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601.
We investigate the formation process and pathways of deep water masses in a coupled ice–ocean model of the Arctic and North
Atlantic Oceans. The intent is to determine the relative roles of these water masses from the different source regions (Arctic
Ocean, Nordic Seas, and Subpolar Atlantic) in the meridional overturning circulation. The model exhibits significant decadal
variability in the deep western boundary current and the overturning circulation. We use detailed diagnostics to understand
the process of water mass formation in the model and the resulting effects on the North Atlantic overturning circulation.
Particular emphasis is given to the multiple sources of North Atlantic Deep Water, the dominant deep water masses of the world
ocean. The correct balance of Labrador Sea, Greenland Sea and Norwegian Sea sources is difficult to achieve in climate models,
owing to small-scale sinking and convection processes. The global overturning circulation is described as a function of potential
temperature and salinity, which more clearly signifies dynamical processes and clarifies resolution problems inherent to the
high latitude oceans. We find that fluxes of deep water masses through various passages in the model are higher than observed
estimates. Despite the excessive volume flux, the Nordic Seas overflow waters are diluted by strong mixing and enter the Labrador
Sea at a lighter density. Through strong subpolar convection, these waters along with other North Atlantic water masses are
converted into the densest waters [similar density to Antarctic Bottom Water (AABW)] in the North Atlantic. We describe the
diminished role of salinity in the Labrador Sea, where a shortage of buoyant surface water (or excess of high salinity water)
leads to overly strong convection. The result is that the Atlantic overturning circulation in the model is very sensitive
to the surface heat flux in the Labrador Sea and hence is correlated with the North Atlantic Oscillation. As strong subpolar
convection is found in other models, we discuss broader implications. 相似文献
602.
Fluctuations in US Freezing Rain Days 总被引:1,自引:0,他引:1
Freezing rain occurrences during a 50-year period, 1949/1950–1998/1999, derived from carefully examined records of 161 first-order stations distributed across the United States, were assessed for temporal fluctuations and trends. Classification of station fluctuations based on five 10-year periods revealed five unique distribution types in areas east of the Rockies. One of these five distributions, for stations located in the western Great Plains, experienced its greatest 10-year value at the end of the 50-year period. The other four regional distributions experienced their highest 10-year value in either of the first two 10-year periods. Nationally, the 10-year period when the greatest number of stations experienced their maximum value was 1949/1950–1958/1959, while the period when the greatest number of stations experienced their minimum value fell near the end of the 50-year record (1979/1980–1988/1989). The 50-year linear trends defined one region, the western Great Plains, with increasing values, while three areas of decreasing trend were identified; the Great Lakes, the eastern Ohio River valley, and southern New England. These analyses also indicate the need to examine and consider such time–space changes in the frequency of climate variables at various spatial scales when assessing weather risks and developing climate change scenarios. 相似文献
603.
David P. Rowell 《Climate Dynamics》2005,25(7-8):837-849
A scenario of European climate change for the late twenty-first century is described, using a high-resolution state-of-the-art
model. A time-slice approach is used, whereby the atmospheric general circulation model, HadAM3P, was integrated for two periods,
1960–1990 and 2070–2100, using the SRES A2 scenario. For the first time an ensemble of such experiments was produced, along
with appropriate statistical tests for assessing significance. The focus is on changes to the statistics of seasonal means,
and includes analysis of both multi-year means and interannual variance. All four seasons are assessed, and anomalies are
mapped for surface air temperature, precipitation and snow mass. Mechanisms are proposed where these are dominated by straightforward
local processes. In winter, the largest warming occurs over eastern Europe, up to 7°C, mean snow mass is reduced by at least
80% except over Scandinavia, and precipitation increases over all but the southernmost parts of Europe. In summer, temperatures
rise by 6–9°C south of about 50°N, and mean rainfall is substantially reduced over the same area. In spring and autumn, anomalies
tend to be weaker, but often display patterns similar to the preceding season, reflecting the inertia of the land surface
component of the climate system. Changes in interannual variance are substantial in the solsticial seasons for many regions
(note that for precipitation, variance estimates are scaled by the square of the mean). In winter, interannual variability
of near-surface air temperature is considerably reduced over much of Europe, and the relative variability of precipitation
is reduced north of about 50°N. In summer, the (relative) interannual variance of both variables increases over much of the
continent. 相似文献
604.
W. David Rust Donald R. MacGorman Eric C. Bruning Stephanie A. Weiss Paul R. Krehbiel Ronald J. Thomas William Rison Timothy Hamlin Jeremiah Harlin 《Atmospheric Research》2005,76(1-4):247
Balloon-borne electric field soundings and lightning mapping data have been analyzed for three of the storms that occurred in the Severe Thunderstorm Electrification and Precipitation Study field program in 2000 to determine if the storms had inverted-polarity electrical structures. The polarities of all or some of the vertically stacked charge regions in such storms are opposite to the polarities observed at comparable heights in normal storms. Analyses compared the charge structures inferred from electric field soundings in the storms with charges inferred from three-dimensional lightning mapping data. Charge structures were inferred from electric field profiles by combining the one-dimensional approximation of Gauss's law with additional information from three-dimensional patterns in the electric field vectors. The three different ways of inferring the charge structure in the storms were found to complement each other and to be consistent overall. Charge deposition by lightning possibly occurred and increased the charge complexity of one of the storms.Many of the cloud flashes in each case were inverted-polarity flashes. Two storms produced ground flash activity comprised predominantly of positive ground flashes. One storm, which was an isolated thunderstorm, produced inverted-polarity cloud flashes, but no flashes to ground. The positive and negative thunderstorm charge regions were found at altitudes where, respectively, negative and positive charge would be found in normal-polarity storms. Thus, we conclude that these storms had anomalous and inverted-polarity electrical structures. Collectively, these three cases (along with the limited cases in the refereed literature) provide additional evidence that thunderstorms can have inverted-polarity electrical structures. 相似文献
605.
Seismic anisotropy beneath Dronning Maud Land, Antarctica, revealed by shear wave splitting 总被引:1,自引:0,他引:1
Bettina Bayer Christian Müller David W. Eaton Wilfried Jokat 《Geophysical Journal International》2007,171(1):339-351
Shear wave splitting analyses have been carried out using teleseismic data from broad-band seismograph stations deployed at temporary and permanent locations in Dronning Maud Land (DML), Antarctica. In most cases, the observed anisotropy can be related to major tectonic events that formed the present-day Antarctic continent. We rule out an anisotropic contribution from recent asthenospheric flow. At the Russian base Novolazarevskaya near the coast in central DML, waveform inversion suggests a two-layer model where the fast direction of the upper layer is oriented parallel to Archean fabrics in the lithosphere, whereas the anisotropy of the lower layer is interpreted to have been created during the Jurassic Gondwana break-up. Recordings at the South African base Sanae IV, however, show enigmatic results. For narrow backazimuthal segments, splitting parameters show strong variations together with a multitude of isotropic measurements, indicative of complex scattering that cannot be explained by simple one- or two-layer anisotropic models. In the interior of the continent, the data are consistent with single-layer anisotropy, but show significant spatial variations in splitting parameters. A set of temporary stations across the Heimefront shear zone in western DML yield splitting directions that we interpret as frozen anisotropy from Proterozoic assembly of the craton. An abrupt change in fast axis direction appears to mark a suture between the Grunehogna craton, a fragment of the Kalahari–Kaapvaal craton in southern Africa and the Mesoproterozoic Maudheim Province. 相似文献
606.
David Al-Attar 《Geophysical Journal International》2007,171(2):755-760
A formal solution to the elastodynamic equation in an anelastic earth model is presented. The derivation also incorporates the effects of aspherical structure, rotation, self-gravitation, and pre-stress. It is found that the solution can be expressed as a sum of the normal modes of the earth model along with additional terms accounting for anelastic relaxation processes. However, the derivation does not assume that such an eigenfunction expansion is possible, and so avoids difficulties previously encountered due to the non self-adjointness of the problem. 相似文献
607.
608.
Nature-triggered hazards and disasters have traditionally been treated only from the lens of geophysical and biophysical processes,
implying that the root cause of large-scale death and destruction lies in the natural domain rather than in a coupled human–environment
system. Conceptually, the physical domain has been seen as discrete and separate from human entities, and solutions were sought
in the technological intervention and control of the physical environment—solutions that often ended up being less effective
than hoped for and sometimes even counter productive. At all levels, institutions have directed and redirected most of their
financial and logistical resources into the search for scientific and engineering solutions without allocating due attention
and resources towards the assessment of effects and effectiveness of the applications of such technological outcomes. However,
over the last two decades, forceful criticisms of the ‘dominant’ technocratic approach to hazards analysis have appeared in
the literature and consequently there has not only been a shift in thinking of causation of disaster loss in terms of human
vulnerability, but also newer questions have arisen regarding distinguishing between the ‘physical exposure’ of people to
threats and societal vulnerability, and linking them with propensity to hazards loss.
Though the vulnerability/resilience paradigm has largely replaced the hazards paradigm within the social sciences and much
of the professional emergency and disaster management communities, this shift of thinking has not progressed to much of the
physical science community, decision-makers and the public, who have not yet accepted the idea that understanding and using
human and societal dimensions is equally or more important than trying to deal and control nature through the use of technology.
This special issue is intended to further the idea that the aspects of community and peoples’ power to mitigate, to improve
coping mechanisms, to respond effectively, and recover with vigor against the environmental extremes are of paramount conceptual
and policy importance. 相似文献
609.
There is an exceptionally rich aquatic fauna in the epikarst, the skin of karst. High species richness in the epikarst, coupled
with its special vulnerability as the first point of entry of nearly all toxic spills, makes its protection especially important.
The epikarst fauna may also be an useful tool in tracing the potential route of pollutants. Copepods in epikarst have extremely
local distributions, and their body size is such that they are largely at the mercy of directional flows. In a series of caves
in southwest Slovenia and West Virginia, a significant fraction of the copepod species occur in less than 100 m of linear
extent. This suggests a pattern of highly restricted lateral flow under normal conditions and the distribution of copepods
could potentially be used to trace water movement. Under high flow conditions as would often be the case with toxic spill,
mounding of water may increase the lateral radius of flow. Nevertheless, copepods may be useful tracers. 相似文献
610.