Velocity observations at Jakobshavn Isbrae 2006-2008

After nearly doubling its speed concurrently with substantial thinning in the late 1990s, Jakobshavn Isbrae is currently one of the fastest flowing glaciers in Greenland. Since summer 2006 we have used GPS to record velocities at four points along the main trunk of the glacier and at several points on the adjacent ice sheet. Velocities in the lower 20-30 km have mostly remained steady at post-speedup values. But the speed up continues to propagate to the upper reaches of the glacier. Measurements from the adjacent ice sheet indicate that since the mid-1980's, the velocity magnitude has increased and that flow has become more convergent towards the main channel. Velocity variations on diurnal and multiday timescales were also observed, but never exceeded 10% of the background velocity. In summer 2007 a speed-up event that may have been caused by a hydraulic event led to an eventual slow down of the ice stream. The slow down was accompanied by a 0.1 m drop in surface elevation, possibly due to the evacuation of basal water and increased efficiency of the subglacial drainage system. In both 2007 and 2008, diurnal velocity variations started appearing after a pronounced speed-up event. Diurnal velocity variations on the ice stream tend to be smaller (in a relative sense) than the variations on the adjacent ice sheet and they tend to lag those variations by 1-2 hours. This is an indication that runoff driven velocity variations propagate from the ice sheet down into the ice stream, while it has been shown that the seasonal velocity variation of the main trunk is strongly coupled to the position of the terminus.