The Turbulent Mixing And Double Diffusion In Yellow Sea And East China Sea And Their Seasonal Variation

Hydrological and microstructure data in Yellow Sea (YS) and East China Sea (ECS) are used to analyze spatial distribution and seasonal variation of local turbulent mixing and double diffusion phenomenon. The result indicates turbulent mixing on the continental shelf of YS and ECS has a strong relationship with local circulation and water mass. The interaction between different water masses may be favorable to the occurrence of double diffusion.For the research on turbulent mixing, the observed area can be divided into three parts from north to south. The observation in YS area shows there is a weak vertical circulation in the Yellow Sea Cold Water Mass (YSCWM), which causes the enhance mixing, whereas Yellow Sea Warm Current (YSWC) is a stable stratified current in which the mixing is quite weak. Only in the boundary of the warm current exists the enhanced mixing due to the shear instability. The depth of upper mixed layer can be restrained by the current in winter. Comparing upper layer mixing in summer and winter, we may see very strong turbulent dissipation occurs in the upper 15 meters and the strength between these two seasons is very close. This may be because of wind-induced turbulent dissipation and surface wave breaking. The more input wind energy in winter goes down to deepen the mixed layer. The mixing in the area of Changjiang Diluted Water (CDW) is mainly on the surface and bottom. Matsuno suggested the dissipation rate and diffusivity are corresponding to the contour of salinity, but it is not so clear in our observation. There the strong mixing in the shelf bottom is resulted from internal wave break due to small-scaled bottom roughness. The section-averaged diffusivity in winter is 100 times as that in summer. The Taiwan Warm Current (TWC) area has an opposite characteristic to Yellow Sea Warm Current in summer. The mixing in the warm current is higher than surrounding water. In winter, the TWC is similar with YSWC but the core of the current is shallower, which causes enhanced mixing in the surface layer by wind. The TWC and Kuroshio intrude each other in winter. The enhanced velocity shear and diffusivity are in the vertical common boundary between water masses; however, the dynamical mechanism is still ambiguous. The shelf break in ECS is an area that tends to induce internal tide, so the enhanced mixing here is mostly originated from the internal wave break during the propagation. The high diffusivity is corresponding to the ray of M2 internal tide. The magnitude in winter is about one order higher than in summer and the mixing can reach up to 100m in winter.The double diffusion phenomenon in YS and ECS occurs in winter but not in summer. The salt finger was observed in the shelf break. Kuroshio water with high temperature and salinity can intrude onto the surface of ECS shelf, which is responsible to the formation of salt finger. The diffusive convection exists in the YSWC. The warmer and saltier YSWC may extend aside in the bottom, which is favorable to the diffusive convection. The other area with diffusive convection is located in the CDW plume and Zhe-Min coastal current. Both of them come from cold and fresh Changjiang water, therefore they have the same mechanism on the formation of diffusive convection. Affected by atmospheric condition and radiation, Changjiang water can favor diffusive convection in winter. With further exploring the temperature microstructure in the sites with salt finger and diffusive convection, it shows salt finger can increase the thermal diffusivity over the eddy diffusivity but diffusive convection cannot. The reason for this is the difference of vertical temperature and salinity structure between them.Wind-generated internal wave can be calculated with the equations in the mixed layer and lower layer. The result shows the wind inputs energy into the ocean with the near inertial frequency, the velocity of water particle also has the near inertial frequency. The estimated kinetic energy is about 9.63×10-3J/kg in summer and 9.99×10-3J/kg in winter. The turbulent kinetic dissipation rate is 3.72×10-1J/kg in summer and 3.96×10-1J/kg in winter. The ratio between dissipation and input in summer and winter are very close, about 38%-40%.