TIDALLY FORCED UNDER-ICE EKMAN LAYERS OBSERVED BY AN ACOUSTIC DOPPLERCURRENT PROFILER

A simple, tidally forced Ekman model is fit to acoustic Doppler curren t profiler observations of the horizontal velocity field sampled durin g the Resolute portion of the SARES project. An assimilation inversion technique is demonstrated that does not require the use of Lagrange m ultipliers or an adjoint equation. The gradient of the cost function i s obtained through direct differentiation of the equations of motion a nd associated boundary conditions. Both the standard depth-independent pressure gradient and a new depth-dependent pressure gradient are ass imilated into the model. The depth-dependent pressure gradient reduced the residual error over the depth-independent case and produced estim ates of eddy viscosities and drag coefficients that were consistent wi th a marked change in the stratification observed during the course of the experiment. The results suggest that a depth-dependent pressure g radient may be more important than a depth-dependent eddy viscosity in describing under-ice tidally forced Ekman layers. A maximum value of 4.75 X 10(-3) m(2) s(-1) is obtained for the vertical eddy viscosity, which implies a nutrient transport 50 times less than that required to meet ice-algal demand. This result suggests that turbulence associate d with the passage of high frequency internal waves, rather than the b ackground tide, is likely responsible for bringing nutrients to the ic e-water interface.