東亞／西北太平洋氣候變遷

Climate Changes in East Asia/Western North Pacific

吳宜昭(Yi-chao Wu) ； 許晃雄(Huang-Hsiung Hsu) ； Joo-Hong Kim ； 劉鵬(Peng Liu) ； 湯寶君(Pao-Chun Tang) ； 黃威凱(Wei-Kai Huang) ； 楊竣凱(Chun-Kai Yang) ； 周佳(Chia Chou) ； 隋中興(Chung-Hsiung Sui)

《大氣科學》 40卷3期 (2012/09) Pp. 215-247

摘要

臺灣位於亞洲季風區中，氣候及天氣主要受到東亞季風和西北太平洋季風的影響。本研究的目的在彙整東亞／西北太平洋季風區過去的年代／年代際變化及長期趨勢，以了解臺灣所在的背景區域氣候變遷的特性。東亞季風變遷的重點如下：◎近50年來（約1950年代之後），東亞夏季季風呈現減弱趨勢。有些研究認爲東亞夏季季風並未減弱，只是夏季的雨帶向南移了。有些研究認為亞洲季風的改變是伴隨著1976/1977年的氣候遷移而發生的，而且與聖嬰在這期間的變化有密切的關聯。◎不論過去一百多年（1873～1995）來或是較近期的五十多年來（約1950年代之後），東亞冬季季風均呈減弱趨勢。在西北太平洋氣候變遷方面，本研究探討西北太平洋熱帶氣旋的生成、強度、路徑及伴隨降雨與登陸各方面之年代／年代際變化與長期趨勢。世界氣象組織（WMO）專責熱帶氣旋與全球氣候變遷的專家小組回顧過去文獻後（Knutson et al. 2010），認為以現有觀測資料分析過去趨勢時，受限於觀測資料的品質與長度，無法確認目前觀測到的這些熱帶氣旋活動的長期變異超過自然氣候變異的幅度，亦即仍無法從過去的熱帶氣旋觀測具體辨識出顯著的人為影響。幾個重點如下：◎過去六十年來（1951年之後），全年熱帶氣旋生成個數具有顯著年代際變異。自1961年之後，全年熱帶氣旋生成個數更隨年份增加而有顯著下降趨勢；但若僅考慮較近時期（1970年代之後），則無顯著線性變化。◎在強度的變化方面，2005年的一些研究認為自1970年以後西北太平洋熱帶氣旋強度及強烈熱帶氣旋個數有增加趨勢，並認為其反應了全球暖化的影響；此看法遭到一些研究的質疑，原因包括資料的長期一致性堪慮，不足以就是否有顯著增加趨勢下結論，還有些研究認為該變化只是長週期多年代變異中的一部分，並非長期線性趨勢。針對以上爭議，目前的共識是現有的觀測不足以讓吾人判斷這些變化是否和人為全球暖化有關。◎過去五、六十年（約1950年代之後）熱帶氣旋的路徑有兩次突然的遷移，均較太平洋年代振盪的兩次氣候遷移（約在1976/1977年及1998年左右）延遲約兩年；路徑變異是否有顯著的線性趨勢則還不清楚。◎最近百年（1902～2005）登陸菲律賓及近六十年來（1945～2004）登陸東亞沿海各區域的熱帶氣旋個數無顯著線性趨勢。東亞區域氣候變遷的重點如下：◎東亞地區的陸地或海面上都有顯著的暖化趨勢。自1948年以來，東亞陸地上（如中國北方及東部）均呈現顯著的增溫；在海面上，副熱帶西太平洋全部呈現增暖趨勢。◎東亞平均降雨的長期趨勢，具有區域性的差異，不過大多數地區的趨勢不顯著。本章最後以日本、韓國、中國為例，進一步說明東亞過去降雨與溫度的平均狀態與極端事件等氣候變遷特徵。

關鍵字

東亞季風 ； 西北太平洋季風 ； 熱帶氣旋 ； 氣候變遷

並列摘要

Located in the Asian Monsoon region, Taiwan's climate and weather are primarily influenced by two monsoon systems, the East Asian monsoon and the western North Pacific monsoon. To realize the climate change in the nearby regions of Taiwan, this study reviews the decadal, interdecadal variability, and the long-term trend of the climate in these two monsoon regions. The climate changes in the East Asian Monsoon are summarized below. ‧ Over the past 50 years (since 1950s), the East Asian summer monsoon (EASM) shows a weakening trend. Some studies argue that the EASM does not actually weaken. Rather, it is the tendency for a southward shift of the summer rain belt that results in the seeming weakening of the EASM. Some studies argue that the EASM changes along with the occurrence of 1976/1977 climate regime shift, and that it is closely related to the variations in ENSO during that period. ‧ No matter over the past 100 years (1873-1995) or over the past 50 years (since 1950s), the East Asian winter monsoon exhibits a weakening trend. The climate changes in the western North Pacific monsoon is represented by the changes in the formation, intensity, track, and associated precipitation and landfall activity of tropical cyclones. In a comprehensive review (Knutson et al. 2010), the World Meteorological Organization's (WMO) expert team on Climate change Impacts on Tropical Cyclones concluded that, given the limitation in the availability and quality of global historical observation of tropical cyclones, it is still unsure whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. Identifying anthropogenic influences in tropical cyclone activity in the past is still unachievable currently. The climate changes in the western North Pacific tropical cyclone activity are summarized below. ‧ The annual number of tropical cyclone exhibits prominent decadal variability over the past 60 years (since 1951) and a significant decreasing trend over the past 50 years (since 1961). However, no significant linear trend is found if a shorter period (since 1970) is considered. ‧ In terms of the changes in tropical cyclone intensity, some observational studies (Webster et al. 2005; Emanuel 2005) reported that the intensity and the number of intense tropical cyclone exhibited prominent upward trends, which were attributed to anthropogenic influences. The findings were contested by other studies, mainly due to the concern of data consistency and the short record length relative to the periods of interdecadal variability. With respect to above disagreements, the current consensus (Knutson et al. 2010) is that it is still not certain whether these past changes in the tropical cyclone intensity result from anthropogenic influences. ‧ Over the past 60 years (since 1960s), the tracks of tropical cyclone underwent two abrupt shifts. Both shifts occurred around two years later than the well-known Pacific climate regime shifts, the1976-1977 event and the 1998 event. ‧ The number of tropical cyclones that make landfalls in the Philippines over the past 100 years (1902-2005) and that in the coastal regions of East Asia over the last 60 years (1945~2004) shows no significant linear trend. The long-term trends of temperature and precipitation in East Asia and the western North Pacific are also examined. Since 1948, the land surface in East Asia (especially northern and eastern China) and the entire subtropical western North Pacific have warmed up significantly. While the temperature over the East Asia and the western North Pacific shows an overall warming trend, the precipitation exhibits remarkable regional differences. The trends are insignificant in most regions, however. The study also reviews how the mean state and extremes of temperature and precipitation have changed in Japan, Korea, and China in the past to further illustrate the regional climate changes in East Asia.

並列關鍵字

East Asian monsoon ； Western North Pacific monsoon ； Tropical cyclone ； Climate change