超級聖嬰

Abstract

本研究中發現，在年代際變異的影響被濾除後，存在著一類相當獨特且具有極強振幅的聖嬰事件。此類聖嬰在本文中被稱為超級聖嬰，發生的年份分別是1972/1973年、1982/1983年以及1997/1998年。超級聖嬰的與眾不同不僅表現在強度上，它們更擁有其他聖嬰事件所沒有的特徵。其中的兩個特徵，在超級聖嬰肇始(onset)前就已出現，它們調整熱帶太平洋的環境，使其具備發展超級聖嬰的條件，因而引發超級聖嬰。第一個必要條件，在超級聖嬰達到最強時期的前13個月，以一特殊類型的反聖嬰事件在秋天展現，在本研究命名為”super-ma” (超級聖嬰之母)反聖嬰。super-ma反聖嬰有著看似矛盾的海氣配置，其沃克環流比平時更強，然中太平洋赤道上的海平面溫度距平(sea surface temperature anomaly, SSTA)卻偏向中性。較強的沃克環流在熱帶西太平洋堆積大量的正海洋表層熱含量距平，這些表層的暖水會逐漸向赤道集中，並在六個月後移動到東太平洋。此外，比起一般的反聖嬰其冷SSTA極大值位在中太平洋赤道的海溫配置，super-ma反聖嬰在此區的微弱SSTA讓其轉相成聖嬰更加容易。因此，上述的海氣配置構成有利聖嬰事件發生的環境。第二個必要條件為超級聖嬰發展年前的冬天於夏威夷以及菲律賓海所共同出現的兩個低壓系統。雖然夏威夷低壓與北太平洋震盪(North Pacific oscillation, NPO)之中的南側低壓看起來很相似，但它的位置卻比起傳統的NPO更加接近赤道，而菲律賓海低壓可視為反聖嬰時在西北太平洋所對應的大氣環流場，與聖嬰時的反氣旋環流相反。從冬天到春天這段時期，夏威夷低壓透過風-蒸發-海平面溫度反饋(wind-evaporation-SST feedback, WES feedback)以及Sverdrup平衡兩種機制擴大其環流場範圍，並且沿著東北-西南的軸線拉長，逐漸靠近中太平洋赤道地區，在春天時與菲律賓海低壓合併成一個大型低壓，盤距在北太平洋熱帶地區上。此大型低壓在赤道北側所帶來強烈的西風帶，加速了向東的表層洋流，將西太平洋的暖水向中太平洋輸送，並且也透過WES反饋機制，快速加熱此區的海表，因此，二、三月時中太平洋赤道地區SSTA已轉正。在四、五月時，東太平洋赤道上SSTA也因super-ma反聖嬰所囤積的暖水移動到此區底下變成暖海溫距平，所以整個赤道太平洋SSTA為正值，超級聖嬰在此刻肇始。同時，中太平洋赤道地區的北側也因海溫的增暖，形成了超級聖嬰發展的關鍵區域(0–7.5°N, 155°E–170°W)。在此區域，暖SSTA與增強的對流互相支持，增強的對流進而引發了超級聖嬰在發展時期最主要的特徵：從赤道到南半球的橫向環流。此橫向環流在北半球的夏季最顯著，其低層風場由澳洲附近的低層高壓旋出，並沿著澳洲東岸向赤道移動，回到中太平洋增強對流；而對流中心所對應的高層環流其中一分支往西南側流出，並連接到澳洲高壓的沉降區，成為橫向環流的高層風場。因此，透過此橫向環流，在熱帶西太平洋低層的西風距平會被加強，因而成為有效幫助超級聖嬰成長的加速器，使這些聖嬰事件發展為超級聖嬰。

A distinct class of El Nino events with extreme magnitude (termed “super El Nino” events in this study) is identified after removing decadal variation. These events occurred in 1972/1973, 1982/1983 and 1997/1998. They are distinguished not only by their size but also by associated features that are not similarly robust in other El Nino events. Two major features emerge before a super El Nino onset and precondition the tropical Pacific, thus triggering a super El Nino. The earliest feature is a special type of La Nina which can be recognized at least 13 months before the peak of super El Nino events. This “super-ma” (the mother of super El Nino events) La Nina drives a highly-organized Walker cell yet holds the central equatorial Pacific SST anomaly in a nearly neutral state. The enhanced Walker cell stashes a large amount of positive heat content anomaly in the western tropical Pacific, which gradually converges into the equatorial Pacific and propagates to the eastern Pacific after 6 months. The small negative SST anomaly in central equatorial Pacific facilitates a fast phase turning in comparison to regular La Nina events. Thus both constitute favorable conditions for the onset of an El Nino event. At the end of the year before super El Nino onset, the simultaneous emergence of two low SLP systems, the Hawaii low and the Philippine Sea low, provides another necessary condition. Although the Hawaii low resembles the southern lobe of the north Pacific Oscillation (NPO), its location is much equatorward than the conventional NPO, and the Philippine Sea low can be regarded as an anti-phase of the low-level Philippine Sea anticyclone during a La Nino event. As winter advancing, the Hawaii Low expands, elongates along NE-SW axis and reaches the central equatorial Pacific following the wind-evaporation-SST (WES) feedback and the Sverdup balance, and two low systems merge in the early spring. With the union of the Hawaii low and the Philippine Sea low, a broad area of low-level westerly winds drives an eastward current along the northern flank of the equator that further warms SST in addition to the WES feedback. As a result, in April-May, a key box region (0–7.5°N, 155°E–170°W) is formed in the northern-central equatorial Pacific when the eastern equatorial Pacific SST anomaly turn positive (super El Nino onset) by the subsurface warm water due to the super-ma La Nina. Within this key box, anomalous warm SST and convection are sustained and the enhanced convection initiates a Southern Hemispheric transverse circulation, another unique feature of a super El Nino when it develops. This transverse circulation is characterized by a low-level equatorward flow, which spins off from a high sea-level-pressure anomaly around Australia and then merges into the deep convection anomalies over the central Pacific, and by a upper-level southward divergent flow, which branches off from the convection center and connects to the subsidence of the Australia high. It is suggested that this transverse cell, peaking in boreal summer, serves as an effective booster during the developing stage of a super El Nino by intensifying the low-level westerly winds in the tropical Pacific.