運用內潮模式、衛星影像及現場觀測預測北南海內孤立波的行進與抵達時間

Abstract

內孤立波(Internal Solitary Waves, ISWs)是全球邊緣海域常見的海洋物理現象，對於海洋環境和生態系統有重要影響。南海(South China Sea, SCS)北部因其強烈的ISWs活動而受到矚目，水平與垂直流速分別可達2 m s-1和0.5 m s-1，振幅更可達100至200 m，因此準確預測其行進與抵達時間相當重要。過去研究指出ISWs的產生與內潮有關，這些內潮是由呂宋海峽南北走向的兩座海脊與正壓潮流作用所產生，內潮進入南海海盆後因非線性頻散(nonlinear dispersion)而演化成ISWs，故內潮預報模式有可能用於預測ISWs，但須釐清內潮訊號與ISWs訊號間的關係。本研究使用MIOST-IT模式預報內潮造成的海表面高度(ITSSH)變化，將其與錨碇觀測、船測以及Himawari-8 (H8)衛星影像所觀測的ISWs事件進行比對。結果發現遙測15次及實測39次中的26次ISWs事件主要發生於ITSSH值由負轉正的零交叉點(zero-crossing point)附近，實測中另外13次ISWs事件因孕育它的內潮受地形影響，其ITSSH值為正值的訊號減弱或產生其他干擾訊號，但後續採用僅有M2分潮的預報結果，則發現所有ISWs事件的主波皆位在ITSSH值負轉正零交叉點附近。ITSSH與其水下內潮造成的等密度面位移(isopycnal displacement)有反相位關係，故推論ISWs是對應內潮的波峰至波谷間的零交叉點附近，誤差約為1.5小時。此對應可運用ISWs的成因與理論行進速度予以解釋，依據過去數值模式研究顯示下沉型內孤立波初始是在內潮的波谷處成形，但因ISWs的行進速度包含線性波速與非線性波速兩分量，假設線性波速與孕育它的內潮的波速等量，加上非線性波速則使得ISWs超前內潮波谷往零交叉點方向行進。

Internal Solitary Waves (ISWs) are ubiquitous in the coastal marginal seas of the world’s oceans, playing a crucial role in shaping marine environments and ecosystems. The northern South China Sea (SCS) has garnered particular attention, as ISWs exhibit horizontal and vertical velocities reaching 2 m s-1 and 0.5 m s-1, respectively, with an amplitude of 100-200 m. Therefore, it is crucial to predict their propagation and arrival time. ISWs evolve from the nonlinear dispersion of internal tides generated by the interaction between barotropic tidal flows and the ridges of the Luzon Strait. The mechanism sheds light on their predictability using an internal tide prediction model. Here, the internal tide- induced sea surface height (ITSSH) predicted from the MIOST-IT model is used to relate with the ISWs detected by the mooring and shipboard measurements and Himawari-8 (H8) satellite images. It is encouraging to reveal that 15 and 26 of 39 ISW events observed by satellite and in situ measurements are primarily located near the transition points where the ITSSH transits from a negative to a positive value. The remaining 13 ISW events observed by in situ measurements, respectively, are associated with their parent internal tides, influenced by topographic, resulting in weakened or disturbed ITSSH signals with positive values. However, using the prediction result of M2 tide, all of the leading waves are located near the zero-crossing points of the ITSSH from negative to positive. As the out of phase relationship between isopycnal displacement of an internal tide and its surface, this suggests that ISWs are located at the zero-crossing point from crest to trough of the internal tide, with an uncertainty of ~1.5 hours. This is plausible because ISWs have an additional nonlinear wave speed compared to their parent internal tides. Once the depressed ISWs are initially formed in the trough of the internal tide, the nonlinear wave speed causes the ISWs to propagate toward the zero-crossing point from crest to trough of the internal tide.