Quantifying and interpreting the changes in forces during the major retreat of Kangerlussuaq Glacier in South-East Greenland using a 2D force balance analysis

Kangerlussuaq Glacier in South-East Greenland is the largest outlet glacier on the east coast of Greenland, draining approximately 3% of the Greenland Ice Sheet (GrIS). In 2004-2005 this glacier underwent a dramatic retreat, acceleration and thinning, indicating a significant change in ice dynamics. During this time, the ice velocity increased from 6-8 kilometers per year to 14 kilometers per year, resulting in an increase of mass loss of 40 Gt/yr by 2005. This accounted for approximately 20% of the mass loss of the whole SE GrIS. Moreover, other SE Greenland outlet glaciers exhibited synchronous acceleration, retreat and thinning, and thus in 2003-05 the mass loss from SE Greenland dominated the overall mass balance of the GrIS. Our study investigates the possible causes of increased outlet glacier mass loss in this sector. Using the force budget technique, we quantify the forces that controlled flow of Kangerlugssuaq Glacier before and after its major speed-up event. Through multiple sets of remotely sensed data, including repeat ASTER stereo imagery and CReSIS radar echograms, we reconstruct the ice thickness, surface slope, and velocity to generate 2003 and 2006 2D force balance models of the glacier. Evaluating the partitioning of flow resistance at different times allows us to identify the mechanisms responsible for glacier changes. Previous work has suggested that speed-up and thinning of Kangerlussuaq Glacier was caused by a collapse of the floating tongue resulting in a loss of back-stress. By computing the force balance near the grounding line at different times we estimate changes in back-stress and longitudinal stresses that are attributed to partial grounding or confinement of the floating tongue and investigate the role of other mechanisms, such the weakening of the ice in the lateral shear margin or at the bed. This study is part of a broader effort examining the basal and marginal stress balance changes of other major Greenland outlet glaciers (Jakobshavn, Helheim, and Upernavik glaciers) to improve the understanding of the physical mechanisms responsible for observed acceleration and increasing mass loss.