1,716 research outputs found

    Investigating the dynamics of Greenland's glacier-fjord systems

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    Over the past two decades, Greenlandā€™s tidewater glaciers have dramatically retreated, thinned and accelerated, contributing significantly to sea level rise. This change in glacier behaviour is thought to have been triggered by increasing atmospheric and ocean temperatures, and mass loss from Greenlandā€™s tidewater glaciers is predicted to continue this century. Substantial research during this period of rapid glacier change has improved our understanding of Greenlandā€™s glacier-fjord systems. However, many of the processes operating in these systems that ultimately control the response of tidewater glaciers to changing atmospheric and oceanic conditions are poorly understood. This thesis combines modelling and remote sensing to investigate two particularly poorly-understood components of glacier-fjord systems, with the ultimate aim of improving understanding of recent glacier behaviour and constraining the stability of the ice sheet in a changing climate. The research presented in this thesis begins with an investigation into the dominant controls on the seasonal dynamics of contrasting tidewater glaciers draining the Greenland Ice Sheet. To do this, high resolution estimates of ice velocity were generated and compared with detailed observations and modelling of the principal controls on seasonal glacier flow, including terminus position, ice mĆ©lange presence or absence, ice sheet surface melting and runoff, and plume presence or absence. These data revealed characteristic seasonal and shorter-term changes in ice velocity at each of the study glaciers in more detail than was available from previous remote sensing studies. Of all the environmental controls examined, seasonal evolution of subglacial hydrology (as inferred from plume observations and modelling) was best able to explain the observed ice flow variations, despite differences in geometry and flow of the study glaciers. The inferred relationships between subglacial hydrology and ice dynamics were furthermore entirely consistent with process-understanding developed at land-terminating sectors of the ice sheet. This investigation provides a more detailed understanding of tidewater glacier subglacial hydrology and its interaction with ice dynamics than was previously available and suggests that interannual variations in meltwater supply may have limited influence on annually averaged ice velocity. The thesis then shifts its attention from the glacier part of the system into the fjords, focusing on the interaction between icebergs, fjord circulation and fjord water properties. This focus on icebergs is motivated by recent research revealing that freshwater produced by iceberg melting constitutes an important component of fjord freshwater budgets, yet the impact of this freshwater on fjords was unknown. To investigate this, a new model for iceberg-ocean interaction is developed and incorporated into an ocean circulation model. This new model is first applied to Sermilik Fjord ā€” a large fjord in east Greenland that hosts Helheim Glacier, one of the largest tidewater glaciers draining the ice sheet ā€” to further constrain iceberg freshwater production and to quantify the influence of iceberg melting on fjord circulation and water properties. These investigations reveal that iceberg freshwater flux increases with ice sheet runoff raised to the power ~0.1 and ranges from ~500-2500 mĀ³ sā»Ā¹ during summer, with ~40% of that produced below the pycnocline. It is also shown that icebergs substantially modify the temperature and velocity structure of Sermilik Fjord, causing 1-5Ā°C cooling in the upper ~100 m and invigorating fjord circulation, which in turn causes a 10-40% increase in oceanic heat flux towards Helheim Glacier. This research highlights the important role of icebergs in Greenlandā€™s iceberg congested fjords and therefore the need to include them in future studies examining ice sheet ā€“ ocean interaction. Having investigated the effect of icebergs on fjord circulation in a realistic setting, this thesis then characterises the effect of submarine iceberg melting on water properties near the ice sheet ā€“ ocean interface by applying the new model to a range of idealised scenarios. This near-glacier region is one which is crucial for constraining ocean-driven retreat of tidewater glaciers, but which is poorly-understood. The simulations show that icebergs are important modifiers of glacier-adjacent water properties, generally acting to reduce vertical variations in water temperature. The iceberg-induced temperature changes will generally increase submarine melt rates at mid-depth and decrease rates at the surface, with less pronounced effects at greater depth. This highlights another mechanism by which iceberg melting can affect ice sheet ā€“ ocean interaction and emphasises the need to account for iceberg-ocean interaction when simulating ocean-driven retreat of Greenlandā€™s tidewater glaciers. In summary, this thesis has helped to provide a deeper understanding of two poorly-understood components of Greenlandā€™s tidewater glacier-fjord systems: (i) interactions between subglacial hydrology and ice velocity, and; (ii) iceberg-ocean interaction. This research has enabled more precise interpretations of past glacier behaviour and can be used to inform model development that will help constrain future ice sheet mass loss in response to a changing climate."I must express my gratitude to the University of St Andrews and to the Scottish Alliance for Geoscience, Environment and Society (SAGES) for funding and supporting me as a research student."-- Fundin

    Dynamics and Modelling of the 2015 Calbuco eruption Volcanic Debris Flows (Chile). From field evidence to a primary lahar model

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    The Calbuco volcanic eruption of 2015, was characterized by two explosive phases with partialand major column collapses that triggered lahars in many of the flanks of the volcano. Large lahar flows descended to the southern flank where highly fractured ice bodies were emplaced on steep slopes.In this study, we present a chronology of the volcanic flows based on a multi parameterdata set that includes social media, reports of authoritative institutions, instrumental monitoringdata and published research literature on the eruption. Our review established thatlahars in the Amarillo river began during the first phase of the eruption due to the sustained emplacement of pyroclastic flows in its catchment. In contrast, we propose that the lahars in theBlanco ā€“ Correntoso river system and the Este river were likely to have been triggered by asudden mechanical collapse of the glacier that triggered mixed avalanches which transitionedinto lahars downstream.Our observations include inundation cross-sections, estimates of flow speeds, and characterization of the morphology, grain sizes, and componentry of deposits.Field measurements are used together with instrumental data for calibrating a dynamic, physics-based model of lahar, Laharflow. We model flows in the Blanco ā€“ Correntoso river system and explore the influence of the model parameters on flow predictions in an ensemble of simulations. We develop a calibration that accounts for the substantial epistemic uncertainties in our observations and the model formulation, that seeks to determine plausible ranges for the model parameters, including those representing the lahar source. Our approach highlights the parameters in the model that have a dominant effect on the ability of the model to match observations, indicating where further development and additional observations could improve model predictions. The simulations in our ensemble that provide plausible matches to the observations are combined to produce flow inundation maps

    2023-2024 Graduate School Catalog

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    You and your peers represent more than 67 countries and your shared scholarship spans 140 programs - from business administration and biomedical engineering to history, horticulture, musical performance, marine science, and more. Your ideas and interests will inform public health, create opportunities for art and innovation, contribute to the greater good, and positively impact economic development in Maine and beyond

    The generation, propagation, and mixing of oceanic lee waves

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    Lee waves are generated when oceanic flows interact with rough seafloor topography. They extract momentum and energy from the geostrophic flow, causing drag and enhancing turbulent mixing in the ocean interior when they break. Mixing across density surfaces (diapycnal mixing) driven by lee waves and other topographic interaction processes in the abyssal ocean plays an important role in upwelling the densest waters in the global ocean, thus sustaining the lower cell of the meridional overturning circulation. Lee waves are generated at spatial scales that are unresolved by global models, so their impact on the momentum and buoyancy budgets of the ocean through drag and diapycnal mixing must be parameterised. Linear theory is often used to estimate the generation rate of lee waves and to construct global maps of lee wave generation. However, this calculation and subsequent inferences of lee wave mixing rely on several restrictive assumptions. Furthermore, observations suggest that lee wave mixing in the deep ocean is significantly overestimated by this theory. In this thesis, we remove some common assumptions at each stage of the lee wave lifecycle to investigate the reasons for this discrepancy and to motivate and inform future climate model parameterisations. Firstly, we investigate the way that seafloor topography is represented in lee wave parameterisations, finding that typical spectral methods can lead to an overestimate of wave energy flux. Next, we make the case for considering lee waves as a full water column process by modelling the effect of vertically varying background flows and the ocean surface on lee wave propagation. Finally, we take a holistic view of topographic mixing in the abyssal ocean, finding that deep stratified water mass interfaces may modify the nature of the lee wave field, and themselves contribute to mixing and upwelling in the deep ocean through topographic interaction.Open Acces

    Sea Ice Extraction via Remote Sensed Imagery: Algorithms, Datasets, Applications and Challenges

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    The deep learning, which is a dominating technique in artificial intelligence, has completely changed the image understanding over the past decade. As a consequence, the sea ice extraction (SIE) problem has reached a new era. We present a comprehensive review of four important aspects of SIE, including algorithms, datasets, applications, and the future trends. Our review focuses on researches published from 2016 to the present, with a specific focus on deep learning-based approaches in the last five years. We divided all relegated algorithms into 3 categories, including classical image segmentation approach, machine learning-based approach and deep learning-based methods. We reviewed the accessible ice datasets including SAR-based datasets, the optical-based datasets and others. The applications are presented in 4 aspects including climate research, navigation, geographic information systems (GIS) production and others. It also provides insightful observations and inspiring future research directions.Comment: 24 pages, 6 figure

    Numerical Studies on Folds and related Deformation Structures in Anisotropic Viscous Materials undergoing Ductile Deformation

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    Folds are common structures in deformed rocks and ice sheets from the microscale to lithospheric scale. This thesis present numerical studies on folds and related deformation structures in anisotropic viscous materials undergoing ductile deformation with various boundary conditions. Mechanical anisotropy considered her is due to a crystallographic preferred orientation (CPO), for example by of alignment of micas or the basal planes of ice crystals. The modelling aims to numerically better understand the various fold geometries that are observed in natural rocks or ice drill cores. This thesis covers two main topics: (i) the influence of an initial CPO and intensity of anisotropy on resulting crenulation geometries in a single-phase material that deforms in moderate strain in dextral simple shear deformation, and (ii) the influence of an initial CPO, intensity of anisotropy and viscosities on evolving fold geometries of single-phase or poly-phase materials that deform in layer-parallel pure shear. The modelling is performed with the Viscoplastic Full-Field Transform (VPFFT) crystal plasticity code coupled with the two-dimensional platform modelling platform Elle. Mechanical anisotropy can enhance shear localisation and redistribute the strain, resulting in localised shear domains with strain concentration and low-strain domains in between. This strain localisation dominates the formation of structures in anisotropic materials and is visualised by foliated layers or foliations. The fold and crenulation geometries displayed in this thesis are made by systematically varying (i) the initial orientation of the anisotropy (CPO), (ii) the intensity of anisotropy, and (iii) the viscous property differences of materials. In simple shear with a CPO in the stretching field from the beginning, three types of localisation behaviour are synthetic shear localisation, antithetic shear localisation and distributed localisation. However, the resulting visible crenulation geometries are very varied and include ā€˜S-Cā€™ structure (C & Cā€™ bands), ā€˜anti S-Cā€™ structure (Cā€™ā€™ bands), or mixes of both, or even in some cases no crenulation at all. This highlights that crenulation geometries are primarily due to the strong mechanical anisotropy of rocks. Mechanical anisotropy also affects layer-parallel pure shear shortening simulations. Here we observe two end-member geometries: The first is buckle folding and thickening of a competent layer similar to classical Biot-type buckle folds. An axial planar crenulation cleavage forms in the anisotropic matrix. In the absence of a competent layer, folds in the anisotropic matrix are self-similar with no characteristic length scale. This is observed in polar ice sheets. In this case it was also observed that fold amplification ceased after some strain, due to the rotation of the CPO. This confirms the hypothesis proposed for the shear margins of the Northeast Greenland Ice Stream (NEGIS), where fold amplification ceased about 2000 a BP. The second end member is layer-extension folding with strong amplification of fold amplitudes due to the formation of conjugate, localised bands in the matrix. Other geometries are in between

    Geological controls on megathrust slip: the 2014 Pisagua, Chile, earthquake sequence as a natural laboratory

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    Most subduction zones remain poorly resolved when it comes to the study of the precise location of the far oļ¬€shore located updip limit of coseismic slip and its controlling parameters, which is an essential component of earthquake hazard assessment. The relative lack of seismicity on the updip limit, combined with laboratory friction studies, suggest the shallow fault is mostly velocity strengthening and likely to creep. This view is reinforced by geodetic inversions, which show low to zero coupling close to the trench. However, these locations are remote from the land; hence the models derived from terrestrial stations are not suļ¬ƒciently well constrained. Moreover, the updip region can also be seismogenic, as demonstrated by tsunami earthquakes and shallow, slow slip events. To better understand the controls on the updip limit, we analyze high-resolution seismic data to image an erosive margin with documented intense seismicity. Developing a high-resolution model of the seismic velocity and reļ¬‚ectivity of a region where we have exceptionally good knowledge of the temporal and spatial distribution of slip will provide new insights into these controls that can be transferred to studies of seismic slip and crustal structure elsewhere. These results will also have a signiļ¬cant impact on statistical forecasts of future earthquake activity, which can aļ¬€ect seismic hazard evaluation and mitigation plans. The thesis is motivated by two themes, which are interrelated and addressed in a holistic seismic approach. One theme is mainly focused on the ļ¬‚uid-pressure variations revealed by the reļ¬‚ectivity of the seismic proļ¬les along the updip limit to identify the shallow, velocity strengthening part of the plate boundary. Another theme is the study of the structure of the updip limit and the hanging upper plate caused by the subducting oceanic ridge

    The Global Riverine Hydrokinetic Resource

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    Modelling Coastal Vulnerability: An integrated approach to coastal management using Earth Observation techniques in Belize

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    This thesis presents an adapted method to derive coastal vulnerability through the application of Earth Observation (EO) data in the quantification of forcing variables. A modelled assessment for vulnerability has been produced using the Coastal Vulnerability Index (CVI) approach developed by Gornitz (1991) and enhanced using Machine learning (ML) clustering. ML has been employed to divide the coastline based on the geotechnical conditions observed to establish relative vulnerability. This has been demonstrated to alleviate bias and enhanced the scalability of the approach ā€“ especially in areas with poor data coverage ā€“ a known hinderance to the CVI approach (Koroglu et al., 2019).Belize provides a demonstrator for this novel methodology due to limited existing data coverage and the recent removal of the Mesoamerican Reef from the International Union for Conservation of Nature (IUCN) List of World Heritage In Danger. A strong characterization of the coastal zone and associated pressures is paramount to support effective management and enhance resilience to ensure this status is retained.Areas of consistent vulnerability have been identified using the KMeans classifier; predominantly Caye Caulker and San Pedro. The ability to automatically scale to conditions in Belize has demonstrated disparities to vulnerability along the coastline and has provided more realistic estimates than the traditional CVI groups. Resulting vulnerability assessments have indicated that 19% of the coastline at the highest risk with a seaward distribution to high risk observed. Using data derived using Sentinel-2, this study has also increased the accuracy of existing habitat maps and enhanced survey coverage of uncharted areas.Results from this investigation have been situated within the ability to enhance community resilience through supporting regional policies. Further research should be completed to test the robust nature of this model through an application in regions with different geographic conditions and with higher resolution input datasets
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