• Non-linear glacier response to calving events, Jakobshavn Isbræ, Greenland

      Cassoto, Ryan; Fahnestock, Mark; Amundson, Jason M.; Truffer, Martin; Boettcher, Margaret S.; De La Pena, Santiago; Howat, Ian (International Glaciological Society, 2018-11-29)
      Jakobshavn Isbræ, a tidewater glacier that produces some of Greenland’s largest icebergs and highest speeds, reached record-high flow rates in 2012 (Joughin and others, 2014). We use terrestrial radar interferometric observations from August 2012 to characterize the events that led to record-high flow. We find that the highest speeds occurred in response to a small calving retreat, while several larger calving events produced negligible changes in glacier speed. This non-linear response to calving events suggests the terminus was close to flotation and therefore highly sensitive to terminus position. Our observations indicate that a glacier’s response to calving is a consequence of two competing feedbacks: (1) an increase in strain rates that leads to dynamic thinning and faster flow, thereby promoting desta- bilization, and (2) an increase in flow rates that advects thick ice toward the terminus and promotes restabilization. The competition between these feedbacks depends on temporal and spatial variations in the glacier’s proximity to flotation. This study highlights the importance of dynamic thinning and advective processes on tidewater glacier stability, and further suggests the latter may be limiting the current retreat due to the thick ice that occupies Jakobshavn Isbræ’s retrograde bed.
    • 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.