Meghana Ranganathan , Brent Minchew, Colin R. Meyer, G. Hilmar Gudmundsson

Drag at the bed and along the lateral margins are the primary forces resisting flow in outlet glaciers. Simultaneously inferring these parame- ters is challenging since basal drag and ice viscosity are coupled in the momen- tum balance, which governs ice flow. Here, 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. We modify existing inversions by including surface strain rates into the regularization of the in- version. Using synthetic data generated with physically-motivated variations in basal drag and ice rheology, we show that this allows for more accurate in- ferences. 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 the former 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 the regularization is informed by strain rates.