Top) The August-2006 speedup (color) relative to the September-2004-to-December-2006 mean speed displayed over SAR imagery (grayscale). Speeds are determined using overlapping tracks that span two 24-day intervals centered on August 5 and 13. Mean speed is shown with blue, 50-m/yr (50-150 m/yr) and magenta, 200-m/yr (200-1000 m/yr) contours. At the lake sites (white circles), the InSAR differences are replaced with corresponding values from GPS observations for the same period (see plot for time series). Surface elevation is contoured at 250-m intervals (black).
Bottom) Speed from GPS stations located south of Jakobshavn Isbrae . We installed one summer continuous (15 s) GPS receiver (NS) and one annual (operating 12-hour/week) GPS receiver (NA). The South Lake had one annual (SA) and two summer (SS1 and SS2) receivers. In summer 2007 NS and SS2 did not resume logging and SA lost power for 3 months. The summer-station data has been smoothed to 2-day resolution and the year-round estimates are based on weekly positions. Positive-degree-day values are included for both sites in 2006 and for the South Lake in 2007. Gray shading indicates when the North Lake drained in 2006.
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Collaborative Research: Behavior of supraglacial lakes and their role in outlet glacier dynamics and mass balance of the Greenland Ice Sheet
Goals
Through this project we are investigating the role of Greenland’s supraglacial lakes in delivering melt water to the ice sheet’s bed and in modulating ice flow on short time-scales. This effort uses a combination of satellite and ground-based observations.
Methods and Observations
To investigate the process through which water hydro-fractures through kilometer-thick ice, we instrumented two supraglacial lakes on the west coast of Greenland, near Illulisat. We documented the rapid (< 2 hours) drainage of a large supraglacial lake down 980 meters through to the bed of the Greenland Ice Sheet initiated by water-driven fracture propagation evolving into moulin flow [Das et al., 2008]. Drainage coincided with increased seismicity, transient acceleration, ice-sheet uplift, and horizontal displacement. During peak drainage, the flow of water into the crack exceeded the flow over Niagara Falls. Subsidence and deceleration occurred over the subsequent 24 hours. The short-lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially. These observations clearly confirm earlier theoretical predictions that hydro-fracturing can breach thick, cold ice to establish a surface meltwater connection to the bed.
It has been widely hypothesized that a warmer climate in Greenland would increase the volume of lubricating surface meltwater reaching the ice-bedrock interface, accelerating ice flow and increasing mass loss. We assembled a satellite data set that provides a synoptic-scale view, spanning ice-sheet to outlet-glacier flow, with which to evaluate this hypothesis [Joughin et al., 2008]. On the ice sheet, these data reveal summer speedups (50 to 100%) consistent with, but somewhat larger than, earlier observations. The relative speedup of outlet glaciers, however, is far smaller (< 15%). Furthermore, the dominant seasonal influence on Jakobshavn Isbrae’s flow is the calving front’s annual advance and retreat. With other effects producing outlet-glacier speedups an order of magnitude larger, seasonal melt’s influence on ice flow is likely confined to those regions dominated by ice-sheet flow.
