Sheltering of Sea Ice Ridges in the Ice-Ocean Drag Force: Implications From Idealized Laboratory Experiments

Abstract

Sea ice ridge keels enhance the turbulent mixing beneath ice and the ice-ocean drag coefficient. However, densely distributed keels suppress drag through the sheltering effect, which has not been specifically examined. To this end, we investigated whether and how this sheltering should be incorporated into form drag parameterization for sea ice-ocean momentum exchange. Through conducted water tank experiments, the form drag on artificial keels with varying shapes was measured at different flow velocities. Particle image velocimetry was employed to capture the wake characteristics and vertical mixing induced by keel-flow interactions. Sheltering reduced the downstream keel drag, which reversed at dimensionless spacing L/H (keel spacing-to-depth ratio) less than 5. Sheltering decreased exponentially with L/H and increased following a power law with the keel slope angle, independent of the flow velocity. We propose a new sheltering function that incorporates the effects of these keel properties, fitting observational data. It affects the ice-ocean drag coefficient in a nonmonotonic way, arising from the competition between the keel form drag and sheltering. Compared with this function, the previous L/H-dependent exponential/power sheltering functions overestimated/underestimated the drag coefficient by 33%/17%, respectively, for L/H < 100 with an angle less than 50°. We present findings from nonstratified flows with a dimensionless water depth-to-keel depth ratio (D/H) ranging from 3.75 to 22.50. Therefore, our results are not applicable to large parts of the polar oceans where D ≫ H. However, our findings provide fundamental insights into the shallow limit case, serving as a benchmark for ice-ocean momentum flux parameterizations.