Modeling the Transition from Inland to Ice-stream Flow in West Antarctica

Fast-flowing ice streams evacuate slow-moving inland ice from the interior of West Antarctica and have a controlling influence on the mass balance of the ice sheet. Discharge through the ice streams may be controlled by the rate at which inland ice passes through the onset regions. Recent studies suggest that the transition from inland- to ice-stream flow may migrate inland over time, although the causes for such behavior are unclear. One result of such migration would be ice stream lengthening and increased mass discharge over time. An improved understanding of the physics that control onset regions seems a necessity for assessing the future balance state of the West Antarctic Ice Sheet. We address the following questions: (1) Are the ice stream onsets "fixed" spatially, or can they migrate over time? (2) What physical controls and processes are important in determining the fixed or migratory nature of the onsets? (3) What range of behaviors might the onsets exhibit over time? (4) What are the relative importances of external conditions (e.g. bed morphology, geothermal flux) versus those related to ice flow (e.g. basal temperature gradient, frictional melting, basal hydraulic potential gradient) in establishing and controlling the inland- to ice-stream-flow transition. We formulate a 2-D "flowband" (vertical and along-flow coordinate) model using Control-Volumes. We account for motion within the ice and at the ice-bed interface; diffusion and advection of heat; frictional-heat generation and basal melting; partitioning of resistive stresses between basal drag, longitudinal-stress gradients, and drag from the lateral margins; and feedbacks between stresses in the ice and conditions at the ice-bed interface. Future advances in modeling will allow ice streams to evolve naturally, from the physics specified in the model, rather than requiring them to be specified a priori.