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標題: | 呂宋海峽內之黑潮不穩定:地形與分層效應 Instability of the Kuroshio in Luzon Strait: Effects of Bottom Topography and Stratification |
作者: | Jen-Hua Tai 戴仁華 |
指導教授: | 唐存勇(Tswen-Yung Tang) |
關鍵字: | 黑潮鋒面,不穩定波,呂宋海峽,地形效應,海水分層, Kuroshio front,Instabilities,Luzon Strait,Topographic effect,Stratification, |
出版年 : | 2010 |
學位: | 博士 |
摘要: | 利用施放於呂宋海峽中長期(1997-1999)觀測的三組錨碇海流資料,探討黑潮於呂宋海峽內的變動。經由分析錨碇的海流資料,顯示呂宋海峽中的流速存在5天週期的震盪;此5天週期震盪具有鋒面不穩定波的特性,即其訊號會向北傳遞(向黑潮下游),且其振幅會隨時間增長。經由曲線擬合(Curve fitting)與頻率域經驗正交函數(Frequency-domain Empirical Orthogonal Functions),顯示此5天週期的不穩定波的特性,其波速約為32至40 cm s-1、波長約為130至150公里、e-folding 時間尺度約為0.8至3天。
以線性兩層解析模式探討呂宋海峽內黑潮鋒面不穩定的特徵,模式結果顯示,呂宋海峽中兩個海脊的存在,為影響黑潮鋒面不穩定波特徵的一個重要因子。當地形加入兩層模式,由於垂直流速流切增強,促使不穩定波的波長向短波偏,此時,模式計算之最不穩定波其週期為4.79天、波長為148公里、其e-folding時間尺度約為9天。兩層解析模式結果,其最不穩定波的波長與週期與觀測結果相近;然而,其e-folding時間尺度較觀測值大。 由於呂宋海峽內的海水,具有強分層且其流速結構複雜,故兩層模式的假設不足以解析最不穩定波的增長率。因此,進一步以數值模式(spectral numerical model),探討較接近實際海水垂直分層與流速結構,對於呂宋海峽內黑潮不穩定波的影響。模式結果與兩層模式相似,但其e-folding時間尺度較小,約為3天。綜合觀測與模式分析結果,本文推論,呂宋海峽內5天週期的海流流速變化可能為黑潮鋒面不穩定所產生之鋒面不穩定波所致。 進一步以兩層模式探討黑潮鋒面相對於海脊的位置、黑潮鋒面的坡度與兩層海水密度差對呂宋海峽內黑潮鋒面不穩定波特性的影響。結果顯示,黑潮不穩定波的特性較受鋒面與海脊相對位置的影響;當鋒面越靠近海脊,則不穩定波波長變短;反之,則波長變長。 Observations of flows in Luzon Strait are used to examine variability of the Kuroshio in this region. Three sets of current velocity mooring were deployed in the central portion of Luzon Strait during 1997 – 1999. The spectral analyses of moored current velocities in the central Luzon Strait reveal northward (i.e. downstream of the Kuroshio) propagation of a frontal wave with a 5-day period, with wave amplitude increasing northward. Estimated from both curve fitting and frequency domain Empirical Orthogonal Function methods, the characteristics of 5-day variations have wave speeds ranging from 32 to 40 cm s-1, wave lengths ranging from 130 to 150 km, and e-folding time scales for growth ranging from 0.8 to 3 days. An analytical two-layer model is used to explore the linear stability characteristics of the Kuroshio in Luzon Strait. The result indicates that the bottom topography (two meridional ridges) is important for the stability characteristics of the Kuroshio in Luzon Strait. In the two-layer model with two ridges, the flow is stabilized for the long-wave mode but destabilized for the short-wave mode (due to increasing vertical shear in the horizontal velocity). The most unstable waves from the analytical model has an e-folding time scale of 9 days, a period of 4.79 days, a phase speed of 35.85 cm s-1 downstream of the Kuroshio, and a wavelength of 148 km. The analytical model produces wavelengths and phase speeds for the most unstable mode which is similar to the observations, but the growth rate is underestimated. Because the water in Luzon Strait are strongly stratified and the current velocity structure is more complicated, the two-layer system assumption is not sufficient for resolving the fastest growth rates of the unstable waves. Therefore, a spectral numerical model was applied with more realistic stratification and velocity structure. The model outputs are similar with two-layer model, but a more rapid instability with an e-folding time scale of three days. From the results of both observations and stability model outputs, the conclusion is made that the 5-day variation of current velocity could be caused by the instability of the Kuroshio front in Luzon Strait. Parameter sensitivity tests were conducted using the analytical model. The characteristics of the most unstable mode are most sensitive to the surface front location relate to bottom topography. The characteristics of the most unstable mode are relatively sensitive to varying the density difference and thickness of the upper layer. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/46140 |
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顯示於系所單位: | 海洋研究所 |
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