Subglacial Drainage Evolution Modulates Seasonal Ice Flow Variability of Three Tidewater Glaciers in Southwest Greenland

B. J. Davison,T. R. Cowton,D. Fahrner,A. J. Sole,D. A. Slater,P. W. Nienow,J. M. Lea

doi:10.1029/2019jf005492

Abstract

AbstractSurface‐derived meltwater can access the bed of the Greenland ice sheet, causing seasonal velocity variations. The magnitude, timing, and net impact on annual average ice flow of these seasonal perturbations depend on the hydraulic efficiency of the subglacial drainage system. We examine the relationships between drainage system efficiency and ice velocity, at three contrasting tidewater glaciers in southwest Greenland during 2014–2019, using high‐resolution remotely sensed ice velocities, modeled surface melting, subglacial discharge at the terminus, and results from buoyant plume modeling. All glaciers underwent a seasonal speed‐up, which usually coincided with surface melt onset, and subsequent slow‐down, which usually followed inferred subglacial channelization. The amplitude and timing of these speed variations differed between glaciers, with the speed‐up being larger and more prolonged at our fastest study glacier. At all glaciers, however, the seasonal variations in ice flow are consistent with inferred changes in hydraulic efficiency of the subglacial drainage system and qualitatively indicative of a flow regime in which annually averaged ice velocity is relatively insensitive to interannual variations in meltwater supply—so‐called “ice flow self‐regulation.” These findings suggest that subglacial channel formation may exert a strong control on seasonal ice flow variations, even at fast‐flowing tidewater glaciers.

Highlights

The dynamic response of Greenland's tidewater glaciers to changes in environmental conditions remains a key uncertainty in predictions of future sea level rise (Nick et al, 2013)
While we argue that the evidence presented here strongly supports the hypothesis that the seasonal velocity variations at our study glaciers are modulated by subglacial hydrology, it is possible that changes in terminus position and mélange buttressing play a role
We infer that surface‐derived meltwater inputs drive pronounced seasonal changes in ice velocity characterized by early‐summer flow acceleration followed by deceleration either to, or below, pre‐acceleration speeds

Summary

Introduction

The dynamic response of Greenland's tidewater glaciers to changes in environmental conditions remains a key uncertainty in predictions of future sea level rise (Nick et al, 2013). At land‐terminating glacier margins, continual subglacial water flow during the summer months causes the formation of hydraulically efficient subglacial channels These enable the rapid evacuation of meltwater, decreasing basal water pressure, and cause the overlying ice to decelerate in late summer to speeds slower than those prior to the melt season (an “extra slow‐down”) (e.g., Sole et al, 2013). This late‐summer extra slow‐down scales with meltwater supply such that annually averaged ice velocity is insensitive to interannual variations in meltwater supply—the so‐called “ice flow self‐regulation” (Sole et al, 2013; van de Wal et al, 2015).

Methods

Results

Discussion

Conclusion