從亞澳及西北太平洋季風觀點探討臺灣氣候年雨型辨識方法

Taiwan Annual Rainfall Pattern and the Asian-Australian and Western North Pacific Monsoons

盧孟明(Mong-Ming Lu) ； 劉品誼(Ping-Yi Liu)

《大氣科學》 45卷3期 (2017/09) Pp. 193-219

https://doi.org/10.3966/025400022017094503001

摘要

聯合國政府間氣候變遷委員會(IPCC)在2013年公佈的氣候變遷科學報告的區域氣候章節指出夏季是豐雨期冬季是枯雨期是判斷季風區域的基本法則。而在全球氣候變遷的影響下，豐枯對比是否日趨強烈？答案關係到全球70%人口的生計與糧食生產。臺灣位處東亞與西北太平洋季風區，須密切監測年雨型變化並適時提供雨型變化的氣候分析詮釋結果供各界參考。為此，本文提出適用於臺灣的多年平均氣候以及單一年份的年雨型定義方法，以6個基本特徵值和一個導出量描述年雨型特性。結果發現臺灣年雨型年際變化明顯，但影響因子甚多，不歸屬於亞澳與西北太平洋季風區內任單一區域季風，因此也不似亞澳與西北太平洋年雨型在夏季的開始和結束都深受聖嬰現象(ENSO)影響。另外還發現臺灣的季風特徵明顯程度與豐雨期開始時間早晚有關，豐雨期開始時間越早乾濕期對比越明顯，但影響豐雨期開始時間早晚的主要因素並非聖嬰現象或與聖嬰現象高相關的亞澳與西北太平洋季風區雨量變異的第一、二主模態，反而是與大氣底層的溫度變化關係不大第三、四個主模態和臺灣雨量的相關性較高，表示影響臺灣的因素以大氣內部擾動為主，了解或推估臺灣年雨量變化無疑是加倍困難。不過，IPCC報告指出的若冬季是ENSO冷相位(La Niña)的該年的豐枯對比往往比冬季是ENSO暖相位(El Niño)的年份更明顯，這方面臺灣有與東亞季風類似的年際變化。建議未來應加強臺灣年雨型監測，分析詮釋臺灣年雨型變動與全球氣候與海表面溫度變化以及亞澳與西北太平洋季風變異主要模態的關係。

關鍵字

臺灣氣候 ； 東亞與西北太平洋季風 ； 亞澳季風 ； 全球季風 ； 年雨型

並列摘要

A distinct difference in summer and winter rainfall is one of the identification characteristics of monsoon climate. Possible change of monsoon rainfall pattern is an extremely important climate issue because it can influence 70% of global population and enormous agriculture productivity. It is suffice to say that monitoring the annual rainfall pattern in Taiwan and understanding its relationship with the Asian-Australian and western North Pacific monsoons is a fundamental part of climate services. This paper presents two methods for identifying the climatological mean Taiwan annual rainfall pattern and individual years of the annual rainfall patterns, respectively. One annual rainfall pattern is described by 6 parameters including the onset and withdrawing time of the wet period, the rainfall peak value and time, the annual percentage amount of the rainfall totals during the wet period. The length of the wet period is calculated as the time duration from the onset to withdraw time. It turned out that the rainfall pattern in Taiwan is not dominated by any single subsystems in the Asian-Australian and western North Pacific monsoon region. Not like the monsoons that are clearly influenced by ENSO, Taiwan annual rainfall pattern only shows a weak relationship with ENSO and the major modes of the Asian-Australian and western North Pacific monsoonal rainfall. The seasonal contrast of the wet and dry period is stronger if the wet period onset time is earlier than normal. However, the onset time is not correlated with the most dominant first and second modes of the Asian-Australian rainfall variability, nor with ENSO, but shows strong correlation with the third and fourth EOF modes which are likely from atmospheric internal variability. Note that Taiwan annual rainfall pattern variability is in line with the signal reported in IPCC AR5 (Box 14.1, Figure 1) that following a cold phase ENSO in winter the wet and dry contrast in the summer is stronger than the summer following a winter with the warm phase ENSO. It is strongly recommended that the annual rainfall pattern and its relationship with the major modes of the monsoon rainfall and the global sea surface temperatures should be regularly monitored and analyzed the information should be disseminated through climate services.

並列關鍵字

Taiwan climate ； East Asia and western North Pacific monsoons ； Asian-Australian monsoon system ； global monsoon ； annual rainfall pattern