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The Effects of Thermodynamic Parameterizations, Ice Shelf

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The Effects of Thermodynamic Parameterizations, Ice Shelf AN ABSTRACT OF THE DISSERTATION OF Rachael D. Mueller for the degree of Doctor of Philosophy in Oceanography presented on May 16, 2014. Title: The Effects of Thermodynamic Parameterizations, Ice Shelf Geometry, and Tides on Modeled Basal Melting of Weddell Sea Ice Shelves. Abstract approved: Eric D. Skyllingstad Laurence Padman Antarctic Ice Sheet mass balance and, hence, sea level change is affected by the floating extensions of outlet glaciers and ice streams that take up about 44% of the coastline (Drewry et al., 1982) and are referred to as \ice shelves". Ice sheet mass loss accelerates when these ice shelves lose mass through basal melting at the ice-ocean interface or calving along the ice shelf front. The focus of this dissertation is to explore the uncertainties in basal melt predictions, as affected by ocean temperatures, ocean currents, and model geometries. Uncertainties in tidal currents and the corresponding affect on sub ice shelf basal melt was explored using the Regional Ocean Modeling System (ROMS 3.2), adapted to represent the thermodynamics of ice shelf basal melt at the ice/ocean interface. Plausi- ble representations of present and future sub ice shelf topographies were used to explore potential errors in tidal forcing and ocean circulation beneath the Larsen-C and Filchner- Ronne ice shelves of the Weddell Sea, Antarctica. The influence of thermal forcing and thermodynamic parameterizations was also explored. The results presented here demonstrate that two plausible Larsen-C Ice Shelf (LCIS) topographies could yield shelf-averaged basal melt rates that differ by nearly a factor of two. The difference in these two cases is due to regional variations in tidal cur- rents. The standard grid topography, based on realistic modern bathymetry and ice draft, supported topographic vorticity waves at diurnal frequencies in the northeast LCIS while an alternate model geometry did not. As such, these two grid topographies not only affected the shelf-averaged value of basal melting but also the regional variation in basal melting. Regional variation is important because it determines whether basal melting will have a greater impact on the rate at which ice moves off-shore, as in grounding line melt, or the rate of calving, as in melting along the ice shelf front. Out of all parameterizations, ground- ing line melt is shown to be largest in a commonly used parameterization that applies a uniform \friction velocity" to estimate basal melting. These model results confirm that both topographic errors and choice of thermodynamic parameterization have a significant influence on the spatial characteristic of basal melt. In a separate study of basal melting of the much larger Filchner-Ronne Ice Shelf (FRIS), simulations shows that a future scenario of warmer ocean conditions may lead to a change in the FRIS cavity shape that strongly affects the map of tidal currents and, hence, regional characteristics of basal melting. In general, the change in FRIS cavity shape due to a warming ocean introduces a negative feedback where increased melting reduces the overall magnitude of tidal currents (by increasing the thickness of the water column) which then results in less basal melting; however, there are large regional variations in these results. In one region, south of Henry Ice Rise, the change in cavity shape reduces basal melting from 5 m a−1 to 1:5 m a−1 due to the corresponding change in tidal forcing. In contrast, basal melting increases from 1 m a−1 to 1:5 m a−1 in the nearby region of the Institute Ice Stream outlet owing to a reduction in the upstream basal melting and, hence, cooling of inflowing water. In summary, uncertainties in cavity geometry have a large impact on the regional characteristics of tidal current predictions and, hence, ice shelf basal melting. These uncer- tainties introduce significant, regional errors to ice shelf mass balance. Critical processes that influence the evolution of the Antarctic Ice Sheet cannot be accurately represented without the inclusion of small grid spacing (∼ 1 km), accurate topography, and tidal forc- ing in the predictions of ice shelf basal melt. c Copyright by Rachael D. Mueller May 16, 2014 All Rights Reserved The Effects of Thermodynamic Parameterizations, Ice Shelf Geometry, and Tides on Modeled Basal Melting of Weddell Sea Ice Shelves. by Rachael D. Mueller A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented May 16, 2014 Commencement June 2014 Doctor of Philosophy dissertation of Rachael D. Mueller presented on May 16, 2014. APPROVED: Co-Major Professor, representing Oceanography Co-Major Professor, representing Oceanography Dean of the College of Earth, Ocean, and Atmospheric Sciences Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Rachael D. Mueller, Author ACKNOWLEDGMENTS When I arrived at Oregon State University eleven years ago, I could not have fathomed the depth of experiences that lay ahead. I started graduate school thinking that I was on a path to get a Ph.D. in physical oceanography. I couldn't anticipate that I would arrive at the end of my degree with the understanding that an acronym is the least important contribution to my life that this degree will impart. Its important, to be sure, but what is of greater value to me is what I will carry forward in the indelible imprinting of other people's influence on my life. I had intended to offer my gratitude to these people in a simple paragraph or two in this section of my dissertation, but it didn't take long for me to realize that my gratitude can't be contained in a couple paragraphs; it needs a bit more space to breath. That is how fortunate I have been here. Some people may cross paths with a few others who forever change their perspective on the world in a meaningful and deeply appreciated way, but I have been fortunate enough over the past seven years of my Ph.D. to have had countless encounters with people who have helped me grow in unanticipated and deeply appreciated ways. This dissertation wouldn't exist without these influences, and this section of gratitude and reflections may be the most important section of my entire dissertation. The person whom I am most grateful to for their mentoring over these past seven years is Dr. Laurence Padman. Laurie, your support and generosity|both in funding and encouraging my Ph.D.|has been unparalleled. It is because of you that I found a way to pursue my passion in polar oceanography, was able to observe the oceans around Antarctica in a way that brought my Masters thesis to life and that inspired my interest in ocean/ice- shelf/atmosphere interactions (which I hope to carry forward), and was able to participate in workshops (WAIS, FRISP, and IGS) in which, for the first time, I finally felt a sense of belonging within a scientific community. You held out a hand, lifted me up, guided me through and offered incredible tolerance and patience as I fumbled through some aspects of development that were difficult for my own self to experience. I had no clue what I was doing when I wrote my first paper on Larsen-C basal melting, and there were times when I handed you righteously crap writing. Even so, you somehow found a will to see me through to the end and, in so doing, help me learn that...well...you were right about...oh so many things! I really admire your ability to advise so many aspects of a research problem and to bring to the table of advising a tremendous depth of knowledge that spans length scales, time scales, research methods, and both liquid and solid phases of water. I also admire your ability to remain engaged and committed through all circumstances, even the difficult ones. Thank you for not giving up on me all these many years. Your encouragement and respectful guidance will always be remembered and appreciated. Dr. Helen Amanda Fricker, I will always appreciate your \can-do" attitude that carried with it both enthusiasm and support for my research. I really respect your ability to lift people up and to offer a full range of guidance, from affirmative to challenging. You have been an incredible role model to me in the way you advocate for women and offer an example for how to sit at a table that is largely dominated by men. I appreciate you as a role model and a mentor, as well as for all the effort that you put in with Laurie to help me work through my growing pains as a scientist and scientific writer. I am also so grateful to you for the opportunities you have extended to me to share my work at AGU and to connect with others both in the field and beyond. Thank you for all the positive and constructive feedback that you have given to me so freely. I am also incredibly grateful to Drs. Eric Skyllingstad, Roger Samelson, and Peter Ruggiero. Thank you, Eric, for your willingness to assume the role of my major-professor in the eleventh hour and for helping me understand that I needed to let go of what I wanted to research (in the short term) in order to sustain my ability to move forward with my degree. I appreciate all your encouragement and support. I will remember you for always bringing to the table more ideas than I could assimilate and for your enthusiasm for me to branch out and explore other areas of possibility, both in research and computing.
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