臺灣地區秋老虎事件之診斷分析與其長期趨勢

Abstract

「秋老虎」現象指的是中國傳統節氣「立秋」後的回暖天氣，在西方則稱為「Indian Summer Day」。各地對其定義有所不同，更沒有明確的操作型定義。本研究基於1979~2022年間17個中央氣象署的臺灣氣象測站的氣溫數據定義了一個篩選秋老虎事件的框架，並利用ERA5、TReAD、NCEP R1等資料庫，探討影響副熱帶高壓的熱帶低頻振盪及區域次環流。透過氣溫變率，本研究發現氣溫在入秋後下降趨勢中於立秋後出現小幅回升或停頓，證實「秋老虎」為一氣候現象。而在本研究建立的篩選框架確定了138個秋老虎事件樣本。這些樣本在臺灣各地有相對均勻的分布，且在21世紀後事件數量顯著增長，顯示副熱帶高壓勢力的西移強化與氣候暖化之間存在一定關聯性。利用擴展經驗正交函數分析揭示了三種主要的副熱帶高壓擾動模式：副高西移型展示了副熱帶高壓的向西延伸；東風駛流型顯示副熱帶高壓位於東北亞並伴隨其南側低壓擾動的東風沉降；副高滯留型則呈現副熱帶高壓在長期趨勢下增強的表現。以上三個模態透過ERA5、NCEP R1、TReAD資料庫組合分析驗證了空間分布特徵與觀測資料的一致性，顯示三個模態具有代表性。本研究進一步探討BSISO1、BSISO2與副熱帶高壓之間的交互作用。本研究推測BSISO1的對流抑制相位可能會通過增強下沉運動來加強副熱帶高壓的穩定性和強度。而BSISO2對副熱帶高壓的影響有限，主要因其10至20天的短週期及活動範圍集中於赤道附近，作用偏重局地對流與短期擾動，對高壓系統的整體影響較小。The "Qiu Laohu" phenomenon refers to the warming weather occurring after the Chinese traditional solar term "Beginning of Autumn" , which is known in the West as "Indian Summer Day." Definitions of this phenomenon vary across regions, and it lacks a clear operational definition. We developed a framework to identify Qiu Laohu events based on temperature data from 17 meteorological stations in Taiwan managed by the Central Weather Bureau during the period from 1979 to 2022. Additionally, we utilized datasets such as ERA5, TReAD, and NCEP R1 to investigate the influence of tropical low-frequency oscillations and regional secondary circulations on the subtropical high.Through temperature variability analysis, we observed a slight rebound or stagnation in temperature after the"Start of Autumn" within an overall declining trend, confirming Qiu Laohu as a distinct climatic phenomenon. Using the selection framework we established, we identified 138 Qiu Laohu event samples. These samples are relatively evenly distributed across Taiwan, and their frequency significantly increased after the 21st century, suggesting a potential link between the westward intensification of the subtropical high and global warming. Using Extended Empirical Orthogonal Function (EEOF) analysis,we revealed three primary disturbance patterns of the subtropical high. The westward extension pattern exhibits the westward elongation of the subtropical high; the easterly drift pattern positions the subtropical high over Northeast Asia, accompanied by easterly subsidence influenced by a low-pressure disturbance to its south; and the stationary pattern reflects the strengthening and static behavior of the subtropical high under long-term trends. These three patterns were validated through composite analysis using ERA5, NCEP R1, and TReAD datasets, demonstrating consistency between their spatial distribution features and observational data, confirming their representativeness.We further examined the interactions between BSISO1, BSISO2, and the subtropical high. We hypothesize that during the suppressed convective phases of BSISO1, enhanced subsidence may contribute to the stability and intensity of the subtropical high. In contrast, BSISO2's influence on the subtropical high is limited, primarily due to its shorter cycle of 10 to 20 days and its activity being concentrated near the equator. BSISO2 mainly affects local convection and short-term disturbances, exerting relatively little influence on the overall subtropical high system.