UNRAVELING SHORT-TERM VARIATIONS IN TIDEWATER GLACIER FLOW: INSIGHTS FROM TERRESTRIAL RADAR INTERFEROMETRIC STUDIES

Tidewater glaciers are fast-flowing valley glaciers that advect ice from the interior of ice sheets to the ocean. Processes along the submarine boundaries of tidewater glacier termini can trigger a dynamic response in glacier ice that can impact stability along the terminus. Predictions of 21st century sea level rise require a comprehensive understanding of tidewater glacier dynamics over a variety of spatial and temporal scales. Perturbations to the calving front, such as iceberg calving, tidal modulations, changes in proglacial ice mélange strength and rigidity, and the subglacial discharge of meltwater occur on time-scales that exceed temporal resolution of satellite measurements; thus, little is known about the dynamic response of glaciers to these processes. Terrestrial radar interferometry is a relatively new technology that measures millimeter scale surface deformation with a spatial resolution comparable to satellites, but at much higher temporal resolution. Here, I use terrestrial radar interferometers to measure short-term variations in speed and surface elevation along Jakobshavn Isbræ, Greenland and Columbia Glacier, Alaska. I find that small calving events can trigger large, dynamic changes in speed and ice thickness. I present observations that show that glacier response to calving events is a consequence of two competing feedbacks: (1) an increase in strain rates leads to dynamic thinning and faster flow, thereby promoting destabilization, whereas (2) an increase in flow rates 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. I also present the first field evidence of a granular ice mélange influence on iceberg calving, which has implications for calving rates, the speed and thickness of the terminus, and consequently tidewater glacier stability. Finally, I present observations of a large increase in speed along Columbia Glacier in response to a precipitation event. The results demonstrate the importance that variations in basal hydrology have on sliding along the bed, and more importantly how changes in the subglacial hydrology can affect the response of a tidewater glacier to tidal fluctuations

Implications of changing winter fjord ice melanges for Greenland outlet glacier dynamics

Recent studies have demonstrated rapid change along the margin of the Greenland Ice Sheet (GIS) over the last decade. In particular, increases in glacier velocities coincident with terminus retreat for many of Greenland\u27s outlet glaciers have effectively increased the amount of ice discharged. Much of this calved ice passes through elaborate fjord systems en route to the ocean. This study utilizes remote sensing observations to investigate the changing conditions in several of Greenland\u27s pro-glacial fjords and changes along glacier termini. The findings indicate that changes in the mix of calved ice and water in pro-glacial fjords have implications for the location of the calving front and for glacier speed and thickness in the near-terminus region on seasonal to interannual time scales. The ability of the fjord ice to influence terminus dynamics and glacier stability has implications for predicting ice loss over much longer time scales

Seasonal and interannual variations in ice melange and its impact on terminus stability, Jakobshavn Isbræ, Greenland

We used satellite-derived surface temperatures and time-lapse photography to infer temporal variations in the proglacial ice melange at Jakobshavn Isbræ, a large and rapidly retreating outlet glacier in Greenland.We used satellite-derived surface temperatures and time-lapse photography to infer temporal variations in the proglacial ice melange at Jakobshavn Isbræ, a large and rapidly retreating outlet glacier in Greenland. Freezing of the melange-covered fjord surface during winter is indicated by a decrease in fjord surface temperatures and is associated with (1) a decrease in ice melange mobility and (2) a drastic reduction in iceberg production. Vigorous calving resumes in spring, typically abruptly, following the steady up-fjord retreat of the sea-ice/ice-melange margin. An analysis of pixel displacement from time-lapse imagery demonstrates that melange motion increases prior to calving and subsequently decreases following several events. We find that secular changes in ice melange extent, character and persistence can influence iceberg calving, and therefore glacier dynamics over daily-to-monthly timescales, which, if sustained, will influence the mass balance of an ice sheet.This research was supported by funds from the Gordon and Betty Moore Foundation (GBMF2627), NASA (NNX08AN74G), the US National Science Foundation (ANT0944193 and ANS0909552) and the New Hampshire Space Grant Consortium (NNX10AL97H). We thank CH2M HILL Polar Services and Air Greenland for logistics support, and PASSCAL (Program for the Array Seismic Studies of theContinental Lithosphere) for the use of seismic instrumentation. Ian Joughin derived TerraSAR-X velocities and terminus positions from images provided by the German (DLR) space agency under NASA grant NNX08AL98A. We acknowledgethe use of Rapid Response imagery from the Land Atmosphere Near-real time Capability for EOS (LANCE) system operated by the NASA/GSFC/Earth Science Data and Information System (ESDIS) with funding provided by NASA HQ. Glacier surface elevations were provided by CReSIS, and bed elevations by CReSIS and Mathieu Morlighem. The manuscript was significantly improved by comments from Tim Bartholomaus and an anonymous reviewer.Ye

Measuring the state and temporal evolution of glaciers in Alaska and Yukon using synthetic-aperture-radar-derived (SAR-derived) 3D time series of glacier surface flow

Climate change has reduced global ice mass over the last 2 decades as enhanced warming has accelerated surface melt and runoff rates. Glaciers have undergone dynamic processes in response to a warming climate that impacts the surface geometry and mass distribution of glacial ice. Until recently no single technique could consistently measure the evolution of surface flow for an entire glaciated region in three dimensions with high temporal and spatial resolution. We have improved upon earlier methods by developing a technique for mapping, in unprecedented detail, the temporal evolution of glaciers. Our software computes north, east, and vertical flow velocity and/or displacement time series from the synthetic aperture radar (SAR) ascending and descending range and azimuth speckle offsets. The software can handle large volumes of satellite data and is designed to work on high-performance computers (HPCs) as well as workstations by utilizing multiple parallelization methods. We then compute flow velocity&ndash;displacement time series for glaciers in southeastern Alaska during 2016&ndash;2021 and observe seasonal and interannual variations in flow velocities at Seward and Malaspina glaciers as well as culminating phases of surging at Klutlan, Walsh, and Kluane glaciers. On a broader scale, this technique can be used for reconstructing the response of worldwide glaciers to the warming climate using archived SAR data and for near-real-time monitoring of these glaciers using rapid revisit SAR data from satellites, such as Sentinel-1 (6 or 12&thinsp;d revisit period) and the forthcoming NISAR mission (12&thinsp;d revisit period).</p

SAR-derived flow velocity and its link to glacier surface elevation change and mass balance

Modern remote sensing techniques, such as Synthetic Aperture Radar (SAR), can measure the direction andintensity of glacier flow. Yet the question remains as to what these measurements reveal about glaciers&rsquo;adjustment to the warming climate. Here, we present a technique that addresses this question by linking the SARderivedvelocity measurements with the glacier elevation change and the specific mass balance (i.e. mass balanceper unit area). The technique computes the speckle offset tracking results from the north, east and vertical flowdisplacement time series, with the vertical component further split into a Surface Parallel Flow (SPF) advectioncomponent due to the motion along a glacier surface slope and a non-Surface Parallel Flow (nSPF). The latterlinks the glacier surface elevation change with the specific mass balance and strain rates. We apply this techniqueto ascending and descending Sentinel-1 data to derive the four-dimensional flow displacement time series forglaciers in southeast Alaska during 2016&ndash;2019. Time series extracted for a few characteristic regions demonstrateremarkable temporal variability in flow velocities. The seasonal signal observed in the nSPF component ismodeled using the Positive Degree Day model. This method can be used for computing either mass balance orglacier surface elevation change if one of these two parameters is known from external observations.</p

Granular decoherence precedes ice mélange failure and glacier calving at Jakobshavn Isbræ

The stability of the world’s largest glaciers and ice sheets depends on mechanical and thermodynamic processes occurring at the glacier–ocean boundary. A buoyant agglomeration of icebergs and sea ice, referred to as ice mélange, often forms along this boundary and has been postulated to affect ice-sheet mass losses by inhibiting iceberg calving. Here, we use terrestrial radar data sampled every 3 min to show that calving events at Jakobshavn Isbræ, Greenland, are preceded by a loss of flow coherence in the proglacial ice mélange by up to an hour, wherein individual icebergs flowing in unison undergo random displacements. A particle dynamics model indicates that these fluctuations are likely due to buckling and rearrangements of the quasi-two-dimensional material. Our results directly implicate ice mélange as a mechanical inhibitor of iceberg calving and further demonstrate the potential for real-time detection of failure in other geophysical granular materials.We thank A. Robel and T. Snow for stimulating conversations. We gratefully acknowledge CH2MHill Polar Service and Air Greenland for logistics support, NASA NNX08AN74G (M.A.F. and M.T.) for funding the field work, financial support from NASA Earth and Space Fellowship NNX14AL29H (R.K.C.), the National Science Foundation grant nos. DMR-1506446 (J.C.B.) and DMR-1506307 (J.M.A. and R.K.C.), and the Gordon and Betty Moore Foundation grants nos. GBMF2626 (M.A.F.) and GBMF2627 (M.T.) for the purchase of the TRIs.Ye

Dynamic jamming of iceberg-choked fjords

We investigate the dynamics of ice mélange by analyzing rapid motion recorded by a time-lapse camera and terrestrial radar during several calving events that occurred at Jakobshavn Isbræ, Greenland. During calving events (1) the kinetic energy of the ice mélange is 2 orders of magnitude smaller than the total energy released during the events, (2) a jamming front propagates through the ice mélange at a rate that is an order of magnitude faster than the motion of individual icebergs, (3) the ice mélange undergoes initial compaction followed by slow relaxation and extension, and (4) motion of the ice mélange gradually decays before coming to an abrupt halt. These observations indicate that the ice mélange experiences widespread jamming during calving events and is always close to being in a jammed state during periods of terminus quiescence. We therefore suspect that local jamming influences longer timescale ice mélange dynamics and stress transmission

Asynchronous behavior of outlet glaciers feeding Godthåbsfjord (Nuup Kangerlua) and the triggering of Narsap Sermia's retreat in SW Greenland

We assess ice loss and velocity changes between 1985 and 2014 of three tidewater and fiveland terminating glaciers in Godthabsfjord (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 melange and conclude that the 2010-14 NS retreat was triggered by a combination of factors but primarily by an increase in submarine melting.We thank W. Dryer and D. Podrasky for assistance with fieldwork and L. Kenefic for assisting with digitizing glacier front positions. CH2 M HILL Polar Services and Air Greenland provided logistics support. The SPOT-5 images used for the 2008 DEM were provided by the SPIRIT program (Centre National d'Etudes Spatiales, France). The DigitalGlobe Worldview images used for the 2014 DEM were obtained from P. Morin. Terminus positions were derived from Landsat images courtesy of the U.S. Geological Survey. Funding was provided by the US National Science Foundation (NSF) Office of Polar Programs (OPP) grants NSF PLR-0909552 and NSF PLR-0909333. Cassotto is supported by NASA under the Earth and Space Science Fellowship Program (Grant NNX14AL29H). K. K. Kjeldsen acknowledges support from the Danish Council Research for Independent Research (grant no. DFF-409000151). K. Kjaer is thanked for his support during the earlier phases of this study. On-ice weather stations are operated by GEUS (Denmark) within the Programme for Monitoring of the Greenland Ice Sheet (PROMICE). J. Mortensen acknowledges support from IIKNN (Greenland), DEFROST project of the Nordic Centre of Excellence program "Interaction between Climate Change and the Cryosphere" and the Greenland Ecosystem Monitoring Programme (www. g-e-m. dk).S. Rysgaard was funded by the Canada Excellence Research Chair Programme. Additional funding was provided by the Geophysical Institute, University of Alaska Fairbanks, and Greenland Climate Research Centre. Scientific editor H. Fricker and reviewers H. Jiskoot and G. Cogley provided very constructive feedback that helped improve the paper.Peer ReviewedRitrýnt tímari