My research projects focus on combining remotely-sensed and in situ observations and numerical ice flow modeling to develop a better understanding of the external triggering mechanisms and internal controls of tidewater glacier behavior (i.e., glacier dynamics). I am particularly interested in ice-ocean interactions, namely submarine melting and iceberg calving, and how changes in these interactions influence the rate of mass loss from the fast-flowing glaciers that drain the Greenland and Antarctic ice sheets. I completed my Ph.D. research under the supervision of Dr. Ian Howat (Glacier Dynamics Group, Byrd Polar and Climate Research Center) in 2013. Since then, I have been working with Dr. Gordon Hamilton at the University of Maine Climate Change Institute. I've included some brief background on several of my major research projects below and more information on my research interests can be found under my Research page. Ongoing Research Projects: As part of a collaborative and interdisciplinary effort to develop a better understanding of ice-ocean interactions in Greenland's glacial fjords, I've developed a method to estimate the submarine meltwater fluxes and melt rates from icebergs using repeat high resolution stereo satellite images. A description of the method and preliminary results for Sermilik Fjord, SE Greenland was recently published in the Journal of Glaciology (http://www.igsoc.org/journal/60/224/j14j085.html). The preliminary analysis suggests that iceberg melt rates can be used as a proxy for winter melt rates along submerged glacier termini and that the volume of meltwater produced by iceberg melting scales with the submerged iceberg surface area. I plan to apply this method to icebergs calved from ~12 large and fast-flowing Greenland outlet glaciers in order to assess the spatio-temporal variations in iceberg meltwater fluxes and melt rates (and winter glacier melt rates) as part of a recent award from the NSF Office of Polar Programs. The controls of meltwater fluxes within these fjords will also be investigated and the results will be shared with oceanographer colleagues to ensure that meltwater fluxes from icebergs are properly accounted for in fjord circulation analyses. I recently received funding from the NASA Cryospheric Sciences program to investigate the spatial and temporal scales over which critical changes in glacier dynamics occur at two well studied glaciers with robust observational datasets: Columbia Glacier in Alaska and Helheim Glacier in SE Greenland. These glaciers have recently undergone a period of rapid dynamic change followed by a relatively stable quiescent period. Although the multi-year changes in behavior at these glaciers have been well documented, the spatial and temporal scales over which rapid dynamic changes are initiated have not been investigated. In collaboration with colleagues at the USGS in Alaska, I plan to leverage a suite of remotely sensed datasets to quantify sub-annual changes in the force balance of the two study glaciers. The relative influence of external triggering mechanisms and internal controls of force balance variations will be assessed by comparing the force balance timeseries with nearly contemporaneous environmental forcing data and by simulating the observed changes using a numerical ice flow model that I developed as part of my dissertation (see below). PhD Research: In order to develop a better understanding of recent mass loss from the Greenland Ice Sheet, I constructed time series of ice discharge change for 178 marine-terminating outlet glaciers from 2000-2012 with help from my colleagues at The Ohio State University. These discharge time series were then compiled to create a record of ice sheet discharge change. A time series of annual surface mass balance change was obtained from the Regional Atmospheric Climate Model for the Greenland Ice Sheet (RACMO/GR). The surface mass balance and discharge time series were combined to create record of ice sheet mass change since 2000. We found that changes in ice sheet discharge were driven by increased discharge from a small number of large and fast-flowing outlet glaciers (50% of cumulative discharge change from 4 glaciers). Despite the observed increase in ice sheet discharge from ~462 Gt/yr in 2000 to ~547 Gt/yr in 2012, ice sheet mass loss since 2000 was primarily due to increased meltwater runoff. The discharge data used for the published analysis (article) can be found here. I have also developed a depth-integrated, width-averaged numerical ice flow model that can be used to efficiently model ice flow for the channelized tidewater glaciers draining much of the Greenland Ice Sheet. As part of my dissertation research projects, I used the model to examine the influence of uncertainty in glacier shape (article), ice rheology and basal sliding parameterizations (article) on predictions of future glacier change. I found that if a glacier is initially grounded across a basal over-deepening, relatively small variations in outlet width will control both the magnitude and timing of grounding line retreat into the over-deepening following a reduction in resistive stress at the terminus. Additionally, changes in the maximum over-deepening depth within the uncertainty of current ice-penetrating radar observations can result in glaciers switching from stable (i.e., ice remains grounded on a marine shoal) to unstable (i.e., rapid retreat into the over-deepening) behavior. My numerical modeling simulations also indicate that a non-unique combination of ice rheology and basal sliding parameterizations can be used to reproduce similar steady-state thickness and speed profiles but that following the onset of a reduction in resistive stress, the dynamic behavior of a simulated glacier will vary with the parameter choice. Based on these results, I suggest that prognostic ice flow models are either accompanied by sensitivity studies that investigate the influence of geometry and parameter uncertainty on predictions of dynamic behavior or that these models are not used as precise predictors of future glacier change. The model that I developed to perform my sensitivity studies is open access (download model demo). The model is set-up to run a demonstration and is accompanied by a user guide that explains the model basics, the demonstration, and how to modify the model for application to real glacier systems. A more complex version of the model can also be obtained via email correspondence (ellyn.enderlin@gmail.com). I also calculated submarine melt rates beneath floating termini for 13 marine-terminating outlet glaciers in Greenland from 2000-2010 in order to develop a better understanding of variability in ocean forcing that may have triggered the observed changes in dynamics. Annual melt season submarine melt rates were calculated by differencing grounding line and front discharge obtained from satellite remotely-sensed ice flow observations, assuming that mass loss between the grounding line and front is due to submarine melting after accounting for mass loss from surface melt. Although my analysis was spatially and temporally limited, I found no correlation between changes in dynamics and submarine melt rates during the study period. Melt rates varied widely between glaciers, but, on average, mass loss from submarine melting was approximately equal to mass loss from the iceberg calving (article). |
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