Plastic bed beneath Hofsjökull Ice Cap, central Iceland, and the sensitivity of ice flow to surface meltwater flux

Abstract

ABSTRACT. The mechanical properties of glacier beds play a fundamental role in regulating the sensitivity of glaciers to environmental forcing across a wide range of timescales. Glaciers are commonly underlain by deformable till whose mechanical properties and influence on ice flow are not well understood but are critical for reliable projections of future glacier states. Using synoptic-scale observations of glacier motion in different seasons to constrain numerical ice flow models, we study the mechanics of the bed beneath Hofsjökull, a land-terminating ice cap in central Iceland. Our results indicate that the bed deforms plastically and weakens following incipient summertime surface melt. Combining the inferred basal shear traction fields with a Coulomb-plastic bed model, we estimate the spatially distributed effective basal water pressure and show that changes in basal water pressure and glacier accelerations are non-local and non-linear. These results motivate an idealized physical model relating mean basal water pressure and basal slip rate wherein the sensitivity of glacier flow to changes in basal water pressure is inversely related to the ice surface slope.

Incorporating modelled subglacial hydrology into inversions for basal drag

“…This is due to the fact that basal shear traction must satisfy the global stress balance (Joughin et al, 2004;Minchew et al, 2016). The basal drag and basal velocities from the linear sliding law used to initiate the subglacial hydrology model are therefore self consistent with the subsequent inversion results of the two non-linear sliding laws.…”

“…Here, we have assumed that the parameters of the sliding law are independent of time. This assumption is better suited for bedrock than till, as properties of till are dependent on saturation and deformational history (Minchew et al, 2016).…”

Assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under Pine Island Glacier

“…For each year Y = (1996, 2007, 2008, 2009,and 2010), a cost function is defined as follows: 2J Y ∕N obs Y . As mentioned previously, when several data sets are available, it is common to invert the basal stress field for each data set and interpret local changes in b [Jay-Allemand et al, 2011;Habermann et al, 2013;Minchew et al, 2016]. Solved independently, the five inverse problems (2) are independent of m as the inverse method should lead to the same effective basal slipperiness could be used to infer mean (time independent) values of C(x, y) and m(x, y) at every mesh node from the five solutions [C eff (Y, x, y), b (Y, x, y)].…”

Mechanisms leading to the 2016 giant twin glacier collapses, Aru range, Tibet

“…Thereby we assumed that basal shear stress is mainly constrained by the global stress balance and should not be influenced by the sliding law that we used (eq. 1) (Joughin et al, 2004;Minchew et al, 2016). The steady-state condition shows a basal shear stress between 100 kPa and 200 kPa in both glaciers with a mean shear stresses of 137 kPa and 150 kPa for Aru-1 and 2, respectively ( Figure 8A).…”

“…Using constant climatic conditions associated with the balanced glacier conditions for the SRTM DEM, we determined the 15 bedrock topography allowing the best match between modeled and observed (i.e., SRTM DEM) surface topography (van Pelt et al, 2013). For this purpose, we run a 3D transient simulation assuming no sliding, fixed surface topography (SRTM DEM), and constant surface forcing (mass balance and enthalpy).…”