Article ID: 2019-057
This paper examines southwesterly flows and their relationship 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, chance of rain greatly rose in Mei-yu and summer and mean rain intensity increased for all three seasons. In northern Taiwan, the percentage of southwesterly flow appearance was the highest in spring and decreased over warm seasons, while the trend reversed in southern Taiwan.Southwesterly flows formed in spring primarily due to a deepening mid-latitude trough over eastern China. 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 formation of southwesterly flows was more equally contributed to by the mid-latitude trough and the southwestward extending Pacific high than in spring. The southwesterly flow axis was located roughly over Taiwan. This flow axis 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 island of Taiwan. As a result, moisture-laden air was transported to the Taiwan area by the strong southwesterly flow, providing favorable conditions for continuous rain in Taiwan. In summer, southwesterly flows formed 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.