| 摘要: | 本研究首先透過El Niño Modoki index (EMI) 區分典型聖嬰 (canonical El Niño) 及非典型聖嬰 (El Niño Modoki) (Ashok et al., 1997)，接著利用南中國降水距平進一步將El Niño Modoki分成Modoki I及II (Wang and Wang, 2013)。在典型聖嬰中，發展時海溫主要集中於赤道東太平洋地區，由東向西發展，並由西向東衰退；Modoki I事件在巔峰時，最大海溫正距平位於中太平洋地區，當年冬天即於東太平洋率先出現海溫負距平，此負距平逐漸向中太平洋發展，減弱中太平洋較暖的特徵，甚至可轉換到反聖嬰事件，傾向於聖嬰的循環發展事件 (Kim and Yu, 2021)；Modoki II事件在發展時，首先由東北太平洋出現海溫正距平，持續發展並向中太平洋延伸，在整個Modoki II事件東北太平洋海溫皆可具有較暖的趨勢，且中太平洋海溫正距平可維持至第二年，傾向於聖嬰的多年發展事件 (Kim and Yu, 2021)。
本研究接著利用充放電振盪機制 (recharge oscillator mechanisms) 分析兩種非典型聖嬰在發展過程的差異，Modoki I事件擁有較弱的充放電過程，但卻能較有效的抑制赤道海溫正距平的發展，Modoki II事件則具較明顯的充放電過程，而其中太平洋海溫距平卻可以持續至第二年，並且，Chen et al.(2021) 表示聖嬰的循環發展事件應會發生在具有較強的充放電振盪機制下，故本研究進一步利用混合層熱量收支分析，了解除了充放電振盪機制外，造成兩種非典型聖嬰事件發展差異的因素。Modoki I發展後期東風距平出現而加強赤道東風，經由wind-evaporation-SST (WES) feedback造成海水冷卻，並藉由Bjerknes feedback持續增強東風距平，伴隨因海溫降低產生的低雲，使得進入海表面的短波輻射減少，故可造成赤道太平洋海溫距平的衰減；Modoki II事件發展時東北太平洋西南風距平出現，減弱此處的貿易風，經由WES feedback升高此處海溫，並向西南傳遞至赤道中太平洋，除了緯向平流項及溫躍層回饋項在衰退期的正貢獻，潛熱通量亦為造成Modoki II事件海溫正距平持續較久的原因。
本研究最後根據海溫模擬的表現，自CMIP6模式中挑選出一個與觀測最為接近的模式—CAMS-CSM1-0，希望能根據此篩選方法，提供未來在利用氣候模式分析不同類型的聖嬰事件時，能擁有一個較佳的依據，儘管此模式可成功模擬三種聖嬰事件在成熟期的海溫差異，但在模擬海溫發展至衰退期的表現卻還有待加強。
;Over the past few decades, El Niño Modoki has been revealed to be distinguished from the canonical El Niño in terms of the El Niño Modoki index. Based on the rainfall patterns in southern China (Wang and Wang, 2013), El Niño Modoki events can be further divided into two types. In El Niño Modoki I, positive sea surface temperature anomaly (SSTA) first emerges in the central equatorial Pacific during spring, develops over the same region, and then damps quickly in the summer of the second year. In El Niño Modoki II, the positive SSTA first appears in the northeastern subtropical Pacific in spring, and then develops toward the equatorial central Pacific. The warm SSTA persists into the second year and slowly decays in the central Pacific during the subsequent seasons.
According to Chen et al. (2021), the recharge oscillator mechanism tends to produce a cyclic transition in the evolution of El Niño. Nevertheless, our results show that El Niño Modoki I features a strong La Nina-like SSTA pattern in the decaying phase with a relatively weaker recharge-discharge process compared to El Niño Modoki II. To clarify the factors except for the recharge oscillator that differs the development process of two types of El Niño Modoki, a mixed layer heat budget analysis is further conducted. In El Niño Modoki I, the easterly wind anomalies appear during the decaying phase, releasing heat from the ocean and resulting in negative latent heat anomalies. Through strong Bjerknes feedback, winds are able to develop continually and cool the eastern Pacific quickly. On the other hand, in El Niño Modoki II, the wind-evaporation-SST (WES) feedback continually provides a positive effect, sustaining warm SSTA in the central Pacific during the entire evolutionary process.
To further explore the dynamics of El Niño Modoki under global warming, CAMS-CSM1-0 was selected from the 15 of CMIP6 models through the Taylor diagram as it can best distinguish the unique patterns of different types of El Niño in the historical simulations. The model does a good job on characterizing the different SSTA patterns during the mature phase between the two types of El Niño Modoki, particularly in the subtropical Pacific. |
