2020年梅雨季提早結束，夏季沒有侵臺颱風，秋冬季雨量亦明顯偏少，2021年春雨也是寥寥可數，就在水庫快要見底之際，望眼欲穿的梅雨終於在5月底到來，臺灣旱象才得以逐漸緩解。此次旱災除在經濟上有許多損失外，亦對民眾生活造成諸多不便，值得深入了解形成旱象的物理機制。統計2020年6月至2021年5月的臺灣雨量，達到1910年有紀錄以來的最少，堪稱為百年大旱。本報告分析此段期間的大尺度環流，推測造成臺灣百年大旱的可能原因，並歸納主要發現如下：①2020年6月至9月的少雨，主要是極端偏強的副熱帶高壓造成，當時的副高不僅在強度上達到最強，持續時間亦為1949年之後的最持久；副高偏強可能與熱帶各洋面同時出現海溫異常有關，包含反聖嬰在中東太平洋的偏冷、印度洋及大西洋的偏暖。②臺灣2020年秋冬季雨量偏少，可能與反聖嬰及菲律賓海偏暖有關。③2021年臺灣春雨偏少，主因應該是菲律賓海溫偏暖，其次為逐漸減弱的反聖嬰，負相位的太平洋年代際震盪亦有少許的貢獻；當菲律賓海偏暖時，會在其西側產生氣旋式環流，為臺灣及東亞沿岸帶來水氣較少的東北風距平，進一步造成臺灣春雨較少；反聖嬰有利上述偶合系統的維持。④2021年3月中旬至5月期間的熱帶季內振盪較為活躍，季內振盪可能使臺灣4月中下旬的西南水氣源減少，5月季內振盪乾區逐漸接近臺灣，進而造成臺灣少雨。簡而言之，2020-2021年的臺灣百年大旱不是單一因素造成的極端事件，而是許多因素同時出現的複合現象。

During 2020, an earlier-ended Mei-Yu season combined with a typhoon-absent summer, followed by notably low rainfall from following autumn to spring in 2021, Taiwan’s accumulated rainfall from June 2020 to May 2021 broke the lowest record since 1910, we report the unique characteristics and physical mechanism of this extreme event. The possible causes of the drought event are presented as follows.① The duration and strength of subtropical high over the Western North Pacific from Jun to September were the longest and strongest since 1949. The abnormal subtropical high maybe caused by the compounding effects from cool sea surface temperature (SST) over the central to eastern equatorial Pacific associated with La Nina, and warm SST in the northern Indian Ocean and the tropical Atlantic.② The warmest SST appeared in the Philippine Sea through October 2020 to Marth 2021, concurrent with La Nina, generated the dry condition near Taiwan.③ The less spring rainfall in Taiwan in 2021 could be directly attributed to the warmest SST in the Philippine Sea and indirectly to the weakening La Nina , whereas negative phase of the Pacific Decadal Oscillation seemed to have little effect. An anomalous cyclone usually appeared in the western side of the warm Philippine Sea and brought dry northeasterly anomaly to the East Asian coast, resulting in less spring rainfall in Taiwan, and the La Nina would favor and maintain this atmosphere-ocean coupled system.④ In April-May 2021, active tropical intraseasonal oscillation caused weaker moist southeasterly in middle to late April, and also resulted in dry condition before the end of May. In conclusion, the most serious drought event in 2020-2021 since 1910 was caused by the compounding effect from various influencing factors that occurred concurrently and sequentially. How much of these abnormal conditions were caused by the warming trend in recent decades, which is not investigated in this study, warrants further studies.