| Summary: | Observations this century have revealed that the Greenland Ice Sheet (GrIS) is losing mass at an accelerating rate. The GrIS contains ~7.4 m of potential global sea level rise if completely melted. About half of the total mass loss (>100 Gt/yr) from 2003-2009 resulted from ice dynamic processes, expressed by rapid outlet glacier thinning. Southeast Greenland contributed a significant portion (~50%) to the total mass loss during this time. The physical forcing mechanisms responsible for the observed acceleration and thinning are not fully understood. The synchronous retreat of several southeast outlet glaciers in 2003 and restabilization in 2005 suggest a common regional forcing. Proposed forcings include oceanic and atmospheric warming of the calving front, as well as glacier/fjord geometry. It is imperative to quantitatively investigate dynamic ice loss processes to improve ice sheet models and sea level rise predictions. Continued monitoring and complete observational coverage are necessary to quantify dynamic loss. This work extends the spatiotemporal record of dynamic elevation changes at fast-flowing Helheim Glacier, which drains 3% of the GrIS. The novel Surface Elevation Reconstruction and Change detection (SERAC) approach utilizes laser altimetry time series to improve the vertical accuracy of existing Digital Elevation Model (DEM) datasets. The improved elevation record includes MicMacASTER (MMASTER) DEMs. This is the first study to combine MMASTER products with the SERAC procedure, yielding highly-accurate DEMs (<7 m vertical error) suitable for numerical modeling treatments. Spatiotemporal variations in ice thickness and velocity are compared against climate data to determine the processes responsible for the observed elevation changes during 1981-2017. Rapid destabilization occurred in late 2001, indicative of an ocean-forcing mechanism. Upon reaching shallower water depths, the glacier then readvanced and retreated across a 4-km region of the fjord several times, likely caused by local bedrock undulations and water storage behind the grounding line (Chapter 2). Evidence is provided for a previously unknown hydrologic process operating in the subglacial environment of the Helheim Glacier, including the detection of numerous (n>8) subglacial lakes within the catchment (Chapter 3). Changes in basal water pressure and subglacial drainage configuration are determined to govern the pulsed glacier behavior observed since 2005. Major findings presented here are consistent with studies that proposed melt-induced acceleration and increased basal water pressure as main drivers for the dynamic behavior observed early this century. This body of work suggests that Helheim Glacier ultimately behaves in response to increased surface ablation from warming atmospheric temperatures and ocean temperatures, and the response is modulated by complex basal topography (i.e., folded and faulted gneiss bedrock). This dissertation advocates that special treatment of specific outlet glacier catchments should be considered when projecting Greenland ice dynamics into future decades. This dissertation significantly advances the current understanding of glacier dynamics and will improve future numerical treatments of outlet glaciers.
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