Turning depths of internal tides in the South China Sea inferred from profile data

Theoretically,propagating internal tides in the ocean may reflect at turning depths,where buoyancy frequencies equal tidal frequencies,before colliding with the air-sea interface or rugged bottom topography.Globally,the internal tide lower turning depths(ITLTDs)in the open ocean have been mapped;however,knowledge of the presence of ITLTDs in the South China Sea(SCS)is lacking.In this study,2125 high-quality temperature-salinity profiles(including 58 deep-sea hydrographic measurements with observational depths exceeding 3000 m)are collected and analyzed to investigate the existence of ITLTDs in the SCS.Furthermore,the concept of the upper turning depth is first introduced in the context of internal tides,and internal tide upper turning depths(ITUTDs)are also investigated.ITLTDs are found to exist at several abyssal stations;these stations are distributed mostly in the southern part of the SCS basin,possibly due to the greater water depths there.Fewer locations show the presence of ITLTDs for K_(1) versus M_(2) tidal frequencies because of the lower tidal frequency.The distance between ITLTDs and the seafloor ranged from 270 m to more than 1200 m,implying the possible existence of multiple internal wave evanescent regions in the abyssal bottom.ITUTDs of tens of meters are ubiquitous in the SCS;stations with the presence of ITUTDs are located mainly in the northeastern SCS due to the intensive observations there.However,the calculated ITUTDs have large uncertainties;they are sensitive to the selected bin values.The horizontal propagation directions of internal tides in the SCS change dramatically,and as a result,the estimated turning depths under the full Coriolis force definition are different compared to that under the traditional approximation.