Supraglacial river forcing of subglacial water storage and diurnal ice sheet motion

Abstract

Surface melting impacts ice sheet sliding by supplying water to the bed, but subglacial processes driving ice accelerations are complex. We examine linkages between surface runoff, transient subglacial water storage, and short-term ice motion from 168 consecutive hourly measurements of meltwater discharge (moulin input) and GPS-derived ice surface motion for Rio Behar, a ∼60 km 2 moulinterminating supraglacial river catchment on the southwest Greenland Ice Sheet. Short-term accelerations in ice speed correlate strongly with lag-corrected measures of supraglacial river discharge (r = 0.9, τ = 0.7, p < 0.01). Though our 7 days record cannot address seasonal-scale forcing, diurnal ice accelerations align with normalized differenced supraglacial and proglacial discharge, a proxy for subglacial storage change, better than GPS-derived ice surface uplift. These observations counter theoretical steady state basal sliding laws and suggest that moulin and proglacially induced fluctuations in subglacial water storage, rather than absolute subglacial water storage, drive short-term ice accelerations. Plain Language SummaryThe importance of surface meltwater runoff to Greenland ice sheet subglacial hydrology and ice sliding dynamics is widely recognized but poorly constrained by field observations. We present 168 consecutive hours of rare in situ discharge measurements in a large supraglacial river draining the ice sheet surface, just upstream of where it plummets into a major moulin. GPS measurements of ice surface motion record brief accelerations in ice sliding speed that follow daily cycles in meltwater entering the moulin. By comparing these measurements with proglacial river discharges leaving the ice sheet, we identify daily fluctuations in subglacial water storage that track shortterm accelerations in ice motion. These findings affirm the importance of supraglacial rivers to subglacial water pressure and ice dynamics, even in relatively thick ice >40 km inland from the ice terminus.

Greenland Ice Sheet runoff reduced by meltwater refreezing in bare ice

“…Our previous field measurements during July 2015 found that regional and global climate models overpredicted Rio Behar runoff by +21-58% 8 . During a 6-13 July 2016 field experiment we revisited the site to collect a seven-day record of 168 consecutive hourly discharges 34 , concurrent with three-hourly ice surface lowering measurements from a network of ablation stakes installed near the gauging site (Methods and Figure S2). Shallow ice cores collected at the same site in July 2016 revealed that the bare-ice surface was porous and saturated with meltwater, with an average bare-ice density of just 690 kg m -3 within the upper 1.1 m of ice 26 .…”

“…8 , two supraglacial lake catchments with satellite lake volume (SLV1 and SLV2) runoff estimates 12 , the Leverett Glacier (LG) catchment 42 , and the Akuliarusiarsuup Kuua River's northern tributary (AK4) catchment 48 sub-hourly measurements and associated quality measures were recorded. These raw data were converted to channel flow rate [m 3 s -1 ] in post-processing following data quality-control workflows optimized for the supraglacial environment 8,34 . The sub-hourly measurements were used to estimate ~±15 m 3 s -1 measurement uncertainty (error bars in Figure 1 representing one standard deviation in sub-hourly discharge).…”