Rapid changes in glacier flow and what they teach us about glacier mechanics

Glaciers flow under their own weight through a combination of internal deformation and slip at the ice-bed interface. Understanding the behavior of internal deformation and slip at glacier beds is challenging because (1) both mechanisms are nonlinear and (2) natural glaciers in diverse geologic and climatic settings are inherently complex. Concerns about the consequences of climate change place a premium on plausible forecasts of the future states of glaciers and ice sheets, projects that require a deeper understanding of the mechanics of these systems. In this talk, I discuss recent work carried out in Iceland and West Antarctica that is aimed at understanding the mechanics of deformable glacier beds and ice rheology. In both study areas, we employ synoptic-scale synthetic aperture radar (SAR) observations collected at multiple times and from several viewing geometries in order to constrain short timescale (hourly to seasonal) variations in glacier flow. These observations provide constraints on numerical ice flow models that help elucidate the mechanics of interest in each region. In Iceland, our interest is in seasonal variations in glacier flow driven by surface meltwater flux to a sediment-mantled bed. Using airborne interferometric SAR data collected in winter and summer, we infer a plastically deforming bed beneath large portions of Hofsjökull—a 40-km-diameter, quasi-circular ice cap in central Iceland—and discuss the implications of basal plasticity for the response of ice flow to meltwater flux. In West Antarctica, we study how ocean tides modulate ice flow tens of kilometers inland using a unique 3D surface velocity field time series derived from nine months of continuous spaceborne SAR acquisitions. We find that the response of horizontal ice flow to ocean tidal uplift is most pronounced over the floating ice shelf and propagates inland at ~30 km/day, decaying quasi-linearly over ~85 km upstream of the area where ice begins to float. These observations provide insight into the rheology of ice and the characteristics of ice stream shear margins.