探討歐亞-北太平洋多年代振盪與其成因

Abstract

歐亞-北太平洋多年代振盪(Eurasia-Pacific Multidecadal Oscillation, EAPMO)是由本研究發現的存在北半球中緯度高對流層的環流結構，呈現多年代振盪特徵。EAPMO位於北半球溫帶氣候噴流軸北側約40°N至60°N的範圍，幾乎緯向均勻對稱的重力位高度場且近似環繞整個北極圈，但以歐亞大陸及北太平洋區域振幅最大。垂直方向為相當正壓且最大振幅位於200hPa的高對流層，並伴隨整層中對流層的溫度變化。EAPMO具有緯向波數為4至5的空間結構，歐洲、東亞、北太平洋為疊加在緯向均勻構造上的三個區域極大值中心，其中位於東亞局地極值中心剛好位於高地上方，隱含地形可加強EAPMO局部強度。不同氣象組織產生的觀測分析場均可診斷出EAPMO，除強化本研究的可靠性外，更將EAPMO存在氣候系統中的歷史證據向前推前至二十世紀初。 由於EAPMO與北大西洋海溫高度相關，且演變趨勢與溫帶大西洋多年代振盪(Atlantic Multidecadal Oscillation, AMO)如出一轍，因此可能是AMO所引發的大氣現象。使用大氣環流模式驗證此一推論，在北大西洋中置入AMO形態的海溫變化，數值模式實驗結果顯示大氣對北大西洋海溫的反應與EAPMO十分相似，證實AMO形態的海溫變化不僅影響到大西洋鄰近地區，也透過大氣擾動的傳播，將影響力帶到東亞與北太平洋，甚至整個北半球氣候。另一組無地形實驗的結果顯示，在地勢平滑無起伏的歐亞大陸中，EAPMO的緯向結構變得非常均勻一致，證明高山地形有強化局部訊號的看法。 北半球中緯度的多年代變異量比熱帶與南半球複雜，它是由暖化趨勢與多年代振盪兩者結合而成的複合體；相對來說，熱帶與南半球僅需考慮暖化趨勢就足夠了。本研究認為，AMO本身與其引發的後續反應是北半球主要(或至少部份)造成北半球地表溫度呈現多年代振盪的原因。人為暖化及多年代振盪的雙重影響，造成全球均溫在1970年代至二十一世紀初的暖化加速。

A multidecadal geopotential height pattern in the upper troposphere of the extratropical Northern Hemisphere (NH) is identified in this study. This pattern is characterized by the nearly zonal symmetry of geopotential height and temperature between 358 and 658N and the equivalent barotropic vertical structure with the largest amplitude in the upper troposphere. This pattern is named the Eurasian–Pacific multidecadal oscillation (EAPMO) to describe its multidecadal time scale and the largest amplitudes over Eurasia and the North Pacific. Although nearly extending over the entire extratropics, theEAPMOexhibits larger amplitudes over western Europe, East Asia and the North Pacific with a zonal scale equivalent to zonal wavenumbers 4 and 5. The zonally asymmetric perturbation tends to amplify over the major mountain ranges in the region, suggesting a significant topographic influence. The EAPMO has fluctuated concurrently with the Atlantic multidecadal oscillation (AMO) at least since the beginning of the twentieth century. The numerical simulation results suggest that the EAPMO could be induced by the AMO-like sea surface temperature anomaly and strengthened regionally by topography, especially over the Asian highland region, although the amplitude was undersimulated. This study found that the multidecadal variability of the upper-tropospheric geopotential height in the extratropical NH is much more complicated than in the tropics and the Southern Hemisphere (SH). It takes both first (warming trend) and second (multidecadal) EOFs to explain the multidecadal variability in the extratropicalNH, while only the firstEOF, which exhibited a warming trend, is sufficient for the tropics and SH.