| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 95 === 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
|
|---|
