An Investigation Of The Turbulent Mixing Characteristics And Induced Vertical Fluxes Adjacent To The Changjiang Estuary

Turbulent mixing is the key process controlling the materials and momentum exchange between different water masses.And this is even more true near the estuary being important for understanding the biogeochemical cycles.The lack of direct observation limits the understanding of the spatial and temporal variability,driving mechanism of turbulent mixing,and the induced vertical fluxes.Based on the hydrography and microstructure observations from six cruises adjacent to Changjiang estuary,we investigated the temporal and spatial variation of turbulent mixing and influencing mechanisms.The effect of turbulent mixing on the vertical transport of Changjiang diluted water（CDW）is also discussed.Based on the microstructure profiler measurements,the spatial distribution of the turbulent mixing and the related driving mechanisms east of the Changjiang estuary are first discussed.（1）The observed turbulent kinetic energy dissipation rate（ε）generally shows a decreasing trend towards the deeper region.The distribution ofεshows a significant response to the seasonal variation of stratification.Strong turbulence penetrates from bottom to a shallower depth northeast of the Changjiang estuary in spring.While in summer,the upward penetration is largely limited by stratification with largeε(～10^-6 W kg^-1)only occurs at the bottom.At deeper regions southeast of the Changjiang estuary,consistent lowε(～10^-9 W kg^-1)can be found in the interior in different seasons as the turbulence generated at surface and bottom can hardly reach there.（2）By comparing the averagedε(log₁₀<ε>)with the barotropic tidal currents at each station,we show a strong positive correlation between log₁₀<ε>and barotropic tidal currents indicating the dominant role of tidal forcing in inducing the turbulence.The effect of wind forcing only exist in the sea surface boundary layer.In addition to the strong tidal forcing,we found that the internal wave（IW）process plays an important role in resulting in the strong pycnocline turbulence(10^-7<ε<10^-5 W kg^-1).Regarding with the driving mechanism,we found that theεand Ri in the interior layer are negatively correlated when the gradient Richardson number is small.The correlation betweenεand Ri satisfiedε=1.6×10^-8+3.0×10^-7(1+Ri/Ri _cr)^-3.（3）In summer,the plume extends to the northeast and the thickness gradually deepens,the vertical salinity gradient varies from 0 to 2.1 m^-1.Pycnocline gradually weakened resulting in the increase of vertical eddy diffusivity()with the expansion of the plume.As influenced by the eddy diffusivity,the vertical salt flux spanned four orders of magnitude(10^-7～10^-3 m s^-1).Based on microstructure measurements from two repeated sampling stations,the temporal variation characteristics and influencing mechanism of turbulent mixing and vertical salt flux in the CDW are next investigated.Stations（Stns.）E2 and MT1 locates at the south（260 km）and northeast（340 km）of the Changjiang esturay,respectively.（4）The largeε(～10^-6 W kg^-1)vary with tidal stage in the bottom layer at both stations because of the bottom friction of tidal currents.Turbulence generally maintains a low level(～10^-9 W kg^-1)at the pycnocline of Stn.E2.In contrast,intermittent intense pycnocline turbulence(～10^-5 W kg^-1)occurred in the pycnocline at the station south of Jeju Island（Stn.MT1）,along with large vertical displacements of the isopycnals.（5）Taken together with satellite MODIS measurements,these features are further demonstrated to be induced by the internal solitary waves（ISWs）then propagate to the measurement location.The observed ISWs caused shear instability in the pycnocline.After examining the spatial distribution of the internal tide-generating body force,we found that the ISWs was likely to be generated at the shelf break of the East China Sea or the surrounding Jeju Island.（6）The daily-mean vertical diffusive salt flux at Stn.E2is 3.9×10^-7 m s^-1,the daily mean vertical diffusive salt flux at Stn.MT1 can reach 4.3×10^-6 m s^-1.The vertical salt flux can be increased by more than one order of magnitude when strong mixing events occurs,increasing from 2.5×10^-7 m s^-1 to 8.4×10^-6 m s^-1.The presented results highlight the importance of ISWs in influencing the vertical diffusion of CDW and other properties.In conclusion,there is strong turbulence dissipation proportional to the velocity at the surface and bottom layer under the influence of tide and wind.Turbulence generally maintains a low level at the interior layer throughout the year.There is intermittent strong pycnocline turbulence induced by the ISWs.The vertical salt flux spanned four orders of magnitude in the plume.Strong mixing induced by ISWs can cause local salt flux pulses.In this study,turbulent mixing in typical tidally energetic,large estuary are investigated by using direct microstructure observation.The complexity of temporal and spatial variation of turbulent mixing caused by topography is investigated.Future work to understand the properties and roles of ISWs in the area is needed.The parameterization of turbulent salt flux caused by ISWs also needs further study based on remote sensing and field observations.