東亞大氣長河與其對臺灣強降雨事件影響

Abstract

大氣長河是強勁且呈狹長帶狀的水氣傳輸帶，在供應中緯度地區水資源的同時，卻也常因行經山區地帶而形成具破壞性的降雨。由於大氣長河在熱帶往中高緯度輸送水氣的水文循環過程中扮演著重要的角色，因此數十年來關於大氣長河的探討，在中緯度始終備受矚目。近年來越來越多的研究，開始以大氣長河的角度討論相對低緯度地區的水氣傳送。本研究的基礎，主要架構在前人設計的大氣長河自動偵測演算法。該客觀偵測法以圖型辨識分析垂直積分水氣傳送距平場，並擷取具大氣長河特徵的水氣輸送。

本文統計在1981年至2015年西北太平洋與東亞地區的大氣長河氣候特性，討論不同時間尺度的氣候振盪對於大氣長河的調控，歸納35年間觸陸大氣長河對於臺灣大雨事件的影響。此研究進而修改部分偵測演算法的辨識過程與輸出，並以NASA MERRA-2再分析資料計算垂直積分水氣傳送。在氣候特性上，東亞和西太平洋地區大氣長河的發生頻率與生成個數，整體而言在北半球夏季均明顯高於北半球冬季，明顯受到東亞夏季季風的影響。兩個主要的大氣長河生成區，分別位於青藏高原東南側與日本東南側的海域，但兩地大氣長河生成的機制與季節有明顯差異。東亞大氣長河的發生頻率在受到聖嬰與太平洋十年振盪等年代際影響外，本文指出北半球夏季季內振盪（Boreal Summer Intraseasonal Oscillation）對於大氣長河季內尺度變異的重要性。同時，北半球夏季季內振盪的第二模態和登陸臺灣的大氣長河數量與降雨強度有顯著關聯。根據35年的統計，在夏季約有20 %-30 %登陸臺灣的大氣長河，會在中南部山區降下大雨等級以上的雨量。在春季與冬季，位於臺灣西半部地區大雨等級以上的降雨事件約有60 %-90 %是由大氣長河貢獻。Atmospheric rivers (ARs) are intense water vapor conveyor belt with filament-like shape contributing most of the water resources to societies within the Midlatitude region but also generating impactful rainfall over mountainous topography. Researches on ARs in the Midlatitude were prevalent throughout these decades since AR acts such an important role in the global hydrological cycle by transporting water vapor from the Tropics to the Midlatitude. Analysis of hydrometeorology in lower latitude basing on the viewpoint of AR appears more frequently in recent years. Fundamentals of this research base on the automatic detection algorithm developed by the previous study on ARs. The objective detection method analyzes the anomalous vertically integrated water vapor transport (IVT) with image identification processes, and acquires the AR-like moisture transport features.

Climatological characteristics of ARs over East Asia and the Western North Pacific (WNP) in 1981-2015 are statisticized. Modulations on AR from climate oscillations with different scale and also the impacts of landfalling ARs on heavy rainfall events in Taiwan are analyzed. Parts of the processes and output of the original detection algorithm are modified in this study, while IVT is computed from the reanalysis data adopted from NASA Modern-Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2).

In general, both the occurrence frequency and genesis numbers of ARs over East Asia and the WNP are apparently higher in boreal summer than winter, which is obviously affected by the East Asian summer monsoon. Two primary AR genesis regions located at the southeastern side of the Tibetan Plateau and the sea areas at the southeastern side of Japan. However, distinct formation mechanism and season of ARs exist between the two regions. Besides the interannual and interdecadal influences of El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on AR frequency over East Asia, this research emphasized the importance of Boreal Summer Intraseasonal Oscillation (BSISO) on the intraseasonal variability of AR. Meanwhile, evident relations of the second mode of BSISO and the numbers of landfalling ARs as well as the intensity of accompanied rainfall in Taiwan are presented. According to the 35-year statistics, 20 %-30 % of landfalling ARs induce heavy rainfall over the mountainous area in the Central and Southern Taiwan in summer. Moreover, 60 %-90 % of heavy rainfall events occurred in the western part of Taiwan during spring and winter were contributed by landfalling ARs.