伴隨聖嬰現象西北太平洋反氣旋演變的研究

An Observational Study of Anomalous Anticyclone in Western North Pacific Associated with El Niño

吳爾東(Erh-Tung Wu)

指導教授 ： 隋中興

國立臺灣大學/理學院/大氣科學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201801242

摘要

聖嬰現象的強度隨著時間逐漸接近北半球冬季而達到高峰，此時由聖嬰現象引發的環流異常和西北太平洋冬季季風交互作用通過wind-evaporation-SST回饋機制(WES feedback)產生西北太平洋異常反氣旋(WNP AAC)(Wang et al. 2000)，而這個反氣旋距平並沒有隨著聖嬰事件消散並維持到聖嬰消退年的夏季JJA(1)。為了解釋聖嬰事件後西北太平洋異常反氣旋的維持機制，Xie et al.2009提出Indian Ocean Capacitor機制，認為此時是由印度洋暖海溫加熱大氣在赤道西太平洋產生Kelvin wave，進而導致西北太平洋低層輻散來維持西北太平洋反氣旋。Chung et al. 2011則從西北太平洋副熱帶高壓的年際變化中分離出兩種時間尺度的振盪，週期2-3年的和反聖嬰現象有關，海洋大陸上的對流異常引發的局地哈德里環流在西北太平洋產生下沉運動；週期3-5年的振盪則是由聖嬰現象透過WES feedback造成。
考慮到聖嬰事件之間消退速度的差異會對熱帶海表溫度、對流活動的分布產生影響，本文將1958-2016年間的聖嬰事件依據聖嬰消退年夏季JJA (1) Niño3.4 (170°-120°W, 5°S-5ׄ°N)海表溫度距平的三個月滑動平均即 Oceanic Niño Index (ONI) 作為指標，將聖嬰事件分為三類：fast-decay, slow-decay 以及prolonged type。並利用合成圖分析大尺度環流和海表溫度、視熱源(Q1)之間的關係，討論維持聖嬰事件後夏季西太平洋反氣旋的機制是否會隨聖嬰事件相位有所不同。
當ONI在JJA(1)下降到負值，意即聖嬰事件已經轉變至反聖嬰相位，稱為fast-decay事件；如果ONI仍然維持正值，則為slow-decay事件。如果到了下一個冬天D(1)JF(2)還是維持聖嬰相位，就定義為prolonged事件。
在fast-decay事件中，WES feedback從D(0)JF(1)維持到MAM(1)，隨著反氣旋南側的東風移入西太平洋，造成聖嬰事件在JJA(1)快速轉變成反聖嬰相位。在反聖嬰事件發生時，海洋大陸上異常旺盛的對流活動在西北太平洋產生對應的沉降區，維持了西北太平洋反氣旋。Slow-decay事件發展時類似fast-decay事件，但是中太平洋上的暖海溫與異常對流持續到了夏天，所以WES feedback能夠維持作用。而相對於前述兩類聖嬰事件，Prolonged事件發展較慢，到了MAM(1)才有明顯的西北太平洋反氣旋，除了透過WES feedback在西北太平洋產生反氣旋環流之外，海洋大陸上對流減弱引發的Gill-Matsuno response使得異常反氣旋延伸孟加拉灣。
從合成分析的結果來看，聖嬰現象的相位決定了JJA(1)西北太平洋異常高壓的維持機制。當Niño3.4 SST仍然是暖海溫異常時，WES feedback就可以持續作用。如果已經轉變成反聖嬰相位，西北太平洋反氣旋則是透過局地哈德里環流維持。印度洋的角色在fast-decay事件中較為清楚，主要是幫助聖嬰現象的相位轉換(Kug and Kang 2006)，到夏天印度洋海溫距平則已經消散。Prolonged事件雖然在夏季伴隨印度洋暖海溫異常，但是印度洋上卻是反氣旋環流，顯示此時海溫距平並沒有達到加熱大氣的作用。唯在Slow-decay事件中，印度洋暖海溫距平有加熱大氣並引發西太平洋東風異常提供西北太平洋負渦度，來維持反氣旋異常。

關鍵字

年際變化 ； 聖嬰現象 ； 西北太平洋反氣旋 ； wind-evaporation-SST回饋機制 ； 局地哈德里環流

並列摘要

This is an observational analysis aimed to classify mechanisms for maintaining anomalous anticyclone (AAC) in western North Pacific (WNP) associated with El Niño events, especially in the El Niño decaying summer JJA(1) following the peak phase in winter D(0)JF(1). El Niño events from 1958-2016 are categorized into fast-decay, slow-decay, and prolonged types based on their Oceanic Niño Indices (ONI) in JJA(1) that are negative, positive, positive and followed by another El Niño event in the following winter, D(1)JF(2), respectively. Composite analysis is applied to anomalous fields of Sea Surface Temperature (SST), 850hPa stream function and vertically integrated apparent heat source 〈Q1〉 of the three types of El Niño events.
In the composite fields of the fast-decay type, the development of warm SST Anomaly (SSTA)/heating over equatorial Pacific is associated with an evolution of SSTA over Indian Ocean (IO) from a dipole pattern in SON(0) to basin wide warming in D(0)JF(1) and MAM(1). The heating associated with the IO SST evolution causes an eastward propagation of equatorial easterly anomalies (equatorial part of AAC) from eastern IO in SON(0)/ D(0)JF(1) to western Pacific in MAM(1) that results in a rapid phase transition into La Niña condition in JJA(1). The most evident maintenance mechanism for the WNP AAC is the wind-evaporation-SST (WES) feedback in D(0)JF(1) and MAM(1), and the strong La Niña heating contrast in JJA(1). The IO capacitor is only evident in MAM(1). The La Niña SST distribution and associated heating over the Maritime Continent in JJA(1) drives an overturning circulation with subsidence over WNP.
The composite fields of the slow-decay type and the prolonged type of El Niño events are generally similar except that the WNP AAC associated with the prolonged type emerges late in MAM(1). The WES feedbacks is the most evident mechanism maintaining WNP AAC in D(0)JF(1) and MAM(1), and IO capacitor is evident in MAM(1) for the slow-decay type but not for the prolonged type. For both types of events, the AAC persists in JJA(1) with a broad zonal extent in the tropics (10°-30°N) from the dateline to the Bay of Bengal. Associated with the AAC for the slow-decay type, a zonal band of anomalous heating (cooling) exists between equator and 10°N (10°-20° N), and warm SSTA and anomalous heating also exist over northern IO. For the prolonged type, the heating over IO and western Pacific is weaker and more localized. The AAC in JJA(1) for the two types of El Niño is similar to the AAC found in Wang et al. (2013), Kosaka et al. (2013) and Li et al. (2017). The AAC appears to be maintained by a broader scale air-sea interaction involving extra tropical influence.

並列關鍵字

interannual variability ； El Niño ； western North Pacific anomalous anticyclone ； wind-evaporation-SST feedback ； local Hadley circulation

參考文獻

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