Revisiting heat and salt exchange at the ice-ocean interface: Ocean flux and modeling considerations

Properly describing heat and salt flux at the ice/ocean interface is essential for understanding and modeling the energy and mass balance of drifting sea ice. Basal growth or ablation depends on the ratio, R, of the interface heat exchange coefficient to that of salt, such that as R increases so does the rate-limiting impact of salt diffusion. Observations of relatively slow melt rates in above freezing seawater (plus migration of summer "false bottoms'') suggest by analogy with laboratory studies that double diffusion of heat and salt from the ocean is important during the melting process, with numeric values for R estimated to range from 35 to 70. If the same double-diffusive principles apply for ice growth as for melting (i.e., if the process is symmetric), supercooling (possibly relieved by frazil crystal production) would occur under rapidly growing ice, yet neither extensive supercooling nor frazil accumulation is found in Arctic pack ice with limited atmospheric contact. Physical properties and turbulent fluxes of heat and salt were measured in the relatively controlled setting of a tidally driven Svalbard fjord, under growing fast ice in late winter. The data failed to show supercooling, frazil production, or enhanced heat flux, suggesting that the double diffusive process is asymmetric. Modeling results compared with measured turbulent fluxes imply that R = 1 when ice freezes; i.e., that double-diffusive tendencies are relieved at or above the immediate interface. An algorithm for calculating ice/ocean heat and salt flux accommodating the different processes is presented, along with recommended ranges for the interface exchange coefficients.