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Ice crystal accretion on the underside of sea ice and ice shelves, a signature of pressure-induced supercooling, has the potential to alter the energy balance in the ocean boundary layer through enhanced hydrodynamic roughness. Here we present estimates of crystal-driven ocean boundary layer roughness in supercooled water beneath sea ice adjacent to the McMurdo/Ross Ice Shelf. Data were collected from four sites in McMurdo Sound, Antarctica, between 2007 and 2015, and represent a range of ice shelf-affected conditions. The results show that drag of the rough ice underside in the presence of platelets is 6-30 times larger than typical levels homogeneously applied in ice-ocean interaction models. The crystal-enhanced drag promotes increased entrainment into the boundary layer from the upper ocean, which has the potential to affect ice shelf evolution and sea ice growth through enhanced turbulent exchange of heat and momentum.

Plain Language Summary Water that includes a component of meltwater can become colder than its freezing point as it ascends the ice shelf base. This can promote the growth of thick layers of ice crystals between the ice and ocean. Owing to a lack of observational evidence, computational models presently make no allowance for this granular type of interface and therefore suppress vertical mixing that may be significant for ice shelf longevity. Over five Antarctic field campaigns, we have made measurements in the ocean beneath sea ice that demonstrate the effect of thin and thick layers of ice crystals beneath the ice. We have found that there may be as much as 5 times greater water exchange near the base of the ice than when the crystals are not present. Depending on the temperature of that water, this effect could lead to either enhanced melt or freeze.