摘要
ECHAM4大氣環流模式使用AMIP2實驗的實際海表面溫度及海冰資料(EAR-40重分析資料)作為邊界條件；初始條件為ECMWF資料在11月26-30日間，每日00Z及12Z的大氣及地表狀況；模式模擬的時間由1955年12月至2000年2月，總共有10個個別模擬，本文針對Z500、海平面氣壓場及降水場進行分析。
利用變異數分析的方法(Rowell，1998)，推估因海表面溫度驅動力所造成的變異數佔全部變異數的百分比，描繪可預報度的分布情況。熱帶地區受到海表面溫度影響較大，存在著較高的可預報度(Charney et al.，1981)，熱帶外地區的大氣主要受到內部動力過程主控，大多為混亂的訊號，可預報度較低，僅在特定的時間及地區有稍大的可預報度存在，如冬季的PNA地區。在不同氣象參數的分布型態上，海平面氣壓場及降水場的可預報度主要集中在熱帶太平洋上，降水場的分布較海平面氣壓場更集中於赤道附近，Z500高度場呈現帶狀分布環繞整個熱帶地區。中緯度地區冬季、初春可以發現PNA波動型態的可預報度分布，夏季、秋季時也可以在南太平洋地區發現可預報度向熱帶外地區伸展的現象。
將系集平均的距平值定義為訊號，系集間的變異數定義為雜訊(Kumar and Hoerling，1998)，來了解訊號、雜訊與可預報度(訊號雜訊比)隨海溫驅動力的變化情況。在12-3月期間，訊號大多依照海溫驅動力的大小產生規則性的變化，而且正的驅動力所產生的訊號大於負驅動力所產生的訊號；雜訊不隨驅動力大小產生規則變化，海平面氣壓場及Z500高度場冬季時的雜訊較大，降水場的雜訊夏季時較大。海平面氣壓場與Z500高度場可預報度的變化上，低緯度地區的可預報度較高，而且El Nino的可預報度高於La Nina；北半球PNA地區最高的可預報度出現在1-3月，其他時間的可預報度較低，季節的差異較低緯度地區明顯，但南半球PSA地區因訊號與雜訊變化的型態與幅度相同，因此可預報度無明顯的季節變化。熱帶太平洋地區降水場的可預報度大於其他地區，最大值出現在12-3月期間，以El Nino最為明顯。東亞地區降水場在El Nino的1-3月期間訊號有增強的現象因此產生較高的可預報度。
利用距平型態相關係數(Anomaly Pattern Correlation)作為評估可預報度的指標(Chen and Van Den Dool，1997)，探討模式在相同的邊界條件驅動下，模式是否能夠產生一致的反應？分析的結果與利用訊號雜訊比定義的可預報度變化型態相當一致，PNA的Z500高度場與海平面氣壓場在1-3月期間可預報度最高，而且El Nino大於La Nina，造成可預報度差異的原因除了El Nino訊號較大外，La Nina雜訊大於El Nino也是重要因素，雜訊來自於動能在渦流與平均流場間的轉換，當較大的能量轉換發生時會產生較大的雜訊(Chen and Van Den Dool，1999；Schubert et al.，2001)；在南半球的7-9月也觀察到El Nino比La Nina有較多的能量轉換發生，因此在南太平洋地區也觀察到El Nino時期有較大的雜訊。東亞地區海平面氣壓場，在ENSO期間的1-4月時有較高的可預報度產生，El Nino可預報度高於La Nina，造成差異的主要原因為El Nino的訊號大於La Nina，訊號的產生與ENSO期間由冬季到春季建立在西太平洋地區的反氣旋(氣旋)有關，而夏季時熱帶海表面溫度驅動力較弱，而且影響東亞夏季氣候的因素不只熱帶地區的海溫還包括青康藏高原加熱等(Wang et al.2000)，因此造成ENSO冬季、春季的可預報度大於夏季。東亞地區降水場在1-3月有較大的可預報度，El Nino可預報度高於La Nina，主要也是因為El Nino年的訊號大於La Nina年，訊號的增加與ENSO在東亞地區產生特定的環流有關。

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