Resumen de Tidewater glacier flow of Helheim Glacier, Greenland, 2006-2008, using high-rate GPS
Greenland's main outlet glaciers are highly sensitive to climate conditions, and have been recently observed to experience rapid dynamic changes. However, they remain poorly monitored and understood. This study presents an analysis of high-rate Global Positioning System (GPS) observations from a dense network installed at Helheim Glacier, east Greenland, during the Arctic summers of 2006-2008, that results in a new data set of glacier flow. To obtain optimal estimates of spatio-temporal variations of glacier flow, an investigation of errors of the high-rate GPS technique in glaciology is undertaken. The resulting high-precision (cm level) and high-rate (15 s) position estimates reveal the surface expression of glaciological signals, occurring from sub-hourly to daily time-scales and beyond, enabling the characterization of glacier deformation at temporal resolutions not achievable with most standard techniques employed in glaciology. Particular emphasis is placed on the study of the response of Helheim Glacier to ocean tidal forcing. This study is part of a multidisciplinary project that investigates the dynamics of tidewater fast-flowing glaciers in Greenland using high-rate GPS along with other geophysical sensors. The new data set of glacier flow contributes to an improved understanding of Helheim Glacier in various ways. First, estimates of mean flow of Helheim Glacier during the 2006-2008 period are presented, which ranged from 24 m/d near the terminus to 4 m/d 37 km upglacier, with 2007 flowing faster, showing more variability, and exhibiting the largest number of glacial earthquakes. Then, sub-seasonal to sub-daily flow variations are presented. In particular, we present a detailed study of the semi-diurnal variations in the flow of the lower 12-16 km of Helheim Glacier due to hydrostatic pressure changes at the glacier terminus caused by ocean tides. An admittance analysis shows that the amplitude of the tidal response decays exponentially with distance from the calving front in the three components of flow, and that the response of the glacier is delayed with respect to the ocean tides by 1-2 h. Moreover, the analysis shows that the tidal response in the along-flow component is highly time varying, both on long timescales, with a non-linear response of ~15 days periodicity, and on short timescales, with abrupt changes associated with calving events and glacial earthquakes. For example, the along-flow tidal response after a glacial earthquake increases abruptly, by a factor of as much as 2.5, and recovers after 1-2 days, which could be explained by the disruption of the subglacial drainage system and a concomitant reduction in flow resistance. Last, variations of Helheim Glacier's flow for two important glacier dynamic processes are also presented. These include flow variations associated with glacial earthquakes and calving events, and with the potential drainage of supraglacial meltwater lakes. This thesis demonstrates that the analysis of the high-rate, high-precision GPS observations acquired at the surface of Greenland fast-flowing outlet glaciers provides valuable information pertaining to their dynamics, including their response to small and continuous forcings, such as the ocean tide, and also to large and sudden changes in the glaciers boundary conditions, such as those related to glacial earthquakes and meltwater pulses.
