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Subseasonal variability in a two-level atmospheric general circulation model
Full text linkThe dynamical processes which maintain atmospheric disturbances
in regions of strong wintertime variability of the northern hemisphere
are examined using data from a GCM simulation. Time series of the
dependent variables and diabatic heating components from 10 Northern
Hemisphere winters simulated by the Oregon State University two-level
GCM are used. Variance and covariance analyses are performed to
determine the geographical distribution of the intensities and transport
properties of high-frequency (periods between 2.5 and 10 days)
and low-frequency (periods between 10 days and a season) eddies.
These are compared with existing observations and the discrepancies
are discussed in terms of their dynamical consistency with the
time-mean circulation.
The energetics of high-frequency and low-frequency eddies are
studied. It is found that the behavior of high-frequency eddies is
consistent with baroclinic instability theory. Low-frequency eddies
appear to be maintained mainly by a high-latitude baroclinic energy
cycle. Energy conversions characteristic of barotropic processes are
also significant at jet-stream-latitudes. The process of wave-energy
dispersion is found to be an important factor governing the geographical
distribution of low-frequency activity at middle latitudes.
The nature of the systems causing low-frequency variability over
the North Pacific Ocean is examined by applying complex EOF analysis
to the time series of geopotential height anomalies. The first eigenmode
of this analysis describes a wave of planetary scale extending
from northeastern Asia to the Gulf of Mexico across the North Pacific
basin. While the phase of this wave retrogrades along the continental
borders of the ocean basin, energy propagates in the opposite direction
and penetrates as far as the central North Atlantic. The dynamical
characteristics of this disturbance are examined by complex covariance
analysis between the first mode's principal component and the
dependent-variable fields. It is found that the disturbance grows
mainly through baroclinic processes with some contribution from barotropic
processes.
On the basis of these results it is proposed that the observed
differences between the high- and low-frequency disturbances result
from their being generated in different geographical regions where
sphericity and the properties of the stationary flow cause baroclinic
growth of structurally different modes
Atmospheric processes governing the northern hemisphere annular mode/north atlantic oscillation
No full textThe North Atlantic Oscillation, referred to herein as the Northern Hemisphere annular mode (NAM), owes its existence entirely to atmospheric processes. In this chapter, we review the structure of the NAM in the atmospheric general circulation, discuss opposing perspectives regarding its physical identity, examine tropospheric processes thought to give-rise to NAM-like variability, and review the role of the stratosphere in driving variability in the NAM. The NAM is characterized by a deep, nearly barotropic structure, with zonal wind perturbations of opposing sign along -55° and -35° latitude. It has a pronounced zonally symmetric component, but exhibits largest variance in the North Atlantic sector. During the Northern Hemisphere (NH) winter, the NAM is strongly coupled to the circulation of the NH stratosphere. The NAM also affects tropical regions, where it perturbs the temperature and wind fields of both the tropical troposphere and stratosphere. The structure of the NAM is remarkably similar to the structure of the leading mode of variability in the Southern Hemisphere circulation. The processes that give rise to annular variability are discussed. In the troposphere, the NAM fluctuates on timescales of -10 days and is associated with anomalous fluxes of zonal momentum of baroclinic waves across ~45°N. It is argued that the tropospheric component of the NAM exhibits largest variance in the Atlantic sector where the relatively weak thermally driven subtropical flow and the relatively warm lower boundary conditions at subpolar latitudes permit marked meridional excursions by baroclinic waves. In the stratosphere, fluctuations in the NAM evolve on timescales of several weeks. Evidence is presented that long-lived anomalies in the stratospheric NAM frequently precede similarly persistent anomalies in the tropospheric NAM. It is argued that variability in the lower stratospheric polar vortex yields a useful level of predictive skill for NH wintertime weather on both intraseasonal and seasonal timescales. The possible dynamics of these linkages are outlined. The recasting of the North Atlantic Oscillation as an expression of an annular mode has generated a debate over the physical identity of the mode in question. This debate attests to the absence of a unique theory for the existence of annular modes in the first place. Our current understanding of the fundamental processes to which the NAM owes its existence is discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Mediterranean climate future: an insightful look into the Basin's precipitation response to greenhouse gas forcing
Full text linkIn a new investigation of model projections of greenhouse gas warming impact on the Mediterranean, Zappa et al (2015 Environ. Res. Lett. 10 104012) find that the decline in basin-wide precipitation scales linearly with the strength of the 850 hPa zonal wind over North Africa. This result supports previous findings that climate change will affect the Mediterranean primarily through changing the regional atmospheric circulation. The results of this study may guide improvements of climate models to better simulate the impact of greenhouse gas warming in this critical world region
Variations on the Author
Full text link“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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North Atlantic influence on 19th–20th century rainfall in the Dead Sea watershed, teleconnections with the Sahel, and implication for Holocene climate fluctuations
Full text linkThe importance of understanding processes that govern the hydroclimate of the Mediterranean Basin is highlighted by the projected significant drying of the region in response to the increase in greenhouse gas concentrations. Here we study the long-term hydroclimatic variability of the central Levant region, situated in the eastern boundary of the Basin, as reveled by instrumental observations and the Holocene record of Dead Sea level variations. Observations of 19th and 20th century precipitation in the Dead Sea watershed region display a multidecadal, anti-phase relationship to North Atlantic (NAtl) sea surface temperature (SST) variability, such that when the NAtl is relatively cold, Jerusalem experiences higher than normal precipitation and vice versa. This association is underlined by a negative correlation to precipitation in the sub-Saharan Sahel and a positive correlation to precipitation in western North America, areas that are also affected by multidecadal NAtl SST variability. These observations are consistent with a broad range of Holocene hydroclimatic fluctuations from the epochal, to the millennial and centennial time scales, as displayed by the Dead Sea lake level, by lake levels in the Sahel, and by direct and indirect proxy indicators of NAtl SSTs. On the epochal time scale, the gradual cooling of NAtl SSTs throughout the Holocene in response to precession-driven reduction of summer insolation is associated with previously well-studied wet-to-dry transition in the Sahel and with a general increase in Dead Sea lake levels from low stands after the Younger Dryas to higher stands in the mid- to late-Holocene. On the millennial and centennial time scales there is also evidence for an anti-phase relationship between Holocene variations in the Dead Sea and Sahelian lake levels and with proxy indicators of NAtl SSTs. However the records are punctuated by abrupt lake-level drops, which appear to be in-phase and which occur during previously documented abrupt major cooling events in the Northern Hemisphere. We propose that the mechanisms by which NAtl SSTs affect precipitation in the central Levant is related to the tendency for high (low) pressure anomalies to persist over the eastern North Atlantic/Western Mediterranean region when the Basin is cold (warm). This, in turn, affects the likelihood of cold air outbreaks and cyclogenesis in the Eastern Mediterranean and, consequently, rainfall in the central Levant region. Depending on its phase, this natural mechanism can alleviate or exacerbate the anthropogenic impact on the regions' hydroclimatic future
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Full text linkWe 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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