Southern Hemisphere Extra-tropical Forcing on ENSO- Observation and Model Comparisons

目前研究對於聖嬰現象的特徵及發展已有相當的了解，關於聖嬰的成因至今依然是科學家研究的重要課題，但是科學界對於聖嬰現象的觸發機制尚不明確。近來許多研究發現聖嬰有兩種不同的型態，一種是海表面溫度距平中心在東太平洋的東太平洋聖嬰，另一種則是海表面溫度距平中心在中太平洋的中太平洋聖嬰。兩種型態的聖嬰有著不一樣的距平分布、時間發展、遙相關和機制。本研究利用觀測資料採Combined EOF的方法，分析中太平洋聖嬰和東太平洋聖嬰兩種不同型態聖嬰現象的特徵和表現，發現1977年之後的中太平洋聖嬰對於整個太平洋的重要性比1977年前高。而且中太平洋聖嬰會領先東太平洋聖嬰約12個月，兩者之間的關係在1977年之後更加的高。另外使用Complex Hilbert EOF討論聖嬰、南極繞極波和全球聖嬰波動系統的相位移動的特徵，找出聖嬰、南極繞極波和全球聖嬰波動之間的關係。發現當東太平洋聖嬰發生2到3年之前，南太平洋會由南極繞極波帶著暖海溫距平和低氣壓距平，穿越中緯度到東太平洋區域驅使東太平洋聖嬰發生。赤道則會由全球聖嬰波動帶著暖海溫距平和低氣壓距平，穿越赤道印度洋和太平洋驅使東太平洋聖嬰發生。其中南極繞極波和全球聖嬰波動的表現由於1976/77的氣候改變而有所改變，對於聖嬰的影響也有所差異。從Taiwan Earth System Model耦合模式的模擬顯示出南極繞極波對東太平洋聖嬰有很大的影響，但是對於中太平洋聖嬰沒有很大的關係，驗證了先前觀測資料中所發現南極繞極波會影響東太平洋聖嬰並且驅使聖嬰產生的結果。The development of ENSO and its impacts on climate are well-known from the literatures, however, the origin of ENSO indeed remains unknown. Recently, two types of ENSO are further defined; Eastern-Pacific ENSO (EP ENSO) and Central-Pacific ENSO (CP ENSO), respectively. Recent studies indicate that they are driven by different driving mechanisms which require better understanding. Here, we discussed these driving mechanisms and how ENSO is generated resulting from the southern hemisphere extra-tropical influences. It is found that CP ENSO becomes more prominent since 1977 and leads EP ENSO for about 12 months with increasing correlation during the recent decades. Around two to three years before the occurrence of EP ENSO, warm Sea Surface Temperature anomaly (SSTa) and negative Sea Level Pressure anomaly (SLPa) propagate eastward from the Southern Pacific toward South America through the ocean-atmosphere coupled Antarctic Circumpolar Wave (ACW). In the tropic, the warm SSTa and negative SLPa also propagate eastward through the low-frequency component of Global ENSO Waves (GEW) and enhances the occurrence of EP ENSO. The evolving of ENSO after 1977 could be due to the large pattern changes of ACW and GEW after the 1976/1977 region shift. Further coupled model results show that the ACW indeed affects the occurrence of EP ENSO while there is no direct link with CP ENSO