Subglacial hydrology modeling predicts high winter water pressure and spatially variable transmissivity at Helheim Glacier, Greenland

Abstract

Water pressure beneath glaciers influences ice velocity. Subglacial hydrology models are helpful for gaining insight into basal conditions, but models depend on unconstrained parameters, and a current challenge is reproducing elevated water pressures in winter. We eliminate terms related to englacial storage, opening by sliding, and melt due to changes in the pressure-melting-point temperature, to create a minimalist version of the Subglacial Hydrology And Kinetic, Transient Interactions (SHAKTI) model, and apply this model to Helheim Glacier in east Greenland to explore the winter base state of the subglacial drainage system. Our results suggest that meltwater produced at the bed alone supports active winter drainage with large areas of elevated water pressure and preferential drainage pathways, using a continuum approach that allows for transitions between flow regimes. Transmissivity varies spatially over several orders of magnitude from 10−4 to 103 m2s−1, with regions of weak transmissivity representing poorly connected regions of the system. Bed topography controls the location of primary drainage pathways, and high basal melt rates occur along the steep valley walls. Frictional heat from sliding is a dominant source of basal melt; different approaches for calculating basal shear stress produce significantly different basal melt rates and subglacial discharge.

Numerical and physical instability of subglacial water flow

“…In equation ( 2), we take the closure lengthscale (average cavity width) as equal to b (Schoof, 2010;Werder et al, 2013;Sommers et al, 2018), but other functions of b have also been proposed, such as l r /(1−b/b r ) by Kyrke-Smith et al (2014). Because opening by sliding may be less active beneath soft-bedded glaciers (Sommers et al, 2023), we include it in our analysis for comparison with I. J. Hewitt (2011), but remove it in our example calculations.…”

“…As a demonstration, we ran SHAKTI in the ISSM framework on a domain including Russell Glacier (RG) and Isunnguata Sermia (IS) in Southwest Greenland under winter (no meltwater input) conditions (per Sommers et al, 2023), using geometry from Bed-Machine v4 (Morlighem et al, 2017) and velocities from MEaSUREs (Joughin et al, 2018).…”

“…This categorisation shows some spatial clustering of seasonal patterns, but also reveals that the response of a single glacier can change year-on-year based on the climatic conditions, and neighbouring glaciers can respond quite differently. Models of summertime hydrology often assume that no channels persist through the winter, but some studies show persistent winter channels (Hager et al, 2022;Sommers et al, 2023). Thus a small velocity response could be attributable either to no channelization during the summer or persistent channelization during the winter.…”

Unique In-situ Measurements from Greenland Fjord Show Winter Freshening by Subglacial Melt

“…As a consequence, model estimates of winter subglacial discharge differ by orders of magnitude (cf. [5,22,23]). One attempt to measure winter subglacial discharge in Kangersuneq (in Nuup Kangerlua, West Greenland) detected no significant freshwater fluxes [1].…”

Seasonal Flow Types of Glaciers in Sermilik Fjord, Greenland, Over 2016–2021

“…If there is some runoff control at Helheim Glacier, this suggests that the subglacial hydrologic system remains inefficient through the melt season. Recent studies using subglacial modeling (Poinar et al, 2019;Sommers et al, 2023) and direct observations of plumes (Everett et al, 2016;Melton et al, 2022), however, suggest a seasonally channelized subglacial environment on the lower reach of Helheim Glacier. These conclusions are not necessarily inconsistent.…”