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Sustainability -Structures Understanding tidal mechanisms in shelf flows

By Sebastian Rosier, Northumbria University, UK, and Andy Bell, MSC Software UK

The continent of covers the south pole of our planet, is about the size of Europe, is covered by a vast up to 3 miles deep and is one of the most bleak, coldest, and yet beautiful places on . It is home to very few people other than a few thousand engineers and scientists, but it is a relatively pristine, untouched environment that contains many ice records that reflect the atmosphere and the climate of our planet stretching back tens of thousands of years. It is also a bell weather for effects in the world and the sustainability of our complex ecosystems upon which we all depend. We saw this most clearly with the Ozone Layer depletion above Antarctica scare in the 1980s when mankind took urgent action to limit the use of chlorofluorocarbon gases this helping to repair the emerging hole in the atmosphere.

A century on from the heroic age of Antarctic exploration, the Antarctic continent remains one of the most hostile and least accessible places on earth. Sitting atop this hidden continent is the ; a vast reservoir of frozen ice up to 4km thick and containing enough water to raise global sea levels by 58m. This landscape, that appears almost entirely flat and dormant from the surface, actually consists of rivers of fast flowing ice, known as ‘ice streams’. These narrow-concentrated regions of ice flow transport ice to the where they thin, eventually lifting off the bed under buoyancy and forming the floating ice shelves that fringe most of the continent.

24 | Engineering Reality Magazine Over the last few decades, the Stream, an ice stream in has been rapidly Antarctica that drains into the large losing mass, largely in response to Filchner-Ronne Ice Shelf which itself warming ocean temperatures leading has an area approximately the size of to increased melting of the ice shelves Sweden (Figure 1). from underneath. Understanding how a changing climate will affect the These GPS systems were far away Antarctic Ice Sheet has become a key from the coast, and yet they showed question in climate science and yet, strong variability in the flow speed of despite the important role it plays in the ice stream at ocean tidal the earth system and the potential frequencies. Even more surprisingly, that a large fraction of the world’s the strongest tidal component was at population could be displaced as it a frequency not even measurable in melts, our understanding of the the neighbouring ocean. More recently, Figure 2: Overview of the Marc finite-element model, showing model resolution, the quadratic physical processes that determine we have found that for every ice pentahedral elements used, and a vertically ongoing and future mass loss remains stream flowing into the Filchner-Ronne exaggerated oblique view of the model showing modelled mean ice velocity for the default remarkably poor (1). Ice Shelf that has been measured, as setup experiment. well as across the entire ice shelf Perhaps the most fundamental issue itself, this same phenomenon exists. In order to explore what was causing for ice science, and the one most Here, then, is an observation that can’t these puzzling observations we problematic for computer modelling be explained by our current created a model of the Rutford ice efforts, is that we have very little grasp understanding of ice stream flows. stream in MSC Marc Mentat using the of how ice streams flow rapidly over built-in meshing options and forced their beds. Drilling to the base of ice In this way, ocean tides and the ice the model with ocean tides (Figure 2). streams to observe this directly is a sheet’s response acts as a natural These tides were found to generate huge technical challenge and only a experiment and by filling this complex flexural and longitudinal few measurements exist. As a result, knowledge gap we can gain new stresses at the hinge line where the ice sheet modellers have to resort to insights into ice flow that, among other ice stream meets the ocean. These parameterisations to describe the things, will help constrain the form of stresses are transmitted upstream numerous possible processes that the basal sliding law. through interactions with the dictate how quickly an ice stream can underlying sediment. slide over, or deform, its underlying Until recently, progress in this area has sediment; known as the ‘basal sliding been limited; these tidal processes are What we found through these law’. Testing how effective these occurring at timescales of hours or simulations was that it was remarkably parameterisations are is therefore days and this poses a real problem. difficult to replicate both the crucial, but until recently no reliable Currently, ice sheet models only model amplitude of the observed signal and method existed for doing this. the viscous component of ice the distance it travels upstream from deformation since ice sheet modelling the coast. In fact, this was only Progress in science is often made efforts are largely focused on possible with the inclusion of a highly through trying to explain observations projections spanning decades to conductive drainage system beneath that do not fit with contemporary centuries that can safely ignore the ice stream. Drainage systems such understanding and in Antarctica there short-term behaviour. Not only that, as these, through which melt water is exists a particularly striking example. but most of them use thin plate type transported to the coast, are readily In the late 2000s several GPS systems approximations to model ice flow and observed in the smaller Ice were deployed on the Rutford Ice ignore certain components of stress Sheet but very little evidence exists that are less important for for them in Antarctica and they are determining ‘secular’ ice flow. Here, rarely included in large scale model the power of non-linear FEA simulations. Our results suggest not simulations using Marc from MSC only that they should be included, but Software came into its own we found. that we can infer their hydraulic With its ability to accurately and conductivity using tidal observations rapidly solve the behaviour of such as these. nonlinear viscoelastic materials without neglecting any stress terms, it The other enigma that has confounded can tackle many problems that would Antarctic ice scientists is that the not otherwise be possible to simulate. horizontal flow of the ice shelf itself is Figure 1 : Map showing the Filchner–Ronne Ice strongly modulated by ocean tides. In Shelf and adjoining ice streams, along with fact, this is so strong that in some locations of GPS measurements. places the tidal variation in flow causes the ice shelf to periodically

VolumeVolume XIXI -- SummerSummer 20202020 | | mscsoftware.com | 25 flow backwards! Ice shelf flow is of ice over large spatial scales and focused on isolated regions and generally thought to be more well processes in the hinge zone - a interactions between different areas understood than that of ice streams, particularly crucial part of the ice have, therefore, not been fully since these regions float on the ocean sheet system. accounted for. In our study we have and the complication of unseen conducted the first largescale ice flow processes at the bed can be Many aspects of these remarkable modelling study (using MSC Marc) to neglected. How, then, do tides that observations remain unexplained and explore these processes using a cause floating ice to move vertically there is no doubt that MSC Marc will viscoelastic rheology and realistic translate to a horizontal motion? Once continue to play an important role in geometry of the entire Filchner–Ronne again, MSC Marc has enabled us to helping us to understand these Ice Shelf, where the best observations address this question. By creating a processes. Complicating factors such of tidal response are available. We model of the entire Filchner-Ronne Ice as ice damage and temperature evaluated all relevant mechanisms Shelf, including all major ice stream variation, which are largely overlooked that have hitherto been put forward to that drain into it, we were able to make in the current generation of ice sheet explain how ocean tides might affect great leaps forward in our models, can easily be added into our ice shelf flow and compared our understanding of the entire coupled model to hopefully answer some of the results with observational data. We system. remaining open questions. As the concluded that, while some models Antarctic continent and the are able to generate the correct It turns out that from our simulations, surrounding ocean continues to warm, general qualitative aspects of the the answer is two-fold for the the need to reduce uncertainty in our tidally induced perturbations in ice Antarctic ice shelf in question. Firstly, projections of the future evolution of flow, most of these mechanisms must spatial variations in tidal amplitude the ice sheet grows increasingly be ruled out as being the primary and phase leads to a slight tilting of urgent. cause of the observed long-period the ice shelf, generating sufficient response. We find that only tidally elastic strain to account for some of Conclusion induced lateral migration of grounding the high frequency large scale motion. lines can generate a sufficiently strong Secondly, the margins of the ice shelf Until our study, no computer model long-period Msf tidal frequency move as the tides lift and drop the ice has yet been able to reproduce the response on the ice shelf to match onto the bed below. For this second quantitative aspects of observed tidal observations. Furthermore, we showed mechanism, the advanced contact modulation across the entire Filchner– that the observed horizontal short- capabilities of MSC Marc were Ronne Ice Shelf in Antarctica. The period semidiurnal tidal motion, indispensable, allowing us to cause of the tidal ice flow response causing twice daily flow reversals at accurately model the tidal migration of has, therefore, remained an enigma, the ice front, were generated through the point at which the ice loses indicating a serious limitation in a purely elastic response to basin- contact with the bed and how the science’s current understanding of the wide tidal perturbations in the ice contact stresses evolve over time. mechanics of large-scale ice flow on shelf slope. Our simulation model has These modelling efforts have led to a the continent. A further limitation of allowed us to quantify the effect of better understanding of the rheology previous studies is that they have all tides on mean ice flow and we found that the Filchner–Ronne Ice Shelf flows, on average, were 21% faster than they would in the absence of large ocean tides. We have therefore found MSC Marc to be a very powerful tool that is well suited to studying processes at shorter timescales where ice behaves viscoelastically. This area of research has important implications for climate science in terms of predicting the future contribution of Antarctica to global .

Reference

1. “Exploring mechanisms responsible for tidal modulation in flow of the Filchner–Ronne Ice Shelf”, S. H. R. Rosier & G. H. Gudmundsson, Department of Geography and Environmental Sciences, Northumbria University, UK, The Journal, 14, 17–37, 2020, https://doi.org/10.5194/tc-14-17-2020

26 | Engineering Reality Magazine