TY - JOUR T1 - Non-linear glacier response to calving events, Jakobshavn Isbræ, Greenland JF - Journal of Glaciology Y1 - 2019 A1 - Cassotto, Ryan A1 - Fahnestock, Mark A1 - Amundson, Jason M. A1 - Truffer, Martin A1 - Boettcher, Margaret S. A1 - De La Peña, Santiago A1 - Howat, Ian KW - calving KW - dynamic thinning KW - Jakobshavn Isbræ KW - terrestrial radar interferometry KW - tidewater glaciers AB - 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 destabilization, 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. VL - 65 ER - TY - JOUR T1 - Acquisition of a 3 min, two-dimensional glacier velocity field with terrestrial radar interferometry JF - Journal of Glaciology Y1 - 2017 A1 - Voytenko, Denis A1 - Dixon, Timothy H. A1 - Holland, David M. A1 - Cassotto, Ryan A1 - Howat, Ian M. A1 - Fahnestock, Mark A. A1 - Truffer, Martin A1 - De La Peña, Santiago KW - glacier flow KW - glacier geophysics KW - glaciological instruments and methods AB - {\textless}p{\textgreater}Outlet glaciers undergo rapid spatial and temporal changes in flow velocity during calving events. Observing such changes requires both high temporal and high spatial resolution methods, something now possible with terrestrial radar interferometry. While a single such radar provides line-of-sight velocity, two radars define both components of the horizontal flow field. To assess the feasibility of obtaining the two-dimensional (2-D) flow field, we deployed two terrestrial radar interferometers at Jakobshavn Isbrae, a major outlet glacier on Greenland's west coast, in the summer of 2012. Here, we develop and demonstrate a method to combine the line-of-sight velocity data from two synchronized radars to produce a 2-D velocity field from a single (3 min) interferogram. Results are compared with the more traditional feature-tracking data obtained from the same radar, averaged over a longer period. We demonstrate the potential and limitations of this new dual-radar approach for obtaining high spatial and temporal resolution 2-D velocity fields at outlet glaciers.{\textless}/p{\textgreater} UR - https://www.cambridge.org/core/product/identifier/S0022143017000284/type/journal{\_}article ER - TY - JOUR T1 - Asynchronous behavior of outlet glaciers feeding Godth\aabsfjord (Nuup Kangerlua) and the triggering of Narsap Sermia's retreat in SW Greenland JF - J. Glaciol. Y1 - 2017 A1 - Motyka, Roman J. A1 - Cassotto, Ryan A1 - Truffer, Martin A1 - Kjeldsen, Kristian K. A1 - Van As, Dirk A1 - Korsgaard, Niels J. A1 - Fahnestock, Mark A1 - Howat, Ian A1 - Langen, Peter L. A1 - Mortensen, John A1 - Lennert, Kunuk A1 - Rysgaard, Søren KW - glacier calving KW - glacier discharge KW - glacier mass balance KW - ice/atmosphere interactions KW - ice/ocean interactions KW - tidewater glaciers AB - We assess ice loss and velocity changes between 1985 and 2014 of three tidewater and five-land terminating glaciers in Godth{\aa}bsfjord (Nuup Kangerlua), Greenland. Glacier thinning accounted for 43.8 ± 0.2 km 3 of ice loss, equivalent to 0.10 mm eustatic sea-level rise. An additional 3.5 ± 0.3 km 3 was lost to the calving retreats of Kangiata Nunaata Sermia (KNS) and Narsap Sermia (NS), two tidewater glaciers that exhibited asynchronous behavior over the study period. KNS has retreated 22 km from its Little Ice Age (LIA) maximum (1761 AD), of which 0.8 km since 1985. KNS has stabilized in shallow water, but seasonally advects a 2 km long floating tongue. In contrast, NS began retreating from its LIA moraine in 2004–06 (0.6 km), re-stabilized, then retreated 3.3 km during 2010–14 into an over-deepened basin. Velocities at KNS ranged 5–6 km a −1 , while at NS they increased from 1.5 to 5.5 km a −1 between 2004 and 2014. We present comprehensive analyses of glacier thinning, runoff, surface mass balance, ocean conditions, submarine melting, bed topography, ice mélange and conclude that the 2010–14 NS retreat was triggered by a combination of factors but primarily by an increase in submarine melting. VL - 63 ER - TY - JOUR T1 - Seasonal and interannual variations in ice melange and its impact on terminus stability, Jakobshavn Isbræ, Greenland JF - Journal of Glaciology Y1 - 2015 A1 - Cassotto, Ryan A1 - Fahnestock, Mark A1 - Amundson, Jason M. A1 - Truffer, Martin A1 - Joughin, Ian KW - arctic glaciology KW - calving KW - ice KW - ocean interactions KW - Remote sensing KW - sea-ice dynamics VL - 61 ER - TY - JOUR T1 - Outlet glacier response to forcing over hourly to interannual timescales, Jakobshavn Isbræ, Greenland JF - J. Glaciol. Y1 - 2012 A1 - Podrasky, David A1 - Truffer, Martin A1 - Fahnestock, Mark A1 - Amundson, Jason M. A1 - Cassotto, Ryan A1 - Joughin, Ian VL - 58 UR - http://openurl.ingenta.com/content/xref?genre=article{&}issn=0022-1430{&}volume=58{&}issue=212{&}spage=1212 ER - TY - JOUR T1 - Outlet glacier response to forcing over hourly to interannual timescales, Jakobshavn Isbræ, Greenland JF - Journal of Glaciology Y1 - 2012 A1 - Podrasky, David A1 - Martin Truffer A1 - Mark Fahnestock A1 - Jason M Amundson A1 - Cassotto, Ryan A1 - Ian Joughin VL - 58 ER -