Tidal movement of Nioghalvfjerdsfjorden glacier, northeast Greenland: observations and modelling

Nioghalvfjerdsfjorden glacier is a >60 km long and 20 km wide floating outlet glacier located at 79°30'N, 22° W, draining a large area of the northeast Greenland ice sheet. Climate, mass-balance and dynamics studies were carried out on the glacier in three field seasons in 1996, 1997 and 1998. As part of this work, tidal-movement observations were carried out by simultaneous differential global positioning system (GPS) measurements at several locations distributed on the glacier surface. The GPS observations were performed continously over several tidal cycles. At the same time, tiltmeter measurements were carried out in the grounding zones along the glaciers margins and upstream, where the glacier leaves the main ice sheet. A tide gauge installed in the sea immediately in front of the glacier front recorded the tide in the open sea during the field seasons. The observations show that the main part of the glacier tongue responds as a freely floating plate to the phase and amplitude of the local tide in the sea. However, kilometre-wide flexure zones exist along the marginal and upstream grounding lines. Attempts to model the observed tidal defectionand tilt patterns in the flexure zone by elastic-beam theory are unsuccessful, in contrast to previous findings by other investigators. The strongest disagreement between our measurements and results derived from elastic-beam theory is a significant variation of the phase of the tidal records with distance from the grounding line (most clearly displayed by the tilt records). We suggest that the viscous properties of glacier ice must be taken into account, and consequently that a viscoelastic-beam model must be used to adequately describe tidal bending of floating glaciers