Characterization of vertical mixing at a tidal-front on Georges Bank

Studies of mixing were done at the northern flank of Georges Bank in the summer and autumn of 1988. Two time-series of the evolution and intensity of microstructure were examined over a tidal period in the context of tidal forcing and the evolution of the density and velocity field at the site. From the CTD, ADCP and microstructure observations (EPSONDE) on Georges Bank, several interesting features of the mixing processes were found. High dissipation and diffusivity regions appear near the bottom of the Bank. Turbulence near the bottom is highest in intensity and reaches farthest from the bottom at peak tidal flow and diminishes in intensity and vertical extent as the flow decreases. The thickness of the bottom turbulent layer has its maximum value when the flow is strongest and the stratification is weakest. Characterization of the dissipation rate and turbulent diffusivities in respect to buoyancy frequency N, current shear S, Richardson Number Ri and ϵνN2 was done. Dissipation and χT showed little dependence on shear or N2 but decreased at larger Ri. χt was found to be higher in regions of higher N2 and increased as ϵνN2 increased. KT, Kϱ and Kν, were all highest near the bottom in excess of 10−2m2s−1 and decreased towards the surface. There was little suggestion of a dependence of mixing efficiency on S2, Ri or ϵνN2, but some indication that Γ decreases with decreasing N2.