Enhanced mixing by patchy turbulence in the northern South China Sea

Enhanced mixing induced by patchy turbulence in the pycnocline was investigated from the turbulent microstructure profiling in the northern South China Sea, where internal waves are active in the pycnocline. Totally 397 turbulence patches have been identified in March 2014 and August to September 2016 for the statistical analyses. The patch Thorpe scale LTp was found to be larger than the patch Ozmidov scales LOp, i.e., LTp=5.56LOp, indicating that the patchy turbulence is mainly driven by the convection. The mean time scale of turbulent patches is 1.4 × 103 s for conversion from potential energy to turbulent dissipation. The mean mixing efficiency of Γ= 0.23 occurs in the patches, approximately twice as large as Γ= 0.12 in the pycnocline. Hence, the patchy turbulence in the pycnocline enhances the intensity of turbulent mixing and increases the mixing efficiency. Turbulent patches in the pycnocline occur mostly near the critical slope at the shelf break or over the sill in the northern SCS. The investigation of the Eulerian strain and available potential energy of turbulent patches indicates that convection induced by high frequency internal waves should be a major mechanism for the formation of turbulent patches.