Mixing Processes Observation On Continental Shelf And Slope Of The Northern South China Sea

On the continental shelf and slope of the northern South China Sea, the topography changes sharply, and the mixing would be very intensive. The special hydrological structure formed by the SCS subsurface water and the SCS intermediate water causes the salt fingering in a certain depth layer, and this would make the local mixing processes more complex. In order to understand the inner mixing processes objectively in this region, an experiment was performed with TurboMap-II ( Turbulence Ocean Microstructure Acquisition Profiler)on board the R/V No.2 Yanping from 20 April to 5 May, 2004, and this was the first time to do the micro-scale observations in SCS.After the data analysis and some calculation, the dissipation rate of TKE, the distribution of the dissipation rate of thermal variance and the diffusivity of heat have been estimated, the salt-finger's influence on the mixing efficiency was also been studied. The results show that the average value of the dissipation rate of TKE and thermal variance on the continental shelf are both one order larger than those on the slope, so the mixing on the shelf was more intensive; in addition, the mixing in the surface layer(less than 30m) is more intensive than in the middle or deeper layers. The vertical distribution of the dissipation rate of thermal variance shows the"low-high-low"character with the largest value in the thermocline. In the doubly stable region, the diffusivity of density calculated with the Osborn(1980) model is 6. 73×10^-4m²s^-1, and the diffusivity of heat get using the Osborn-Cox(1972) model is 2. 52×10^-4m²s^-1, the rough equivalence of them indicates that the mixing processes in the doubly stable region is almost dominated by turbulence. Besides, the heat flux, salt flux and buoyancy flux were estimated to analyze the differences between doubly stable region and finger favorable region. In the doubly stable region, the direction of heat flux and salt flux are opposite, but the direction of buoyancy flux and salt flux are both upward. In the finger favorable region, the directions of heat flux, slat flux and buoyancy flux are all downwards, furthermore, the salt flux is lager than the heat flux evidently.The statistical distribution of the dissipation rate of TKE in the finger favorable region shows the bimodal structure, the reason is that both the turbulence and salt finger mechanism contribute to the mixing processes; furthermore, it's more evident when the strength of salt fingering is comparatively strong. For the mixing efficiency, the values on the shelf are larger than that on the slope. In the section, its value is 0.18 in the doubly stable region, and this is very close to 0.2 which was used widely in the ocean mixing study; but in the finger favorable region, its mean value is 0.81, and it's also consistent with the former results get from experiments in the laboratory or observations in the ocean. On the other hand, when Ri is less than 1, the mixing efficiency is close to 0.2, especially in the doubly stable regions; but when Ri is larger than 1, it increases with Ri .