北南海陸坡海域之第二斜壓模內孤立波

Abstract

一組分別掛載溫度計串和都普勒流剖儀並施放於北南海陸坡區的錨錠，由其溫度和流速資料發現了為數不少的第二斜壓模之內孤立波。這種形態的內孤立波在自然界中很少被觀測到，一般而言，第二斜壓模之內孤立波會造成等溫面在上部(下部)水層往上(下)的位移。經分析，在上部和下部水層相對應的等溫面位移分別是33±4 m和28±4 m。特徵時間尺度大約是18分鐘而其最大振幅可達80公尺。波大多往西傳遞且經常在小潮的時候出現。在夏天，第二斜壓模之內孤立波的出現與全日潮有著相位上的一致，約24小時。在冬天，第二斜壓模之內孤立波則是隨機且經常性地出現。這種季節上的差異性可能和垂直分層的變化有關。分析冬夏兩季分層的情況可以發現，斜溫層在冬天時比較深。因此，可視為有一層溫度變化的薄層位於上下兩層溫度均的水層之間。這種密度垂直結構近似於雙曲正切函數，理論上可以求得一個在斜溫層傳遞且形狀不變的第二斜壓模之內孤立波的解析解 (Benjamin, 1967 和 Davis and Acrivos, 1967, 等人)。楊(2007)利用三層海洋推導出第二斜壓模之內孤立波的解析解，其結果推論只要中間層為薄層的時候，第二斜壓模之內孤立波就有比較高的機率存在。因此，冬天時第二斜壓模之內孤立波的生成可能與斜溫層比較深有關。然而，夏天時第二斜壓模之內孤立波的生成機制是不相同的，未來仍需要進一步的研究。 關鍵字：內波，孤立波，第二斜壓模，南海。

A mooring located on the shelf break of northern South China Sea (SCS) measured temperature and current velocity data by thermistor chain and an Acoustic Doppler Current Profiler (ADCP), respectively. Both temperature and current velocity records revealed a number of signals of second baroclinic mode (mode-2) internal solitary wave (ISW), which are seldom observed in nature. Typically, the mode-2 ISWs show the upward/downward displacement of isotherms in the upper/lower water column, respectively. Accordingly, the displacement of isotherms induced by mode-2 ISWs are 33±4 m and 28±4 m in the upper and lower water column, respectively. The characteristic time scale is about 18 minutes as well as the maximum displacement of isotherms is ~80 m. The westward propagating mode-2 ISWs emerge generally during the neap tide period. In summer, mode-2 ISWs could have phase-locked with the diurnal tide about 24 hours. In winter, mode-2 ISWs emerge randomly but frequently. The seasonal difference could be associated with the vertical stratification. The thermocline is deeper in winter. Therefore, the water column has one thin layer in middle with temperature varied and two thick layers having near uniform temperature above and below it, respectively. This thermal structure is approximate to a hyperbolic tangent profile, which allows theoretically that a mode-2 ISW propagates along the thermocline with permanent form (Benjamin, 1967 and Davis and Acrivos, 1967, etc). Yang (2007, personal communication) derives a mode-2 ISW analytical solution based on the 3-layer ocean. The solution infers that a high possibility of existence of mode-2 ISWs as long as the middle layer is thin. Therefore, the generation of mode-2 ISW in winter may correlate with the deeper thermocline. Nonetheless, the generating mechanism of mode-2 ISW in summer is different, future studies are needed. Key words: Internal wave, Solitary wave, mode-2, South China Sea