1,721,091 research outputs found
ENSO and Paraná flow variability: Long-term changes in their connectivity
No full textThe Paraná River of South America is one of the largest rivers in the world. Although the connection between Paraná streamflow and El Niño–Southern Oscillation (ENSO) has been studied before, little is known about the long-term (interdecadal) changes of this connectivity. Here we reveal these changes by analysing long instrumental records of Paraná flow, ENSO indices and other climate variables. We find that flow and ENSO were connected in 1876–1940 and 1983–2016, but disconnected in 1941–1982. Flow variability was more related to central Pacific ENSO fluctuations in 1876–1940 but more linked with eastern Pacific ones in 1983–2016, probably because the ENSO–Paraná basin teleconnection was different in the two periods. ENSO-related Pacific climate anomalies exhibited their smallest amplitudes during the disconnection period 1941–1982, suggesting that the ENSO–flow link vanishes when ENSO is weak. We also find that the ENSO–flow disconnection coincided with the coldest phase of the Interdecadal Pacific Oscillation (IPO) in the study period. Thus, the IPO may be modulating the ENSO–flow connectivity. Interestingly, during the disconnection years (1941–1982), flow was related to climate variability observed in sectors of the southern South Atlantic and tropical North Atlantic oceans. This and the other empirical results presented here provide new insights into the Paraná response to ENSO and thus provide valuable information for future mechanistic studies on this response.Fil: Antico, Andres. Universidad Nacional del Litoral. Instituto de Humanidades y Ciencias Sociales del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Humanidades y Ciencias Sociales del Litoral; ArgentinaFil: Vuille, Mathias. State University Of New York. University At Albany.; Estados Unido
The Effect of Moisture Sources and Convective Activity on Stable Isotopes in Precipitation and Tree Rings in Southern Ecuador
No full text10.3389/feart.2020.604804 ; 10.1029/2023JD040630Extreme and heavy precipitation events cause disastrous floods threatening human lives. Such events are projected to occur more frequently and with a higher intensity in a warming climate. The future projections of precipitation are marked by a high degree of uncertainty. This uncertainty is particularly pronounced in the tropics, as climate observations are scare. To overcome this issue and improve future projections, climate reconstructions are essential. They contribute to a better understanding of the natural variability of precipitation in the past and in the present. A promising tool for climate reconstructions is the use of stable water isotopes in tree rings. In the tropics, the amount of precipitation is a well-known driver of isotopic variability. Although the relationship is well-established, it still occurs that some years and seasons cannot be explained. Hence, other mechanisms are also relevant for the isotopic signal. Recently, different moisture sources for precipitation and convective activity are frequently discussed to influence stable water isotopes in the tropics. To increase the reliability of climate reconstructions, it is essential to not only use one climate parameter but to understand the responsible atmospheric processes recorded in the stable water isotopes of tree rings.
The analysis of atmospheric processes requires spatially and temporally coherent data, which can be provide by climate or atmospheric models. For investigating convective processes and their effects on stable water isotopes in precipitation, a high-resolution isotope-enabled atmospheric model that resolves these processes is indispensable. The implementation of stable isotopes in the model is very precious as observations of stable isotopes in precipitation are very costly and spatially limited as they are point measurements. Moreover, it enables a comparison with stable water isotopes in tree rings based on the same unit, which does not hold for example for precipitation amount.
In my dissertation project, I tried to better understand the effect of atmospheric processes such as moisture sources and convective activity on stable water isotopes in tree rings of Bursera graveolens collected on the western flanks of the Andes in southern Ecuador, South America. To determine moisture source characteristics, I calculated trajectories for the study site from 2008 to 2017, which form the basis for the subsequently accomplished moisture source diagnostic. For the assessment of the influence of convective activity on stable water isotopes, I used a high-resolution and isotope-enabled atmospheric model over the study region for the rainy season (January – April) of 2012. To determine the influence of convective activity, I classified precipitation by prevailing rain types and analyzed its effect on the stable water isotopes in precipitation. At the end of my dissertation project, I assessed the relative importance of moisture sources and convective activity for stable isotopes in precipitation on the western flanks of the Andes. Additionally, I evaluated the possibility of inferring the effects of atmospheric processes recorded in the stable water isotopes in precipitation on the stable water isotope variations in tree rings of Bursera graveolens.
I discovered that moisture for precipitation arises from both sides of the climate divide, with contributions from the Pacific Ocean, so from west of the climate divide, intensifying with increasing daily precipitation. Interannually, positive sea surface temperature anomalies related to El Niño Southern Oscillation events enhance the influence of moisture from the Pacific Ocean. Regarding convective activity, I found that in January 2012, precipitation was primarily generated by shallow convection on the western flanks of the Andes. Between February and April many heavy precipitation events occurred and were characterized by deep convective precipitation. Stable water isotopes in precipitation show a relatively enriched (positive) signal during shallow convective compared to depleted (negative) signals during deep convective conditions.
Assessing the relative importance of moisture sources and convective activity revealed that both mechanisms are determined by atmospheric conditions. Strong upper-tropospheric easterly winds suppressed the development of deep convection, allowing only shallow convection, which was associated with a relatively enriched isotopic signal in precipitation during January 2012. A near-surface westerly moisture flux from the Pacific Ocean was a prerequisite for heavy precipitation events on the western flanks of the Andes from February to April 2012. Prevailing upper-tropospheric easterlies enabled the spillover of easterly convective cloud clusters to the west of the climate divide. The combination of extensive easterly cloud clusters spilling over to the west and an intense westerly moisture flux throughout the lower troposphere caused the highest daily precipitation amounts and the strongest depleted isotopic signals. In contrast, westerly conditions throughout the entire troposphere—which, in our study period, consistently prevailed after very heavy precipitation events—were associated with westerly convective cloud clusters of lesser extent than the easterly ones. The respective daily precipitation amounts were moderate, while the isotopic signals in precipitation showed moderately depleted values or even a progressively enriching signal.
I conclude that moisture sources and convective activity are responsible for stable water isotopic variability in precipitation, with convective activity having the greatest influence. Both mechanisms are intimately linked and determined by the atmospheric conditions. The interpretation of stable water isotope variations in tree rings in relation to these atmospheric processes remains limited. This limitation can be attributed to insufficient knowledge of tree growth patterns in Bursera graveolens, highlighting the need for further field research and additional data collection campaigns.Extreme Niederschlagsereignisse führen oft zu gefährlichen Überschwemmungen, die das Leben von Menschen gefährden. Diese Extremereignisse sollen in einem sich erwärmenden Klima häufiger und intensiver auftreten. Allerdings sind die Zukunftsprojektionen für Niederschlag mit großen Unsicherheiten behaftet. Diese Unsicherheiten sind vor allem in den Tropen stark ausgeprägt, da nur wenige lange Klimamessreihen vorhanden sind. Um diesem Umstand entgegenzuwirken und die Zukunftsprojektionen zu verbessern, sind Klimarekonstruktionen essenziell. Sie tragen dazu bei, die natürliche Variabilität des Niederschlags in der Vergangenheit und Gegenwart besser bestimmen zu können. Insbesondere Messungen stabiler Isotope des Wassers in Baumringen sind eine vielversprechende Methode zur Erstellung von Klimarekonstruktionen. In den Tropen wird das Isotopensignal maßgeblich von der Niederschlagsmenge bestimmt. Obwohl dieser Zusammenhang schon vor langer Zeit nachgewiesen wurde, gibt es Jahre und Monate, in denen das Isotopensignal und die Niederschlagsmenge deutlich divergieren. Folglich müssen noch weitere Mechanismen existieren, die das Isotopensignal beeinflussen. In jüngerer Vergangenheit wurden in der Wissenschaft vor allem die Herkunft der Feuchtigkeit für den Niederschlag sowie die konvektive Aktivität als mögliche Mechanismen diskutiert. Um Klimarekonstruktionen zu verbessern, ist es folglich unumgänglich, nicht nur einzelne Klimaparameter zu betrachten, sondern stattdessen atmosphärische Prozesse zu analysieren, die in den stabilen Isotopen von Baumringen aufgezeichnet werden.
Für die Analyse atmosphärischer Prozesse sind räumlich und zeitlich zusammenhängende Datensätze notwendig, die durch Klimamodelle bereitgestellt werden können. Zur Untersuchung von konvektiven Prozessen und deren Auswirkungen auf die stabilen Isotope im Niederschlag sind hochauflösende und isotopenfähige Klima- oder Atmosphärenmodelle unerlässlich. Die Implementierung stabiler Isotope in solchen Modellen ist besonders wertvoll, da Messdaten stabiler Isotope im Niederschlag kostenintensiv und durch die punktuelle Messung räumlich limitiert sind. Zudem ermöglicht die Modellausgabe der stabilen Isotope im Niederschlag einen Vergleich mit den stabilen Isotopen aus den Baumringen auf der Basis derselben Einheit. Dies trifft zum Beispiel nicht für den Vergleich mit Niederschlagsmengen zu.
In meinem Dissertationsprojekt habe ich den Einfluss von atmosphärischen Prozessen, wie die Herkunft der Feuchtigkeit sowie die konvektive Aktivität, auf die stabilen Isotope von Baumringen der Bursera graveolens an den Westhängen der Anden in Südecuador untersucht. Zur Bestimmung der Herkunft der Feuchtigkeit berechnete ich Trajektorien von 2008 bis 2017, die von meinem Studienort ausgingen, an dem die Baumproben entnommen wurden. Die Herkunft der Feuchtigkeit wurde anschließend mit einem Algorithmus zur Identifikation der Feuchtigkeitsquellen bestimmt. Für die Analyse der konvektiven Aktivität simulierte ich mit einem hochauflösenden und isotopenfähigen Atmosphärenmodell die Regensaison von Januar bis April 2012 über der Studienregion. Zur Bestimmung des Einflusses der konvektiven Aktivität auf das Isotopensignal im Niederschlag klassifizierte ich den modellierten Niederschlag anhand des vorherrschenden Regentyps und untersuchte die Auswirkungen auf die stabilen Isotope im Niederschlag. Zum Schluss meines Dissertationsprojekts versuchte ich, die relative Wichtigkeit beider atmosphärischer Prozesse für das Isotopensignal im Niederschlag an den Westhängen der Anden in Südecuador zu bestimmen und abzuschätzen, wie sich dies im Isotopensignal der Baumringe von Bursera graveolens manifestiert hat.
Die Ergebnisse für die Analyse der Herkunft der Feuchtigkeit zeigen, dass die Feuchtigkeit jeweils von beiden Seiten der Anden zum Regen am Studienort beiträgt. Der Einfluss des Pazifischen Ozeans, also der westliche Einfluss, nimmt zu, wenn die täglichen Niederschlagsmengen steigen. Eine Variabilität des westlichen Einflusses lässt sich auch auf jährlichen Zeitskalen feststellen. Der Einfluss nimmt mit positiven Anomalien der Meeresoberflächentemperatur im östlichen Pazifischen Ozean zu, die der warmen Phase des „El Niño Southern Oscillation“-Phänomens zugeordnet werden können. Was die konvektive Aktivität betrifft, konnte ich zeigen, dass im Januar 2012 der Niederschlag an den Westhängen der Anden in Südecuador vor allem durch niedrige Konvektion erzeugt wurde. Zwischen Februar und April ereigneten sich viele Starkregenereignisse, die durch tiefe Konvektion charakterisiert waren. Die stabilen Isotope im Niederschlag zeigten ein verhältnismäßig angereichertes (positives) Signal im Januar während der Phase der niedrigen Konvektion im Vergleich zu dem abgereicherten Signal während der restlichen Regenzeit, die von tiefer Konvektion geprägt war.
Die Einschätzung der relativen Bedeutung der Herkunft der Feuchtigkeit und konvektiver Aktivität für das Isotopensignal im Niederschlag hat ergeben, dass beide Mechanismen maßgeblich von der atmosphärischen Dynamik beeinflusst werden. Starke Ostwinde in der oberen Troposphäre, die bis in die mittlere Troposphäre vordrangen, unterdrückten im Januar 2012 die Entwicklung tiefer Konvektion. Dadurch konnte sich lediglich niedrige Konvektion mit leicht angereicherten Isotopensignalen im Niederschlag entwickeln.
Im Gegensatz dazu waren Starkregenereignisse an einen verstärken Feuchtigkeitseintrag vom Pazifischen Ozean durch Westwinde in der unteren Troposphäre sowie mit vorherrschenden Ostwinden in der oberen Troposphäre verbunden, die den Übergang von konvektiven Wolkencluster zu den Westhängen der Anden in Südecuador ermöglichten. Das Zusammenwirken großräumiger Wolkencluster aus dem Osten und eines starken westlichen Feuchtigkeitseintrags vom Pazifischen Ozean führte zu den intensivsten Niederschlagsereignissen und zur stärksten Abreicherung im Isotopensignal des Niederschlags im Untersuchungszeitraum.
Westwindbedingungen, bei denen sich Westwinde bis in die obere Troposphäre ersteckten, waren hingegen durch mäßige Niederschläge und eine moderate Abreicherung des Isotopensignales im Niederschlag gekennzeichnet. Diese Gegebenheiten waren von kleinräumigen konvektiven Wolkenclustern geprägt und traten im analysierten Zeitraum ausschließlich nach einem Starkregenereignis auf.
Aus den Ergebnissen schlussfolgere ich, dass sowohl die Herkunft der Feuchtigkeit als auch die konvektive Aktivität - insbesondere Letzteres – die Variabilität der stabilen Isotope im Niederschlag bestimmen. Beide Mechanismen werden maßgeblich von der zugrunde liegenden atmosphärischen Dynamik bestimmt. Die Interpretation der stabilen Isotope der Baumringe ist durch das Fehlen von Informationen zu den Wachstumsmustern von Bursera graveolens während der Regenzeit limitiert, was die Notwendigkeit für weitere Feldarbeit und wissenschaftliche Betrachtungen in der Zukunft verdeutlicht
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
Synoptic and Mesoscale atmospheric features associated with an extreme Snowstorm over the Central Andes in August 2013
Get PDFWe study the synoptic and mesoscale characteristics of a snowfall event over the Bolivian Altiplano in August 2013 that caused severe damage to people, infrastructure and livestock. This event was associated with a cold front episode following the eastern slope of the Andes-Amazon interface and a cut-off low pressure system (COL) over the Pacific Ocean. Large scale analyses suggest a two-stage mechanism: The first phase consisted of a strong cold surge to the east of the Andes inducing low level blocking of southward moisture transport over the SW Amazon basin due to post-frontal high-pressure up to 500 hPa synchronized to a Rossby wave train. The second stage was initiated by the displacement of 500 hPa anticyclone over the Andes due to a Rossby wave passage and a subsequent increase in north-easterly moisture transport, while another cold front along the eastern Andes provided additional lifting. We analyse an analog event (July 2010) to confirm the influence of these large-scale features on snow formation.
We conduct a mesoscale analysis using the Weather Research and Forecasting (WRF-ARW) model. For this purpose, we perform a series of high-resolution numerical experiments that include sensitivity studies where we apply orographic and lake Titicaca temperature modifications. We compare our findings to MODIS snow cover estimates and in-situ measurements. The control simulation is able to capture the snow cover spatial distribution and sheds light over several aspects of the snowfall dynamics. In our WRF simulations, daytime snowfall mainly occurs around complex orography whereas nocturnal snowfall is concentrated over the plateau due to a combination of nocturnal winds and complex orography inside the plateau. The sensitivity experiments indicate the importance of the lake and mountain for thermal wind circulation affecting the spatial distribution of snowfall by shifting the position of the convergence zones. The influence of the lake's thermal effect is not evident around the regions surrounding the lake
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
Editorial: Observed and Predicted Changes in Climate in the Andes Cordillera
Get PDFThe Andes Cordillera forms a continuous high elevation range along the western margin ofSouth America and constitutes a fundamental component in the environmental, cultural, andsocio-economic fabric of the continent. Over its ca. 7,000 km-long extent, this mountain rangecontains an immense variety of climates and biomes and provides natural resources and ecosystem services to a population of almost 100 million people in Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile, and Argentina.Over the past decades various ecological, glaciological, and hydrological assessments haveidentified noticeable changes in vegetation dynamics, glacier cover and water availability indifferent sectors of the Andes. Although these findings can be at least partially associated withthe occurrence of warmer and/or drier conditions across the Andes, additional, more detailedclimatological assessments are needed to improve understanding of the complex patterns of climate change that have occurred in recent decades across this extensive mountain range. Given the predominant role of the Andean climate system on many socio-economic activities and on themost important hydrological and ecological processes in this region, further detailed studies arealso needed to identify the most likely future climate scenarios for high elevation areas at differentlatitudes in the Andes. This Frontiers Research Topic compiles, in only a few studies, a greatdiversity of innovative meteorological, climatological, and glaciological analyses in different sectorsof the Andes from Colombia to southern Chile and Argentina.Fil: Masiokas, Mariano Hugo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Luckman, Brian H.. University of Western Australia; AustraliaFil: Vuille, Mathias. State University of New York; Estados UnidosFil: Villalba, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Poveda, Germán. Universidad de Medellin; Colombi
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