東亞冬季季風：年代際變化、突變與減弱趨勢

The East Asian Winter Monsoon: Decadal Variation, Abrupt Change and Weakening Trend

高培根(Pei-ken Kao)

指導教授 ： 許晃雄 ； 洪致文

國立臺灣大學/理學院/大氣科學研究所/博士(2016年)

https://doi.org/10.6342/NTU201601377

摘要

在過去的許多研究往往使用單一指數來定義東亞冬季季風的強度，本論文找出高緯度至低緯度地區，其北風分別有年代際變化、突變與減弱趨勢。第一個機制為阿留申低壓的東移，而使得高緯度東亞沿岸地區的冬季向西海平面氣壓梯度與冬季低層北風減弱，而阿留申低壓的東移是由於太平洋年代際震盪的相位轉換所致。第二個機制為位於中緯度北風的突變，其原因為北極震盪在1980年代晚期後轉為正相位並持續十幾年，使得東亞主槽減弱，並使得中緯度東亞沿岸地區的冬季溫度增加與低層北風減弱。第三個機制為，在低緯度地區的東北季風，其年代際變化之部份是由於太平洋年代際震盪轉為正相位，並在菲律賓產生一反氣旋式環流距平，進而使得東亞沿岸低緯度地區的北風減弱。而在長期的減弱趨勢方面，第四個機制可能是因為大西洋多重年代際震盪在1990年代初期轉為正相位，使得中太平洋聖嬰發生頻率增加，進而讓位於南中國海北部與臺灣東方海面的冬季反氣旋式環流的位置發生改變，低緯度地區的低層冬季北風在1990年代之後轉為南風。本研究認為單一指數與單一機制是不足以描繪及解釋跨越低緯度至高緯度地區的東亞冬季季風系統，本研究提出的四種機制才可解釋位於不同緯度的東亞冬季季風，其年代際變化、突變以及減弱趨勢。

關鍵字

東亞冬季季風 ； 年代際變化 ； 突變 ； 趨勢 ； 太平洋年代際震盪 ； 北極震盪 ； 中太平洋聖嬰

並列摘要

The intensity of East Asian winter monsoons (EAWM) is represented by a single EAWM index in many previous works. This study finds a decadal variation, abrupt change and a weakening trend of northerly wind in EAWM in different areas over latitudes along the East Asia Coast. The first mechanism of the northern part of the East Asia Coast is influenced by the forcing from the middle latitudes during 1950s to 2000s and related to the Pacific Decadal Oscillation (PDO). I propose that the decreased EAWM in the recent decades is associated with the change of pressure gradient along the East Asia coast. This mechanism accounts for the change of the westward sea-level pressure (SLP) gradient along the Northeast Asia coast, and is affected by the location of Aleutian low, which is associated with the phases shift of the PDO. As the Aleutian low is influenced by the negative PDO phase and moves westward, the SLP gradient between the Siberian high and the Aleutian low can increase and the northerly wind at 850 hPa will be enhanced. The second mechanism accounts for the phase change of the Arctic Oscillation (AO) after the late 1980s. When the AO is in its positive phase, the low-level northerly wind from South Korea to northern Taiwan and the East Asian trough weaken. The third mechanism of the decadal variation of the northerly wind in the lower latitude is associated with the positive phase of PDO and the location change of the anti-cyclonic circulation center over the southern China. The positive phase of the PDO induces a low-level anti-cyclonic anomalous circulation against the EAWM in the lower latitudes. On the other hand, the forth mechanism accounts for the increased occurrence of Central-Pacific El Niño copes with the phase change of Atlantic Multi-decadal Oscillation (AMO) in early 1990. The anti-cyclonic circulation moves eastward and produces southwesterly wind over the northern part of the South China sea when the Central-Pacific El Niños increase. Therefore the EAWM has weakened in recent decades. This study argues that neither a single EAWM index nor only one mechanism can explain the whole process of the weakening of EAWM over the East Asia Coast from the tropics to the middle latitudes. The four mechanisms provided by this study can explain the decadal variation, abrupt change and the weakening of EAWM at different latitudes.

並列關鍵字

East Asian winter monsoon ； decadal variation ； abrupt change ； trend ； Pacific Decadal Oscillation ； Arctic Oscillation ； Central-Pacific El Niño

參考文獻

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