近年來，透過分析海溫等的觀測資料，聖嬰現象(El nino and southern oscillation, ENSO)之強度與週期有不規則的特徵。對於ENSO已有相當程度之研究，但是如何影響與導致ENSO不規則變化之研究目前仍在進行。本研究透過分析1951至2010年NOAA海溫觀測資料發現，太平洋地區海溫之年際震盪特徵有明顯之太平洋年代震盪(Pacific decadal oscillation, PDO)特徵。因此，我們依照PDO時間定義之冷暖年代區間來做分析，分析結果顯示1975-2000年(暖年代)之ENSO事件特徵較1975年前(冷年代一)之ENSO事件強，而2000後(冷年代二)之ENSO事件特徵又較暖年代之ENSO事件弱。此外，1970年後之ENSO事件於ENSO發生前之春季，海溫在中太平洋地區皆有增暖之現象。許多研究指出，該現象為春季於熱帶西太平洋地區發生之西風爆發所導致。因此，本研究分析冷年代一、暖年代、冷年代二這三個年代尺度中，各四個顯著ENSO個案的風場資料，結果顯示暖年代西風爆發個數較冷年代密集並且次數增加，而冷年代一與冷年代二時期之間，西風爆發也有成長的趨勢。然而，是什麼原因造成西風爆發?
過去研究指出，有許多因素影響西風爆發，其中以熱帶地區之季內震盪(Madden and Julian Oscillation, MJO)、熱帶氣旋討論最多。然而，本研究想釐清是否也有中緯度系統之影響?Chu(1988)在文獻中亦指出因此除熱帶地區影響之外，中緯度寒潮爆發亦會影響熱帶西風爆發。故本研究規劃分析中緯度天氣系統與熱帶西風爆發之關聯性，透過海氣動力耦合模式(Hybrid coupled model)實驗模擬來釐清中緯度系統與西風爆發和ENSO事件的關係，進而解析其對ENSO之影響。其中實驗之控制組為規律的ENSO年際震盪，實驗組其實驗設計為加入寒潮爆發進行模擬。實驗結果顯示中緯度寒潮爆發南下使得熱帶西太平洋海水溫變冷，與熱帶中太平洋的暖海水之間的溫度梯度增強，冷海溫上空的下沉氣流與暖海溫上空的上升氣流形成垂直環流，在熱帶西太平洋地區誘發出西風爆發，進而增強中太平洋地區的聖嬰現象。
Our study analysis 1951-2010 NOAA sea surface temperature observations found that interannual Pacific SST has obvious characteristics of the Pacific decadal oscillation (PDO). Therefore, in accordance with the time section of PDO to distinguish cold(warm) epoch, the analysis showed that the ENSO event features of 1975-2000(warm epoch) is stronger than 1951-1975(cold epoch), while the ENSO event features of 2000 (cold epoch 2) is weaker than warm epoch. In addition, our analysis found that most of ENSO events after 1970 in spring have common features that before the ENSO occurs there are warming SST in the central Pacific. Many studies have pointed out that this phenomenon resulted from westerly wind bursts in the spring in the tropical western Pacific.
Westerly wind bursts in the tropical Pacific often occurred during the early spring in most ENSO events and are believed to be a key factor triggering an ENSO event. Our study found that the mid-latitude system will affect the tropical westerly wind bursts, and the cold-air outbreak is most obvious influence in the mid-latitude.
In this study, a simple hybrid coupled model was used to examine the effects of cold-air outbreak on ENSO evolution. The experiment results show that the cold-air outbreak causes SST become cooler in the tropical western Pacific, and enhance SST gradient between warmer central and cooler western Pacific. The downdraft over cold SST and the updraft over warm SST combine into a vertical circulation, which may cause westerly wind bursts over the western Pacific in the subsequent early spring, and the associated anomalous westerlies then induce an ENSO event in the coming winter.
