Effects of keels on ice bottom turbulence exchange

Abstract

The effects of ice keels on the upper ocean are examined using a combination of turbulence measurements and output from a large-eddy simulation (LES) model. Two cases are examined, one during the winter when the under-ice boundary layer is relatively deep (∼20 m) and near the freezing point and a second during the summer when the ice is melting and the boundary layer consists of a shallow (∼0.5 m), highly stratified fresh layer. In the winter case, measurements show that flow disruption by a 10-m-deep keel causes enhanced vertical mixing, increasing the heat flux from a background value of ∼5 W m-2 to values averaging ∼25 W m-2. Simulations using the LES model are in good agreement with the measurements and indicate that the keel generates a turbulent wake region extending hundreds of meters downstream from the keel. Elevated heat fluxes in the wake region are generated by increased entrainment of warmer water from beneath the mixed layer. Simulations of summer cases demonstrate that shallow keels (∼0.5 m) generate strong turbulence that is able to rapidly mix the fresh layer in the lee of keels. However, this effect decreases quickly as the fresh layer accelerates to match the ice velocity. Deeper keels (1 m) follow a similar pattern but generate more mixing as the fresh layer is forced under the keel. Simulated ice melt heat fluxes are similar to estimates made from ice balance measurements taken during the Surface Heat Budget of the Arctic Ocean summer field program. Copyright 2003 by the American Geophysical Union.