南海大陸斜坡上沙丘海域之內孤立波特性

Abstract

本研究使用現場錨碇觀測及衛星影像資料研究內孤立波經過海床巨大沙丘後的能量消散變化。北南海是一個內孤立波活動旺盛區域，曾經有振幅大於150 m的觀測紀錄。這些內孤立波因潮流經過呂宋海峽時受水下海脊影響而產生，向西北傳遞、橫越北南海到達大陸坡。大陸坡上有些區域擁有一些巨大沙丘，根據多波束測深儀的海底地形調查，沙丘高度大約10 m ~ 20 m，相距約300 m，這些沙丘會影響到內孤立波的性質。本研究主要使用了2014年6月3至19日在此區域5串錨碇的現場觀測資料，並配合衛星影像資料、海底地形資料及大氣模式資料，以深入瞭解內孤立波的能量變化情形。每一錨碇串上包含了數量不等的海流儀和溫壓儀，資料長度包含了一個大小潮週期。錨碇放置在深度380 m ~ 260 m的區域內，連成的測線長度大約13 km且幾乎垂直於等深線。在此假設海水鹽度均為34.5 psu後，可以由溫度、壓力及鹽度計算出海水密度。在內孤立波經過錨碇時，可以由等密線垂直方向上的變化找出內孤立波的振幅。流速資料部分，ADCP在測量海流時，會因為儀器限制產生音束擴散效應影響資料品質，利用Chang et al. (2011)的校正方法並進加以改良後則可以校正音束擴散效應。結合海流垂直剖面資料並利用 DJL (Dubreil-Jacotin-Long Equation)和連續方程式後，可以去除內孤立波的背景流場資料，並估算出內孤立波的波向、瞬時波速和每一內孤立波於不同位置的動能、位能及能量消散變化。此外，利用Thorpe scale方法可以由下放溫深鹽儀的資料求得紊流動能消散率。內孤立波在水深較趨緩的386 m至342 m區間，總能下降了約60%，平均能量消散為21.8 W m-2，平均能量消散率為約6.2×〖10〗^(-5) W kg-1。而在實驗位置中地形最陡峭的342 m至262 m區間，總能則無明顯的變化。在此巨大沙丘的大陸斜坡上的能量消散約為北南海東沙島北方100 km、水深426 m至103 m處的600倍，推測可能和深度淺化、海底沙丘有關。而內孤立波的紊流動能消散率主要在內孤立波的後半段最為明顯，可達6×〖10〗^(-5) W kg-1，而在內孤立波經過40分鐘後紊流動能消散率很小。

In the northern South China Sea, the internal solitary wave (ISW) is extremely active and has the largest amplitude (> 150 m) in the world. The ISWs generate from tidal forcing on the ridges in the Luzon Strait, propagate northwestward, and traveling across the deep basin to the continental shelf. There are a few very large subaqueous sand dunes distribute on the upper continental slope and the ISWs could passage these sand dunes. According to the multi-beam bathymetry survey, the amplitude of sand dunes were about 10 m and crest-to-crest wavelength were about 300 m. These sand dunes could influenced the aspects of ISW. Five temperature and current velocity moorings deployed in the sand-wave area on the upper continental slope in June 3-19, 2014, which was contain a spring/neap tide period in order to investigate the properties of ISWs in this area. The transect of these moorings was almost parallel to the slope and the length of mooring array was about 13 km from local depth 380 m to 260 m. In traditional method, the average phase speed of ISWs could be estimated by the arrival time and the distance between different moorings. However, the directions of the ISWs were not parallel to the mooring transect, and therefore the phase speed estimated from arrival time could overestimated. Based on the DJL theory, using the temperature and current profiles observation data to estimate properties of ISWs including vertical displacement, instantaneous phase speed, direction, and energy flux. Moreover, based on Thorpe scale, the turbulence kinetic energy dissipation rates are estimated from shipboard CTD data by Thorpe scale. The total energy of ISWs reduce about 60 % and the average dissipation reach about 21.8 W/m2 and the average dissipation rate is about 6.2×〖10〗^(-5) W kg-1 in water depth from 286 m to 342 m which is relatively flat. However, the total energy of ISWs change insignificantly in water depth from 342 m to 262 m which is the steepest part of experimental area. The dissipation rate in the sand-wave continental slope is about 600 times as Dongsha plateau. It may because of the shoaling and the large subaqueous sand dunes. Moreover, the peak turbulence kinetic energy dissipation rate occur at the trailing edge of the apparent core, with the value of 6×〖10〗^(-5). Forty minutes later, the turbulence kinetic energy dissipation rate was small.