西北太平洋上大尺度環境與颱風特性的關係

Abstract

在西北太平洋上，颱風是一重要的劇烈天氣現象，而透過大尺度環境場中的氣候因子與海洋因素，在動力機制與熱力機制的驅動下，將會對颱風特徵帶來影響。隨著全球暖化日益嚴重，大尺度環境場也會隨之改變，為此，本研究除了對長期氣候場中颱風對環境場的影響做討論之外，也進一步檢視其年際變化上的情形，以瞭解大尺度環境場如何影響颱風的特徵，特別是移動速度，進而能提供一些瞭解颱風因全球暖化後可能產生改變的資訊。 颱風移動速度的空間分布上，在長期氣候平均場中，30°N 以南在季風槽伸入之處的颱風移動速度最慢。而年際變化上，強冷年在季風槽上颱風的移動速度大致會變快，太平洋副熱帶高壓脊上則變慢；強暖年相反，但主要區域有些許改變。並且不論是長期氣候平均或年際變化上，在 30°N 以北移動速度皆與中層（500hPa）重力位高度場梯度有關，即梯度較大（小）且環流較強（弱）時，颱風移動速度較快（慢）；而在 30°N 以南則與低層（925hPa）重力位高度場梯度有關，即當季風槽變弱（強），氣壓梯度變大（小）時，颱風的移動速度會變快（慢）。 颱風強度的空間分布上，在長期氣候平均場中，最強處會落在太平洋副熱帶高壓脊與季風槽之間。而年際變化上，強冷年季風槽西退處的颱風強度會減弱；強暖年在季風槽與太平洋副熱帶高壓脊間的颱風強度會增強。並且不論是長期氣候平均或年際變化，主要皆會受到垂直風切場大小的影響，即垂直風切較小時，颱風的強度較容易增強。而颱風強度與海水溫度之間則是颱風強度較強的情形大都發生在海水溫度較高的區域，但不具絕對一對一關係，且在年際變化上兩者的變化會呈反比，代表海水溫度會受到颱風的影響。另外，在長期平均場中還可以看到颱風強度對重力位高度場的影響，即颱風強度的增加，會使中低層重力位高度場減少，高層重力位高度場增加。 最後，颱風移動方向的空間分布上，在長期氣候平均場中，30°N 以南颱風朝西北方移動，在約 30°N 處向北轉後，再轉朝東北方移動。而在年際變化上，強冷年颱風呈現往北轉向者較多，使南北向分量增強；強暖年颱風向西繼續移動者較多，使東西向分量增強。並且不論是長期氣候平均或年際變化，主要皆會受到太平洋副熱帶高壓環流的影響，即太平洋副熱帶高壓較強（弱）且向西延伸（向東退）時，颱風會呈較偏東西向（南北向）的移動。

Typhoon is an important severe weather phenomenon over the Western North Pacific. Through large-scale environmental field of climatic and sea oceanic factors, mechanical and thermal mechanism would have impact on typhoon characteristics. As global warming becomes seriously, large-scale environmental field may also change. This research would discuss about the large-scale environmental field effects for typhoon characteristics during long-term climatology. Also the interannual variations of the large-scale environmental field, especially for typhoon translation speed, would be discussed. And then we also want to get more information to know how the typhoon will change under global warming. According to long-term climatology analysis, typhoon has the slowest translation speed on the monsoon trough, South of 30°N. In the aspect of interannual variance, the typhoon translation speed would be faster on the monsoon trough and would be slower on the subtropical ridge during strong cold years. Process would be opposite during strong warm years. Either long-term climatology or interannual variations, typhoon translation speed may associate with mid-level (500hPa) geopotential height gradient of North of 30°N, and associates with low-level (925hPa) geopotential height gradient of South of 30°N, which means typhoons move faster (slow) when the large (small) gradient. According to long-term climatology analysis, typhoon intensity is strongest among the Pacific subtropical ridge and monsoon trough. In the aspect of interannual variance, the typhoon intensity will weaken with monsoon trough move western during strong cold years, and enhance among the Pacific subtropical ridge and monsoon trough during strong warm years. Either long-term climatology or interannual variations, the main effect of typhoon intensity is vertical wind shear, and also have a relationship between sea temperatures, especially reverse on interannual. So we also know that not only sea temperatures will affect typhoon intensity, but also typhoon intensity will affect the sea temperatures. In addition, strong typhoon intensity will decrease mid-level and low-level geopotential height and increase high-level geopotential height. According to long-term climatology typhoon translation direction is northwestward of South of 30°N, northward around 30°N and northeastward of North of 30°N. In the aspect of interannual variance, typhoon translation direction will be more northward when the subtropical ridge weakens during strong cold years and will be more westward when the subtropical ridge enhances during strong warm years.