Distribution Of Environmental Factors And Mixing Of Turbulence Out Of The Yangtze Estuary

The estuary of the Yangtze river and its adjacent sea are the shallowest,nutrient-rich and biodense areas in the east China sea.Because the Yangtze river sends billions of cubic meters of diluted water into the east China sea each year,the area forms a double-layered structure of diluted water and seawater within a few hundred square kilometers of the estuary.This strong layered structure forms a natural"barrier",which on the one hand blocks the transport of dissolved oxygen and chlorophyll from the upper layer to the lower thermocline,and on the other hand blocks the upward transfer of nutrients from the seabed.Therefore,the study of turbulent mixing mechanism and regulating ability has become the core problem to understand the physical and biochemical processes in this area.In the first part of this paper,temperature,salinity,dissolved oxygen and seawater velocity data obtained from 25-hour continuous observation in July 2016 in the Yangtze river estuary were used to study the transport volume of dissolved oxygen across the thermocline in daily period in combination with the simplified vertical one-dimensional material transport model.The results are as follows:?1?this site belongs to the diluted water control area of the Yangtze estuary,with a typical"double-layer"stratification structure.The average thickness of the thermocline is 2-3 meters,and the core position varies up and down depending on the strength of the tidal current.The tidal property is irregular semi-diurnal,and the upper mixing layer is controlled by the high temperature and low salt diluted water of the Yangtze river.The lower mixing layer is relatively low temperature,high salt seawater.?2?the floating frequency square?N²?was between 10^-1.5-10^-4.5s^-22 and the shear energy square?S²?10^-1-10^-5s^-2 within 25 hours.The distribution of Richardson number is similar to the thermohaline structure.It is divided into two parts by the contour line Ri=0.25,and the location of Ri=0.25 is basically consistent with the location of the thermohaline core.Above the boundary,Ri>0.25,the stratification strength is stronger than the shear strength,and the water is in a stable state.Below the boundary,Ri<0.25,shear strength greater than stratification,flow rate will promote the mixing of substances in the water.?3?the turbulence dissipation rate during observation was 10^-4-10^-1w/kg,and the vertical turbulence diffusion rate Kz was10^-4-10^-2m²/s.On this basis,combined with a simplified one-dimensional transport model,the downward transport volume of dissolved oxygen across the saltlayer is calculated to be 4.75 mmol/?m2·d?in daily period,and the reliability of the calculated results in this chapter is discussed by combining with the conclusions of other scholars on sediment oxygen consumption and water oxygen consumption in the Yangtze river estuary.Finally,based on the calculated results,the occurrence process of hypoxia in the Yangtze river estuary is deduced.In the second part,the turbulence data and environmental factor data obtained by turbulence instruments MSS 90 and ADCP were used to study the influencing factors and mechanism of turbulence mixing in estuarine waters under the environment of strong stratification and strong tidal current.The specific research results are as follows:?1?the upper mixing layer is subject to the control of sea surface wind,solar radiation and day and night changes,resulting in different degrees of fluctuations.During the whole observation process,the turbulent dissipation rate of the thermocline is in a low state in all time periods,which inhibits the transport of environmental elements across the layer.?2?the change of tidal current is one of the most important factors causing turbulent mixing in the Yangtze estuary.The dissipation rate of turbulent kinetic energy was>10^-6.5w·kg^-1 during the ebb and flow period.The dissipation rate of turbulent kinetic energy decreases to 10^-7.5w·kg^-1 when the tide stops.It is found from the Ri number variation that the strong turbulent mixing caused by the tidal current controls the whole water body,and the turbulent kinetic energy dissipation rate and turbulent diffusion coefficient both have obvious semi-diurnal changes,and the cycle length is close to the main tidal component?M2 tidal component?frequency of the observation site.?3?in addition to being affected by tidal current,internal wave is another factor affecting turbulence.