以臺灣中部雲杉樹輪重建三百年古氣候：利用傳統樹輪及總體經驗模態分解法

300-year dendroclimatic reconstructions based on conventional methods and Ensemble Empirical Mode Decomposition using Picea morrisonicola tree rings from central Taiwan

陳姿彤(Tzu-Tung Chen)

指導教授 ： 魏國彥

共同指導教授 ： 李紅春(Hong-Chun Li)

國立臺灣大學/理學院/地質科學研究所/碩士(2011年)

https://doi.org/10.6342/NTU.2011.01116

摘要

本研究以塔塔加山區的臺灣雲杉 (Picea morrisonicola) 樹輪寬度資料建立輪寬年表，作為探討過去氣候變化的依據，結果重建了 373 年 (1636-2008) 的樹輪寬度年表。根據氣候對樹木生長影響機制，將樹輪寬度與氣候因子進行相關及反應函數分析，結果顯示晚材輪寬主要反映阿里山 7-9 月平均最高溫 (maximum temperature, Tmax)；而早材輪寬主要反映 4-6 月日較溫差 (Diurnal Temperature Range, DTR)。

為了改善傳統樹輪年表建立方法 (ARSTAN) 中，主觀去除樹木生長趨勢的問題，本研究以黃鍔院士在 1998 年所提出「希爾伯特—黃轉換 (Hilbert-Huang Transform, HHT) 」中之總體經驗模態分解法 (Ensemble Empirical Mode Decomposition, EEMD) 法作為建立年表的替代方法，將樹輪寬度序列分解為具有不同頻率訊息的分量，並以與氣候反應最佳的分量組合做為新的分量總和年表，提供一個有別於傳統樹輪建立年表的替代方法。EEMD 與 ARSTAN 年表所重建之 Tmax 與 DTR 皆通過重建模式技巧檢測且結果相似，但 EEMD 總和年表在 DTR 重建上結果較佳。

由修正晚材殘差年表所重建之阿里山山區過去 373 年的 7-9 月 Tmax，重建結果可解釋觀測期間 (1951-2008) 7-9 月 Tmax 之 23% 變異 (r=0.48)，並顯示幾個較明顯的暖期，分別是 1718-1726、1908-1916 與 2002-2008。由 NCEP/NCAR 再分析資料顯示 Tmax 反映臺灣夏季受西太平洋夏季季風 (Western North Pacific Summer Monsoon, WNPSM) 的影響，在少雨年時，WNPSM 其中一個環流系統—西太平洋副熱帶高壓 (Western Pacific Subtropical High, WPSH) 規模擴大並往西延伸，阻擋西南氣流及梅雨鋒面，並伴隨著東海的反氣旋活動，因而產生異常溫暖且乾燥的夏季，並對應到較大輪寬發生的年份。此外，重建的 Tmax 中最溫暖的 10 年有 8 年與 El Niño 事件的年份相符合，表現出臺灣夏季 Tmax 與 ENSO 活動的關聯性。

以早材 IMF 總和年表重建之阿里山山區過去 373 年 4-6 月日較溫差 (DTR) 可解釋觀測期間 (1934-2000) DTR 之 28% 變異 (r=0.53)。阿里山日照時數與 DTR 的變化趨勢一致 (r=0.87)，而日照時數減少是日間雲層覆蓋量增加的結果，由 DTR 與雲量間顯著的負相關，可證實阿里山山區 DTR 的變小確實與增加的中高雲量有關。此外，重建之 DTR 顯示過去地表接受到的太陽輻射量變化週期約為 28 年；可反映其變化所引起的 global dimming/brightening 及推測 DTR 開始大幅變化的起始時間點。

關鍵字

樹輪氣候學 ； 臺灣雲杉 ； 總體經驗模態分解法 ； 最高溫 ； 日較溫差

並列摘要

Virtually very little dendrochronology data have been reported internationally from Taiwan, despite the existence of many dendrochronologically appropriate tree species. In this study, the potential for reconstruction of local paleoclimate was investigated using multi-century tree-ring chronologies developed from Picea morrisonicola (the endemic Taiwan Spruce). Significant correlations were found against the mean April-June diurnal temperature range (DTR) and against the mean July-September maximum temperature (Tmax). Both of these climate parameters were reconstructed based on the regression relationships.

In a related study, a new frequency decomposition method called empirical mode decomposition (EMD), one part of the Hilbert-Huang Transform (HHT), was investigated as an alternative to standard methods of chronology generation in terms of climate signal. A noise assisted version of EMD called ensemble empirical mode decomposition (EEMD) was used to decompose the tree-ring time series into a series of quasi-periodic modes from high to low frequency. Consecutive modes were combined from high to low frequency and compared with the climate data. The combination with the most significant climate relationships was then used to reconstruct the climate parameters. As with the reconstructions using traditional methods of chronology generation, statistics from the reconstructions of DTR and Tmax also passed tests for model skill. The reconstruction statistics and variance explained were similar for both methods of chronology generation, with EEMD chronology having better results in the DTR reconstruction and the traditional chronology having better results in the Tmax reconstruction.

Adjusted latewood ring widths show significant (p<0.01) positive correlation against Alishan July-September Tmax. Linear regression of the Alishan Tmax on the tree-ring chronology produced a calibration model that accounted for 23% of the actual Tmax variance. This model was used to reconstruct the July-September Tmax back to A.D. 1636. The reconstruction shows warm periods during 1718-1726, 1908-1916, and 2002-2008. Evidence from comparisons with NCEP-NCAR reanalysis data indicates that the summer climate variability in Taiwan is regulated by processes associated with changes in the Western Pacific Subtropical High (WPSH). In years with less precipitation the WPSH reduces the southwesterly monsoonal flow by extending further westward than in other years. This appears as an anomalous warm and dry summer accompanied with anti-cyclonic motion over the East China Sea. In addition, eight of the ten warmest summers (July-September Tmax) in central Taiwan occurred during El Niño years, indicating a link between Taiwan summer maximum temperatures and ENSO dynamics.

The earlywood mean chronology was calibrated against April-June DTR. A calibration model that accounted for 28% of the actual DTR variance was then produced to reconstruct the DTR. The increasing Tmin, which can be attributed to locally increased cloud cover, contributed to the reduction of DTR. The reconstructed DTR has a cycle of period 28 years, showing the variations in solar irradiance possibly due to cloudiness changes.

並列關鍵字

dendrochronology ； Picea morrisonicola ； Ensemble Empirical Mode Decomposition (EEMD) ； maximum temperature ； diurnal temperature range

參考文獻

邦卡兒．海放南 (2007a)，論塔塔加地區高山植物的物候期，林業研究專訊，14 
 (5)。

邦卡兒．海放南 (2007b)，塔塔加地區植物相調查與解說規劃，靜宜大學生態學系

呂勝由、陳舜英 (1996)，本省雲杉林生態之研究，臺灣省林業試驗所簡訊，17-20

張琬玉 (2004)，台灣颱風季乾旱與大尺度環流場的關係，國立臺灣大學大氣科學

劉昭民 (2000)，清代台灣強烈地震記錄及奇聞資料，中華科技史同好會會刊，1