呂宋海峽東邊的內潮

Abstract

於呂宋海峽(Luzon Strait)東邊(西北太平洋)海域之錨碇觀測發現，此海域存在有顯著的內潮(Internal tides)運動，14組溫度計於上層500公尺量測之溫度資料經由能譜分析(spectral analysis)顯示，此內潮以全日(diurnal)及半日(semidiurnal)週期為主，其中，又以全日潮較大，錨碇流速資料經旋轉能譜分析(rotary spectral analysis)亦顯示出此海域有顯著的內潮波運動，全日內潮與半日內潮皆主要在東西方向上傳遞，顯示呂宋海峽可能為重要的內潮產生處。經頻帶過濾後之溫度資料顯示，全日內潮造成之溫度變動存在不同樣式(pattern)，於大多數觀測期間，內潮引起的溫度變動隨深度改變明顯存在時間差，而於其餘觀測期間，溫度變動隨深度改變則無明顯時間差存在，前者顯示於垂直方向上明顯存在內潮tidal beam傳遞，因此溫度變動以行進模態結構(propagating mode)為主，後者則顯示於垂直方向上沒有明顯內潮tidal beam傳遞，溫度變動則主要以單一駐模態(standing mode)結構，即正模(normal mode)結構為主。內潮造成之溫度變動存在不同樣式與海水當時之成層效應有關，當內潮tidal beam結構明顯且以行進模態結構存在時，較大浮力頻率值於垂直深度上之所在區間範圍較大且浮力頻率值較小，而當內潮tidal beam結構相對較不明顯且以駐模態結構存在時，浮力頻率於垂直深度上之狹窄區間範圍則明顯存在峰值。由簡單理論計算tidal beam傳遞路徑顯示，海水成層效應之強弱對於內潮tidal beam之傳遞路徑有很大的影響，溫度變動差異可以此tidal beam傳遞路徑的差異解釋，而由iTides數值模式(由美國麻州理工學院非線性動力實驗室提供)模擬全日內潮運動則解釋內潮tidal beam存在的垂直分佈情形。 錨碇溫度觀測顯示於特定時間範圍可以觀察到相對高頻之內波運動存在，此期間之浮力頻率於狹窄深度區間存在峰值；垂直浮力頻率分佈於錨碇觀測期間存在不同變化與觀測區域附近存在數個冷渦旋有關，當冷渦旋存在時會造成浮力頻率於此期間相對較大並於狹窄深度區間存在一浮力頻率峰值；由Gerkema(2001)所做之數值模擬顯示，當海水成層強度適中時，於分層介面將產生相對高頻之內波運動，此理論結果與觀測結果頗為相符，顯示冷渦旋可能會造成浮力頻率於狹窄深度區間存在峰值，可能提供較易內波生成之分層環境。

The observative data from moorings deployed in the east of the Luzon Strait show that internal tides are active in the Northwestern Pacific. Spectral analysis of the temperature data of upper 500m shows that diurnal tides and semidiurnal tides are the two primary kinds of tides. In contrast to semidiurnal tides, diurnal tides are dominant. The rotary spectral analysis of the velocity data shows that both of them are propagating in the East-West direction, and it means that internal tides generated from the Luzon Strait. There are different patterms about temperature variations which induced by diurnal internal tides. In most of the observational time, there are obvious time lags about the variations of temperature between different depths, while in some observational time, there is no time lag about the temperature varibations between different depths. The former means that internal tidal beams propagate in the vertical direction so the propagating mode is dominant, and the latter means that the internal tidal beams do not propagate in the vertical direction so the standing mode is dominant. There are some obvious relationships between the stratification and temperature variations which induced by diurnal internal tides. When there are obvious time lags about the temperature varibations between different depths and the propagating mode is dominant, there is no peak value about buoyancy frequency in a narrow vertical range, on the contrary, when there is no obvious time lag about the variations of temperature between different depths and the standing mode is dominant, there is a peak value about buoyancy frequency in a narrow vertical range. According to the theoretical interpretations and the model simulations, the different patterns about the variations of temperature which induced by diurnal internal tides are caused by the propagating internal tidal beams which was affected by the stratifications of the ocean and the simulations from the iTide model explains the internal tidal beam structure in the vertical direction. There are some high-frequency internal solitary waves can be observed from the data. The high-frequency internal solitary waves occur during the time when bouyance frequency has a peak value in a narrow vertical depth range. It means that the high-frequcncy internal solitary waves appear if the stratification is stronger. The satellite data illustrates that the stronger stratification may be caused by the cyclonic cold eddies near the moorings. The model simulations, Theo Gerkema [2001] show that internal solitary waves only appear when the thermocline is moderated developed. Under the moderated developed thermocline, the high-frequency internal waves will emerge from the interface. As above, the cyclonic cold eddies may make the stratification to be stronger and provide the ‘better’ stratification for generating high-frequency internal solitary waves.