| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 103 === 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.
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