Catching the Wave: Observing the Inland Propagation of Dynamic Thinning in Greenland

Abstract:

Thinning due to ice flow acceleration, or dynamic thinning, caused by abrupt retreat of Greenland’s coastal outlet glaciers earlier in the century continues to propagate inland through diffusion. The rate of diffusion increases as the ice thickens and surface slopes decrease further into the interior, causing the mass loss to be spread over an ever increasing area, and the rate of thinning to decrease. Measurement of dynamic thinning and resulting mass loss with altimetry is thus becoming ever more challenging as the signal decreases relative to other potential sources of surface elevation change, particularly pre-existing dynamic imbalances and variations in surface and firn density, both of which are poorly constrained.

We combine observations from field campaigns, multiple air and spaceborne sensors, firn cores and climate model predictions to assess our ability to remotely measure dynamic thinning. Field measurements obtained in the percolation zone above Jakobshavn Isbrae indicate dynamic thinning has propagated nearly 250 km, or twice as far as detected by altimetry. This signal, however, is complicated by other variations that both mask the dynamic thinning signal and create substantial errors in the conversion from surface elevation change to mass change. We find a recent, rapid compaction of the firn layer due to increased melt has caused a surface elevation change of similar magnitude and spatial pattern as dynamic thinning. This compaction has occurred during a longer-term increase in accumulation, causing an inverse relationship between firn layer thickness and mass. We also detect a pre-existing, positive dynamic imbalance (i.e. flux convergence) in the interior of the catchment that offsets, and therefore masks, the inland propagation of thinning. Based on these findings, we discuss the limitations of altimetry for recovering the evolution of dynamic thinning and mass change and make recommendations for future improvements.