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

Abstract

 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, while 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 weakens and moisture is 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 results in high equivalent potential temperature near Taiwan, which in turn creates a large equivalent potential temperature gradient to the north of Taiwan. This setting favors the activity of mei-yu fronts and produces a low pressure environment. The pressure gradient thus increases, supporting the formation of southwesterly flows. The southwesterly flows then help to transport 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 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.