西北太平洋長期波候變遷之研究 ; A Study on wave climate variation in Tropical North Western Pacific

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Title:	西北太平洋長期波候變遷之研究;A Study on wave climate variation in Tropical North Western Pacific
Authors:	王韻圓;Yun-Yuan Wang
Contributors:	水文與海洋科學研究所
Keywords:	長期波候變遷、年際震盪、大波極端事件;extreme wave events;wave climate variation;annual oscillation
Date:	2010-06-29
Issue Date:	2010-12-09 10:50:51 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究針對台灣周邊及西北太平洋海域波候的變異性進行探討，應用SWAN波浪模式，以NCEP提供之全球歷史海表面風速分析場推算1948~2008年近60年西北太平洋之波浪，其所重建的波浪資料與中央氣象局、經濟部水利署及美國大氣海洋總署之國家資料浮標中心設置的海氣象資料浮標觀測資料進行比對和驗證，用於探討長期波候變遷之趨勢。本研究將波候變遷趨勢分成兩部份討論，第一部份探討台灣周邊海域波高、波能、波浪尖銳度與波向等波浪特性之長期變異震盪與趨勢，了解不同時間尺度下台灣波候之震盪特性及其變異趨勢；第二部份為波高極端事件與統計特性分析。第一部分研究結果發現台灣波候變異存在三種不同時間尺度之震盪，分別為季節性震盪、受到聖嬰與反聖嬰現象影響的數年震盪、以及十年際週期的震盪。季節震盪方面，冬季波能與夏季波能之比值約在3~3.6倍，其中東岸比值比西岸高；在年際震盪方面，聖嬰與反聖嬰現象對於年際波能震盪具有強烈影響因素，反聖嬰年時，波高與波能變大，冬季的大波會晚到，約在1月發生；聖嬰年時，波高與波能變弱，大波提早於11月發生。此外，台灣海域波能震盪與南方震盪指數(Southern Oscillation Index, SOI)沒有存在相位延遲，但西北太平洋海域則存在一年的相位延遲。十年際震盪方面，將十年際之波候震盪與太平洋年代際震盪(Pacific Decadal Oscillation, PDO)進行相關分析，發現十年際的震盪與具有中度相關。整體趨勢方面，台灣周邊海域之波高有下降的趨勢，平均每年下降0.3公分。整體而言，冬季波高與波能有逐漸下降的趨勢，平均每年波高下降0.8公分，每年波能平均下降0.2千瓦/公尺，此現象與西伯利亞高壓的東北季風強度減弱有關。值得注意的是，夏季有逐漸上升的趨勢，平均每年波高上升0.5公尺，每年波能上升0.1千瓦/公尺，可能與西北太平洋颱風強度增加有關。第二部分研究探討台灣週邊海域各種大波極端事件發生機率，結果發現近60年來極端事件集中發生於1967至1974年以及2000至2008年間，其中後者所發生的極端事件波高大、延時長。進一步發現，台灣週邊海域所發生的極端大波事件中，冬季的極端大波事件有減少趨勢，夏季的極端大波事件有增加的趨勢，其中夏季發生之大波極端事件完全由颱風所造成，若以1985年為界，之後颱風所造成極端事件的比例，較東北季風為高，且逐年增加。另一方面，波浪極端值之統計分佈在聖嬰年與反聖嬰年也有極大差異，反聖嬰年之波高極端值約為聖嬰年的2倍，反聖嬰年之波能極端值約為聖嬰年的5倍，此結果可提供海岸工程設計的參考。We investigated the variation of wave climate around Taiwan Waters and over the Tropical Northwestern Pacific. The global sea surface wind fields from NCEP reanalysis data were used to drive SWAN wave model to re-construct the historical records of wave from 1948 to 2008. The reconstruct wave data was compared and verified with observation from the Central Weather Bureau (CWB) , Water Resources Agency (WRA) and the NDBC/NOAA. The discussion of the trend of long-term wave climate change was categorized into two parts. In order to clarify the oscillations and trend of wave climate in different time scale, the first part was focused on the oscillations of wave heights, wave energy, wave steepness, and wave direction in the Taiwan Waters. The second part is the analysis of extreme wave events and its statistical characteristics. The results of the first part showed that the oscillations consisted of three different time-scale oscillations in Taiwan wave climate variations. There were seasonal oscillation, interannual oscillation affected by ENSO, and interdecadal oscillation. In the seasonal oscillation, the wave energy in winter was about 3~3.6 times than in summer. The ratio in east coast was higher than in west coast. The influence of ENSO was significant in interannual oscillation of wave energy. Wave height and wave energy became large in La Niña years. The time of the occurrences winter extreme wave events in La Niña years was several weeks late, which was in January. Wave height and wave energy weakened in El Niño years. The occurrence of winter extreme wave events in El Niño years was in November. In addition, there was no phase lag between wave climate oscillation in the seaaround Taiwan and Southern Oscillation Index (SOI). But there existed one year phase lag in the Northwest Pacific. The interdecadal oscillation of wave climate correlated medially with Pacific Decadal Oscillation (PDO). A decrease trend of wave height was in the sea area nearby Taiwan, which reduced 0.3 cm per year. In winter, the decrease trend of wave height and wave energy was due to decreasing the intensity of Siberia Heighs and associated winter monsoon. It is noted that increase trend is obvious in summer which might indicate the increasing strength of typhoon in this region. The occurrence probability analysis of extreme wave events in the sea area nearby Taiwan was discussed in the second part. In recent 60 years, most extreme wave events happened in two periods: 1967-1974 and 2000-2008. The larger wave height and longer duration of extreme wave events occurred in the latter than in the former. The extreme wave events in summer (winter) had the increase (decrease) trend in the sea area nearby Taiwan. All the extreme wave events in summer were caused by typhoon. After 1985, the number of extreme wave event caused by typhoon was more than caused by winter monsoon. The difference increased year by year. But the total number of extreme wave event in one year did not have any significant change. In addition, the statistical distribution of wave extreme value had large difference between El Niño years and La Niña years. The wave height extreme value in La Niña years was about double in El Niño years. The wave energy extreme value was about five times. This result can be provided for the design of coastal engineering.
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