內潮傳播受地轉流影響之數值研究

Numerical study of internal tide propagation under the influence of a Geostrophic flow

王仲安(Chung-An Wang)

指導教授 ： 詹森

國立臺灣大學/理學院/海洋研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.00218

摘要

本研究利用理想化設置的三維數值模式，模擬內潮從海脊產生以後由東向西傳播受到不同流向的斜壓地轉流之影響。並比較西傳的M2內潮沒有通過地轉流以及通過三種不同地轉流流向的模擬結果：(1)無地轉流(Case M)；(2)地轉流南北走向(Case M0)；(3)地轉流自正北逆時針旋轉22° (Case M22)；(4)地轉流自正北逆時針旋轉45° (Case M45)。而地轉流流量設為10 Sv (1 Sv = 106 m3/s)。
結果顯示地轉流會改變內潮的水平流速結構，且當地轉流的流速分量與內潮傳遞同方向時，因都普勒效應會增加內潮傳播的水平相速度，經過地轉流後的相速度約提升3-7 %。另外從動能與有效位能的分析結果都顯示地轉流會減弱內潮能量的傳遞，其中M45的流向使內潮能量密度衰減的程度最高，約衰減51 %。比較內潮能量通量的改變程度，M、 M0、M22、M45流前後的衰減百分比分別為15.2 %、21.8 %、22.4 %、30.9 %。透過內潮傳播於背景流場的能量收支方程式分析，在海脊地區內潮能量的收支主要由能量通量在空間中的散度與壓力變化做功為主。地轉流區域的能量收支則以內潮與背景流場間的能量交換與背景流場對內潮動能的平流作用和能量通量在空間中的散度為主。

關鍵字

數值模式 ； 內潮 ； 斜壓地轉流 ； 能量 ； 相速度

並列摘要

This study evaluated the effect of the baroclinic geostrophic flow on the westward propagation of semidiurnal M2 internal tides by using a three-dimensional model with idealized settings. The variability of M2 internal tides under the absence of a geostrophic and the influence of the presence of a geostrophic flow with three different flowing directions were investigated. The four numerical experiments are (1) Case M with horizontally homogeneous initial field, (2) Case M0 with a northward geostrophic flow, (3) Case M22 with a geostrophic flow flowing towards 22° counterclockwise from the north, and (4) Case M45 with a geostrophic flow flowing towards 45° counterclockwise from the north. The volume transport of the geostrophic flow is kept at 10 Sv (1 Sv = 106 m3/s).
The results indicate that the velocity structures of internal tide are modified by the geostrophic flow via the modulation of the energy flux distribution. Since the westward velocity component of the geostrophic flow is at the same direction as the propagation of the internal tide, the phase speed of internal tide increase 3-7% after passing through the geostrophic flow due to the Doppler effect. The results of kinetic energy and available potential energy analysis indicate that the geostrophic flow acts as a damper for propagation of internal wave energy. Among all the cases, the decrease of energy density of Case M45 is the profoundest one. Its energy density decreases about 51% by the impediment of the background flow. The variation of M2 tidal cycle averaged, vertical integrated energy flux was also investigated. Due to the difference of both horizontal velocity shear and density gradient in each case, after internal tides passed through the flow area, the energy flux decreases by 15.2% (M0), 21.8% (M0), 22.4% (M22), and 30.9% (M45), respectively.
The energy exchange between the internal tides and background flow was analyzed by using the internal tide energy budget equation. After encountering the background flow, the internal tides tend to transfer energy into the flow through the redistribution of the energy flux. The energy exchange in the geostrophic flow region is governed by the advection of the mean flow, the divergence of internal tide energy flux and the interaction between the internal tide and the mean flow.

並列關鍵字

numerical model ； internal tide ； baroclinic geostrophic flow ； energy ； phase speed.

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