The effects of keels and frozen leads on under ice turbulence

Turbulence produced by ice bottom roughness elements is examined using a large-eddy simulation (LES) model. We utilize a standard ocean LES model modified so that the upper boundary can vary in depth, thereby resolving under ice features. Experiments are performed to examine the turbulent structure under ice that is aerodynamically rough (via a surface roughness parameterization) and under ice with large scale thickness variations representing keels or frozen leads. Observations of the temperature and salinity structure taken from the SHEBA field experiment are used to initialize the model for both low-wind stable summer melt conditions and more fully mixed high wind cases. Results show that fields of ice ridges can trap stable fresh water, preventing rapid mixing as ice velocity is increased. Below the fresh layer, however, turbulence is enhanced by the motion of the keel in the less stable mixed layer interior. Frozen leads also trap near surface water, causing a displacement of the under-ice shear layer away from the bottom of the ice. With well-mixed boundary layer conditions, keels cause a large increase in the local turbulence strength by directly forcing strong vertical motions. Keels contribute to a more rapid breakdown in the summer fresh layer and can cause enhanced mixed layer entrainment