摘要
亞洲夏季季風肇始同時受到季節變化與季內振盪影響,其中季內振盪是指時間尺度小於100天的變化,主要有30~60天與10~20天準週期變化。為了探討季內對流振盪的變化對季風對流變化的貢獻,我們採用Daubechies(1988)所提出之小波轉換為濾波工具,將OLR原始值過濾出30~60天與10~20天波動,並計算季內對流振盪變異量與總變異量。結果顯示, 10~60天OLR波動變異量的高值區分佈與總變異量一致。亞洲季風區, OLR的變化以30~60天波動的貢獻較大。東亞地區之季內對流振盪變異量最大的地區為南海/西太平洋地區,而南海地區之季內對流變化主要是由30~60天振盪的貢獻而來。
進一步分析各年南海地區季內對流振盪與季風對流之關係,我們發現雖然各年季風對流開始時間存在很大的年際變化,然而各年30~60天OLR波動低值與各年OLR原始值降低的時間一致,各年10~20天OLR波動與各年OLR原始值的相位較不一致,這說明了小波轉換所過濾出來的30~60天波動,的確可以掌握東亞夏季季風對流開始時間。
分析各年東亞地區季內對流振盪與各年季風對流(OLR)時間序列之相關係數得知, 兩者相關係數高達0.7以上,即各年東亞地區季風對流之肇始時間、活躍與中斷的變化,與各年東亞地區季內對流振盪開始、活躍與中斷的時間一致。季內對流振盪中又以30~60天OLR波動與OLR原始值相關係數較大(兩者相位較一致) 。
在30~60天對流振盪年際變化的研究方面, 本文以南海地區30~60天對流波動的強度作依據,將30~60天對流波動分類為南海地區30~60天OLR波動強年與弱年。比較強、弱年東亞地區季內對流振盪與夏季季風對流之關係,結果發現強年季內對流振盪中以30~60天OLR波動與OLR原始值之相位關係較一致。而弱年30~60天與10~20天OLR波動的變化,對OLR原始值的變化都很重要,即弱年東亞夏季季風對流與10~60天對流振盪(包含30~60天與10~20天振盪)的相位一致。
為了了解強、弱年南海地區30~60天OLR波動來源, 我們計算5月至7月,強、弱年南海地區30~60天OLR波動與各地30~60天OLR波動之時間序列延遲相關,結果得知強、弱年之波動來源不完全相同。強年南海地區主要的波動來源為,赤道印度洋向東傳的波動,而弱年則為中太平洋向西傳的波動。
由延遲相關分析,只能得知南海地區30~60天OLR波動之來源存在年際變化,若要了解30~60天OLR波動強度隨時間的演化,需作進一步分析。由10~15°N緯向剖面時間序列顯示, 強年各年南海地區第一波30~60天OLR負值主要來源,為從印度洋向東傳遞的波動,與合成後結果相似,而弱年則各年不一致。強年,印度洋上至南海/西太平洋向東傳的訊號可從赤道附近往北擴展到北邊(10~15°N)。此外,強年赤道附近向東傳的波動,在東半球的強度比西半球強,並且具有繞全球一周的現象。
本文繼續探討SST、季內振盪與東亞夏季季風之關係。經由分析強、弱年各年孟加拉灣與南海地區之30~60天OLR與SST之關係, 發現強年各年此兩區域30~60天SST正值相位領先30~60天OLR負值,與合成後結果相似。而弱年則各年不一致,表示弱年各年波動性質有很大的差異。接著探討強年30~60天對流振盪與SST、低層環流之互動關係,我們發現30~60天對流振盪強年, 對流中心移動前緣為30~60天SST正值區、正渦度區。當對流波動抵達時,渦度由負轉正、風向由東風轉為西風,有利於該區季風肇始。然而季內對流生成後,可能使太陽輻射減少, 30~60天SST由正轉負,季內對流衰減。
由上述結果得知,SST與低層環流影響30~60天OLR波動之傳送方向,除此之外,我們亦想了解影響30~60天對流振盪成長之氣候條件。分析結果顯示30~60天對流振盪較強的地區（印度西岸、孟加拉灣、南海及東太平洋）,SST較高,並且處於垂直東風風切區。而30~60天對流振盪較弱的赤道中東太平洋,SST較低,且處於垂直西風風切區。即SST高有利於30~60天對流振盪成長,而亞洲夏季季風區與東太平洋的平均環流所形成之垂直東風風切,亦提供一個有利於30~60天對流波動成長的環境。
比較強年各年5月至7月30~60天OLR波動與SST、垂直風切之水平分佈。結果得知, 強年各年孟加拉灣與南海地區30~60天OLR波動變異量較大,SST較高,且處於垂直東風風切區。西太平洋地區,雖然波動變異量強弱情形,各年不一致,然而當該年波動變異量較大,則該區域SST較高,且處於垂直東風風切區。由強年各年波動變異量異常與SST異常之水平分佈顯示,強年各年孟加拉灣之30~60天OLR波動變異量與SST皆比氣候值大;而熱帶西太平洋波動變異量異常與SST異常,各年無一致關係,表示SST異常並非影響西太平洋30~60天對流振盪成長的唯一因素,垂直東風風切亦有利於向西傳送的波動之成長。

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