1,721,018 research outputs found
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Motion of ice sheets at their margins: Modeling studies
Get PDFIce sheets interact dynamically with the ocean, losing mass through ocean-induced melt and the calving of icebergs. At ice sheet margins, ice shelves buttress flow of tributary glaciers making them a critical component of the overall ice sheet stress balance and, consequently, mass budget. With retreat of ice shelves seen in both Antarctica and Greenland, accurate modeling of ice shelf motion is important for forecasting future sea-level rise. Presently, most glacier models leave the calving front fixed in time with the implicit assumption that flow at the calving front is balanced by calving resulting in a stationary front. Numerically evolving a calving front is challenging in two ways, the first is due to the difficulty in applying the correct calving front boundary condition as the front shifts in time and the second lies in utilizing an appropriate numerical treatment for the calving process. Many factors influence calving including ocean-induced melt, surface melt-water hydrofracturing of crevasses, and wave energy triggering calving events. Of importance at some Greenland glaciers, melange, a mixture of icebergs and sea ice, can act to suppress calving rates by either preventing calved bergs from rotating away from the front or directly buttressing front strain rates.In this dissertation we utilize two mathematical techniques, the level-set method and diffuse-domain method, to develop an ice shelf model that allows for the temporal evolution of a calving front. Our ice-shelf model allows for mesh independent enforcement of the appropriate boundary condition thus allowing for continuous motion of the front within mesh elements. We validate this model in both a 1D domain and 2D idealized domain. We then apply these front-motion methods to the evolution of the Ross ice shelf and examine characteristics of flow through time. Our model captures key features of ice-shelf flow and produces a smooth evolution of the front. Finally, we use our ice shelf model to examine the potential for melange to buttress ice shelf calving front strain rates. We find that melange, if rheologically strong enough, of 100 m thickness shearing over 1 km can initiate a calving front advance of 490 m/a
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Deconvolution of the Labrador Sea Transit-Time Distribution from Combined Measurements of CFC-11 and CFC-12
Get PDFThe Labrador Sea is an important deep water formation site where a large fraction of theocean's deep waters had their last contact with the atmosphere. Consequently, quantifying the ventilation of the Labrador Sea is crucial for understanding the global ocean circulation. In this study, I use yearly hydrographic measurements of CFC-11 and CFC-12 conducted along WOCE repeat section AR7W from 1991 to 2009. I present improvements to previous application of Bayesian evidence framework to 3-dimensional interpolation of CFC-11, CFC-12, temperature and salinity data. The Bayesian evidence framework is also used to infer the Labrador Sea's climatological and interannual transit-time distribution (TTD) with a maximum entropy deconvolution method. We present improvements to the ventilation problem to allow for measurement errors and to better quantify the uncertainty in the estimated TTDs. These improvements include the introduction of an intrinsic correlation function to explicitly take into account correlations in adjacent portions of the TTD and the choice of hyperparameters in our model. Our results provide a baseline from which changes in theventilation of the Labrador Sea can be quantified
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Novel Tools for Ocean Biogeochemical Models
Get PDFOcean general circulation models of the IPCC class have biases even when simulatingpresent-day conditions, which may bring into question their predictions of futureconditions. This dissertation is about tools for, and results from assessing biases in theCommunity Earth System Model (CESM) ocean component, by itself and when combinedwith the Biological Ecosystem Cycling (BEC) model. Newly developed tools and theirapplications are listed.1. An offline matrix tracer transport model for the ocean component of CESM.2. A fast Newton-Krylov implicit tracer equilibrium solver for both the annually-averagedand the seasonally-varying circulation.3. An effective preconditioner for the solver simulating radiocarbon. Application results: For a natural radiocarbon simulation, an equilibrium solution was obtained in 23 model-years, a dramatic decrease from the 4000 model-years reported for time-stepping. The modeled circulation in the deep Pacific Ocean produced radiocarbon ages twice those of observations.4. A capability for computing the surface origin of water mass fractions as well as theage of the various water masses. Application results: The North Atlantic was the major supplier of ventilated water to not only the Atlantic, but also the Pacific and Indian Oceans. A lack of formation of bottom water in the Southern Ocean was discovered.5. A capability for restricting the tracer simulation domain to a limited region of theocean while retaining the effectiveness of advection and diffusion fields on theboundary. This reduces computational costs and allows separating local versusremote impacts of tracer sources on the biogeochemical tracer concentrations. Thiscapability has the potential to provide a platform for further biogeochemical studies. Application results: The Indian Ocean region was isolated. Global versus regional circulation effects were determined using radiocarbon. Most of the bias within the region was eliminated by using observational, rather than globally calculated values, on the boundaries. Oxygen production and consumption from a CMIP5 BEC simulation were used to drive a regional oxygen model. Boundary values of oxygen from the CMIP5 BEC simulation were replaced with observations, resulting in less bias within the region. However, significant bias in the location of the Arabian Sea oxygen minimum zone remained
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Causes and consequences of the ocean's biological pump
Get PDFThe production of organic matter at the ocean's surface, its transport, and subsequent remineralization in the ocean interior maintains a vertical carbon and nutrient gradient that sequesters atmospheric CO2, controls the distribution of dissolved oxygen, and modulates the flux of nutrients to the euphotic zone that fuels marine ecosystems. Collectively we refer to the set of processes maintaining these gradients as the biological pump and they are the central focus of this thesis. To advance our understanding of the biological pump, this thesis uses models to analyze and quantify individual processes within the larger biological pump system. Chapter 1 introduces the key processes at question and outlines the specific goals of each research chapter. Chapter 2 uses empirical models and a global database of in-situ observations to examine the environmental drivers of carbon export efficiency, defined as the fraction of organic matter production that is transported from the surface to the interior. We find significant regional variability in the relationship between export efficiency and environmental parameters that are shown to have important consequences for the prediction of global export. Chapter 3 further develops empirical models of carbon export efficiency with a focus on regional export variability in the Southern Ocean. We find new evidence that silica controls Southern Ocean export efficiency via interactions with ocean temperature. Chapter 4 uses a global ocean circulation and biogeochemistry model to examine the sensitivity of the global oxygen distribution to changes in the depth at which sinking organic matter is remineralized. Model experiments demonstrate that raising the remineralization depth of organic matter leads to net oxygen uptake and simultaneous expansion of oxygen minimum zones. Chapter 5 examines the consequences of long term climate-driven changes in the biological pump for the productivity of marine ecosystems and fisheries. The thesis concludes in Chapter 6 with a discussion of the challenges and opportunities in understanding the biological pump in a changing climate, along with my recommendations for follow up research
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Characterizing Transport between the Surface Mixed Layer and the Ocean Interior with a Forward and Adjoint Global Ocean Transport Model
Get PDFThe theory of first-passage time distribution functions and its extension to last-passage time distribution functions are applied to the problem of tracking the movement of water masses to and from the surface mixed layer in a global ocean general circulation model. The first-passage time distribution function is used to determine in a probabilistic sense when and where a fluid element will make its first contact with the surface as a function of its position in the ocean interior. The last-passage time distribution is used to determine when and where a fluid element made its last contact with the surface. A computationally efficient method is presented for recursively computing the first few moments of the first- and last-passage time distributions by directly inverting the forward and adjoint transport operator. This approach allows integrated transport information to be obtained directly from the differential form of the transport operator without the need to perform lengthy multitracer time integration of the transport equations. The method, which relies on the stationarity of the transport operator, is applied to the time-averaged transport operator obtained from a three-dimensional global ocean simulation performed with an OGCM. With this approach, the author (i) computes surface maps showing the fraction of the total ocean volume per unit area that ventilates at each point on the surface of the ocean, (ii) partitions interior water masses based on their formation region at the surface, and (iii) computes the three-dimensional spatial distribution of the mean and standard deviation of the age distribution of water
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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