| Summary: | An essential component of an ice sheet model is its description of how ice deforms under applied stresses itsmaterial constitutive relation. Current large-scale ice sheet models routinely rely on Glens flow relation, whichis an isotropic material constitutive relation that is not dependent on the character of the stress applied. However,laboratory experiments subjecting ice to simultaneous simple shear and compressive stresses (a typical situationin ice sheets) show that with sustained deformation under constant stresses, steady state viscous creep becomesanisotropic. For various combinations of simple shear and compression, results show that flow enhancement furtherincreases as the stress configuration becomes dominated by simple shear. The empirical, scalar, tertiary, anisotropicrheology (ESTAR) is a computationally-efficient flow relation that incorporates anisotropic effects through a parameterisationfor a flow enhancement factor that takes into account the proportion of simple shear in the overallstress regime. Here, we use the Ice Sheet System Model to investigate the impact of anisotropy on the dynamics ofan idealized ice shelf by comparing simulated flow fields using ESTAR with those of the standard (isotropic) Glenflow relation. When enhanced to match simple shear flow rates, the Glen flow relation overestimates velocities atthe ice-ocean front by up to 36%. Significantly, no single Glen enhancement factor accurately captures the spatialvariations in flow over the ice shelf produced by ESTAR. Our results have implications for reconstructions andprojections of sea level using ice sheet models that do not account for anisotropy.
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