南海中部全日內潮次諧波之研究

Abstract

本研究利用在太平島北部近東西向水道所觀測到的長期溫度與海流資料，研究週期約為全日潮週期兩倍的全日內潮次諧波運動。 觀測資料顯示四個主要特徵:第一、在深度-頻譜圖中，在潮汐頻率和近慣性頻率之間有一峰值，且非當地中尺度渦漩所造成；第二、頻帶濾波分析顯示全日內潮頻率一半的東西向海流和海溫在颱風雷伊經過後有顯著的變化，並且東西向海流垂直結構呈雙向傳播且高模態結構；第三、在颱風經過後，全日內潮次諧波深度積分動能的最大值與全日內潮變化相互對應，推測該次諧波增強可能和颱風增強當地全日內潮有關。第四、雙頻譜分析(bispectrum)證實在颱風經過後有非線性效應存在，推測與全日內潮參數次諧波不穩定(PSI)運動有關，使得全日內潮傳遞能量至其次諧波，增強颱風後全日內潮次諧波運動。 綜上所述，本研究觀測到頻率為全日內潮頻率一半的次諧波運動於颱風經過後增強，推測與颱風增強當地全日內潮以及全日內潮PSI運動有關。但本研究僅一觀測位置，後續須更多觀測與較大範圍的數值模式研究，才可了解颱風過後如何在該海域透過PSI增強全日內潮次諧波運動促進海水混合的整體動力機制。

This study investigates the subharmonic motions of diurnal internal tides, with periods approximately twice that of the diurnal internal tides, using long-term temperature and current data observed in the east-west oriented channel north of Taiping Island. The observed data revealed four main characteristics. First, the depth-frequency map showed a peak between tidal and near-inertial frequency, which was not caused by local mesoscale eddies. Second, bandpass analysis revealed significant changes in temperature and east-westward currents at half the diurnal internal tide frequency after the passage of Typhoon Rai. The vertical structure of east-westward currents displayed a bidirectional propagation and high-mode pattern. Third, the peaks of subharmonic motions in depth-integrated kinetic energy coincided with peaks of diurnal internal tides after the typhoon, suggesting the enhancement of subharmonic motions might be related to typhoon-induced local diurnal internal tides. Last, bispectrum analysis confirmed the presence of nonlinear interactions after the typhoon, suggesting the involvement of parametric subharmonic instability (PSI), which transfers energy from diurnal internal tides to its subharmonics, enhancing the subharmonic motions following the typhoon. In conclusion, this study observed an enhancement of subharmonic motions at half of diurnal internal tide frequency after typhoon, might be related to the typhoon-induced intensification of local diurnal internal tides and PSI of the diurnal internal tides. However, due to the limitation of a single-station observation, further observations and modelling work is needed to understand the complete dynamical mechanism through which PSI enhances subharmonic motions to facilitate ocean mixing in the region after typhoon.