• Ice me ́lange dynamics and implications for terminus stability, Jakobshavn Isbræ, Greenland

      Amundson, Jason M.; Fahnestock, M.; Truffer, M.; Brown, J.; Luthi, M. P.; Motyka, R. J. (American Geophysical Union, 2010-01-21)
      We used time-lapse imagery, seismic and audio recordings, iceberg and glacier velocities, ocean wave measurements, and simple theoretical considerations to investigate the interactions between Jakobshavn Isbræ and its proglacial ice me ́lange. The me ́lange behaves as a weak, granular ice shelf whose rheology varies seasonally. Sea ice growth in winter stiffens the me ́lange matrix by binding iceberg clasts together, ultimately preventing the calving of full-glacier-thickness icebergs (the dominant style of calving) and enabling a several kilometer terminus advance. Each summer the me ́lange weakens and the terminus retreats. The me ́lange remains strong enough, however, to be largely unaffected by ocean currents (except during calving events) and to influence the timing and sequence of calving events. Furthermore, motion of the me ́lange is highly episodic: between calving events, including the entire winter, it is pushed down fjord by the advancing terminus (at 40 m d1), whereas during calving events it can move in excess of 50 103 m d1 for more than 10 min. By influencing the timing of calving events, the me ́lange contributes to the glacier’s several kilometer seasonal advance and retreat; the associated geometric changes of the terminus area affect glacier flow. Furthermore, a force balance analysis shows that large-scale calving is only possible from a terminus that is near floatation, especially in the presence of a resistive ice me ́lange. The net annual retreat of the glacier is therefore limited by its proximity to floatation, potentially providing a physical mechanism for a previously described near-floatation criterion for calving.
    • The influence of ice melange on fjord seiches

      MacAyeal, Douglas R.; Freed-Brown, Julian; Zhang, Wendy W.; Amundson, Jason M. (International Glaciological Society, 2012)
      We compute the eigenmodes (seiches) of the barotropic and baroclinic hydrodynamic equations for an idealized fjord having length and depth scales similar to those of Ilulissat Icefjord, Greenland, into which Jakobshavn Isbræ (also known as Sermeq Kujalleq) discharges. The purpose of the computation is to determine the fjord’s seiche behavior when forced by iceberg calving, capsize and melange movement. Poorly constrained bathymetry and stratification details are an acknowledged obstacle. We are, nevertheless, able to make general statements about the spectra of external and internal seiches using numerical simulations of ideal one-dimensional channel geometry. Of particular signifi- cance in our computation is the role of weakly coupled ice melange, which we idealize as a simple array of 20 icebergs of uniform dimensions equally spaced within the fjord. We find that the presence of these icebergs acts to (1) slow down the propagation of both external and internal seiches and (2) introduce band gaps where energy propagation (group velocity) vanishes. If energy is introduced into the fjord within the period range covered by a band gap, it will remain trapped as an evanescent oscillatory mode near its source, thus contributing to localized energy dissipation and ice/melange fragmentation.