Seasonal to decadal scale variations in the surface velocity of Jakobshavn Isbrae, Greenland: Observation and model‐based analysis

Abstract

[1] Using new data, we build upon the nearly two-decade long record of observations from Jakobshavn Isbrae to investigate the processes driving its dynamic evolution. While winter flow speed has not increased substantially over the last three winters, there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening. We relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses, using ice-flow models constrained by satellite and airborne observations. These results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus. While the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup, other processes are of comparable significance at seasonal to decadal time scales. From analysis of the models, we hypothesize that thinning-induced change in basal effective pressure is the dominant process influencing near-terminus behavior, while diffusive processes drive the upstream response. The apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs. Our analysis of the processes controlling the speed suggests little potential for further large acceleration. Thinning and elevated speeds may continue at rates similar to present, however, putting the glacier on course to retreat to the head of its deep trough in about a century, at which point it likely would stabilize with a thinner terminus.

Ice–ocean interaction and calving front morphology at two west Greenland tidewater outlet glaciers

“…For example, in west Greenland the sustained attrition of Jakobshavn Isbrae, observed since 1998 (Joughin et al, 2012), has been attributed to warming of subsurface water in Disko Bay and adjacent coastal seas (Holland et al, 2008). Similarly, AW was identified circulating within Sermilik and Kangerdlugssuaq fjords in east Greenland and is implicated in the retreat of Helheim and Kangerdlugssuaq glaciers over the last decade (Straneo et al, 2010(Straneo et al, , 2011.…”

“…significantly contribute to the general circulation across and along the glacier front. Indeed, both processes produce vertical entrainment of ambient AW water in direct contact with the ice front, which is a major driver of enhanced melting at the calving front (Jenkins, 1991(Jenkins, , 2011Josberger and Martin, 1981;Xu et al, 2012Xu et al, , 2013. …”

The sensitivity of the Greenland Ice Sheet to glacial–interglacial oceanic forcing

“…The complex mechanisms that lead to ice-shelf thinning, loss of buttressing and potential grounding-line instability have been studied largely for the Antarctic Ice Sheet (AIS) (DeConto and Pollard, 2016;Favier et al, 2014;Hanna et al, 2013;Joughin et al, 2014a;Pritchard et al, 2012;Rignot et al, 2004;Shepherd et al, 2004;Wouters et al, 2015). The thinning of the Larsen C ice shelf and its recent calving event (Hogg and Gudmundsson, 2017;Jansen et al, 2015), the collapse of Larsen B and the melting of the Antarctic Peninsula glaciers (Cook et al, 2016), the widespread retreat of Pine Island and other glaciers in West Antarctica (Alley et al, 2015;Joughin et al, 2014b;Rignot et al, 2014) and the thinning of some East Antarctica ice shelves are notable examples of the direct connection between changes in oceanic forcing and glacier-termini adjustment (Alley et al, 2015).…”

“…The thinning of the Larsen C ice shelf and its recent calving event (Hogg and Gudmundsson, 2017;Jansen et al, 2015), the collapse of Larsen B and the melting of the Antarctic Peninsula glaciers (Cook et al, 2016), the widespread retreat of Pine Island and other glaciers in West Antarctica (Alley et al, 2015;Joughin et al, 2014b;Rignot et al, 2014) and the thinning of some East Antarctica ice shelves are notable examples of the direct connection between changes in oceanic forcing and glacier-termini adjustment (Alley et al, 2015). Only in the last several years has the scientific community also focused its attention on the ice-ocean interaction in Greenland, motivated by the observed acceleration and retreat of major GrIS outlet glaciers.…”

Spatial and temporal variability of water-filled crevasse hydrologic states along the shear margins of Jakobshavn Isbrae, Greenland

“…Clearly, the impact of terminal perturbations on strain rates in the vicinity of the water-filled crevasse groups are negligible over much of the analysis period. It is only during recent seasons that the front has reached a position such that the lower elevation systems (CV1, 2, and 4) are within the 10-15 km range (Joughin et al, 2012) where longitudinal coupling from calving events could 30 be influential. This may change as the terminus of Jakobshavn continues its rapid retreat.…”