Observational Analysis Of Internal Tides Modulated By Mesoscale Eddies In The Northwestern South China Sea

The South China Sea（SCS）is an ocean area with energetic mesoscale eddies and internal tides.The eddies and internal tides both propagate westward along the slope and encounter each other in the northwestern SCS.Revealing the modulation effects of mesoscale eddies on internal tides is of great significance for understanding the evolution and interaction of multi-scale processes.Using the in-situ mooring observations on the continental slope in the northwestern SCS from October 2018 to February 2020,this study analyzes the variations of the frequency,the horizontal kinetic energy（HKE）,the available potential energy（APE）and the energy flux of internal tides in mesoscale eddy processes.The possible mechanism of the modulation effects are discussed.The main results are as follows:（1）The O₁ and M₂ tides dominate the tidal currents at the observation station.There were 9 warm eddies and 4 cold eddies occurred during the observation.According to the eddy vorticity and sea level anomaly（SLA）,the eddy processes are divided into three stages as the approaching,controlling,and leaving,of which 7,9,and 4 cases are found,respectively.（2）The spectral analysis of baroclinic velocity show that the semidiurnal period remains almostly unchanged in eddy processes,and the peak power spectral density gets larger as eddy approaching and leaving（compared to the period without eddy）,and smaller as eddy controlling.For diurnal tide,the frequency has red shift（with period geting longer）in the stage of eddy controlling and leaving,and its peak power spectral density gets smaller in all eddy processes.（3）The variation of the HKE shows that the incoherent diurnal and semidiurnal tides get weaker in eddy controlling stage,while the values in the approaching and leaving stages are larger or close to that without eddy.The HKE of the diurnal（incoherent）tides is larger than that of the semi-diurnal（incoherent）tides in all stages.（4）The mode decomposition shows that the HKE of mode 3 is dominant for diurnal tide,and mode 2 for semidiurnal tide.On the other hand,the APE of mode 1 is dominant both for diurnal and semidiuranl tides.In eddy process,the proportion of mode-4 HKE of diurnal tides decrease as mode 2 increase in eddy controlling stage,while the HKE of semidiurnal tide has larger variation in eddy leaving stage with energy migration from mode4 to mode 3.The APE of both the diurnal and semidiurnal internal tides migrates from high modes to low modes.（5）The diurnal tidal energy fluxes show significant seasonal variability,with larger magnitude and southward direction in autumn and winter.The semidiurnal tidal energy flux varies with period of about a month.In eddy processes,the magnitudes of diurnal tidal energy flux are larger than that without eddy.Meanwhile,the flux in the controlling stage is lower than in the stages of approaching and leaving.For semidiurnal tides,the energy flux also gets larger in eddy processes,while the difference between three eddy stages are not evident.（6）The variation of tidal HKE vs.eddy properties reveals that the internal tides depend less on eddy polarity and intensity,but associated with the distance of the station to the eddy center.In eddy processes,the HKE of dirunal internal tides have larger values at ranges of100-150 km from eddy center,idicating internal tides are enhanced at eddy edge and decreased in eddy center.