Momentum exchange and Reynolds stress estimates in an underice boundary layer during SHEBA

In the Beaufort Sea, the surface boundary layer is not only driven from above by ice motion, but from below as well, by internal waves [McPhee, 1992, 2001 (submitted)] and baroclinic vortices. An opportunity to observe the combined influence of all of these phenomena was provided at the SHEBA ice camp in early December 1997, when an atmospheric storm passing above the camp was followed immediately by an energetic baroclinic eddy passing beneath. The complex exchanges of momentum following these events, as well as the larger-scale background currents, are documented using both a 600 kHz Doppler sonar (1.5 m resolution, 10 second sampling time, 30-60m profiling depth) and a 140 kHz sonar (3 meter resolution, 2 min averaging time, 300 meter profiling depth). Profiles of average Reynolds stress document the penetration of turbulence through the 30 meter layer just below the ice. Friction velocities peaked at ~ 2 cm/s. As the storm subsided, turbulent stresses at the base of the layer (u* ~ 1 cm/s) began to be influenced by the eddy. Turbulent stresses in the lower part of the boundary layer, above the eddy, increased with depth. In addition to affecting the boundary layer stresses, the eddy also altered the momentum exchange between the upper ocean and the deeper pycnocline by inhibiting the downward propagation of newly generated near-inertial waves. Indeed, waves propagating upward out of the eddy dome were seen. Given the large numbers of baroclinic vortices in the Canadian Basin [Manley and Hunkins, 1985], this complicated picture may be typical of the entire Beaufort Sea.