台灣百年來的乾旱分析研究 III

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. Through the TMD index, several historical severe drought cases in Taiwan are detected and characterized by significant seasonal variability in the annual cycle. Composites for large-scale atmospheric and oceanic environments over different periods within the dry season are conducted. From October to December, the colder sea surface temperature (SST) pattern of Pacific Meridional Mode (PMM) and the PMM-induced local anomalous anticyclones over the South China Sea are both in charge of the extremely dry conditions in Taiwan. From January to February, cold SST in the South China Sea and its adjacent oceans dominates local atmospheric conditions above these regions and creates an unfavorable environment for convection systems. From March to May, a massive anomalous anticyclonic circulation centering beside Alaska and extending its properties to East Asia and Taiwan generates a descending environment and in turn suppresses convection systems to develop. Therefore, the extremely dry conditions under this system are expected.