台灣百年來的乾旱分析研究 II

Drought is one of important natural disasters in Taiwan. Numerous severe drought cases in the past few decades in Taiwan caused tremendous economic loss and great inconvenience in livelihood. After comparing with two other recent drought studies in Taiwan, a relatively reasonable definition, Taiwan Meteorological Drought index (TMD index), was established in this study. The TMD index considers rainfall data on the plains of Taiwan (altitude≦150m). Within a 90-day moving window, the maximum number of consecutive dry days needs to be larger than the criteria which are the one percentile of the historical records to be identified as a drought case, and the actual dry days in the 90-day windows are marked as drought periods to calculate the TMD index. The 1950-2015 TMD index are calculated in this study and show the severe dry cases in Taiwan tend to happen from the end of typhoon season to the next spring before Meiyu season. The severe drought frequency is discussed in this study. The frequency changed in the past decades. More severe drought cases in fall occurred after the 1980s, but more cases happened in winter-spring season before the 1970s. This may be related to the impacts of the Pacific Decadal Oscillation (PDO) from the 1950s to the 1970s. The result for this year is to compare the dry events and the large-scale environment condition, the drought seasons are divided into three groups: fall and winter period (October – next January), spring period (February - April) and Meiyu period (May). The analysis of reanalysis data shows that the atmospheric environment around Taiwan during severe droughts is under unfavorable conditions for convection systems to develop, and that could benefit droughts to occur in Taiwan. The result for the previous year was published in the article of “Construction of the Taiwan Meteorological Drought Index and the Analysis of Severe Drought Cases”, and the result for this year is ready for the submission as an article titled “Analysis of Severe Droughts in Taiwan and its Related Atmospheric and Oceanic Environments”.