圣嬰現象與1998年台灣異常氣候：全年偏暖與春雨偏多現象

El Nifño and 1998 Climate Variability of Taiwan: Persistent Warming and Excessive Spring Rains

陳昭銘(Jau-Ming Chen) ； 汪鳳如(Feng-Ju Wang) ； 呂芳川(Fang-Chung Lu) ； 郭漱泠(Shu-Lin Kuo)

《大氣科學》 30卷4期 (2002/12) Pp. 331-349

摘要

本世纪最強的聖嬰現象發生於1997-98年間，台灣地區於1998年亦發生顯著氣候變異現象，本文主要目的即是探討上述二氣候現象之間的關聯性。分析結果顯示，1998年台灣地區全年偏暖與春雨偏多現象均是與聖嬰現象有所關聯的全島性氣候異常變化，其相關動力機制的主要特性概述如下：
(1)全年偏暖現象：當聖嬰現象於1997／98冬季發展成熟後，於南海和台灣-日本間海域形成暖SST距平，於熱帶西太平洋區形成冷SST距平，此二SST距平調節低層大氣於台灣鄰近區域形成大尺度輻合中心，於熱帶西太平洋形成大尺度輻散中心，此輻合-輻散中心配置，激發低層環流於台灣東南側熱帶洋面形成強烈反氣旋距平，此反氣旋距平搭配南海暖SST距平-熱帶西太平洋冷SST距平持續存在於整個1998年，並經由其西側環流系統影響台灣地區風場，造成暖平流增強或冷平流減弱效應，形成台灣地區1998年全年偏暖現象。
(2)春雨偏多現象：在1970-98年間十個聖嬰現象春季（二、三、四月）中，台灣春雨有7年偏少，3年偏多（包括1998年），偏多年所對應熱帶太平洋SST距平強度較偏少年明顯增強，且其西太平洋冷距平中心區與熱带東太平洋暖距平中心區均較偏少年東移。偏少年時，西太平洋冷SST距平中心區西北側形成反氣旋距平，且籠罩在台灣上空，提供不利降雨環境；偏多年時，反氣旋距平隨著熱帶西太平洋冷SST距平區東移而東偏，不再籠罩台灣，取而代之為氣旋距平外圍環流，提供有利降雨環境。隨著反氣旋、氣旋距平的空間分佈變化，台灣地區於偏少年時位於區域性南北哈德里環流距平的下沈氣流帶，偏多年時則位於上升氣流帶，另哈德里環流距平於偏多年時伴隨較強SST距平而具有較大強度，故台灣地區於偏多年之上升氣流距平較偏少年之下沈氣流距平強度明顯增強，因此造成偏多年雨量增加平均多達一倍，但偏少年雨量減少幅度平均約僅三成的不對稱變化結果。

關鍵字

圣嬰現象 ； 氣候變化 ； 全年偏暖 ； 春雨偏多

並列摘要

The purpose of this study is to delineate the relationship between the 1997-98 El Nino and the 1998climate variability of Taiwan. Two major abnormal climate phenomena of Taiwan are selected for analysis: persistent warming throughout the 1998 and excessive rains in spring(Feb.-Apr.). Our results show that occurrences of these two abnormal phenomena are associated with the El Niño. Their dynamic relationships are outlined as follows:
(1)Persistent warming: Anomalous warm SSTs over the South China Sea and the open ocean between Taiwan and Japan and anomalous cold SSTs in the tropical western Pacific persistently existed throughout the 1998.These SST anomalies forced lower-tropospheric atmosphere to form a convergent center at the vicinity of Taiwan and a divergent center over the tropical western Pacific. These anomalous divergent circulations further induced an anomalous anticyclone over the ocean southeast of Taiwan. This anomalous anticyclone altered surface wind pattern over Taiwan, either increasing the warm advection or reducing the cold advection. As a result, a persist warming occurred in Taiwan for the entire 1998.
(2)Excessive spring rains: In the ten El Nino springs during the period 1970-98,Taiwan had excessive rains in 3 years and suppressed rains in 7 years. Composite analyses reveal that SST anomalies associated with El Niño events with excessive rains in Taiwan were stronger in magnitude and more eastward in spatial position of anomalous SST centers than those with suppressed rains. During the suppressed rain springs, an anomalous anticyclone was overlying Taiwan that was accompanied with the downward branch of the local Hadley circulation anomaly, suppressing the precipitation mechanism. This anomalous anticyclone moved eastward during the excessive rain springs following the eastward displacement of SST anomalies. Taiwan was then underlying an anomalous cyclone. These changes led to an upward branch of local Hadley circulation anomaly to locate over Taiwan, providing a favorable environment for precipitation. The Hadley circulation anomaly is stronger in excessive rain springs than suppressed ones in correspondence to stronger magnitude of SST anomalies. Thus, increase of rain in the former is much more than decrease in the latter.

並列關鍵字

El Niño ； Climate variability ； Persistent warming ； Excessive spring rains