Abstract
AbstractA new modelling framework using Lagrangian particle tracking has been developed to assess dynamic and thermodynamic effects of underwater frazil ice. This frazil-ice model treats a Lagrangian particle as a bulk cluster of many frazil crystals, and calculates the thermodynamic growth of each particle and the corresponding budget of latent heat and fresh water. The effective density and viscosity of sea water depend on the mass fraction of underwater frazil ice, and hence affect ocean convection. An idealized experiment using our model successfully reproduces the formation of underwater frazil ice and its transition to grease ice at the surface. Because underwater frazil ice does not reduce the atmosphere/ocean heat exchange, surface heat flux and net sea-ice production in the experiment with frazil ice are relatively high compared with the experiment where surface cooling directly leads to columnar growth of a solid ice cover which effectively insulates the heat flux. These results suggest that large-scale sea-ice models which do not take account of the effects of frazil ice might underestimate atmosphere/ocean heat exchange, particularly at times of active new ice formation.
Highlights
It is widely recognized that sea ice controls atmosphere/ ocean heat exchange in the polar regions, and plays a very important role in the Earth’s climate system
As the volume fraction of frazil ice at the surface increases, wind stirring becomes less active, due to the damping effect from enhanced viscosity, and the occurrence of frazil particles suspended in the ocean interior begins to decrease
After 72 hours, the ocean surface has been completely covered by grease ice and underwater frazil particles are almost eliminated from the ocean interior
Summary
It is widely recognized that sea ice controls atmosphere/ ocean heat exchange in the polar regions, and plays a very important role in the Earth’s climate system. The melt/ freeze cycle of sea ice has a critical impact on the density structure and circulation of the world ocean through brine rejection and freshwater release. The state of newly formed sea ice depends significantly on local and temporal conditions at the ocean surface. The higher surface heat loss results in more sea-ice production and greater buoyancy forcing on the ocean surface. In spite of its substantial importance, even present-day numerical sea-ice models developed to study ocean circulation and the climate system do not take sufficient account of such differences in the seaice state (Hibler, 1979; Hunke and Dukowicz, 1997)
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