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The effects of Arctic sea-ice thickness loss and stratospheric variability on mid-latitude cold spells
Get PDFThe Arctic is a key indicator of global climate change. Annual surface temperatures are increasing at more than twice the rate of the global average, and passive microwave satellite observations of Arctic sea-ice extent show a loss of nearly 40% over the last few decades. Despite recent advances in climate models, availability of observations, and statistical analysis, our understanding of ice-ocean-atmosphere interactions, and the teleconnections between the Arctic and other regions, remains incomplete. Assessing the atmospheric response to the rapid changes in Arctic sea ice will help to determine future societal impacts from climate change, including extreme weather events in the densely populated mid-latitude regions.The broad objective of this thesis is to improve our understanding of Arctic climate variability. To be more specific, this dissertation will explore (i) the internal variability of Arctic sea-ice thickness (SIT), (ii) the atmospheric response to thinning sea ice, and (iii) the role of stratospheric and tropospheric pathways in modulating Arctic-mid-latitude teleconnections. SIT has an important effect on the Arctic energy budget, and therefore it is critical to represent it accurately in global climate models. However, limited observations of SIT (satellite-derived and in situ) have prevented a robust analysis of the atmospheric response to its spatial and temporal variability. Here, I use perturbation experiments in a high-top atmospheric global climate model to examine the importance of stratosphere-troposphere coupling and other causal pathways for teleconnections between Arctic sea ice and the mid-latitudes.Internal variability contributes to an uncertainty of 10-20 years in the timing of future mean SIT falling below 0.5 m in a large ensemble of simulations of a fully-coupled global climate model. This loss of SIT is found to reinforce the large-scale tropospheric response to Arctic sea-ice concentration and contributes up to one third of the surface warming response. Using a high-top model, the Quasi-biennial Oscillation (QBO) is found to significantly modulate the response to Arctic sea-ice decline. The stratospheric polar vortex weakens in response to sea-ice forcing during easterly QBO winters. However, there is little-to-no stratospheric response to sea-ice loss during the westerly phase of the QBO. Finally, in a series of coordinated coupled and uncoupled global climate model perturbation experiments, the “warm Arctic, cold Siberia” temperature anomaly pattern is found to be closely related to the strength of the Siberian High and mid-tropospheric warming response. By comparing simulations that are directly forced with warmer temperature in the Arctic region (corresponding to projected changes), to sea-ice forced experiments (where the forcing corresponds to the same future time), it is shown that sea-ice loss alone is insufficient to bring about the entire thermal signature of Arctic amplification
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The effect of low-frequency climate variability on stratosphere-troposphere coupling during boreal winter
Get PDFThe exchange of heat, momentum, and chemical species between the stratosphere and troposphere affects weather and climate. Stratosphere-troposphere coupling often refers to weather events. As a result, there are many studies focusing on dynamical coupling between the two layers at daily to seasonal timescales. Less is known about how low frequency climate variability in the Earth system affects stratosphere-troposphere coupling.The purpose of this dissertation is to assess how three forms of low frequency climate variability, the Quasi Biennial Oscillation (QBO), Interdecadal Pacific Variability (IPV), and Atlantic Multidecadal Variability (AMV), influence stratosphere-troposphere coupling. The main tools used are multi-century, high-top, atmospheric global climate model perturbation experiments and reanalysis. Two specific goals are to (1) assess what teleconnections are elicited by each mode of variability and (2) identify how these teleconnections enhance or suppress stratosphere-troposphere coupling. The stratospheric circulation is often associated with wintertime extreme weather events. Benchmarking how these modes of variability affect stratosphere-troposphere coupling may enhance predictability of these events.Studies have assessed the atmospheric response to IPV and AMV individually, but not together. Here, all combinations of IPV and AMV are imposed in a set of perturbation experiments to assess their combined effects on the boreal winter atmosphere. The atmospheric response to IPV dominates the response forced by AMV. In nature, the AMV is currently in its positive phase and the IPV is thought to be transitioning to its positive phase. Therefore, more targeted study is done for the positive AMV experiment, positive IPV experiment, and their combination. Positive IPV promotes a deeper Aleutian Low, which is partially cancelled by the atmospheric response to positive AMV, which promotes ridging over the North Pacific. While the polar stratospheric response to positive IPV is broadly similar with or without positive AMV, the downward propagation of the polar stratospheric warming during positive IPV is significantly reduced when the positive AMV is included.Since the QBO has a known teleconnection with the polar stratosphere, the Holton-Tan effect, a brief attempt is made to subsample the IPV and AMV results by QBO phase. It becomes unclear how the QBO is suppressing or enhancing the IPV and AMV teleconnections. As a result, I pivot towards using a hierarchy of model simulations to isolate the impact of the QBO on planetary waves and the tropospheric and stratospheric circulations. The QBO is found to promote amplification of planetary waves at the tropopause, contrasting with the amplification of planetary waves in the troposphere promoted by IPV and AMV. The QBO forces regional teleconnections. There is consistent evidence that the North Pacific atmosphere is more tightly coupled with the QBO compared to other longitudes. The QBO promotes latitudinal shifting of the tropospheric jet stream, especially in the North Pacific. However, the polar stratospheric response to the QBO moderates this effect. While the QBO promotes an interesting set of teleconnections in the lower stratosphere, the dominant teleconnection between the QBO and polar vortex occurs in the middle stratosphere (30 kilometers) particularly over mid-latitude Asia (60°E-120°E)
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Interannual Variability of the South American Monsoon in a climate change context
Get PDFThe South American Monsoon System (SAMS) is characterized by a well-defined wet and dry season, which is connected to the reversal of anomalous low-level winds east of the Andes. Driven by local and remote processes, the intensity and duration of the SAMS can vary considerably from year-to-year. Improving the understanding of the variability in the SAMS is important not only for the planning of water resources, but also for the global carbon budget due to the presence of the Amazon rainforest. Here, I investigate the mechanisms associated with interannual variability and long-term trends of the SAMS, which can impact the local circulation and precipitation anomalies over South America.The broad objective of this dissertation is to understand the physical mechanisms of the intraseasonal and interannual variability of the South American Monsoon and how it may change in a warmer climate. This dissertation explores three different aspects of that problem: (1) how the length and intensity of the South American Monsoon may change in a future climate by examining projections in a large ensemble of climate model simulations; (2) how the Indian Ocean Dipole affects the large-scale circulation over South America through extratropical teleconnections and changes in the local Walker circulation; and (3) the role of the Quasi-Biennial Oscillation, a regular mode of atmospheric variability, in modulating the remote effects of the Madden Julian Oscillation in the Southern Hemisphere.The results shown in this dissertation find that the response of the SAMS to climate change in most areas of South America may be characterized by a dryer dry season and a wetter wet season. However, the Amazon basin is projected to become dryer in all seasons, with possible impacts to forest productivity. Extreme events of rainfall are also projected to become more frequent and intense by the end of the century.Next, by comparing available observations to a set of perturbation experiments in an atmospheric global climate model forced with the SST anomalies due to both signs of the Indian Ocean Dipole (IOD), and a historical simulation from a fully-coupled large ensemble, I analyze the circulation anomalies associated with each phase of the IOD. The simulations reveal that both phases of the IOD can redirect the low-level winds over South America, which changes the advection of moisture to Southeastern Brazil and Southeastern South America. As a result, the response to the positive phase of the IOD is characterized by a dryer-than-usual South Atlantic Convergence Zone, while wet anomalies are found in the La Plata basin during the negative phase of the IOD. This occurs through an extratropical mechanism, composed of Rossby waves excited by the anomalous convection in the tropics that reach South America and redirect the low-level flux in the area. During the positive phase of the IOD, the displacement of the local Walker circulation results in wetting over the Amazon basin.Finally, I show that the Quasi-Biennial Oscillation (QBO) can modulate the response of the SAMS to the Madden-Julian Oscillation (MJO). When there are easterly winds in the lower stratosphere, the convection anomalies within the MJO are intensified and there are significant anomalies in structure of the extratropical wave train. Therefore, the anomalies associated with the MJO in South America are intensified and extend to greater areas of the subtropical South Atlantic. These results may highlight a new mechanism that can improve subseasonal to seasonal forecasts in South America
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An Analysis of the Behavior and Intensity of Extreme Atmospheric Moisture Transport Events over the North Pacific Basin
Get PDFAtmospheric rivers (ARs) are filamentary features that play a leading role in the poleward transport of atmospheric moisture and in the global redistribution of heat from the tropics. When they cross over land (so-called landfall), they are a major source of wintertime precipitation, particularly over the western coastline of North America. The extreme precipitation and flooding that sometimes accompany landfalling ARs can have severe socio-economic consequences. Despite advances in observational networks on land, the large-scale mechanisms influencing AR behavior and landfalling intensity are poorly understood. This dissertation aims to better characterize their present-day behavior and projected response to climate change over the North Pacific basin so as to improve forecasts of their impact at landfall.Composites of dynamical fields using thirty years of the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis are made following the eastward progression of ARs. A close relationship exists between the extratropical upper tropospheric dynamics, particularly anticyclonic Rossby wave breaking, and lower tropospheric moisture transport. Comparison between the strongest and the weakest ARs show consistent differences in both the intensity of moisture transport and the scale and rate of development of anticyclonic Rossby wave breaking. The strong relationship of landfalling ARs to anticyclonic Rossby wave breaking persists in a case study analysis of long-duration landfalling events.Landfalling ARs are evaluated in historical (1980 - 2004) simulations from 28 models participating in fifth phase of the Coupled Model Intercomparison Project (CMIP5) and compared to the MERRA and ERA-Interim reanalyses. Few models correctly resolve the frequency distribution, interannual variability in number and amplitude of moisture flux, and median landfalling latitude. The response of a subset of high performing models to projected warming is investigated using Representative Concentration Pathway (RCP) 8.5 (2070 - 2099) projections. Selected models show a broadening of the frequency distribution, with the largest increase in frequency occurring equatorward of peak historical frequency. The equatorward increase in peak historical frequency is co-located with increases in the 850- and 250-hPa zonal winds. The moisture flux response to warming is mostly thermodynamic, but equatorward of its peak distribution, it is dominated by a dynamic response
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Variability of Pacific tropical convergence zones in observations
Get PDFThe instantaneous South Pacific Convergence Zone (SPCZ) and east Pacific Intertropical Convergence Zone (ITCZ) are identified in long-term satellite observations using two automated statistical models. The statistical models are designed to emulate visual identification of convergence zones, balancing the complex definition of a convergence zone as an elongated envelope of convection including clouds of varying heights as well as clear sky, against the need for automatic detection in large amounts of data. Identification occurs in 3-hourly infrared (IR) images from geostationary satellites from 1980-2012. For the SPCZ, the study is limited to November through April but the east Pacific ITCZ is identified year round. Interannual variability, seasonal evolution, and intraseasonal variability of the SPCZ are quantified using 3-houlry SPCZ labels. The SPCZ is found to have two distinct parts: a tropical segment which is more active, particularly in December through February, having a mostly zonal orientation and a subtropical segment which is less active and has a tilted orientation. The El Niño Southern Oscillation (ENSO) influences the SPCZ on interannual time scales as the SPCZ shifts equatorward during El Niño and poleward during La Niña. On the intraseasonal time scale the SPCZ changes intensity and location according to various phases of the Madden Julian Oscillation (MJO). The SPCZ also has a distinct diurnal cycle in area, mean IR temperature, and cloud height, which changes throughout the season and is influenced by ENSO and the MJO. In the east Pacific, the ITCZ can take on several configurations. A statistical model is used to automatically assess the daily state of the east Pacific ITCZ based on the location of cloud bands: north of the equator (nITCZ), south of the equator (sITCZ), simultaneously north and south of the equator (dITCZ), and over the equator (eITCZ). A fifth state describes when no cloud bands exist in the east Pacific (aITCZ). Most of the year is dominated by the nITCZ state but in the boreal spring all states occur and variability is high. In March and April the dITCZ occurs, on average, 34% of the time, indicating that the double ITCZ occurs frequently in instantaneous data
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Variability of subseasonal and seasonal teleconnections that affect wintertime Southwestern United States Precipitation
Get PDFThe Southwestern United States (SWUS) is a region with large intraseasonal and interannual variance in wintertime precipitation. However, forecasts of subseasonal and seasonal SWUS precipitation lack sufficient skill to aid water managers and other decision makers. This lack of skill is partly due to the inherent chaotic nature of the atmosphere, whose future state diverges exponentially in response to errors in the initial forecast state, but is also attributable to model errors that may diminish forecast fidelity.The primary goal of this dissertation is to identify and analyze the atmospheric circulation patterns and teleconnections that lead to subseasonal, and to a lesser extent seasonal, SWUS wintertime precipitation. Specifically, this thesis aims to (i) ascertain model biases that affect the teleconnections and thus forecast skill, (ii) assess SWUS precipitation sources of predictability and potential predictability limits, and (iii) estimate the role of climate change in modulating the teleconnections over the historical period and into the future. Previous research has highlighted the role of the El Niño Southern Oscillation (ENSO), the Madden Julian Oscillation (MJO), and internal midlatitude atmospheric variability in driving teleconnection patterns that affect SWUS precipitation across multiple timescales. As such, these phenomena and their interplay with other climate modes form the focus for this study. To accomplish this, I utilize data from both ensemble climate experiments and subseasonal hindcast experiments, which I compare with observations and reanalysis.
Both types of experiments indicate that there are three main teleconnection patterns associated with excess SWUS precipitation on subseasonal and seasonal timescales. These are an El Niño-like meridional teleconnection pattern, a Pacific North American-type (PNA) arching pattern, and a waveguide-trapped zonal pattern over the East Asian jet stream and Pacific jet. Of these, only the meridional pattern is primarily associated with tropical convection and forcing. Although the arching and zonal patterns both may be forced by tropical convection, they most often result from internal midlatitude variability and are thus likely unpredictable on subseasonal and longer timescales. Furthermore, dynamical models contain significant errors in the midlatitude atmospheric mean state that affect Rossby wave propagation and source, negatively affecting the ability of dynamical models to faithfully represent these identified patterns, which may lead to decreased forecast skill. Changes in the mean state due to interannual ENSO variability or due to the global warming trend also may impact these teleconnection patterns by altering either the position of the Pacific jet or the preferred locations for tropical convective activity. Lastly, changes in the MJO associated with either decadal SST variability or global warming result in significant changes in MJO teleconnection patterns and “windows of opportunity” for SWUS precipitation forecasts. However, dynamical models still do not realize the potential skill from an accurate representation of the MJO and its teleconnections
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
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Building resources for MT: What the user hasn’t got we have to provide
Get PDFThe greatest sources of language data for natural language processing are
held by the machine translation development community. That data is
potentially more in demand than the MT-systems themselves. The
defensive attitude of not making these data available for further
development is damaging the natural evolution in the field.
Activation generates users and those in turn the number of systems to be
bought. However, that activation is stalled primarily by the cost of building
an MT-system, i.e. the lack of language data available, and secondly by the
fact that the potential buyers of machine translation systems lack the
knowledge needed for tuning the system to fit the in-house environment
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
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