| 摘要: | 颱風需要從海洋中獲取成長所需要的能量並且透過大氣過程逐漸壯大，因此颱風在大氣條件和海洋條件都處於有利的情況下才能夠持續發展。而構成有利的海洋條件需要有溫暖的海水，當海水溫度越高，颱風從海洋所能汲取到的能量也就越多，因此颱風就更有機會發展得更好。然而，當颱風經過海水表面時，其帶來的強風會導致海表溫度冷卻(Sea Surface Temperature cooling；SST cooling)。一般來說，當颱風擁有較大的大小、較慢的移動速度以及較冷的海水次表面條件，會使海水出現較強的冷卻，其海水冷卻的幅度與颱風大小成正比。由於海水的冷卻降低了海洋和大氣的溫度差，使海水能夠輸送到颱風的能量受到影響，對颱風形成負反饋機制，進而使颱風能汲取到的能量減少影響颱風的強度發展。因此，從海氣能量傳輸的角度而言，颱風大小在颱風的增強過程中扮演著非常重要的關鍵因子。
本研究利用美國國家環境預測中心(National Centers for Environmental Prediction；NCEP)氣候預測系統再分析(Climate Forecast System Reanalysis；CFSR)的風場資料對西北太平洋1980~2018年共計39年的颱風進行大小計算，採用颱風中心與颱風外核(outer core)達到17 m/s風速的距離(Radius of 17 m/s；R17)作為颱風的大小，通過使用兩個回歸方程並區分三種情況，即CFSR強度大於或等於17 m/s、CFSR強度低於17 m/s、CFSR大小修正後為負值，並進行相對應的大小計算，最後算出從1980~2018年所有颱風個案(16658筆)的大小，建立了長達39年的颱風大小氣候值，然後使用此大小氣候值進行研究。
從1980~2018年所有颱風個案的統計，得出西北太平洋的平均大小為2.03度。在月平均大小上出現了季節性的大小變化，2月份最小，9和10月最大，在年平均大小上則出現了下降的趨勢。年平均大小與較強的聖嬰和反聖嬰現象則有較高的相關性，兩者的相關係數為0.68。對極端的大小個案進行趨勢檢測發現極端大小的個案數量呈現顯著的下降，趨勢檢測的P值達到0.005。對各個強度的年平均大小進行趨勢檢測發現Cat.1、Cat4、Cat.5的颱風大小出現顯著的年平均大小下降，趨勢檢測的P值分別為0.042、0.031、0.039。對各個移動速度的年平均大小進行趨勢檢測發現僅移動速度 12m/s的颱風大小出現顯著的年平均大小下降，趨勢檢測的P值為0.018。對各個緯度的年平均大小進行趨勢檢測發現在緯度30~35度的年平均大小出現顯著的下降，趨勢檢測的P值為0.003。;The typhoon needs to obtain the energy for growth from the ocean through the atmospheric process. Therefore, the typhoon can continue to develop only when the atmospheric and ocean conditions are favorable. Favorable ocean conditions require warm sea water. The higher sea water temperature, the typhoon can extract more energy from the ocean, so the typhoon has a better chance to develop better. However, when a typhoon passes over the surface of the sea, the sea surface wind will cause Sea Surface Temperature cooling(SST Cooling). Generally speaking, when a typhoon has a larger size, slower moving speed and colder seawater subsurface conditions, the seawater will be strongly cooled, and the extent of the seawater cooling is proportional to the size of the typhoon. Since the cooling of sea water reduces the temperature difference between the ocean and the atmosphere, the energy that the sea water can transport to the typhoon is affected, and a negative feedback mechanism is formed for the typhoon, thereby reducing the energy extracted by the typhoon and affecting the intensity of the typhoon. Therefore, from the perspective of air-sea energy transmission, the size of the typhoon plays a very important key factor in the strengthening process of the typhoon.
This study uses the wind field data reanalyzed by NCEP CFSR (National Centers for Environmental Prediction Climate Forecast System Reanalysis) to calculate the size of typhoons in the Northwest Pacific from 1980 to 2018 for a total of 39 years. The distance between the typhoon center and the 17m/s wind of the typhoon outer core (R17) is used as the typhoon size. By using two regression equations and distinguishing three situations, that is, the CFSR intensity is greater than or equal to 17 m/s, the CFSR intensity is less than 17 m/s, and the CFSR size is negative after correction, and then the typhoon size is calculated. the size of all typhoon cases (16,658 cases) from 1980 to 2018 was finally calculated, and a 39-year climatology of typhoon size was established, then use the climatology of typhoon size for this study.
From the statistics of all typhoon cases from 1980 to 2018, the average size of the Northwest Pacific Ocean is 2.03 degrees. On the monthly average size, there has been a seasonal change in size, the smallest in February, the largest in September and October; on the annual average size, there has been a downward trend. The annual average size is higher correlation with stronger ENSO events, the correlation coefficient is 0.68. The trend detection of extreme size cases found that the number of extreme size cases showed a significant decrease, and the P-value of the trend detection reached 0.005. The trend detection of the typhoon annual average size of different intensities found that Cat.1、Cat.4、Cat.5 showed a significant decrease in annual average size. The trend detection of the typhoon annual average size of different translation speeds found that only the annual average size with a translation speed of 12m/s showed a significant decrease, the P-value of the trend detection was 0.018. The trend detection of the typhoon annual average size of different latitudes found that the annual average size of the latitude 30~35 degrees showed a significant decrease, and the P-value of the trend detection was 0.003. |
