南海北部深水區非線性內波之傳播特性

Abstract

由歷年衛星影像資料中可發現南海北部的非線性內波運動十分活躍，從東邊的呂宋海峽到南海西北邊的大陸棚可以明顯的觀察到非線性內波訊號。關於南海非線性波的研究多集中於南海大陸棚上，對於非線性內波在此深水區的傳遞特性上的研究相對較少。目前，大多數研究的結果皆顯示南海北部非線性內波起源於南海東緣的呂宋海峽區域，但是沒有明確的結論。 為瞭解非線性內波的起源與傳遞時演化的過程，在2005年與2006年的實驗中，在南海北部先後施放五組溫度計串錨碇進行短期的觀測，藉由量測海水等溫線隨時間變化的情形，研究非線性內波在此深水區中傳遞時隨距離演化的情形，由溫度計串錨碇上觀測到最大振幅約140公尺的非線性內波。 本文中利用在此區量測的溫鹽深儀 (CTD) 資料，解其特徵方程式計算此區垂直速度隨深度分佈情形、內波傳遞的線性內波相速度以及各項參數，將求得的參數進一步代入KdV方程式中，模擬此深水區中非線性內波傳遞時隨距離演化的情形，並將模擬結果與現場溫度計串錨碇所記錄到的波形變化情形做比較，就波包中波的數量在相對時間與距離的演化情形上，模擬結果與錨碇觀測資料相當吻合。由模擬結果與錨碇資料兩者間的相對距離推算，推測南海北部的非線性內波的起源可能在距離FB2測站 (20.4N, 120E) 以東約100多公里的位置，即121E附近。 非線性內波在深水區傳遞時，水平空間上波前的演化特性也會顯示在衛星影像中。結合衛星合成孔徑雷達影像 (SAR) 與衛星光學影像 (MODIS)，經由二階線性擬合方式，擬合出完整的波前形狀，再推算波前的形狀及波前隨時間移動的情形，再推算非線性內波傳遞的相速度，利用此方式所估算出此區非線性內波傳遞的平均速度約為2.76(m/s)。

From satellite images over the years, one can find that nonlinear internal waves (NLIWs) are very active in the northern South China Sea (SCS), from Luzon Strait in the east to the continental shelf at northwest of SCS (Liu and Hsu, 2004). But, most of the research of NLIW concentrate over the shelf of SCS, not much on the propagation of NLIW through the deep water region of SCS. So far, most the research suggest that the source of NLIW is in the Luzon Strait, but, there is definitive conclusion on the exact location of the source regions. To understand the generation, propagation and evolution process of NLIW in SCS, we deployed 5 T-strings along 20.4N, between 119E & 121E, in 2005 and 2006 field experiment, to observe the response of isotherms during the passage of NLIW at different locations, and to infer the propagation and evolution of NLIW. Based on the mode theory of linear internal wave, and the collected CTD data, one may derive various parameters to simulate the propagation and evolution of NLIW vs. time and distance. We found that the predicted evolution of NLIW agrees quite well with the mooring observed evolution of NLIW at all locations, and we may conclude that the generation site of NLIW is near 121E. . Satellite images can also show the propagation and evolution of these NLIWs in the deep zone of SCS. Second order regression equation was used to simulate the wave front as seen in satellite radar images from SAR and optical images from MODIS. The displacement of these wave fronts over time, permit us to estimate the NLIW propagation speed to be 2.76 (m/s).