Momentum, Heat, and Salt Budgets to Estimate Drag and Transfer Coefficients inside an Ice Shelf Basal Channel

Understanding melting and freezing at the ice–ocean boundary is crucial for predicting ice shelf stability and, ultimately, sea level rise. This study investigates ice–ocean interactions within a basal channel under the Milne Ice Shelf, Nunavut, Canada, using water temperature, salinity, and velocity data from vertical profiles and moorings, along with ice-penetrating radar observations. Data reveal a freshwater outflow ∼8 (6) m thick in summer (winter), with velocities around 0.2 (0.1) m s−1. Moving down channel (toward the ocean), salinity increases and temperature decreases, often nearing or falling below the freezing point. A momentum budget indicates a subcritical flow driven by buoyancy and yields a minimum ice–ocean drag coefficient Cd of 0.0065. This relatively high value is attributed to high ice roughness and frazil ice accumulation. Heat and salt budget analyses show significant seasonality, with enhanced melting from July to mid-September and both melting and freezing from mid-September to June. The inclusion of frazil ice formation is essential to close the heat budget. The estimated maximum heat transfer coefficient ΓΘ is 0.0062. This relatively low value is attributed to the high-density stratification within the basal channel. This research enhances our understanding of basal melting mechanisms by providing rare estimates of drag and transfer coefficients, by providing a new method to estimate these parameters (momentum, heat, and salt budgets) and by highlighting the role of ice roughness, stratification, and frazil ice, which should be incorporated in improved ice–ocean melt/freeze parameterizations. Significance Statement In this study, we use ocean temperature, salinity, and current observations under the Milne Ice Shelf to estimate the amount of melting and freezing along an under-ice trough (basal channel). Because of the logistical difficulties in accessing the ocean under ice shelves, such observations are extremely rare. Our results show that the amount of melting or freezing varies greatly along the channel, over its depth, and over time. This variability is explained by density stratification, frazil ice formation, and high ice roughness. Therefore, these three aspects should be further investigated and quantified to improve ice–ocean melt/freeze parameterizations and thereby better predict the retreat of ice shelves and, ultimately, sea level rise.