Abstract:
Outlet glaciers in Greenland, and elsewhere, have recently shown large variations in terminus position and ice flux. One example is the tidewater retreat of Jakobshavn Isbr�_, which began in the late 1990s with high thinning rates, acceleration and collapse of the floating glacier tongue. The retreat has continued to the present, with glacier speeds more than doubling in two decades' time. A campaign of in-situ measurements was initiated in 2006 with the aim of determining the importance of short-term forcing as a control on the continuing evolution of the glacier. Three years of continuous GPS measurements along the centerline of Jakobshavn Isbr�_ reveal seasonal velocity variations due to seasonally varying terminus position. The relationship between glacier speed and surface melt is complex, with both speed-up and slowdown events in response to variations in the rate of surface melt. During a particularly long and intense melt season in 2007, a series of melt-driven slowdowns effectively reduced the mean ice flow over the whole year. On shorter timescales, the response to surface meltwater input is more predictable with diurnal velocity variations of 1-2 % that closely match changes in meltwater input. The influence of iceberg calving and tidal forcing is restricted to the lower 10 km of the glacier, imposing an upper limit on longitudinal stress coupling length of a few ice thicknesses. The response to these forcings does not exceed 5 % of mean flow. This is consistent with a glacier operating under high driving stresses. Ice sheet velocities as far as 120 km inland of the margin have responded to the continuing retreat with increases in speed. The flow has also rotated toward the centerline of the main channel. This speedup and channelization of flow are the result of evolving ice surface gradients as the glacier continues to respond to changes initiated at the periphery. This shows that ocean driven changes have led to increased ice flux far inland on the Greenland Ice Sheet, implying a continuing large-scale evolution of the Jakobshavn Isbr�_ drainage basin.
