Turbulence And Mixing In Tidally Energetic Shelf Seas

The shelf seas have an importance that is out of proportion to the relativelysmall fraction of the area of the global ocean they occupy (～8%).It is area ofintense physical and biological activity.In shelf seas,turbulent mixing is central tomany physical and biological processes,as it is often the key determinant of watercolumn structure and nutrient fluxes,and hence the rate of primary production,and also the settling,resuspension,aggregation and disaggregation of particulatematter.In this dissertation,turbulence and mixing in typical tidally energeticshelf seas are investigated by using direct measurements of turbulence parametersin the Yellow Sea and Clyde Sea.By analyzing the characteristics of turbulence at two comparative stations(the fows are reversing and rotating tidal currents,respectively) in the YellowSea,it is found that the reversing and rotating tidal flows affect small-scale near-bottom dynamics differently.In reversing flows,the near-bottom shear and theshear at the upper levels are almost in phase,while in rotating flows,the shear gen-crated near the scafloor propagates slowly to the water interior.The log-layer andskin-layer estimates of the bottom friction velocity show close correspondence forthe reversing tidal flows,but when the tidal vector rotates over a sloping bottomthe log-layer estimate is approximately twice of the skin-layer one.The rotation,form drag due to local topographic inhomogeneities,and weak but appreciablestratification are suggested to be possible sources for this discrepancy.The clas-sical wall-layer parameterization of the turbulent dissipation rate is found to holdwell for reversing flows,while modifications are needed for rotating flows.Therelationship between the turbulent kinetic energy and its dissipation rate,whichis widely used to parameterize the dissipation rate in turbulence closure models,is found to hold well for both reversing and rotating flows,but with differentcoefficients.Microstructure profiling measurements at two comparative stations (a deepercentral basin and a local shelf break) in the stratified Yellow Sea are analyzed,with emphasis on tidal and internal-wave induced turbulence near the bottomand in the pyenocline.The water column has a distinct three-layer thermohaline structure,consisting of weakly stratified surface and bottom boundary layers anda narrow sharp pycnocline.Turbulence in the surface layer is controlled by thediurnal cycle of buoyancy flux and wind forcing at the sea surface.while thebottom stress induced by barotropic tidal eurrents dominates turbulence in thebottom boundary layer.The maximum level at which the tidally enhanced mixingcan affect generally depends on the magnitude of the tidal current,and it canbe up to 10-15 m in the Yellow Sea.This suggests that,in the deeper regionsof the shelf seas,turbulent dissipation and mixing are very weak at the levelsbetween the near-bottom tidally enhanced layer and the pycnocline.Therefore,these levels provide a significant bottle neck for the vertical exchanges.In theshallow regions,however,the tidally-induced turbulence can occupy the wholewater colum below the pycnocline.A quarter-diurnal periodicities of the turbulentdissipation rate and eddy diffusivity are found at different heights with evidenttime lag.In the relatively flat central basin,the pycnocline is essentially non-turbulent and internal-wave activity is very weak.Therefore,vertical fluxes acrossthe pycnocline decreased to molecular levels.In contrast,internal waves of variousperiods can be always found near the local shelf break.Particularly,on the passageof internal solitary waves,turbulence in pycnocline can be increased by orders.The linear theory of dynamic instability is used to investigate the stability ofbaroclinic tidal flows in shelf seas.It is found that the flows are not generally ina state of marginally stable,but either very unstable or very stable.Although forsome flows the critical gradient Richardson number is close to the Miles-Howardlimit of 0.25,for others it is substantially less.The e-folding period of the mostunstable disturbances is found to be generally close to the buoyancy period at thelevels where the disturbance has the largest amplitude.This suggests that thegrowth rate of small disturbances and the consequent turbulence in the flow arelargely under a physical control of the st ratification.A mechanistic link betweenthe turbulent dissipation and the instability of the flow is reveale d,based on whicha new paramerization formula for turbulent dissipation is devised.