Surge Initiation and Termination Dynamics of Sortebrae, East Greenland

Glacial surge mechanisms are poorly understood, in part due to a lack of sufficiently extensive and timely observations of ice flow. The manner in which a surge initiates and propagates, the flow characteristics during the surge, and the mode and timing of termination all provide evidence of the nature of the surge trigger, and the mechanism by which fast flow occurs. The collection of such data is, however, hampered by the unpredictability of surge events, the remote location of most surge-type glaciers, the impracticality of working on a heavily crevassed and rapidly deforming surface, and the large spatial and temporal scale of the measurements required. Synthetic Aperture Radar (SAR) intensity tracking is a simple, robust technique employing automated image cross-correlation to determine two-dimensional surface displacements. This technique allows measurement of velocity even through cloud cover or during the polar night. In this study, the tracking technique was optimised to give relatively high-resolution, extensive measurements of flow through surge initiation, full surge and termination, at speeds which at times exceeded 20 md-1 and over intervals as long as 105 days, for the 65 km long tidewater-terminating glacier Sortebrae in East Greenland. These measurements could not have been made using SAR interferometry or optical image tracking. The surge of Sortebrae began in late 1992 in the lower glacier, which is grounded below sea level. The zone of accelerated flow propagated up-glacier, confined initially within the central of three flow units, as a front with sharp lateral boundaries along medial moraines. On reaching a tributary confluence, up-glacier propagation occurred rapidly in a diffuse manner, incorporating the major upper-glacier tributaries into the surge through the spring of 1993. Termination occurred very rapidly between May and July 1995, and propagated downglacier as a widespread deceleration from between 10 and 22 m per day to 0 to 6 m per day over much of the glacier. A small upper tributary persisted in surging through to the following spring.