Ocean tides modulation of flow at Helheim Glacier, East Greenland, observed using GPS

Observations at high spatial and temporal resolution could be key for improving our understanding of the physical processes that govern outlet-glacier flow variations. We collected simultaneous high-rate GPS observations at several locations distributed along and across Helheim Glacier, East Greenland, during the four Arctic summers of 2006-2009, along with other geophysical observations, to study glacial earthquakes and glacier dynamics. GPSderived position estimates of centimeter-level precision reveal the surface expression of glaciological signals, occurring from sub-hourly to daily time-scales and beyond. We find that the flow velocity of Helheim Glacier is modulated by ocean tides in a region including both sides of the grounding line. An admittance analysis of the tidal signal shows an exponential decrease in amplitude with distance from the calving front, along with increasing time delays in both the vertical and horizontal components. Moreover, the along-flow admittance amplitude and time delay estimates are highly time dependent, both with a periodicity of ~15 days, thus suggesting a response to a fortnightly tidal frequency. In addition, we observe transitory increases in the amplitude of the tidally-driven flow response coinciding with glacial earthquakes events, which are associated to large losses of mass at the calving front and step-like increases in flow speed. We will present an analysis of the tidal modulation signals observed at Helheim and discuss how those signals may place new constraints on models for glacier flow variations.