Abstract
Abstract. Due to increasing surface melting on the Greenland ice sheet, better constraints on seasonally evolving basal water pressure and sliding speed are required by models. Here we assess the potential of using inverse methods on a dense time series of surface speeds to recover the seasonal evolution of the basal conditions in a well-documented region in southwest Greenland. Using data compiled from multiple satellite missions, we document seasonally evolving surface velocities with a temporal resolution of 2 weeks between 2015 and 2019. We then apply the inverse control method using the ice flow model Elmer/Ice to infer the basal sliding and friction corresponding to each of the 24 surface velocity data sets. Near the margin where the uncertainty in the velocity and bed topography are small, we obtain clear seasonal variations that can be mostly interpreted in terms of an effective-pressure-based hard-bed friction law. We find for valley bottoms or “troughs” in the bed topography that the changes in modelled basal conditions directly respond to local modelled water pressure variations, while the link is more complex for subglacial “ridges” which are often non-locally forced. At the catchment scale, in-phase variations in the water pressure, surface velocities, and surface runoff variations are found. Our results show that time series inversions of observed surface velocities can be used to understand the evolution of basal conditions over different timescales and could therefore serve as an intermediate validation for subglacial hydrology models to achieve better coupling with ice flow models.
Highlights
In recent decades, the Greenland ice sheet (GrIS) has been losing mass, reaching a negative mass balance of about −286 ± 20 Gt yr−1 in 2010–2018 (Mouginot et al, 2019)
This is 10 years earlier than our velocity observations. Thickness changes in this area are about −1 m yr−1 near the margin (Helm et al, 2014; Csatho et al, 2014; Yang et al, 2019), which is relatively small compared to the ice thickness; thereby we use it as it is
The areas of friction close to zero along the margin could be due to underestimation induced by BedMachine as well, as here, on the steep slopes of thin ice, the reported error-to-thickness ratio is larger than 50 % (Fig. A2a)
Summary
The Greenland ice sheet (GrIS) has been losing mass, reaching a negative mass balance of about −286 ± 20 Gt yr−1 in 2010–2018 (Mouginot et al, 2019). Increased water pressure as melt drains through an inefficient drainage system is assumed by existing theories to be the mechanism driving the acceleration during the beginning of the melt season. Increased drainage efficiency during the late melt season leads to a decrease in water pressures and causes a commensurate glacier deceleration. During high melt years, higher early summer velocities are supposed to be responsible for the slower velocities during the late melt and winter seasons, offsetting the higher initial ice flux (Tedstone et al, 2015).
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