Observations of amplified roughness from crystal accretion in the sub‐ice ocean boundary layer

Abstract 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.