COMBINED: Channelized Ocean Melting Beneath Ice shelves: Nonhydrostatic Ice and Estimating shelf Densities

DESCRIPTION: Ice shelves in Antarctica buttress the ice discharge from the continent into the oceans, mediating ice–ocean interactions and regulating Antarctic contributions to sea level rise. Ice shelf thinning reduces the buttressing of the adjacent grounded ice sheet, leading to accelerated ice flow. One of the main drivers of ice thinning and mass loss from the Antarctic Ice Sheet is the ice shelf basal melt, which occurs across scales, from narrow channels to distributed melting. Basal channels have a large potential impact on the stability of ice shelves by focused thinning, altering stress distributions, and making the ice shelf more susceptible to basal and surface fractures and atmospheric warming.

The only viable approach to continent-scale quantification of basal melt rates uses satellite data to estimate ice thickness, flow, and thickness change. Because these satellite data observe only the ice surface, quantification of basal melting requires simplifying assumptions about ice-shelf density and stresses. These assumptions yield accurate melt rates in areas where spatial gradients in ice-shelf properties are small but introduce uncertainty in areas where gradients are large. Where melt rates vary over small scales, like a basal channel, non-hydrostatic englacial stresses cannot be ignored, affecting satellite-derived estimates of ice thickness and basal melt. Here we aim to understand how each of these assumptions influences satellite ice thickness estimates and derived basal melt rates, and develop methods to improve calculations and predictions of channelized basal melting.

We will leverage high resolution aerogeophysical data collected over Antarctic ice shelves over the last two decades, using the differences between the altimeter-derived ice thicknesses and those interpreted from ice penetrating radar to indicate where a failure of one or more of the assumptions exists. We will assess different time series of overflown surveys as independent evidence for basal channel formation, advection, and evolution. Capitalizing on coincident observations of airborne lidar and ice penetrating radar, we will test the assumptions of constant density and hydrostatic equilibrium as they relate freeboard to calculated and observed ice thickness. We will use physics-based models to identify regions where the ice shelf is not in hydrostatic balance to constrain and fine-tune this new ice shelf density model. On well-constrained ice shelves, additional density variations arising from the surface processes of snow accumulation and firn densification will be incorporated using a suite of atmospheric reanalysis and 1-D firn models.

The resulting new ice shelf density model, maps of basal channels and hydrostatic-disequilibrium, and improved quantification of uncertainties across a wide range of scales, will provide spatial and temporal corrections to altimetry-based estimates of localized basal melting in key areas of ice shelves. Our findings will partition the processes that control the origin, development, and fate of surface and basal topography across ice shelves, revealing relationships and controls on channelization that can be applied to ice shelves around both Antarctica and Greenland to improve projections of ice shelf strength and stability.