Shelf ice - ocean dynamics and their interaction in the vicinity of ice rumples: A coupled 3D-model and application to selected Antarctic regions
Final Report Abstract
Ice shelves represent the major outflow basins of Antarctic inland ice and are therefore essential for the overall mass balance and dynamics of the Antarctic ice masses. Given the potential implications of substantial rises in global sea level as a result of melting of major parts of the ice shield, a better understanding of the pivotal role played by ice shelves in light of anticipated climate change is of crucial importance. The stability of ice shelves depends on the existence of embayments and is largely influenced by ice rises and ice rumples, which act as “pinning-points” for ice shelf movement. Of additional critical importance are interactions between ice shelves and the water masses underlying them in ice shelf cavities, particularly melting and refreezing processes. Due to their smaller spatial extent, ice rumples react more sensitively to climate change than ice rises. This study aimed to elucidate the role of ice rumples in the context of climate change impacts on Antarctic ice shelves. Different forcings are at work and need to be considered separately as well as synergistically. An increased surface accumulation will likely lead to thickening and thus more stable conditions of the ice shelves, while ocean warming and increasing surface temperatures will presumably lead to thinning ice shelves. In addition, sea-level rise can cause a complete disintegration as a result of changed conditions at the contact between ice-shelf base and bedrock at the grounding line. This project strives to investigate the combined effects of these processes on the overall behavior of ice shelves and the resultant consequences for Antarctic ice sheet mass balance. In order to address these issues, we have decided to deal with the following three issues explicitly: oceanographic-, cryospheric and general topics. In so doing, we paid particular attention to possible interrelationships and feedbacks in a coupled ice-shelf-ocean system. With regard to oceanographic issues, we have applied the ocean circulation model ROMBAX to ocean water masses adjacent to and underneath a number of idealized ice shelf configurations. This includes: wide and narrow as well as laterally restrained and unrestrained ice shelves. These idealized configurations are based on natural ice shelve characteristics in Antarctica. Simulations were performed with and without small ice rises located close to the calving front. In the case of the two larger configurations, the impact of the ice rises on melt rates at the ice shelf base is negligible. In contrast, the two smaller configurations show significant differences. Net melting rates at the ice-shelf base differ by a factor of up to eight depending on whether ice rises are considered or not. These simulations highlight the hypothesis that ice rises are indeed important mass balance triggers for an ice shelf-ocean system. Besides their important role for the ice-shelf-mass balance, ice rises are considered responsible for the dynamics and stability of an ice shelf. However, exact locations and in particular, information on their geometry are scarce. It is only larger ice rises/islands, which are likely less affected by climate warming, that are well surveyed. Therefore, highresolution TerraSAR-X scenes were used in combination with remotely sensed data to detect new ice rise locations. Since the study focused on smaller ice rises we provide 22 new ice rise locations with outlines (in the Fimbul-, West- and Shackleton ice shelf) in addition to the already well-known ice rises (either large or clearly identifiable due to the shape of the calving front). In order to estimate the temporal development of groundingline retreat rates, an analytic approach, which had been adapted to a small idealized ice rise has been employed. This analysis has revealed that the grounding line retreat influences most sensitively the ice-shelf-mass balance if ice rises with steeply sloping flanks are considered. We observe a significant increase in the rate of grounding line retreat, if we assume that climate warming reduces precipitation and enhances basal melting. Sea level rise variations only play a role when both ice-shelf-surfaces and bedrock geometries are gently sloping. Specifically addressing cryospheric issues, we aimed to demonstrate the effect of ice rises and ice rumples as pinning points for ice shelf flow through extensive numerical model simulations. We employed the thermo-coupled ice flow model TIM-FD3 to simulate the effects of several ice rises and one ice rumple. In a first set of experiments, we considered the complete un-grounding of the ice shelf in a stepwise approach in order to investigate the effect of pinning points of different characteristics (interior or near calving front, small and medium sized) on the resulting flow and stress fields. We have concretely focused on the floating ice parts of the Brunt and Riiser-Larsen ice shelves, where the effect of ungrounded pinning points is expected to be maximal. The abrupt change approach has been chosen to investigate the temporal evolution after un-grounding events without superimposed effects of partial grounding, reduced basal friction or temporal change of the grounded area. In our simulations, the major response of the ice is observed instantaneously and is caused by the time independent nature of the Stokes equations and the used Glen-type rheology. The influence of ice temperatures and therefore the time-dependent effect on the flow-rate are small, given a 100 year time frame and applying a fixed-geometry setting. We expect a different behavior of the full system consisting of floating- and grounded ice, where the grounding line position plays a major role in the ice flow dynamics. It is long known that key processes at the grounding line take place over length scales of several hundred meters (i.e., on the order of the local ice thickness). However, observed ice thickness or bedrock topography data are presently only very sparsely available (approx. 50km profile distance in the Stancomb-Wills Ice Stream area) and do not allow verification of this conclusion. A particularly important result of the current project lies in the fact that we have numerically simulated the three-dimensional stress fields in an ice shelf by employing the TIM-FD3 flow model. Common numerical models that utilize a vertically integrated Shallow Shelf Approximation (SSA-models), do not provide that information. Due to the detailed horizontal resolution of 1km in our models, we were able to also model the observed heavily fractured areas in the vicinity of McDonald Ice Rise, a region that is characterized by simulated tensile stresses reaching maximum vertical extension in the ice column. However, the TIM-FD3 model used in the project does not capture grounding line migration. Since it is based on the finite difference method, a new finite element full-Stokes model, allowing for higher spatial resolution in the vicinity of the grounding line, was developed. Instead of the commonly used floatation criterion it solves the evolution equations for the air ice– and sea ice–free interfaces. The grounding line position is determined by solving the contact problem between the ice and the bedrock.
Publications
- 2007; Numerische Untersuchungen zur Dynamik temperierter Eiskappen am Beispiel von King George Island, Antarktis; Dissertation, Westf. Wilhelms-Universität Münster
B. Breuer
- 2009; Entwicklung eines dreidimensionalen numerischen full-Stokes- Fließmodells und seine Anwendung auf ein Inlandeis-Schelfeissystem der Antarktis im Bereich des westlichen Dronning Maud Lands; Dissertation, Westf. Wilhelms-Universität Münster
T. Kleiner
- Ice rises under rising sea level; IUGG Melbourne; 28. June – 7. July 2011
M. Rückamp, T. Kleiner, A. Humbert, M.A. Lange
- The effect of sea-level rise on the stress regime of the Brunt and Riiser-Larsen ice shelves; EGU General Assembly 2011; Wien; 3. – 8. April
T. Kleiner, A. Humbert, M.A. Lange
- Benchmark experiments for evaluating grounding line migration in numerical ice sheet models – Preliminary results; 72. Jahrestagung der Deutschen Geophysikalischen Gesellschaft in Hamburg; 5. – 8. March 2012
N. Wilkens, M. Thoma, M. Rückamp, A. Humbert, K. Grosfeld
- Ice rise inventory using high resolution TerraSAR-X imagery; 72. Jahrestagung der Deutschen Geophysikalischen Gesellschaft in Hamburg; 5. – 8. March 2012
M. Rückamp, S. Beyer, A. Humbert, M.A. Lange, R. Metzig
- Ice rise under rising sea level; 72. Jahrestagung der Deutschen Geophysikalischen Gesellschaft in Hamburg; 5. – 8. March 2012
M. Rückamp, T. Kleiner, M.A. Lange, A. Humbert
- Numerical techniques for resolving grounding line migration in full-Stokes models using finite elements; Geophysics of the Cryosphere, London, 9. – 10. February 2012
N. Wilkens, T. Kleiner, M. Rückamp, A. Humbert
