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排序方式: 共有316条查询结果,搜索用时 171 毫秒
61.
62.
Hu Dunxin Wang Fan Sprintall Janet Wu Lixin Riser Stephen Cravatte Sophie Gordon Arnold Zhang Linlin Chen Dake Zhou Hui Ando Kentaro Wang Jianing Lee Jae-Hak Hu Shijian Wang Jing Zhang Dongxiao Feng Junqiao Liu Lingling Villanoy Cesar Kaluwin Chalapan Qu Tangdong Ma Yixin 《中国海洋湖沼学报》2020,38(4):906-929
The Western Tropical Pacific(WTP) Ocean holds the largest area of warm water(28℃) in the world ocean referred to as the Western Pacific Warm Pool(WPWP),which modulates the regional and global climate through strong atmospheric convection and its variability.The WTP is unique in terms of its complex 3-D ocean circulation system and intensive multiscale variability,making it crucial in the water and energy cycle of the global ocean.Great advances have been made in understanding the complexity of the WTP ocean circulation and associated climate impact by the international scientific community since the 1960 s through field experiments.In this study,we review the evolving insight to the 3-D structure and multi-scale variability of the ocean circulation in the WTP and their climatic impacts based on in-situ ocean observations in the past decades,with emphasis on the achievements since 2000.The challenges and open que stions remaining are reviewed as well as future plan for international study of the WTP ocean circulation and climate. 相似文献
63.
Influences of the unsaturated,saturated, and riparian zones on the transport of nitrate near the Merced River,California, USA 总被引:1,自引:0,他引:1
Joseph L. Domagalski Steven P. Phillips E. Randall Bayless Celia Zamora Carol Kendall Richard A. WildmanJr Janet G. Hering 《Hydrogeology Journal》2008,16(4):675-690
Transport and transformation of nitrate was evaluated along a 1-km groundwater transect from an almond orchard to the Merced River, California, USA, within an irrigated agricultural setting. As indicated by measurements of pore-water nitrate and modeling using the root zone water quality model, about 63% of the applied nitrogen was transported through a 6.5-m unsaturated zone. Transport times from recharge locations to the edge of a riparian zone ranged from approximately 6 months to greater than 100 years. This allowed for partial denitrification in horizons having mildly reducing conditions, and essentially no denitrification in horizons with oxidizing conditions. Transport times across a 50–100-m-wide riparian zone of less than a year to over 6 years and more strongly reducing conditions resulted in greater rates of denitrification. Isotopic measurements and concentrations of excess N2 in water were indicative of denitrification with the highest rates below the Merced River. Discharge of water and nitrate into the river was dependent on gradients driven by irrigation or river stage. The results suggest that the assimilative capacity for nitrate of the groundwater system, and particularly the riverbed, is limiting the nitrate load to the Merced River in the study area. 相似文献
64.
65.
Segmentation of alluvial fans in death valley,california: New insights from surface exposure dating and laboratory modelling 总被引:1,自引:0,他引:1
Laboratory experiments, recent paleoenvironmental analyses of rock varnish, and surface exposure dating of geomorphic units have led to new insights into the process of entrenchment and segmentation of alluvial fans, and into the history of Quaternary sedimentation in Death Valley. Entrenchment begins at the fanhead. As the trench deepens, its down-slope end migrates down-fan, taking several tens of thousands of years to reach lower parts of the fan. Laboratory experiments suggest, however, that a new segment begins to grow at the toe long before the trench reaches this part of the fan. Furthermore, the initial slope of the segment is not the equilibrium slope. Field evidence supports this model. The tectonic tilting that caused entrenchment and segmentation in Death Valley may have been triggered by loading of the valley with water. Sedimentation on the salt pan in southern Death Valley is not, at present, in equilibrium with that on the fans. Rather, it seems to be adjusting to an increase in the amount of fine material reaching the playa, due in part to breaching of the outlet of Lake Tecopa somewhat after 600 ka BP, and in part to subsidence of different parts of the valley at different rates. Failure to recognize this disequilibrium resulted in errors in earlier estimates of the age of the segmentation events. 相似文献
66.
A distinct genetic form of the eucalanid calanoid copepod Eucalanus hyalinus s.l. was identified in subtropical and temperate waters of the Pacific, Atlantic, and Indian Oceans. Results from mitochondrial (16S rRNA, COI) and nuclear (ITS2) genetic markers suggest that the two E. hyalinus forms are reproductively isolated. Eucalanus spinifer T. Scott is taken out of synonymy with E. hyalinus and is applied to the smaller of the two genetic forms, increasing the number of species in the genus Eucalanus to a total of six. Adult females of the sister species E. hyalinus s.s. and E. spinifer can be distinguished morphologically by the shape of the anterior portion of the head in lateral and ventral views, asymmetry in the length of antennule segments 22 and 23, length ratios of 22/5 and 23/5 of the longer of the antennules, the length of the caudal rami, and total body size, as measured by total length or prosome length. We illustrate and describe both adult females and males of E. spinifer, and discuss aspects of sexual dimorphism. The sister species are sympatric throughout much of their circumglobal biogeographic distributions, but appear to have centers of abundance in slightly different water mass types. 相似文献
67.
Joseph E. Salisbury Douglas Vandemark Christopher W. Hunt Janet W. Campbell Wade R. McGillis William H. McDowell 《Estuarine, Coastal and Shelf Science》2008
The partial pressure of carbon dioxide (pCO2) in estuary-plume systems is related to the internal processes of net organic metabolism and physical mixing, but is also strongly influenced by biogeochemical inputs from the land and ocean. Surface layer pCO2, stimulated fluorescence of chlorophyll (f-chl) and colored organic matter (f-com), and beam attenuation at 660 nm (c-660) were measured during three seasonal surveys of the Kennebec (ME) and Merrimack (MA) estuary-plume systems. These estuaries are both supplied by large New England Rivers and separated by less than 150 km, but significant differences were often observed in the distributions of surface pCO2 and optical variables. High pCO2 concentrations were generally associated with high f-com, while lower pCO2 concentrations were associated with high f-chl and c-660. Using simple regression models, optical measurements were used to estimate chlorophyll and total organic carbon concentrations. Results suggest that labile riverine carbon is responsible for sustaining supersaturated pCO2 conditions and that phytoplankton productivity, likely driven by inputs of riverine dissolved inorganic nitrogen, is responsible for pCO2 undersaturation. Although optical variables are often related to surface pCO2, the results suggest that efforts to retrieve pCO2 in complex waters using optical data may be enhanced using a site-specific, multivariate approach. 相似文献
68.
Stephanie Kusch Ole Bennike Bernd Wagner Matthias Lenz Ilona Steffen Janet Rethemeyer 《Boreas: An International Journal of Quaternary Research》2019,48(2):273-286
In this study, we use a combined biomarker and macrofossil approach to reconstruct the Holocene climate history recorded in Trifna Sø, Skallingen area, eastern North Greenland. Chronological information is derived from comparison of lithological, biogeochemical and macrofossil characteristics with a well‐dated record from nearby Lille Sneha Sø. Following local deglaciation around c. 8 cal. ka BP, the local peak warmth occurred between c. 7.4 and 6.2 cal. ka BP as indicated by maximum macrofossil abundances of warmth‐demanding plants (Salix arctica and Dryas integrifolia) and invertebrates (Daphnia pulex and Chironomidae). Warm conditions were dominated by terrestrial organic matter (OM) sedimentation as implied by the alkane‐based Paq ratio, but increased aquatic productivity is indicated when temperature was highest around 6.5 cal. ka BP. The n‐C29/n‐C31 alkane ratio shows that vegetation in the catchment was dominated by shrubs after deglaciation, but shifted towards relatively more grassy/herbaceous vegetation during peak warmth. After 5.4 cal. ka BP, the disappearance of warmth‐demanding plant and invertebrate macrofossils indicates cooling in the Skallingen area. This cooling was characterized by a significant shift towards dominance of aquatic OM sedimentation in Trifna Sø as implied by high Paq ratios. Cooling was also associated with a shift in vegetation type from dwarf‐shrub heaths towards relatively more herbaceous vegetation in the catchment, stronger erosion and more oligotrophic conditions in the lake. Our data show that mean air temperatures inferred using branched glycerol dialkyl glycerol tetraethers (brGDGTs) do not seem to accurately reflect the local climatic history. Irrespective of calibration, methylation of branched tetraethers (MBT) palaeothermometry cannot be reconciled with the macrofossil evidence and seems to be biased by either changing brGDGT sources (in situ vs. soil‐derived) or changing species assemblages and/or an unknown physiological response to changing environmental conditions at high latitude. 相似文献
69.
Natural Hazards - Ethnic and racial minority communities often suffer more from disasters due to a lack of access to resources from governmental agencies and other organizations that provide... 相似文献
70.
A wind-tunnel study was conducted to investigate ventilation of scalars from urban-like geometries at neighbourhood scale
by exploring two different geometries a uniform height roughness and a non-uniform height roughness, both with an equal plan
and frontal density of λ
p
= λ
f
= 25%. In both configurations a sub-unit of the idealized urban surface was coated with a thin layer of naphthalene to represent
area sources. The naphthalene sublimation method was used to measure directly total area-averaged transport of scalars out
of the complex geometries. At the same time, naphthalene vapour concentrations controlled by the turbulent fluxes were detected
using a fast Flame Ionisation Detection (FID) technique. This paper describes the novel use of a naphthalene coated surface
as an area source in dispersion studies. Particular emphasis was also given to testing whether the concentration measurements
were independent of Reynolds number. For low wind speeds, transfer from the naphthalene surface is determined by a combination
of forced and natural convection. Compared with a propane point source release, a 25% higher free stream velocity was needed
for the naphthalene area source to yield Reynolds-number-independent concentration fields. Ventilation transfer coefficients
w
T
/U derived from the naphthalene sublimation method showed that, whilst there was enhanced vertical momentum exchange due to
obstacle height variability, advection was reduced and dispersion from the source area was not enhanced. Thus, the height
variability of a canopy is an important parameter when generalising urban dispersion. Fine resolution concentration measurements
in the canopy showed the effect of height variability on dispersion at street scale. Rapid vertical transport in the wake
of individual high-rise obstacles was found to generate elevated point-like sources. A Gaussian plume model was used to analyse
differences in the downstream plumes. Intensified lateral and vertical plume spread and plume dilution with height was found
for the non-uniform height roughness. 相似文献