CAREER: Taking Process-based Models to the Field to Understand the Possibility and Implication of an Internal Shear Band Forming in Ice Flowing over Rough Topography

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Ice sheets in Antarctica and Greenland lose the vast majority of their ice mass through narrow drainage routes that are embedded into comparatively stagnant ice. The sharp difference in ice speed inside and outside of these drainage routes is comparable in magnitude to the speed difference between a race car and a pedestrian, begging the question how ice with relatively similar properties across the entirety of the ice sheet behaves so differently. The most convincing explanation to date is that rapidly moving ice has lost direct contact with the bedrock below and is slipping towards the ocean while slowly moving ice remains connected to the bed and is moving through internal deformation. Compelling as this explanation might be, it does not address where in the ice column the slip occurs that enables fast motion, but this location is essential for assessing how sliding, and the associated ice motion, responds to climate change. If slip occurs where ice and rock meet at the base of the ice sheet, water infiltration from a warming ocean could directly affect ice speed. If slip occurs close to the base of the ice sheet but within the ice itself, this feedback would likely be less immediate. The research plan is integrated with a collaborative educational program that aims to increase participation in polar science for high-school students from historically disadvantaged communities that are prone to be impacted by sea-level rise in the near future.The goal of this CAREER proposal is to understand the possibility and implications of an internal shear band forming in ice flowing over rough topography by developing a suite of customized numerical models that will be available as open-source tools to the glaciological community for usage and further development. The team hypothesizes that ice flowing over a hard bed with rough topography in the non-cavitating limit may experience sufficient strain localization in the ice just above topographic highs to form an internal shear band through creep instability despite advective cooling as the ice thins and accelerates. The deformation in this internal shear band is highly localized and reminiscent of sliding, highlighting that flow and sliding may be end members that are connected by a spectrum of increasingly localized englacial deformation. The team will test its hypothesis through a three-step process by (1) identifying the nondimensional conditions that are required for internal-shear band formation for a bed with an idealized topography, (2) generalizing insights to beds with more realistic, fractal bed topography and comparing results to existing field observations, and (3) producing continental-scale assessments of where an internal shear band could form for both Greenland and Antarctica.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.