Is ocean heat flux enhanced under rapidly growing ice?

Direct measurements of turbulent heat flux in the boundary layer under sea ice have shown that the rate of heat transfer at the ice/water interface is controlled by molecular processes. Evidence suggests that heat flux to melting ice in above freezing seawater is rate limited by salinity, since salt diffusivity is smaller than thermal diffusivity by a factor of nearly 200. Similar considerations of the double-diffusive character of the interface during freezing suggest that heat might be extracted from the water column faster under thin, rapidly growing ice, which would lead to substantial supercooling and frazil production if the mixed layer is near freezing. In models with several different ice thickness categories, this effect can have an appreciable impact on overall thickness if the frazil is mixed vertically in the upper ocean, then distributed evenly to the ice cover.

A SHEBA Intensive Observation Period program was designed to evaluate the hypothesis that rapid growth enhances ocean heat flux by simultaneously measuring turbulent fluxes under thin (0.5 m) and thick (2 m) ice during freezup in late Oct, 1997. Given straightforward application of a theory for heat and mass transport expounded by Yaglom and Kader, and given the temperature gradient observed in nearby ice of similar thickness, enhanced heat flux due to supercooling should have been readily observable, if present. Results were negative: observed friction velocity and heat flux were appreciably smaller in the growing internal boundary layer under the smooth, new ice surface than at the control (main) mast under thick ice. Interpretation of the present measurements is complicated by heterogeneity of the under-ice surface in the transition from old to new ice; nevertheless, it appears that the supercooling tendency in growing ice is relieved locally by uneven growth. From the point of view of upper ocean dynamics, this would not differ from uniform congelation growth.