Long-term Trends and Interannual Variability of Southwesterly Flows around Southern Taiwan during 44 Mei-yu Seasons

This study investigates the characteristics and long-term trends of southwesterly flows around southern Taiwan (hereafter, SWs) during mei-yu seasons (15 May–15 June) from 1979 to 2022. The results show that the occurrence number of SWs in general exhibited an increasing trend over this 44-year period, with a decadal oscillation starting from a relatively small number in the 1980s and reaching a relative peak in the 2000s. This tendency posts a potential threat to Taiwan due to the increasing trend of heavy rainfall associated with higher moisture flux of the SWs events. The SWs activity was influenced by the long-term increasing trend of geopotential height gradients and their decadal variability near Taiwan. When the intra-seasonal oscillation was evident, the weather system mainly affecting the occurrence of SWs was the low pressure system to the north of Taiwan; when it was weak, the intensity and location of the western North Pacific subtropical high to the south of Taiwan was relatively more important. In addition, the SWs index which was highly correlated with the precipitation during mei-yu seasons can effectively reflect the interannual variability of precipitation in Taiwan in periods of different lengths. These findings indicate that the SWs index can be used as a monsoonal precipitation index for Taiwan, especially southern Taiwan.