1979-2017年西北太平洋颱風年代際變化探討;Interdecadal change of tropical cyclones characteristic over Western North Pacific during 1979-2017

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题名:	1979-2017年西北太平洋颱風年代際變化探討;Interdecadal change of tropical cyclones characteristic over Western North Pacific during 1979-2017
作者:	林冠揚;Lin, Kuan-Yang
贡献者:	大氣科學學系
关键词:	颱風;年代際變化;太平洋年代際震盪;西北太平洋;Tropical cyclone;Interdecadal change;Pacific Interdecadal Oscillation;Northwestern Pacific ocean
日期:	2019-07-17
上传时间:	2019-09-03 12:19:45 (UTC+8)
出版者:	國立中央大學
摘要:	西北太平洋颱風活動以長時間尺度來看有著年代際震盪的變化，而最近一次較大的變化發生在1997/98年時，颱風的平均數量在此之後出現顯著的減少，而此低靡時期一路持續到2017年左右。因此便好奇1997/98年前後西北太平洋颱風除了生成數量有顯著改變外，是否其他特性也有著明顯的改變。首先統計1951-2017年的颱風發生日期，定義了平均颱風旺盛時期，並透過該時期進行1979-2017年颱風路徑及移速上的探討。定義1979-1997為 Phase 1(P1)時期，1998-2017為Phase 2(P2)時期。先將颱風路徑分為轉向型及直西型，而直西型又另外分出西北型及平西型。發現轉向型颱風在P2時期平均路徑有向西調整的趨勢，日本南方及台灣東北方海域的颱風發生頻率也有顯著增加的趨勢。而直西型颱風則是在P2時期平均路徑有北偏的趨勢，另外分類出西北型及平西型更可以看出西北型颱風在後期的發生頻率明顯增加，而平西型颱風則是明顯減少的。在移速方面的研究，發現轉向型颱風在P2時期平均移速較P1時期慢，且隨著緯度升高移速也有持續減慢的趨勢。而直西型颱風則在P2時期出現明顯加速及減速特徵，在東側遠洋海域約134-143°E時移速明顯較P1時期快，而在漸漸西移的過程中出現明顯減速，在120-135°E時移速較P1時期慢，隨後兩時期移速趨於接近。造成轉向型颱風路徑偏移的主要原因在於中高層中緯度一帶出現明顯的東風異常量，使得路徑向西偏移外移速也隨之減慢。而直西型颱風的部分，是因為在低層低緯洋面有較強勁東風異常，使得颱風初期移速較快，之後在菲律賓東方近海開始出現南風分量，使得路徑北偏外移速也跟著減慢。最後透過經驗正交函數分析發現，海表面溫度與重力位高度場及流函數場的時間序列間有很高的正相關性，且發生明顯變化的時間點也在1997/98年間，顯示此時期大氣環境的變化一定程度的影響了西北太平洋的颱風活動。 ;Tropical cyclones (TC) genesis amount has been detected to have both interannual and interdecadal oscillations in the Northwestern Pacific Ocean. The latest and significant change appeared in 1997/98 indicates that TC amount began to decrease dramatically after 1997/98 and persisted to around 2017. Beside the TC genesis amount change, whether the TC characteristic altered is also drawn to our attention. First, we defined the relatively active TC genesis period of a year based upon TC genesis historical dates over 1951-2017 and then intensively investigated both TC tracks and their speeds during 1979-2017 as 1979-1997 represented for phase I (P1) and 1998-2017 for phase II (P2). Second, two types of TC movement were identified: recurving and straight-moving. Furthermore, the straight-moving TCs could be split into northwestward- and westward-moving categories. Interestingly, it is found that the recurving TC in P2 not only has a tendency of moving farther west than those in P1 but also significantly increase the TC occurrence frequency over south of Japan and northeast of Taiwan area. On the other hand, the straight-moving TCs apparently shift northward in P2 with greater occurrence frequency for northwestward-moving one and less for westward-moving TCs. In regard to the moving speed, it appears that the recurving TCs in P2 are not only slower than those in P1 but continue to decrease when they move further north. On the contrary, the straight-moving TCs happen to be more fluctuated that they move a bit faster over 145°E to 135°E then slower between 135°E and 120°E in P2. However, their speeds are somehow in agreement with each other both in P2 and P1 when they move into South China Sea. Based upon the composite analysis, it turns out that the anomalous easterlies over the region between 25°N-35°N on upper troposphere not only force the recurving TCs moving farther west but also reduce their moving speeds in P2. While in the low latitude area, the anomalous easterlies induced by the Pacific subtropical High anomaly could enhance the speeds of both the recurving and straight-moving TCs during their developing stage in P2. Meanwhile, the mild anomalous southerlies near east of Philippines might contribute to both track northward shifting and speed decreasing for straight-moving TCs over 135°-120°E region. Through the empirical orthogonal function analysis on sea surface temperature (SST), geopotential height and stream function for multi levels, it shows that the first principle component time series of SST has a high correlation coefficient with each individual variable ones. Besides, all of them demonstrates a profound phase change between 1997/98. Thus, we could hypothesize that the large scale circulation make the adjustment and leading toward the TC genesis amount abruptly decreasing and characteristic change in response to the long term time variation of SST during 1979-2017.
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