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Installation of continuous GPS data on and near glaciers, ice streams and ice sheets, which, when linked with bedrock reference GPS, is enabling a new understanding of ice flow and dynamics. The previously held view of ice-sheet flow as varying only slowly in time and space is rapidly being replaced by a more nuanced view in which different regions of an ice sheet may exhibit orders of magnitude differences in response time and spatial gradient. For instance, although the inland East Antarctica has been remarkably stable for the last million years (EPICA, 2004), ice shelf collapse caused an order-of-magnitude acceleration over months in the Antarctic Peninsula (Rignot et al., 2004; Scambos et al., 2004). The Whillans Ice Stream varies even more strongly, stopping and starting in minutes twice every diurnal period (Bindschadler et al., 2003). Switching in response time is of great importance. Sea-level projections (e.g., IPCC, 2007) typically have assumed that the response times will remain unaltered, an assumption that may be unjustified. Recent geodetic measurements have shown that glaciers and ice streams change velocity and seismicity in response to ocean and tidal forcings, including ice streams deep in the Antarctic interior. (Anandakrishnan et al., 2003; Bindschadler et al., 2003 ). Determining whether variations in sea ice conditions can cause fluctuations in ice sheet discharge will provide constraints for models of ice sheet response as ongoing ocean warming and climate alter sea/ice configurations. Also, CGPS measurements using so-called “Super poles” (where the GPS antenna mount is effectively buried to depths of 20 or 30 m by freezing in a marker at that depth; see Hamilton et al., 1998) are leading to a better understanding of the balance between accumulation rate variability and long-term ice-dynamics flow variability. These data are critical for interpreting shallow ice cores and ice-penetrating radar data. |
Figure 1 - Field results showing the northing-easting positions (determined by GPS) at 5-minute intervals on Whillans Ice Stream. Upper inset shows enlargement of five slip events. Note the cluster of stagnant positions followed by rapid slip of 10-20 cm in 15 minutes. (b) The displacement history indicates the relationship between the tide (lower plot) and the slip speed (upper plot is displacement in 5 minute intervals). Note that displacement magnitude scales with tidal magnitude (spring vs. neap), but with a possible long-period offset of 1-3 days, which our current models do not capture. Winberry, J.P., S. Anandakrishnan, R.B. Alley, R.A. Bindschadler and M.A. King, Basal mechanics of ice streams: Insights from the stick-slip motion of Whillans Ice Stream, West Antarctica. J. Geophys. Res. 114, F01016, DOI:10.1029/2008JF001035. 
Figure 1 - This image is an interferogram of the Rutford Ice Stream in Antarctica. Both the rapid flow of the ice stream and the location of the grounding line are visible. More recent InSAR measurements of the velocity field of glaciers in Greenland suggest that the mass flux due to melting at the base of the floating glacier, relative to calving of icebergs at the end of the glacier, is much greater than previously believed and may contribute to global sea level rise. Monitoring around the periphery of the West Antarctic ice sheet by means of InSAR can provide an important new method for tracking changes in the ice sheet that may presage unstable behavior. From a purely research perspective, InSAR-derived velocity field will provide a dramatic improvement in our ability to constrain the parameters controlling the dynamics of the flow of glacier ice (e.g., ice rheology and basal boundary conditions). The fringes show displacements over a 6 day period with each color cycle representing 28~mm of LOS displacement. (B) Location map for (A). Downstream from the grounding line, the ice stream is afloat. Courtesy D. Goldstein, JPL. Richard M. Goldstein, Hermann Engelhardt, Barclay Kamb, and Richard M. Frolich, Satellite Radar Interferometry for Monitoring Ice Sheet Motion: Application to an Antarctic Ice Stream, Science 3 December 1993: Vol. 262. no. 5139, pp. 1525 - 1530DOI: 10.1126/science.262.5139.1525. More information is available here. |
Last modified Friday, 28-Jan-2011 21:10:24 UTC