A composite study of southwesterly flows and rainfall in Taiwan

This study examines southwesterly flows and their association with rainfall in Taiwan during the warm seasons: Spring, Mei-yu, and summer. We found that the percentage of southwesterly flow events in the lower troposphere was the highest in the Mei-yu season, followed by summer. When southwesterly flows occurred, the chance of rain greatly increased in Mei-yu and summer and the mean rain intensity increased for all three seasons. In northern Taiwan, the percentage of southwesterly flow events was the highest in spring and decreased over warm seasons, whereas the reverse scenario occurred in southern Taiwan. Southwesterly flows occurred in spring primarily due to a deepening midlatitude trough over eastern China. The chance of rain in Taiwan increased during southwesterly flow events when the Pacific subtropical high retreated eastward and the trough moved closer to Taiwan. In the Mei-yu season, there was greater moisture, and the occurrence of southwesterly flows was more equally caused by the midlatitude trough and the southwestward extending Pacific high than in spring. The southwesterly flow axis was located roughly over Taiwan, which then shifted southeastward as the Pacific subtropical high weakened. At the same time, the high-moisture zone covered the northern South China Sea and the entire Taiwan. As a result, moisture-laden air was transported to the Taiwan area by the strong southwesterly flow, providing favorable conditions for continuous rain to occur in Taiwan. In summer, southwesterly flows occurred when the Pacific high extended southwest and a low/tropical cyclone moved over southeastern China. Rain tended to be more intense when the low was stronger and closer to Taiwan.