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標題: | 西北太平洋颱風最大可能強度長期趨勢及年際變化之探討 Investigation of the Inter-annual Variability and Long term trend of Typhoon’s Potential Intensity in the Western North Pacific Ocean |
作者: | Jui-Yu Chan 詹睿瑜 |
指導教授: | 林依依(I-I Lin) |
關鍵字: | 颱風強度,西北太平洋,颱風最大可能強度,海洋耦合最大可能強度, typhoon intensity,western pacific,typhoon potential potential intensity,ocean coupling potential intensity, |
出版年 : | 2014 |
學位: | 碩士 |
摘要: | 本研究主要探討颱風可能強度 (Potential Intensity;颱風最大可到達強度),於西北太平洋地區的長期趨勢變化及年際變化,過去的研究顯示颱風在此區的增強主要在 (4°N-26°N, 122°E-180°E) 上,並將此區域定為颱風主要發展區域 (Main Development Region, MDR),因此我們將專注於此區域颱風強度的變化。
Emanuel [1986] 年提出一個估計颱風最大可能強度的計算方式:Potential Intensity (PI),此方法用於計算在一環境熱力條件下,颱風能到達的最大強度。Lin et al. [2013] 的文章中改進PI的計算,提出一個修正後的指標:Ocean Coupling Potential Intensity (OCPI)。OCPI加入了海水上表層熱力資訊到舊的PI中,原始PI只考慮海表溫度 (SST)對颱風強度影響,但是颱風經過洋面時,會和整個上表層海水作用,因此OCPI包含上表層海洋的資訊,得以對颱風最大強度有更精確的估計。Emanuel et al. [2013] 中發現在北大西洋,PI值在近30年有上升趨勢,大約有6m/s的上升,而影響PI上升的因素,主要為對流層頂溫度近30年來在北大西洋有下降趨勢。在本研究中,我們想要探討PI在西北太平洋MDR上的趨勢。 在西北太平洋MDR區域,PI從1979年至今同北大西洋有上升的趨勢,上升幅度約為3m/s,將PI量值分成兩項:熱力不平衡 (thermodynamic disequilibrium) 項和熱力效率項 (thermodynamic efficiency) 做更進一步的探討,研究結果發現影響PI有上升趨勢是由熱力不平衡項所致,熱力不平衡項在三十多年來呈現上升趨勢,使得PI有增加現象,而影響熱力不平衡項增加的因素為SST量值有上升趨勢,以及低層大氣變乾的趨勢,這兩個現象使得西北太平洋區域的熱力條件,於三十年來變得更有利於颱風,此趨勢可能為年代際變化或長期趨勢變化。 除了較長期的趨勢外,本研究關於年際變化的探討專注於ENSO現象發展期PI和OCPI的變化,在陳 [2011] 探討兩種聖嬰現象和反聖嬰現象時,西北太平洋區域的海洋熱力結構變化,其研究發現在此三種不同類型的ENSO時,暖水層的厚度有明顯變化。本研究結果顯示在西北太平洋區域,OCPI在三種不同ENSO狀態下有較顯著的差異,而PI值的差異則較小。將PI和OCPI分為能量項和效率項做更進一步的探討,發現在三種不同ENSO的狀態下,不論是PI或者OCPI的變化都是由於能量項的變化造成。 The research objective of this study is to investigate the long-term change (or trend) and the inter-annual variability of typhoon’s potential intensity in the Western North Pacific Ocean (WNPO). In this work, the study domain is on typhoon’s main development region (MDR; 4°N-26°N, 122°E-180°E). In 1986, Emanuel proposed the theory of Potential Intensity (PI), which is an estimate of a tropical cyclone’s intensity upper bound. PI is estimated based on the thermal conditions of ocean and atmosphere. In traditional PI calculation, sea surface temperature (SST) was used to represent ocean’s thermal condition. However, tropical cyclones interact not only with sea surface, but with the entire upper ocean column, it is thus desirable to include ocean’s subsurface contribution to obtain more complete characterization of ocean for PI calculation. To address this limitation, Lin et al., 2013 thus proposed a revised PI index, called Ocean Coupling Potential Intensity (OCPI), so that complete ocean subsurface contribution can be included. It has been found that OCPI much improves the estimation of typhoon’s intensity upper bound. Emanuel et al., 2013 investigated the long term trend of PI over the North Atlantic Ocean (NAO). Emanuel et al. found that PI has a rising tendency (6m/s increase) in the recent 3 decades over NAO. They further suggest that the increase in PI is attributed to the reduction in temperature at tropopause in the recent three decades. In this study, the long-term change of PI over the WNPO MDR is analyzed. It is found that the PI over the WNPO MDR has also increased by about 3 m/s in the past 3 decades. Specifically, the 2 contributing terms of PI, i.e. thermodynamic disequilibrium and thermodynamic efficiency, were analyzed. Results show the rising tendency is due to thermodynamic disequilibrium, possibly associated with rising SST and drying of the lower atmospheric humidity. As for the inter-annual variability part, it was found that the dramatic change in ocean’s subsurface variability during ENSO and La Nina events have strong impact on OCPI, while the traditional SST-based PI has much less sensitivity. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/58597 |
全文授權: | 有償授權 |
顯示於系所單位: | 大氣科學系 |
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