Portland State University PDXScholar Dissertations and Theses Dissertations and Theses Winter 3-20-2017 A Numerical Model Investigation of the Role of the Glacier Bed in Regulating Grounding Line Retreat of Thwaites Glacier, West Antarctica Michael Scott Waibel Portland State University Follow this and additional works at: https://pdxscholar.library.pdx.edu/open_access_etds Part of the Environmental Sciences Commons, and the Geomorphology Commons Let us know how access to this document benefits ou.y Recommended Citation Waibel, Michael Scott, "A Numerical Model Investigation of the Role of the Glacier Bed in Regulating Grounding Line Retreat of Thwaites Glacier, West Antarctica" (2017). Dissertations and Theses. Paper 3467. https://doi.org/10.15760/etd.5351 This Dissertation is brought to you for free and open access. It has been accepted for inclusion in Dissertations and Theses by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. A Numerical Model Investigation of the Role of the Glacier Bed in Regulating Grounding Line Retreat of Thwaites Glacier, West Antarctica by Michael Scott Waibel A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Sciences and Resources: Geology Dissertation Committee: Scott F. Burns, Chair Christina L. Hulbe Charles S. Jackson Andrew G. Fountain Brittany A. Erickson Portland State University 2017 ABSTRACT I examine how two different realizations of bed morphology affect Thwaites Glacier response to ocean warming through the initiation of marine ice sheet instability and associated grounding line retreat. A state of the art numerical ice sheet model is used for this purpose. The bed configurations used are the 1-km resolution interpolated BEDMAP2 bed and a higher-resolution conditional simulation produced by John Goff at the University of Texas using the same underlying data. The model is forced using a slow ramp approach, where melt of ice on the floating side of the grounding line is increased over time, which gently nudges the glacier toward instability. Once an instability is initiated, the anomalous forcing is turned off, and further grounding line retreat is tracked. Two model experiments are conducted. The first experiment examines the effect of different anomalous forcing magnitudes over the same bed. The second experiment compares the generation and progress of instabilities over different beds. Two fundamental conclusions emerge from these experiments. First, different bed geometries require different ocean forcings to generate a genuine instability, where ice dynamics lead to a positive feedback and grounding line retreat becomes unstable. Second, slightly different forcings produce different retreat rates, even after the anomalous forcing is shut off, because different forcing magnitudes produce different driving stresses at the time the instability is initiated. While variability in the retreat rate over time depends on bed topography, the rate itself is set by the magnitude of the forcing. This signals the importance of correct knowledge of both bed shape and ocean circulation under floating i portions of Antarctic ice sheets. The experiments also imply that different ocean warming rates delivered by different global warming scenarios directly affects the rate of Antarctic contribution to sea level rise. ii ACKNOWLEDGEMENTS Writing a dissertation is a long, difficult endeavor. Fortunately, I had help and support from individuals and organizations along the way. This is where I acknowledge the assistance and support provided to me. However, I want to be clear that the following paragraphs do not form a comprehensive list. There are many people and conversations that have been influential to my studies over the years, and I’m grateful to have had the opportunity of being in an environment that encourages scholarly discourse as well as achievement. Thank you Portland State and the community that is Portland. No matter what the future holds, I will always have fond memories of my time here. I’m going to begin my list of specific acknowledgements with funding and supporting organizations that enabled me to conduct this research. NASA award number NNX11AH89 to the University of Texas and Portland State funded this work while additional support was provided SciDac’s PISCEES project. A profuse thanks to both NASA and PISCEES is in order, and I laud these organizations’ efforts contributing to our increasing understanding of the past and future evolution of ice sheets as well as their influence on climate and sea level. Next, I would like to collectively thank my dissertation committee: Dr. Scott Burns, Dr. Christina Hulbe, Dr. Charles Jackson, Dr. Andrew G. Fountain, and Dr. Brittany Erickson. I appreciate all members’ questions and guidance throughout this process. Additionally, I appreciate the thoughtful edits provided to improve the dissertation. iii A special thank you goes to Dr. Scott Burns, the chair of my dissertation committee. Scott is a knowledgeable, dedicated geologist and emeritus professor at PSU who is frequently consulted even in his retirement. There is no one quite like Dr. Burns when it comes to navigating PSU’s bureaucracy. Furthermore, Scott is an incredibly kind and compassionate human being. If there were more people in this world like Dr. Burns, the world would be a happier, kinder place. A special thank you goes to Dr. Charles Jackson as well. Dr. Jackson was essentially a co-advisor (along with Dr. Christina Hulbe) for me and ultimately this research would not have been possible without his guidance. I appreciate the opportunity he provided me for travelling to the University of Texas so that I could learn how to use the computing resources provided by the Texas Advanced Computing Center. Moreover, my other trips to UTIG for research were invaluable, and I was pleased to meet so many knowledgeable and gifted scholars there. I’d like to acknowledge Dr. John Goff at the University of Texas Institute for Geophysics. Dr. Goff created a topographic bed model that is an integral part of this research. The conclusions reached by this research certainly validate his effort. I would like to thank Nancy Eriksson, PSU Geology Department’s former office coordinator. Like Dr. Burns, Nancy was adept at navigating the bureaucracy and cutting through red tape, and I am indebted to her. Another fact about Nancy is that she is an excellent cook. The newer geology students at PSU don’t know what they are missing. I owe a debt of gratitude to Dr. Dan Martin at Lawrence Berkeley National Laboratory. As a model developer, Dan provided invaluable guidance for the numerical iv model used in this research. I learned much from Dan, and he was an integral part of the effort. I would like to thank my family for their support. Erin, my wife, and Pat, my mother, are both strong women; and they probably deserve a medal for putting up with me. Erin has often been a sounding board for my ideas, and I hold her insights in high esteem. In response to my wishes, my mother began teaching me to read when I was four years old and has always been extremely supportive of my academic endeavors ever since. Additionally, I would like to say that I consider Erin’s family as my own, and I sincerely appreciate their encouragement. I would also like to thank Alisa Humphrey, the current office coordinator at PSU’s Geology Department, and Paul Brooks, a computer systems analyst. I judge Alisa to be a fine addition to the office staff as she undertakes administrative tasks allowing students to focus on their work. Paul was a tremendous help as I transitioned to Unix-like systems. I’ve saved my most profuse and heartfelt thanks for last. Thank you, Dr. Christina Hulbe, for everything that you have done to further my career. There really are no words that can amply express my gratitude. Christina was my original advisor and the one who set me on the path to this academic achievement. Christina’s faith in me gave me the confidence to carry on and overcome obstacles along the way (and there were some big ones). I feel very privileged and humbled to have had the opportunity to work with one of the most highly respected minds in ice dynamics. v TABLE OF CONTENTS ABSTRACT ......................................................................................................................... i ACKNOWLEDGEMENTS ............................................................................................... iii LIST OF FIGURES ......................................................................................................... viii LIST OF ABBREVIATIONS ............................................................................................ xi LIST OF SYMBOLS ........................................................................................................ xii 1.0 INTRODUCTION ........................................................................................................ 1 1.1 Physical Setting ......................................................................................................... 2 1.2 Climate Context ........................................................................................................ 7 1.3 Glaciological Context ............................................................................................... 8 1.4 Research Question ...................................................................................................