Moulin density controls drainage development beneath the Greenland ice sheet

Abstract

Uncertainty remains about how the surface hydrology of the Greenland ice sheet influences its subglacial drainage system, affecting basal water pressures and ice velocities, particularly over intraseasonal and interseasonal timescales. Here we apply a high spatial (200 m) and temporal (1 h) resolution subglacial hydrological model to a marginal (extending~25 km inland), land-terminating,~200 km 2 domain in the Paakitsoq region, West Greenland. The model is based on that by Hewitt (2013) but adapted for use with both real topographic boundary conditions and calibrated modeled water inputs. The inputs consist of moulin hydrographs, calculated by a surface routing and lake-filling/draining model, which is forced with distributed runoff from a surface energy-balance model. Results suggest that the areal density of lake-bottom moulins and their timing of opening during the melt season strongly affects subglacial drainage system development. A higher moulin density causes an earlier onset of subglacial channelization (i.e., water transport through channels rather than the distributed sheet), which becomes relatively widespread across the bed, whereas a lower moulin density results in a later onset of channelization that becomes less widespread across the bed. In turn, moulin density has a strong control on spatial and temporal variations in subglacial water pressures, which will influence basal sliding rates, and thus ice motion. The density of active surface-to-bed connections should be considered alongside surface melt intensity and extent in future predictions of the ice sheet's dynamics.

Incorporating modelled subglacial hydrology into inversions for basal drag

“…Many of the parameters are the same as published in Banwell et al (2016) andHewitt (2013). However, testing of the reported optimal parameters for the Paakitsoq region reported by Banwell et al (2016) using the integrated model showed poor agreement with GPS measurements due to insufficient volumes of water being evacuated from mid-elevations.…”

“…The model is written in MATLAB, using a finite difference numerical grid and an implicit forward time step method. For full details, consult Hewitt (2013) and Banwell et al (2016).…”

“…This subglacial hydrology model used is described in detail in Hewitt (2013) and Banwell et al (2016) and is similar conceptually to the model presented in Werder et al (2013). Here, the version employed in Banwell et al (2016) is applied.…”

“…Distributed flow is described by an average thickness and flux over a representative area. As in Banwell et al (2016), the distributed system is composed of two components: a cavity sheet and an elastic sheet. The elastic sheet is included to simulate "hydraulic jacking" from lake hydrofracture events and is activated only when the effective pressure drops to zero and below.…”

Basal drag of Fleming Glacier, Antarctica, Part B: implications of evolution from 2008 to 2015

“…It arises from two main sources in polar regions: either surface melt water draining into the subglacial hydrologic system via crevasses or moulins or in-situ melting at the bed (Banwell et al, 2016;Dunse et al, 2015;Hoffman and Price, 2014). Hoffman and Price (2014) also found a positive feedback between the basal melt and basal sliding through 245 the frictional heating on an idealized mountain glacier using coupled subglacial hydrology and ice dynamics models.…”