Pacific subtropical high, sustained intensity of the South Asian high, and persistent low-pressure systems over East Asia. These pressure patterns collectively establish sustained southwesterly winds, leading to significant wind speed intensification and moisture accumulation around Taiwan over an extended period (84–126 h). These findings highlight the crucial role of synoptic-scale pressure patterns in maintaining PSW and facilitating PHR occurrence, offering quantitative criteria for forecasting PSW during the mei-yu season.