The impact of spatially-variable basal properties on outlet glacier flow

The spatially variable basal flow law suggested by geophysical data from Thwaites Glacier, West Antarctica produces modeled ice-flow response to warming that differs notably from the response of commonly assumed spatially uniform flow laws, with implications for future sea-level rise. Unlike many ice-sheet outlets, Thwaites flows across rather than along prominent topography, with hard stoss faces where form drag is understood to give a low-stress-exponent basal flow law and soft lee-side tills likely to give nearly-plastic behavior. Applying the PSU 2-D higher-order flowline model to a range of idealized Thwaites-like topographies, we test the impact of nearly plastic, nearly viscous, and spatially variable basal rheology on forced grounding-line evolution. In agreement with previous findings, for spatially uniform basal rheology, the timing and rate of grounding-line retreat depend strongly on the bed exponent, with plastic rheology promoting longer stability at the current grounding line but faster retreat once destabilized. Additionally, we find that retreat over mixed-rheology beds is sensitive to the wavelength of the basal topography. For outlet-glacier flow across long-wavelength bumps (≳10 km), behavior falls between that for uniform viscous and plastic beds, yet closer to plastic. However, over shorter-wavelength topography, at times, grounding-line retreat is slower than for either uniform-rheology end-member. Accurately accounting for variable basal conditions in ice-sheet models thus is critical for improving projections of both the timing and magnitude of retreat.