• Glacier, fjord, and seismic response to recent large calving events, Jakobshavn Isbræ, Greenland

      Amundson, Jason M.; Truffer, M.; Luthi, M. P.; Fahnestock, M.; West, M.; Motyka, R. J. (American Geophysical Union, 2008-11-18)
      The recent loss of Jakobshavn Isbræ’s extensive floating ice tongue has been accompanied by a change in near terminus behavior. Calving currently occurs primarily in summer from a grounded terminus, involves the detachment and overturning of several icebergs within 30 – 60 min, and produces long-lasting and far-reaching ocean waves and seismic signals, including ‘‘glacial earthquakes’’. Calving also increases near-terminus glacier velocities by 3% but does not cause episodic rapid glacier slip, thereby contradicting the originally proposed glacial earthquake mechanism. We propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving.
    • Outlet glacier response to forcing over hourly to interannual timescales, Jakobshavn Isbræ, Greenland

      Podrasky, David; Truffer, Martin; Fahnestock, Mark; Amundson, Jason M.; Cassoto, Ryan; Joughin, Ian (International Glaciological Society, 2012-09-07)
      The loss of the floating ice tongue on Jakobshavn Isbræ, Greenland, in the early 2000s has been concurrent with a pattern of thinning, retreat and acceleration leading to enhanced contribution to global sea level. These changes on decadal timescales have been well documented. Here we identify how the glacier responds to forcings on shorter timescales, such as from variations in surface melt, the drainage of supraglacial lakes and seasonal fluctuations in terminus position. Ice motion and surface melt were monitored intermittently from 2006 to 2008. Dual-frequency GPS were deployed 20–50 km upstream of the terminus along the glacier center line. Gaps in surface melt measurements were filled using a temperature-index model of ablation driven by surface air temperatures recorded during the same time period. Our results corroborate the premise that the primary factors controlling speeds on Jakobshavn Isbræ are terminus position and geometry. We also observe that surface speeds demonstrate a complex relationship with meltwater input: on diurnal timescales, velocities closely match changes in water input; however, on seasonal timescales a longer, more intense melt season was observed to effectively reduce the overall ice flow of the glacier for the whole year.