Abstract
Abstract. Ice surface topography controls the routing of surface meltwater generated in the ablation zones of glaciers and ice sheets. Meltwater routing is a direct source of ice mass loss as well as a primary influence on subglacial hydrology and basal sliding of the ice sheet. Although the processes that determine ice sheet topography at the largest scales are known, controls on the topographic features that influence meltwater routing at supraglacial internally drained catchment (IDC) scales (<10s of km) are less well constrained. Here we examine the effects of two processes on ice sheet surface topography: transfer of bed topography to the surface of flowing ice and thermal–fluvial erosion by supraglacial meltwater streams. We implement 2-D basal transfer functions in seven study regions of the western Greenland Ice Sheet ablation zone using recent data sets for bed elevation, ice surface elevation, and ice surface velocities. We find that ∼1–10 km scale ice surface features can be explained well by bed topography transfer in regions with different multiyear-averaged ice flow conditions. We use flow-routing algorithms to extract supraglacial stream networks from 2 to 5 m resolution digital elevation models and compare these with synthetic flow networks calculated on ice surfaces predicted by bed topography transfer. Multiple geomorphological metrics calculated for these networks suggest that bed topography can explain general ∼1–10 km supraglacial meltwater routing and that thermal–fluvial erosion thus has a lesser role in shaping ice surface topography on these scales. We then use bed topography transfer functions and flow routing to conduct a parameter study predicting how supraglacial IDC configurations and subglacial hydraulic potential would change under varying multiyear-averaged ice flow and basal sliding regimes. Predicted changes to subglacial hydraulic flow pathways directly caused by changing ice surface topography are subtle, but temporal changes in basal sliding or ice thickness have potentially significant influences on IDC spatial distribution. We suggest that changes to IDC size and number density could affect subglacial hydrology primarily by dispersing the englacial–subglacial input of surface meltwater.
Highlights
During warmer months on the Greenland Ice Sheet, surface melting in the ablation zone generates a large volume of water
We identify bed topography transfer as the primary control on internally drained catchment (IDC)-scale (∼ 1–10 km) surface topography and meltwater routing, and we use bed topography transfer functions to predict how Greenland surface IDC configuration and subglacial hydraulic flow pathways would change in response to varying ice flow conditions
Given moulin locations and ice flow conditions, our results imply that bed topography transfer should generally explain IDC configurations, such as the trend observed by Yang and Smith (2016) in which average IDC area increases with increasing ice surface elevation and thickness
Summary
During warmer months on the Greenland Ice Sheet, surface melting in the ablation zone generates a large volume of water. Some meltwater is stored in or flows through porous firn or weathered ice, but most flows across the ice surface, forming networks of supraglacial streams and lakes (such as the stream network shown in Fig. 1b) (Fountain and Walder, 1998; van den Broeke et al, 2009; Andersen et al, 2015) The majority of these streams feed into the englacial and subglacial hydrological systems by either flowing directly into open moulins (e.g., Chu, 2014; Smith et al, 2015) or flowing into supraglacial lakes, which can drain when enough water pressure builds up to hydraulically fracture the ice (Das et al, 2008; Selmes et al, 2011; Stevens et al, 2015). Subglacial water and temporal variations in subglacial water flux can significantly influence ice advection by modulating basal sliding resistance (e.g., Zwally et al, 2002; Schoof, 2010; Sole et al, 2011; Shannon et al, 2013; Tedstone et al, 2014).
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