1. Model investigations of the transition from tributary to ice-stream flow
The study was completed in January 2007. The project was funded by the National Science Foundation (#OPP-0125610).
Contributors:Much has been learned about the physics that controlling streaming flow over the past 20 years, but relatively little is known about how inland ice (moving mostly by internal deformation with drag at the bed) makes the transition to streaming flow (with motion primarily at the bed and drag at the lateral margins). It is likely that these transitional or "onset" regions control the rate at which inland ice enters the ice streams. Further, some studies suggest that the onset regions could migrate inland, which could result in lengthening of ice streams and subsequent increases in mass discharge.
Predictive models of large-scale ice sheet behavior can now simulate the flow of both inland ice and ice streams fairly well. However none include the important processes that control how slow inland flow transitions to streaming flow. We have been making steps toward rectifying this problem by investigating flow through onset regions using a transient ice-flow model. Our overarching goal is to improve predictions of how ice sheets with complex internal dynamics are likely to respond to future environmental changes.
We have developed a two-dimensional full-stress model to (1) investigate the controlling processes in onset regions and (2) examine how those processes might affect the evolution of the onset region over time and space. We distinguish between processes that are "external" from those that are "linked" to the ice flow, and examine the relative importance of each in the inland- to ice-stream flow transition. External processes that could help enhance basal sliding and weaken the connection between the glacier and the bed (such as the morphology of the subglacial bed or the geothermal flux) are not likely to change over the short timescales of interest when studying onset-migration (i.e. 10's-100's of years). If external processes are the primary control on where ice streams can initiate, no migration or only a limited amount might be expected. Processes that are linked to the ice flow may also influence the strength of the ice bed connection, and help to facilitate the rapid-basal motion necessary for an ice stream to develop. Examples of some of these processes are the rate of frictional melting (linked to the basal drag and the basal sliding speed), the basal temperature gradient (which controls the rate at which heat at the bed is conducted upwards into the ice), and the hydraulic potential gradient (which controls the direction of basal water flow, and is dominated by the ice sheet surface slope). Non-linear feedback between these processes might allow for the position of the ice stream onset to migrate over time.
Our new flow model was used during site selection for the inland WAIS ice-core project. Details of the site selection work and our report to the community and metadata can be found here.
Publications:
Thesis:
Price, S.F. 2006. Development and Applications of a Full-Stress Flowband Model for Ice. Ph.D. thesis, University of Washington.
Selected presentations: