Abstract
AbstractDrag at the bed and along the lateral margins are the primary forces resisting flow in outlet glaciers. Simultaneously inferring these parameters is challenging since basal drag and ice viscosity are coupled in the momentum balance, which governs ice flow. We test the ability of adjoint-based inverse methods to infer the slipperiness coefficient in a power-law sliding law and the flow-rate parameter in the constitutive relation for ice using a regularization scheme that includes coefficients weighted by surface strain rates. Using synthetic data with spatial variations in basal drag and ice rheology comparable to those in West Antarctic Ice Streams, we show that this approach allows for more accurate inferences. We apply this method to Bindschadler and MacAyeal Ice Streams in West Antarctica. Our results show relatively soft ice in the shear margins and spatially varying basal drag, with an increase in drag with distance upstream of the grounding line punctuated by localized areas of relatively high drag. We interpret soft ice to reflect a combination of heating through viscous dissipation and changes in the crystalline structure. These results suggest that adjoint-based inverse methods can provide inferences of basal drag and ice rheology when regularization is informed by strain rates.
Highlights
Mass loss from outlet glaciers in Antarctica is a primary source of uncertainty in sea-level rise projections (Cornford and others, 2015; DeConto and Pollard, 2016)
Lower values of slipperiness along MacAyeal correspond to sticky spots that have been identified by previous inversions (MacAyeal, 1993; MacAyeal and others, 1995; Joughin and others, 2004) and are colocated in areas with steep surface slopes (Figs 2a, h)
As more satellite data become available, inverse methods are increasingly critical in accurately modeling ice flow from outlet glaciers
Summary
Mass loss from outlet glaciers in Antarctica is a primary source of uncertainty in sea-level rise projections (Cornford and others, 2015; DeConto and Pollard, 2016). A prerequisite to reliable projections of mass loss from Antarctica is understanding what sets the flow speed of ice streams and outlet glaciers. The resisting forces of ice flow in ice streams are not fully constrained in Antarctica, and this contributes to the uncertainty of the response of outlet glaciers to changes in climate. The flow of ice streams is resisted in part by drag at the bed (Echelmeyer and others, 1994), commonly related to the basal velocity through a sliding law. A typical form of the sliding law, and the form taken in this study, relates the basal drag to a power of the basal velocity with a prefactor denoted here as basal slipperiness (Weertman, 1957). Inversions for the sliding-law prefactor in ice streams in Antarctica suggest that the prefactor in the sliding law can vary spatially and temporally over a wide range of values due to variations in bed composition, roughness, and water pressure (Joughin and others, 2004; Morlighem and others, 2013; Isaac and others, 2015)
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