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71.
A regional climate model is used to investigate the mechanism of interdecadal rainfall variability, specifically the drought
of the 1970s and 1980s, in the Sahel region of Africa. The model is the National Center for Environmental Prediction’s (NCEPs)
Regional Spectral Model (RSM97), with a horizontal resolution of approximately equivalent to a grid spacing of 50 km, nested
within the ECHAM4.5 atmospheric general circulation model (AGCM), which in turn was forced by observed sea surface temperature
(SST). Simulations for the July–September season of the individual years 1955 and 1986 produced wet conditions in 1955 and
dry conditions in 1986 in the Sahel, as observed. Additional July–September simulations were run forced by SSTs averaged for
each month over the periods 1950–1959 and the 1978–1987. These simulations yielded wet conditions in the 1950–1959 case and
dry conditions in the 1978–1987 case, confirming the role of SST forcing in decadal variability in particular. To test the
hypothesis that the SST influences Sahel rainfall via stabilization of the tropospheric sounding, simulations were performed
in which the temperature field from the AGCM was artificially modified before it was used to force the regional model. We
modified the original 1955 ECHAM4.5 temperature profiles by adding a horizontally uniform, vertically varying temperature
increase, taken from the 1986–1955 tropical mean warming in either the AGCM or the NCEP/National Center for Atmospheric Research
Reanalysis. When compared to the 1955 simulations without the added tropospheric warming, these simulations show a drying
in the Sahel similar to that in the 1986–1955 difference and to the decadal difference between the 1980s and 1950s. This suggests
that the tropospheric warming may have been, at least in part, the agent by which the SST increases led to the Sahel drought
of the 1970s and 1980s. 相似文献
72.
Climate change and population history in the Pacific Lowlands of Southern Mesoamerica 总被引:1,自引:0,他引:1
Hector Neff Deborah M. Pearsall John G. Jones Brbara Arroyo de Pieters Dorothy E. Freidel 《Quaternary Research》2006,65(3):390-400
Core MAN015 from Pacific coastal Guatemala contains sediments accumulated in a mangrove setting over the past 6500 yr. Chemical, pollen, and phytolith data, which indicate conditions of estuarine deposition and terrigenous inputs from adjacent dry land, document Holocene climate variability that parallels the Maya lowlands and other New World tropical locations. Human population history in this region may be driven partly by climate variation: sedentary human populations spread rapidly through the estuarine zone of the lower coast during a dry and variable 4th millennium B.P. Population growth and cultural florescence during a long, relatively moist period (2800–1200 B.P.) ended around 1200 B.P., a drying event that coincided with the Classic Maya collapse. 相似文献
73.
Michael?W.?LomasEmail author T.?Mark Trice Patricia?M.?Glibert Deborah?A.?Bronk James?J.?McCarthy 《Estuaries and Coasts》2002,25(3):469-482
We examined the temporal and spatial variability of urea concentrations and urea uptake and regeneration rates collected on cruises along the longitudinal axis of the Chesapeake Bay between 1972 and 1998. Interannually, mean Bay-wide surface urea concentrations ranged between 0.49 and 0.91 μg-at N l?1 with a nearly 50% decrease in surface concentrations observed between 1988 and 1998. Concentrations of urea from samples collected within ~1 m of the bottom were generally higher and much more varable than surface samples. Seasonally, two different patterns were observed in mean Bay-wide surface urea concentrations. Urea concentrations from near surface waters exhibited a clear summer peak for 1988 through 1994, while for 1973 and 1996 to 1998 a distinct winter-spring peak in concentration was observed. Urea concentrations from deeper waters showed a similar seasonal trend each year with peak concentrations measured in spring. Spatially, urea concentrations in the surface waters decreased in a conservative-type pattern from 0.91 μg-at N I?1 at the freshwater end member to 0.46 μg-at N I?1 at the ocean end member. Mean Bay-wide surface urea uptake rates displayed a seasonal pattern throughout the data set with maximum uptake rates (up to 0.33 μg-at N I?1 h?1) consistently observed during summer. Mean Bay-wide surface regeneration rates were highest but most variable during fall (1.63±0.82 μg-at N I?1 h?1). Mean urea uptake and regeneration rates displayed opposing spatial trends along the axis of the Bay with uptake rates being lowest in the North Bay where regeneration rates were highest. The average temporal and spatial patterns of urea concentration in Chesapeake Bay appear to reflect a balance between external inputs and internal biological recycling. 相似文献
74.
For qualitative researchers, selecting appropriate sites in which to conduct interviews may seem to be a relatively simple research design issue. In fact it is a complicated decision with wide‐reaching implications. In this paper, we argue that the interview site itself embodies and constitutes multiple scales of spatial relations and meaning, which construct the power and positionality of participants in relation to the people, places, and interactions discussed in the interview. We illustrate how observation and analysis of interview sites can offer new insights with respect to research questions, help researchers understand and interpret interview material, and highlight particular ethical considerations that researchers need to address. 相似文献
75.
Morgan T. Jones Deborah J. Hembury Martin R. Palmer Bill Tonge W. George Darling Susan C. Loughlin 《Bulletin of Volcanology》2011,73(3):207-222
The eruptions of the Soufrière Hills volcano on Montserrat (Lesser Antilles) from 1995 to present have draped parts of the
island in fresh volcaniclastic deposits. Volcanic islands such as Montserrat are an important component of global weathering
fluxes, due to high relief and runoff and high chemical and physical weathering rates of fresh volcaniclastic material. We
examine the impact of the recent volcanism on the geochemistry of pre-existing hydrological systems and demonstrate that the
initial chemical weathering yield of fresh volcanic material is higher than that from older deposits within the Lesser Antilles
arc. The silicate weathering may have consumed 1.3% of the early CO2 emissions from the Soufrière Hills volcano. In contrast, extinct volcanic edifices such as the Centre Hills in central Montserrat
are a net sink for atmospheric CO2 due to continued elevated weathering rates relative to continental silicate rock weathering. The role of an arc volcano as
a source or sink for atmospheric CO2 is therefore critically dependent on the stage it occupies in its life cycle, changing from a net source to a net sink as
the eruptive activity wanes. While the onset of the eruption has had a profound effect on the groundwater around the Soufrière
Hills center, the geochemistry of springs in the Centre Hills 5 km to the north appear unaffected by the recent volcanism.
This has implications for the potential risk, or lack thereof, of contamination of potable water supplies for the island’s
inhabitants. 相似文献
76.
The seasonal and year-to-year variation of bulk precipitation as a source of total N, nitrate and ammonia is reported for a 7-year period at the Rhode River, a subestuary of Chesapeake Bay, U.S.A. These values are placed in perspective by comparison with analogous nitrogen loading due to watershed discharges.Although year-to-year variations were important, in an average year about the same amount of readily available nitrogen entered the system via precipitation as entered in land runoff and, during the summer and fall, precipitation was the largest source. 相似文献
77.
A preliminary methods comparison for measurement of dissolved organic nitrogen in seawater 总被引:2,自引:0,他引:2
Jonathan H. Sharp Kathrine R. Rinker Karen B. Savidge Jeffrey Abell Jean Yves Benaim Deborah Bronk David J. Burdige Gustave Cauwet Wenhao Chen Marylo D. Doval Dennis Hansell Charles Hopkinson Gerhard Kattner Nancy Kaumeyer Karen J. McGlathery Jeffrey Merriam Nick Morley Klaus Nagel Hiroshi Ogawa Carol Pollard Mireille Pujo-Pay Patrick Raimbault Raymond Sambrotto Sybil Seitzinger Georgina Spyres Frank Tirendi Ted W. Walsh C. S. Wong 《Marine Chemistry》2002,78(4)
Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods.This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses.This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology. 相似文献
78.
Deborah G. Martin 《Urban geography》2013,34(4):523-540
Abstract Conflicts over siting of group homes for people transitioning out of homelessness or struggling with addiction illustrate the multiscalar dynamics at play between locally-based activism and supra-local laws. In Massachusetts, the state-scale legal environment limits the effectiveness of opposition to group housing; nonetheless, such opposition occurs at the municipal level. Drawing on media accounts, official documents, and interviews with stakeholders in conflicts over group home siting in Worcester, Massachusetts, I investigate the dynamics between neighborhood place claims, the responses of the local state, and the state-wide laws governing group home siting. Examining the legal frameworks relevant to social movement grievances demonstrate that law structures the political opportunities available for protest and local government response. 相似文献
79.
Kenneth W. Able Deborah N. Vivian Gina Petruzzelli Stacy M. Hagan 《Estuaries and Coasts》2012,35(3):743-753
We examined connectivity among marsh subhabitats to determine the structural limits and important components of a polyhaline
salt marsh by studying the patterns of abundance, residency, and movement of a numerically and ecologically dominant nektonic
fish (mummichog, Fundulus heteroclitus). We captured, tagged (n = 14,040 individuals, 30–110 mm), and recaptured from Feb 2001 to Jul 2002, although most recaptures (75–95% by tagging location)
occurred within 150 days. Seasonal residency and movements were common among most subhabitats based on catch per unit effort
and recapture per unit effort. Thus, these (marsh pools, intertidal and subtidal creeks, and marsh surface) should be considered
natural subhabitats within New England type salt marshes. Further, all these subhabitat types should be included in studies
of salt marsh nekton and marsh restoration and creation activities. 相似文献
80.
Patrizia Fiannacca Deborah Lo Pò Gaetano Ortolano Rosolino Cirrincione Antonino Pezzino 《Mineralogy and Petrology》2012,106(3-4):151-171
An ilmenite-garnet-bearing schist from the medium-grade metapelite complex of the Mandanici Unit in the Peloritani Mountains has been investigated to constrain the P-T conditions attained in this sector of the southern European Hercynian chain. Microprobe investigations assisted by statistical handling of X-ray maps via principal component analysis allowed us to better elucidate the porphyroblast-matrix relationships, the geometry of the elemental distribution in garnet porphyroblasts and the average volume percentage of the reactant garnet during retrograde metamorphic evolution. Selected microprobe data were then used to constrain, by means of P-T pseudosections, the main P-T stages of the metamorphic evolution, using the XRF bulk-rock chemistry as the equilibrium chemical composition for the prograde and peak stages and an effective bulk-rock composition for the retrograde one. Peak metamorphic P-T estimates (~530?°C; 0.9?GPa) are consistent with a Hercynian crustal thickening stage at middle-lower crustal conditions, while subsequent evolution, constrained at 420–460?°C; 0.30–0.60?GPa, depicts a retrograde clockwise P-T trajectory linked to exhumation under likely extensional shearing conditions. The results obtained in this paper lead to envisage a new scenario for the crustal evolution of the Peloritani Mountains and stimulate a revision of previous interpretations in the light of the new investigation techniques. 相似文献