1,721,106 research outputs found
Variability of thermocline and intermediate waters in the South Atlantic
No full textThis thesis investigates the variability of the intermediate and thermocline water mass properties in the South Atlantic, with a particular focus on salinity. These water masses form the upper branch of the Meridional Overturning Circulation in the Atlantic and have an important position in global ocean climate. Variability on various timescales is investigated: decadal changes are investigated using repeat hydrographic surveys, interannual variability is investigated using Argo float data and the two timescales are united by investigating a 40 year modern ocean model run. Salinity of thermocline water masses is shown to vary slowly on decadal timescales. Basinwide changes in salinity are evident at 24ºS over periods of 25 years. That these changes are representative of decadal changes and not merely aliasing shorter term variability is supported by analysis of the model. Increases in salinity at 24ºS from 1958 to 1983 and at 30?S from 1993 to 2003 are linked with influence from the Indian ocean while a freshening at 24ºS from 1983 to 2009 may be linked with an intensified hydrological cycle. Antarctic Intermediate Water (AAIW) is seen to increase in salinity at 24ºS from 1958 to 1983 and in the eastern basin at 30ºS from 1993 to 2003. This is different from the Indian and Pacific Oceans where AAIW has been observed to be freshening – a change linked with the intensification of the hydrological cycle. Using the highly correlated relationship between salinity and oxygen in AAIW, these changes are linked to the influence of the Indian Ocean. Further investigation of the changes in AAIW in the South Atlantic using Argo data and model data show that the variability of this water mass is dominated by westward propagating salinity anomalies. These anomalies originate in the Agulhas influenced Cape Basin of the South Atlantic and propagate westwards with speeds similar to those predicted for second mode baroclinic Rossby waves. The techniques developed for analysis of AAIW salinity using oxygen data are expanded to the other oceans of the southern hemisphere. The conclusions drawn from this analysis put the AAIW in the South Atlantic in a global context
Decadal Variability of Thermocline and Intermediate Waters at 24°S in the South Atlantic
Get PDFNew data are presented from 24°S in the South Atlantic in an investigation of the decadal variability of the intermediate and thermocline water masses at this latitude. Variation of salinity on neutral density surfaces is investigated with three transatlantic, full-depth hydrographic sections from 1958, 1983, and 2009. The thermocline is seen to freshen by 0.05 between 1983 and 2009. The freshening is coherent, basinwide, and of a larger magnitude than any errors associated with the datasets. This freshening reverses a basinwide, coherent increase in salinity of 0.03 in the thermocline between 1958 and 1983. Changes in apparent oxygen utilization (AOU) are investigated to support the salinity changes. In the thermocline of the eastern basin, a correlated relationship exists between local AOU and salinity anomalies, which is consistent with the influence of Indian Ocean Water. This correlated relationship is utilized to estimate the magnitude of Indian Ocean influence on the salinity changes in the thermocline. Indian Ocean influence explains half of the salinity changes in the eastern thermocline from 1958 to 1983 but less of the salinity change in the eastern thermocline from 1983 to 2009. Antarctic Intermediate Water properties significantly warm from 1958 through 1983 to 2009. A significant salinification and increase in AOU is evident from 1958 to 1983. Changes in the salinity of AAIW are shown to be linked with Indian Ocean influence rather than changes in the hydrological cycle. Upper Circumpolar Deep Water is seen to be progressively more saline from 1958 through 1983 to 2009. Increased Agulhas leakage and the intensification of the hydrological cycle are conflicting influences on the salinity of thermocline and intermediate waters in the South Atlantic as the former acts to increase the salinity of these water masses and the latter acts to decrease the salinity of these water masses. The results presented here offer an interpretation of the salinity changes, which considers both of these conflicting influences
The water mass transformation framework and variability in hurricane activity
Get PDFHurricane activity has been higher since 1995 than in the 1970s and 1980s. This rise in activity has been linked to a warming Atlantic. In this study, we consider variability of the volume of water warmer than 26.5 ºC, considered widely to be the temperature threshold crucial to hurricane development. We find the depth of the 26.5 ºC isotherm better correlated with seasonal hurricane counts than SST in the early part of the Atlantic hurricane season in some regions. The volume of water transformed by surface heat fluxes to temperatures above 26.5 ºC is directly calculated using the Water Mass Transformation framework. This volume is compared with the year-to-year changes in the volume of water of this temperature to see how much of the volume can be explained using this calculation. In some years, there is notable correspondence between transformed and observed volume anomalies, but anomalies in other years must be largely associated with other processes, such as the divergence of horizontal heat transport associated with the AMOC. This technique provides evidence that, in a given year, coordinated physical mechanisms are responsible for the build-up of anomalous ocean heat; not only net surface heat exchange but also the convergence of horizontal heat transport from ocean currents, to provide fuel for larger numbers of intense hurricanes
South Atlantic overturning circulation at 24°S
Get PDFTo estimate the size of the meridional overturning circulation and the meridional heat and freshwater transports in the South Atlantic, we made a new transatlantic hydrographic section along 24°S in 2009 and we compare the resulting transports with those estimated for a historical section made in 1983. For the two sections, the overturning is estimated to be 21.5 Sv (2009) or 16.5 Sv (1983), the heat transport is northward at 0.7 PW (2009) or 0.4 PW (1983), and the freshwater transport is small but northward at 0.04 Sv (2009) or 0.17 Sv (1983). The differences in transports are primarily due to the different strengths of the southward Brazil Current transport during the occupation of the sections, 4.9 Sv (2009) or 12.3 Sv (1983). The freshwater transport associated with the meridional overturning circulation is estimated by two different methods for each of the two sections and is always southward ranging from -0.09 Sv to -0.34 Sv which means that the Atlantic meridional overturning circulation transports freshwater southward at 24°S. On the basis of theoretical studies, such southward freshwater transport at the southern boundary of the Atlantic Ocean means that the present Atlantic circulation has multiple equilibrium states, and that the one it occupies at present may be unstable to a sufficiently large freshwater event
A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline
Get PDFA decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Previous studies have used linear models based on upper-layer temperature anomalies to extend AMOC estimates back in time; however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26∘ N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline
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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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