| Summary: | 博士 === 臺灣大學 === 海洋研究所 === 98 === 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.
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