西北太平洋地區熱帶氣旋活動對氣候變異度的影響

Abstract

氣候為多重尺度的系統，過去許多學者探討大尺度環境場對颱風的生成、移行及強度的影響；相對的，針對熱帶氣旋對大尺度環流的影響之研究則較少。因此本研究針對西北太平洋地區，欲探討熱帶氣旋活動對氣候平均場與變異度的影響。 首先我們使用Kuruhara et al. (1993) GFDL模式初始化中的方法，人為的移除環流場中熱帶氣旋的訊號，分離出背景環流場，再利用850hPa渦度場分析原始環流場與背景環流場的差異性，而這個差異就視為熱帶氣旋的影響。結果發現在颱風季節時，沿著颱風路徑的氣候平均場，正渦度有一大部分為熱帶氣旋所加強；而44年間的年際變化與季內尺度的變異度增強了高達六成的比例。Hsu et al. (2006) 研究中指出2004年颱風個數多且32-76天頻段的季內震盪訊號強，此研究利用波譜分析研究2004年的狀況，結果顯示在32-76天頻段的強振幅，在背景場的分析中強度明顯減弱，季內震盪的訊號有五成以上來自於熱帶氣旋的貢獻，此外熱帶氣旋也加強了此頻段的氣候變異度。 為了更進一步瞭解熱帶氣旋的影響性是否存在，我們使用普渡降尺度區域氣候模式(PRM)與台灣大學全球氣候模式(NTU-GCM)進行實驗，模擬2004年6-10月份主要颱風季節的情況。分別就原始模擬和加強颱風的模擬進行比較，PRM模擬結果分析顯示，在32-76天頻段的季內變化明顯受到颱風訊號加強，而小波分析中則看到30-70天頻段在加強颱風的模擬中振幅強度最大；而NTU-GCM不同的實驗設計中，原始模擬中颱風強度衰減，加強颱風後其氣候變異度和季內尺度的訊號強度都較接近真實狀況，所以兩個模式模擬結果都顯示颱風影響氣候變異變異度甚鉅。 由以上結果可知熱帶氣旋和大尺度環流之間有多重尺度的交互作用，因此我們需要瞭解對熱帶氣旋與不同尺度間的交互作用，及其對大尺度環境場動力機制的影響，才能夠更正確的分析夏季的氣候特徵，改善氣候模式在夏季的模擬情況。

Climate system includes multi-scale interaction. Many scientists have found that typhoons occur mostly in an active monsoon trough environment and are modulated by intraseasonal oscillation. The influence of the large-scale environment on tropical cyclones has been much studied. But relatively little is known about how tropical cyclones affect the large-scale environment. In this study, we attempt to understand the roles of tropical cyclones in affecting the climate variability in the Western North Pacific region. We remove the TC signals from the original circulation during the typhoon season (JJASO). By comparing the differences between the original and TC-removed fields, we find that TC enhances the positive vorticity collocated with the TC tracks, especially in the typhoon-active years and contributes significantly to the seasonal mean vorticity. The contribution of TC accounts for 60-70% of interannual variability. On the intraseasonal time scale, the 2004 typhoon season is studied because of the strong intraseasonal oscillation signal and large number of TC. It is found that the TC contributes a substantial amount to the 32-76-day variance of 850 hPa vorticity. A study on other years also indicates that the intraseasonal variability is enhanced by the TC activity. Further, we design two experiments with Purdue Regional Model (PRM) and four experiments with NTU-GCM, which were carried out for the entire JJASO period in 2004. Heating, vortex-enhanced, and vortex-removed experiments were then performed for the same period. The contrast between the experiments of different designed was then analyzed to investigate whether and how the intraseasonal variability is affected. The ensemble effect of TC on the intraseasonal variability is indicated by the difference between simulations. Comparing the non-heating and heating simulation results in PRM, the 32-76-day variance of 850hPa vorticity is obviously larger in the later case, and the wavelet analysis shows stronger amplitude in the 30-70-day period band. Also, similar effect shows up in the NTU-GCM simulation, especially in the vortex-enhanced simulation. The results suggest significant contribution to climate variability. There must be some interaction between tropical cyclones and large-scale environment. Furthermore, if tropical cyclones indeed influence certain climate processes, we need to understand the impact.