三極降水結構的年代際變化 及其對年際與次季節變異之影響

Abstract

東亞地區夏季年際降水變化，存在一個三極結構(Tripole Pattern)：當華中、日本一帶多雨時，華南及華北地區少雨；相反的，當華中、日本一帶少雨時，華南及華北地區雨量較多，此三極結構為東亞地區夏季降水的年際變化的重要型態之一。本研究的主要目的為探討東亞地區三極結構的年代際變化以及其對更短的時間尺度（如年際與次季節）的影響。研究結果發現三極降水結構在1980年左右有著突然的轉變，降水由南北方向負正負的距平轉變為正負正的距平，年際變化也以該年為分界，後期振輻增加週期加長，年際與次季節變異也在1980年後有所轉變，且訊號整體分布也呈現類似三極結構的分布。針對1980年前後環流場進行分析，發現在1980年後，海溫與熱容量的正距平提供較多的能量給低層氣旋式環流距平幫助其輻合，使對流增強降水增加，整體低層與高層結構配置更有利於三極降水結構的形成與加強，且對流的增強有效增加可用位能，再次加強整體三極結構，異常的海溫增加，也會加強激發的羅士培波，向北傳送的羅士培波使後期華北與台灣以及華南一帶降水增加，而日本與華中則是降水減少，呈現正負正的降水距平結構。菲律賓海比較溫暖的海溫，提供較容易發生對流的環境，也可能因此放大年際與季節內擾動的變異量。

The tripole pattern is an important structure in East Asia during the northern summer. It is characterized by a zonally elongated and meridionally banded structure with signs changing alternatively from 0°N to 60°N along the East Asian coast. This study investigates the inter-decadal fluctuation of tripole rainfall pattern and its effect on inter-annual and sub-seasonal variability in 1958 to 2002. In 1980, the inter-decadal and inter-annual variations of tripole rainfall pattern changed from negative to positive phase. After 1980, there was more rainfall in northern and southern China, while there was less rainfall in central eastern China, Japan, and South Korea. The period and amplitude of inter-annual variation also increased after this year, so did the subseasonal variability. The spatial pattern of the change in interannual and subseasonal variance is similar to the tripole pattern. Both the positive SST and heat content increased in the northern Philippine Sea after 1980. It is suggested that the direct SST effect enhanced the cyclonic circulation and convergence in the lower troposphere in the tropical Western North Pacific, and in turn enhanced the tripole pattern through northward Rossby wave dispersion. It is hypothesized that the warmer Philippine Sea is able to provide an environment more favorable for the occurrence of convection activity in shorter time scale and therefore led to larger interannual and subseasonal variability.