A Climatological Study of Southwesterly Flows and Heavy Precipitation in Taiwan during Mei-yu Seasons from 1979 to 2018

This paper examined southwesterly flows and rainfall around the Taiwan area during the mei-yu seasons from 1979 to 2018. The occurrence percentage of the southwesterly flow events in southern Taiwan was highly correlated with 6-h accumulated rainfall and heavy precipitation in Taiwan, and those in northern Taiwan showed little correlation. Low pressure to the north of Taiwan and high pressure to the south exerted a large northward pressure gradient force across the Taiwan area, favoring the formation of southwesterly flows and rainfall in southern Taiwan. During an active year of southwesterly flow events, the Pacific high weakened and moisture was transported along two paths in the early mei-yu season: one from the Bay of Bengal and the other from the south of the Pacific high. The moisture-laden air resulted in a high equivalent potential temperature near Taiwan, which, in turn, created a large equivalent potential temperature gradient to the north of Taiwan. This setting favored the activity of mei-yu fronts and produced a low-pressure environment. The pressure gradient thus increased, supporting the formation of southwesterly flows. The southwesterly flows then helped in transporting more moisture toward the Taiwan area, resulting in heavy rainfall as well as a further increase of equivalent potential temperature. This kind of positive feedback produces more fronts, stronger southwesterly flows, and heavier rainfall during the mei-yu season. The study also suggests that the meridional component of the vertically integrated water vapor transport over the South China Sea and the Philippines in the early mei-yu season can be used to predict the occurrence of southwesterly flows and heavy rain for the entire mei-yu season.