Estimates of mixing on the South China Sea shelf

Mooring observations aimed at understanding the vertical mixing were carried ont on the outer shelf of the South China Sea from April to May in 2002. Temporal and vertical distributions of horizontal velocity shear and Brunt-Vaisala frequencies are calculated with these observations. Dissipation rate and diapycnal diffusivity are then inferred from the fine-scale parameterization. The temporally and vertically averaged dissipation is 15 nW/kg and the associated diapycnal diffusivity is 2×10-5 m2/s. Daily-averaged diapycnal diffusivity is well related to the tides, larger during the spring tide, and smaller during the neap tide. Depth-averaged diapycnal diffusivity, which is as larger as 5×l0-5 m2/s during the spring tide, is 8.3 times that of the neap tide, which is only 6×l0-5 m2/s. This is in proportion to the vertical energy flux from barotropic tide to baroclinic tide. During the spring tide, the energy flux from the semi-diurnal and diurnal barotropic tide to the internal tide is 160 mW/m, while it is only 35 mW/m during the neap tide. Vertically, monthly-averaged dissipation rate and associated diapycnal diffusivity are large near the upper mixing layer and the bottom boundary. Dissipation rate is about 30～100 nW/kg, diapycnal diffusivity is about 4×10-5～10×l0-5 m2/s. However, both of them are quite small in the mid column, where dissipation rate is 3～10 nW/kg and diapycnal diffusivity is 4× 10-640× 10-6 m2/s.