Ocean Dynamics of Greenland’s Glacial Fjords at Subannual to Seasonal Timescales PDF Download

Author: Robert M. Sanchez
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Languages : en
Pages : 0

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Mass loss of the Greenland Ice Sheet is expected to accelerate in the 21st century in response to both a warming atmosphere and ocean, with consequences for sea level rise, polar ecosystems and potentially the global overturning circulation. Glacial fjords connect Greenland's marine-terminating glaciers with the continental shelf, and fjord circulation plays a critical role in modulating the import of heat from the ocean and the export of freshwater from the ice sheet. Understanding fjord dynamics is crucial to predicting the cryosphere and ocean response to a changing climate. However, representing glacial fjord dynamics in climate models is an ongoing challenge because fjord circulation is complex and sensitive to glacial forcing that is poorly understood. Additionally, there are limited observations available for constraining models and theory. This dissertation aims to improve our understanding of fjord dynamics, focusing on key aspects (heat variability, freshwater residence time, and fjord exchange) which need to be included in glacial fjord parameterizations. We use three approaches combining novel observations, idealized, modeling and numerical simulations to investigate the dynamics of fjord circulation at different spatial scales. First, we investigate the heat content variability in the fjord using acoustic travel time (Chapter 2). We demonstrate that acoustic travel time can be used to model fjord stratification during winter months and monitor heat content variability at synoptic and seasonal timescales. Secondly, we use a combination of in situ observations and an idealized box model to evaluate freshwater residence time in a west Greenland Fjord (Chapter 3). We find that meltwater from the ice sheet is mixed downward across multiple layers near the glacier terminus resulting in freshwater storage and a delay in freshwater export from the fjord. Finally we analyze a multi-year realistically forced numerical simulation of Sermilik Fjord in southeast Greenland and identify the impact of shelf and glacial forcing on fjord exchange (Chapter 4). We show that the glacial-driven circulation is more efficient at renewing the fjord and that the sign of the exchange flow is related to the along-shelf wind stress. This dissertation strengthens our understanding of the fundamental connections between oceans and glaciers, and will lead to improved representation of ice-ocean interactions in climate models.

Author: Rebecca Harding Jackson
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Languages : en
Pages : 172

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Glacial fjords form conduits between glaciers of the Greenland Ice Sheet and the North Atlantic. They are the gateways for importing oceanic heat to melt ice and for exporting meltwater into the ocean. Submarine melting in fjords has been implicated as a driver of recent glacier acceleration; however, there are no direct measurements of this melting, and little is known about the fjord processes that modulate melt rates. Combining observations, theory, and modeling, this thesis investigates the circulation, heat transport, and meltwater export in glacial fjords. While most recent studies focus on glacial buoyancy forcing, there are other drivers -e.g. tides, local wind, shelf variability - that can be important for fjord circulation. Using moored records from two major Greenlandic fjords, shelf forcing (from shelf density fluctuations) is found to dominate the fjord circulation, driving rapid exchange with the shelf and large heat content variability near the glacier. Contrary to the conventional paradigm, these flows mask any glacier-driven circulation in the non-summer months. During the summer, when shelf forcing is reduced and freshwater forcing peaks, a mean exchange flow transports warm Atlantic-origin water towards the glacier and exports glacial meltwater. Many recent studies have inferred submarine melt rates from oceanic heat transport, but the fjord budgets that underlie this method have been overlooked. Building on estuarine studies of salt fluxes, this thesis presents a new framework for assessing glacial fjord budgets and revised equations for inferring meltwater fluxes. Two different seasonal regimes are found in the heat/salt budgets for Sermilik Fjord, and the results provide the first time-series of submarine meltwater and subglacial discharge fluxes into a glacial fjord. Finally, building on the observations, ROMS numerical simulations and two analytical models are used to investigate the dynamics of shelf-driven flows and their importance relative to local wind forcing across the parameter space of Greenland's fjords. The fjord response is found to vary primarily with the width relative to the deformation radius and the fjord adjustment timescale relative to the forcing timescale. Understanding these modes of circulation is a step towards accurate modeling of ocean-glacier interactions.

Author: Margaret Ruth Lindeman
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Category :
Languages : en
Pages : 160

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The glacial fjords that connect the Greenland Ice Sheet to the North Atlantic control ocean heat transport toward the ice sheet and the downstream fate of glacier meltwater. This thesis builds on a growing body of research into Greenland fjord dynamics, focusing on aspects of glacier-fjord systems that are especially challenging to observe: sub-annual ocean variability beneath a floating ice tongue; iceberg meltwater properties and distribution; and the distribution and cycling of environmental mercury. Ice discharge to the ocean can be moderated by ice tongues, floating extensions of glaciers that buttress the upstream ice flow. In Chapter 3, an ice-tethered mooring record from beneath the 79 North Glacier ice tongue shows that ocean warming observed on the continental shelf is advected into the fjord and reaches the glacier grounding line within 6 months, indicating that basal melt of the ice tongue is sensitive to regional ocean variability. Icebergs calved from tidewater glaciers are a major component of fjord freshwater and heat budgets in fjords, but there are few observations to constrain iceberg melt models. In Chapter 4, meltwater plume intrusions are identified based on their temperature and salinity properties in two surveys of a large iceberg in Sermilik Fjord in southeast Greenland. The intrusions are distributed around the iceberg between 80-250 m depth and drive upwelling over vertical scales averaging 15-50 m, with the plume height primarily controlled by stratification. A standard melt plume model does not recreate the observed melt concentrations even with adjustments to the model coefficients, suggesting that more substantial modifications to the model physics are needed to accurately simulate iceberg melt and upwelling. In Chapter 5, results from a recent survey in Sermilik Fjord show that glacially modified waters are depleted in the toxic trace element mercury relative to regional ocean waters, indicating that glacier melt is not a significant source of environmental mercury in that system. We hypothesize that mercury is removed from the water column in the ice melange region near the glacier terminus through scavenging and settling of suspended sediments from iceberg melt and runoff.

Author: Ken Zhao
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ISBN:
Category :
Languages : en
Pages : 260

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Melting at the submerged faces of marine-terminating glaciers at the fringes of Antarcticaand Greenland has increased dramatically in recent decades. This acceleration has been driven in part by the ocean circulation within ice-shelf cavities and fjords through the increased access of warm, salty water masses and a presumed amplification of the heat flux towards these glaciers. However, the dynamics of the ocean circulation within fjords and ice-shelf cavities are poorly understood and require the representation of scales of motion that range seven orders of magnitude, from 100s of kilometers (circulation on the adjacent coastal shelves) down to centimeters (at the ice-ocean inner boundary layer interface). This presents unique challenges for existing models, which underpredict melt rates by an order of magnitude compared to recent observations at vertical glacial faces. The work in this dissertation seeks to improve the agreement of models and theory with observations and provide a better understanding of the dynamical processes within fjords and ice-shelf cavities. To accomplish this, a series of high-resolution numerical simulations of increasing complexity is presented. In Chapters 2 and 3, 2- and 3-layer isopycnal model configurations with idealized geometry and forcing are used; subsequently in Chapters 4 and 5, z-coordinate models with idealized and semi-realistic regional configurations are used. Inspired by the simplest models, theories of the overturning and horizontal recirculation (the two primary bulk measures of circulation strength within fjords and ice-shelf cavities,) are developed and tested and used to make predictions for the glacial melt rate. These theories are then tested in increasingly complex models, which reveal new features and factors that also should be taken into account. Three important features presented in this dissertation include the identification of cavity/fjord geometry as a critical constraint on heat transport, melt-circulation feedbacks in fjords, and the existence of standing eddies, which can both further amplify glacial melt rates. This work advances the understanding of the dynamics within fjords and ice-shelf cavities and many promising avenues of future work have emerged as a result. Future work will likely continue to provide critical improvements to our understanding of ocean circulation near the margins of ice sheets and improve our projections of future sea level rise and glacial retreat in a changing climate.

Author: Kaitlin M. Walsh
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Category :
Languages : en
Pages :

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Abstract: Outlet glaciers and ice caps on the periphery of the Greenland Ice Sheet have been observed to be extremely sensitive to climate. The limited studies conducted on the marine-terminating glaciers of eastern Greenland's Geikie Plateau and Blosseville Coast suggest exceptionally rapid rates of mass loss and short-term variability in ice dynamics. This study is targeted at a region of central east Greenland for which GRACE mass-anomaly observations show substantial recent mass-loss since its launch in March 2002. Additionally, glaciers in this region terminate into Denmark Straight, which is a thermodynamic transition zone between the Arctic and North Atlantic oceans. Extensive glacial change has been more pronounced through the Denmark Straight and south of the straight, which supports the hypothesis that ocean dynamics, specifically the Irminger Current and East Greenland Current, are supporting increased melt at the ice-ocean interface. It is possible that an appreciable amount of melt and ice loss south of Kangerdlugssuaq is occurring as a result of warmer subpolar water flowing into glacial fjords. We present changes to 38 marine-terminating glaciers as observed using Landsat-7 ETM+ imagery to develop a time series of changing front positions and flow speeds of these glaciers from 2000 to 2010. ASTER DEMs were used to quantify elevation change and thinning. Additionally, we examine sea surface temperatures at five sites along the east Greenland coast to identify possible correlations between warming of the sea surface and increased melt at the glacier termini.

Author: Benjamin Joseph Davison
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Category : Fjords
Languages : en
Pages : 229

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Author: Twila Moon
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Category : Ice sheets
Languages : en
Pages : 116

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Outlet glacier ice dynamics, including ice-flow speed, play a key role in determining Greenland Ice Sheet mass loss, which is a significant contributor to global sea-level rise. Mass loss from the Greenland Ice Sheet increased significantly over the last several decades and current mass losses of 260-380 Gt ice/yr contribute 0.7-1.1 mm/yr to global sea-level rise (~10%). Understanding the potentially complex interactions among glacier, ocean, and climate, however, remains a challenge and limits certainty in modeling and predicting future ice sheet behavior and associated risks to society. This thesis focuses on understanding the seasonal to interannual scale changes in outlet glacier velocity across the Greenland Ice Sheet and how velocity fluctuations are connected to other elements of the ice sheet-ocean-atmosphere system. 1) Interannual velocity patterns Earlier observations on several of Greenland's outlet glaciers, starting near the turn of the 21st century, indicated rapid (annual-scale) and large (>100%) increases in glacier velocity. Combining data from several satellites, we produce a decade-long (2000 to 2010) record documenting the ongoing velocity evolution of nearly all (200+) of Greenland's major outlet glaciers, revealing complex spatial and temporal patterns. Changes on fast-flow marine-terminating glaciers contrast with steady velocities on ice-shelf-terminating glaciers and slow speeds on land-terminating glaciers. Regionally, glaciers in the northwest accelerated steadily, with more variability in the southeast and relatively steady flow elsewhere. Intraregional variability shows a complex response to regional and local forcing. Observed acceleration indicates that sea level rise from Greenland may fall well below earlier proposed upper bounds. 2) Seasonal velocity patterns. Greenland mass loss includes runoff of surface melt and ice discharge via marine-terminating outlet glaciers, the latter now making up a third to a half of total ice loss. The magnitude of ice discharge depends in part on ice-flow speed, which has broadly increased since 2000 but varies locally, regionally, and from year-to-year. Research on a few Greenland glaciers also shows that speed varies seasonally. However, for many regions of the ice sheet, including wide swaths of the west, northwest, and southeast coasts where ice loss is increasing most rapidly, there are few or no records of seasonal velocity variation. We present 5-year records of seasonal velocity measurements for 55 glaciers distributed around the ice sheet margin. We find 3 distinct seasonal velocity patterns. The different patterns indicate varying glacier sensitivity to ice-front (terminus) position and likely regional differences in basal hydrology in which some subglacial systems do transition seasonally from inefficient, distributed hydrologic networks to efficient, channelized drainage, while others do not. Our findings highlight the need for modeling and observation of diverse glacier systems in order to understand the full spectrum of ice-sheet dynamics. 3) Seasonal to interannual glacier and sea ice behavior and interaction Focusing on 16 northwestern Greenland glaciers during 2009-2012, we examine terminus position, sea ice and ice m??lange conditions, seasonal velocity changes, topography, and climate, with extended 1999-2012 records for 4 glaciers. There is a strong correlation between near-terminus sea ice/mélange conditions and terminus position. In several cases, late-forming and inconsistent sea ice/mélange may induce sustained retreat. For all of the 13-year records and most of the 4-year records, sustained, multi-year retreat is accompanied by velocity increase. Seasonal speedup, which is observed across the region, may, however, be more heavily influenced by melt interacting with the subglacial hydrologic system than seasonal terminus variation. Projections of continued warming and longer ice-free periods around Greenland suggest that notable retreat over wide areas may continue. Sustained retreat is likely to be associated with multi-year speedup, though both processes are modulated by local topography. The timing of seasonal ice dynamics patterns may also shift.

Author: Vena Chu
Publisher:
ISBN:
Category :
Languages : en
Pages : 221

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The current need for forecasting Greenland Ice Sheet contributions to global sea level rise is complicated by the lack of understanding of ice sheet hydrology. The proportion of meltwater contributing to sea level rise, as well as the pathways transporting meltwater on, through, and out of the ice sheet, are not well understood. Remote sensing of hydrologic dynamics in combination with small-scale fieldwork allows examination of broad spatial and temporal trends in the Greenland hydrologic system responding to a changing climate. This dissertation reviews the current state of knowledge on Greenland Ice Sheet hydrology, and examines three components of the Greenland hydrologic system: (1) fjord sediment plumes as an indicator of meltwater output, (2) supraglacial streamflow as an indicator of meltwater input to the ice sheet, and (3) moulin distribution and formation as a mechanism diverting meltwater from the surface of the ice sheet to the bed. Buoyant sediment plumes that develop in fjords downstream of outlet glaciers are controlled by numerous factors, including meltwater runoff. MODIS retrievals of sediment plume concentration show a strong regional and seasonal response to meltwater production on the ice sheet surface, despite limitations in fjords with rapidly calving glaciers, providing a tool for tracking meltwater release to the ocean. Summertime field observations and high-resolution satellite imagery reveal extensive supraglacial river networks across the southwestern ablation zone transporting large volumes of meltwater to moulins, yet these features remain poorly mapped and their discharges unquantified. A GIS modeling framework is developed to spatially adapt Manning's equation for use with high-resolution WorldView-2 imagery to map supraglacial river discharge. Moulins represent connections between surface meltwater on the Greenland ice sheet and subglacial drainage networks, where increased meltwater can enhance ice sliding dynamics. A new high-resolution moulin dataset in western Greenland created from WorldView-1/2 imagery in the 2012 record melt year is used to assess moulin distribution and formation. Moulin locations show a significantly different distribution compared to geospatial variables in the entire study area, with moulins forming in areas of thinner ice, higher velocity and extensional strain rate, as well as lower surface elevation and slope, and higher bed elevation and slope.

Author: Theresa Jayne Morrison
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Category :
Languages : en
Pages : 0

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The increased presence of warm Atlantic water on the Greenland continental shelf and within glacial fjords has been connected to the accelerated melting of the Greenland Ice Sheet, particularly in the southwest and southeast continental shelf regions. Meltwater is transported into the Labrador Sea where it may disrupt deep convection, a process that has been linked to the variability of the Atlantic Meridional Overturning Circulation (AMOC). To study the transport of heat onto the continental shelf and freshwater into the Labrador Sea, we use coupled ocean/sea-ice simulations with horizontal resolutions that permit or resolve mesoscale eddies. We compare the on shelf transport of heat in two mesoscale eddy-permitting simulations. In both simulations, the region of greatest heat flux onto the shelf is southeast Greenland and south of the Denmark strait, where there is a seasonally persistent pattern of multi-day variability in the cross-shelf heat flux. This high-frequency variability is associated with Denmark Strait Overflow eddies propagating along the shelfbreak. Using mesoscale eddy-resolving simulations to compare the off-shelf transport of meltwater and impacts on the West Greenland Current, we find that vertically distributed meltwater results in an increase in total freshwater flux into the Labrador Sea. Eddy kinetic energy and baroclinic conversion in the West Greenland Current along the continental shelf break also increase with the addition of vertically distributed meltwater. Finally, we derive and test a box model to represent the mixing within fjords. This Fjord Box Model merges buoyant plume theory with the estuarine mixing that occurs along fjords. The Fjord Box Model provides a more realistic boundary condition for meltwater forcing in ocean models that cannot resolve fjords.

Author: Laura A. Stevens
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Category : Glaciers
Languages : en
Pages : 220

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Seasonal fluxes of meltwater control ice-flow processes across the Greenland Ice Sheet ablation zone and subglacial discharge at marine-terminating outlet glaciers. With the increase in annual ice sheet meltwater production observed over recent decades and predicted into future decades, understanding mechanisms driving the hourly to decadal impact of meltwater on ice flow is critical for predicting Greenland Ice Sheet dynamic mass loss. This thesis investigates a wide range of meltwater-driven processes using empirical and theoretical methods for a region of the western margin of the Greenland Ice Sheet. I begin with an examination of the seasonal and annual ice flow record for the region using in situ observations of ice flow from a network of Global Positioning System (GPS) stations. Annual velocities decrease over the seven-year time-series at a rate consistent with the negative trend in annual velocities observed in neighboring regions. Using observations from the same GPS network, I next determine the trigger mechanism for rapid drainage of a supraglacial lake. In three consecutive years, I find precursory basal slip and uplift in the lake basin generates tensile stresses that promote hydrofracture beneath the lake. As these precursors are likely associated with the introduction of meltwater to the bed through neighboring moulin systems, our results imply that lakes may be less able to drain in the less crevassed, interior regions of the ice sheet. Expanding spatial scales to the full ablation zone, I then use a numerical model of subglacial hydrology to test whether model-derived effective pressures exhibit the theorized inverse relationship with melt-season ice sheet surface velocities. Finally, I pair near-ice fjord hydrographic observations with modeled and observed subglacial discharge for the Saqqardliup sermia–Sarqardleq Fjord system. I find evidence of two types of glacially modified waters whose distinct properties and locations in the fjord align with subglacial discharge from two prominent subcatchments beneath Saqqardliup sermia. Continued observational and theoretical work reaching across discipline boundaries is required to further narrow our gap in understanding the forcing mechanisms and magnitude of Greenland Ice Sheet dynamic mass loss.