Collaborative Proposal: Joint US/Norwegian Studies of Ice/Ocean Interaction in Frozen Fjords

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Heat and salt exchanges at the ice/ocean interface play a key role in the annual cycle of sea-ice growth and ablation. Recent observations of significant change in the extent and thickness of the Arctic ice cover have focused attention on factors that control the mass balance, and treatment of ice/ocean exchanges in numerical models is becoming increasingly sophisticated. Direct measurements of turbulent heat flux under pack ice (by covariance techniques) have shown that it is nearly proportional to the product of friction velocity (square root of the kinematic Reynolds stress) and the elevation of mixed layer temperature above freezing, but that the exchange coefficient is an order of magnitude smaller than the corresponding exchange coefficient for momentum. This implies that unlike momentum flux, heat flux at the interface is rate-limited by molecular processes in thin sublayers adjacent to the surface. At low temperature, the molecular diffusivity of salt in seawater is only about 0.6% of that for heat, thus the possibility exists for double diffusive effects, where the transfer rate is ultimately controlled by salt diffusion. There is evidence that this holds when ice is melting, according to measurements in relatively warm water in the marginal ice zone. A model developed by Yaglom and Kader to describe laboratory studies of heat and mass transfer over hydraulically rough surfaces considers the difference in diffusivities between scalar properties, and has been applied in several numerical ice/ocean models. The models do not distinguish between melting and freezing (although the YK studies did not consider crystalline growth at the solid/liquid interface), consequently, heat is diffused through the interface faster than salt is ejected during ice growth. The models thus predict enhanced ocean heat flux under thin ice, and produce a significant amount of supercooled water, which is generally assumed to nucleate as frazil crystals distributed in some fashion throughout the ocean boundary layer. Observations do not support the modeled response: there is little evidence of extensive frazil production under growing ice in the Arctic, nor do the limited data from under thin ice indicate larger heat flux. The proposed work aims to understand ice/ocean heat and mass exchange, particularly during freezing conditions, by collaborating with Norwegian and US scientists in making detailed measurements of ice/ocean interaction in tidally active fjords in Svalbard. Fast ice will serve as a laboratory for carefully controlled experiments measuring exchanges both in the ice cover and in the ocean boundary layer. Work proposed here deals mainly with the latter, including measuring momentum, heat, and salinity flux in the boundary layer near the ice undersurface. A specific goal is to improve parameterization of ice/ocean heat and mass transfer for numerical models.