利用衛星測高觀測西北太平洋的上層海洋溫度結構

Abstract

近年來有很多研究指出颱風的強度變化是跟上層海洋(0-200m)的溫度結構有密切的關系。但是要在大洋中取得上層海洋溫度結構的資訊是非常困難的，傳統上只可以透過實測的方法。而傳統實測資料的時空分佈並不足以代表整個大洋的變化，對颱風強度的研究與預報造成相當大的阻礙。因此，本研究利用先進的衛星測高技術去估計西北太平洋的上層海洋溫度結構，目的是要去彌補海洋觀測資料的不足。 首先，本研究利用測高衛星的海水面高度資料透過簡單的兩層海洋橂式去計算上層海洋的溫度結構，並利用這些資料去研究西北太平洋的超級颱風Dianmu (2004)的強度變化。發現Dianmu的強度變化對海表面溫度冷卻(SST cooling)和動態上層海洋熱容量(dynamic UOHC)最敏感。在2010年臺灣美國合作的ITOP實驗期間，我們就利用這個兩層海洋模式透過衛星的測高資料去提供每天西北太平洋上層海洋溫度結構的資訊，對整個實驗有相當大的幫助。 由於兩層模式相對簡單，而且並不適用在整個西北太平洋上。所以本研究利用>38,000個Argo實測的海洋溫度剖面去建立西北太平洋的線性回歸方法(REGWNP)，目的是要去改進衛星對上層海洋溫度結構的估計。之後，再利用>7,000個獨立的Argo剖面去驗證REGWNP的估計。驗證的結果顯示REGWNP可以準確地估計西北太平洋的上層海洋溫度結構。在颱風季節，由REGWNP估計出來的20°C等溫線深度(D20)的誤差為30m、26°C等溫線深度(D26)的誤差為20m、UOHC的誤差為20 kJ cm-2、上層100m平圴溫度(T100)的誤差為 1.5°C。另外也發現用REGWNP對比用傳統兩層模式估計出來的上層海洋溫度結構有顯著的改進；同時也非常接近用完整海洋模式計算出來的結果。 最後，研究過去西北太平洋颱風增強區域的海洋條件變化，發現從1993年到2010年之間，海洋暖渦(warm eddy)增加，冷渦(cold eddy)減少。水下溫度變化方面，在過去18年之間D20、D26和UOHC增加了9-17%，而T100增加了0.16-0.35°C。另外，正海水面異常(SSHA ≧ 10 cm)區域增加，負海水面異常(SSHA ≦ -10 cm)區域減少。這個結果顯示西北太平的海洋條件變得越來越有利颱風的增強作用。

This thesis firstly demonstrates the importance of satellite-derived upper ocean thermal structure (UOTS) in typhoon research. Using a simple two-layer model, UOTS can be roughly derived from satellite altimetry and the intensity change of the Supertyphoon Dianmu (2004) is investigated. It is found that Dianmu’s intensity is very sensitive to the during typhoon SST cooling and upper ocean heat content (UOHC). During 2010, UOTS was estimated on a daily basis from satellites for the use in the large international field experiment, Impact of Typhoons on the Ocean in the Pacific (ITOP), showing the advantage of satellite-derived UOTS in typhoon-ocean research. Secondly, using >38,000 Argo temperature profiles, a linear regression method for the western North Pacific (i.e., REGWNP) is developed. Then, >7,000 in situ profiles are used to assess accuracy of REGWNP-derived UOTS. The results show that REGWNP is able to produce rather reliable UOTS. During the typhoon season, the rms difference for the depth of 20°C isotherm (D20), depth of 26°C isotherm (D26), UOHC and averaged temperature of the upper 100 m (T100) is less than 30 m, 20 m, 20 kJ cm-2, and 1.5°C, respectively. Also, it is found that REGWNP outperforms the traditional two-layer approach and is comparable to a sophisticated full ocean model for producing real-time UOTS field. Finally, based on the sea surface height anomaly (SSHA) record between 1993 and 2010, the long-term changes in ocean conditions in the western North Pacific main typhoon intensification region are investigated. It is found that the activity of warm eddies enhanced while cold eddies weakened. In terms of subsurface variability, D20, D26 and UOHC increased by 9-17%, meanwhile, T100 warmed by 0.16-0.35°C over the 18 years period. Furthermore, it is also found that the total area of positive SSHA (≧ 10 cm) features substantially increased while negative SSHA (≦ -10 cm) features deceased. These results suggest that the western North Pacific ocean conditions are becoming more favorable to typhoon intensification.