Evidence for Non-tidal Diurnal Velocity Variations of Helheim Glacier, East Greenland

Space geodetic observations acquired on glaciers provide a detailed record of flow variations in response to a wide range of dynamic processes, including ocean-tidal forcing. Understanding a glacier's response to tides yields insight into its elastic and rheological properties as well as coupling to the glacier bed. We have used GPS data acquired on Helheim Glacier in East Greenland and tide-gauge data from its fjord to investigate the spectra of tidal forcing and flow response. For the semidiurnal tidal components, the observed glacier flow variations can be related to the tidal force by assuming a linear delayed response with a single admittance and time lag. However, we find that there are diurnal variations in the glacier flow that are not consistent with this model of glacier response to tidal forcing. The figure shows a phasor diagram of the observed S₁ and M₂ tidal components, as well as a prediction from the tide-gauge spectra based on the admittance and lag derived from the semidiurnal components. This figure illustrates the inconsistency of the tidal forcing and glacier flow at the S₁ period of 1 day. We conclude that this inconsistency represents non-tidal diurnal velocity variations of the flow of Helheim Glacier that account for a peak-to-peak variation in glacier flow speed at a site located close to the terminus of ~0.8 m/d. At this site, the mean flow rate was ~20 m/d. The speed variations have a 9-day modulation and reach their daily maximum value 4.0-4.5 hours after local noon. Based on these results, we hypothesize that melting associated with solar heating causes diurnal lubrication of the glacier bed. Phasor diagram of the S₁ and M₂ tidal components for the GPS time series of along-flow position (blue) and a prediction (red) based on the tide-gauge data using an admittance and lag calculated from the semidiurnal components.