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141.
Interannual climate variability in South America: impacts on seasonal precipitation, extreme events, and possible effects of climate change 总被引:8,自引:7,他引:1
Interannual variability is an important modulator of synoptic and intraseasonal variability in South America. This paper seeks
to characterize the main modes of interannual variability of seasonal precipitation and some associated mechanisms. The impact
of this variability on the frequency of extreme rainfall events and the possible effect of anthropogenic climate change on
this variability are reviewed. The interannual oscillations of the annual total precipitation are mainly due to the variability
in austral autumn and summer. While autumn is the dominant rainy season in the northern part of the continent, where the variability
is highest (especially in the northeastern part), summer is the rainy season over most of the continent, thanks to a summer
monsoon regime. In the monsoon season, the strongest variability occurs near the South Atlantic Convergence Zone (SACZ), which
is one of the most important features of the South American monsoon system. In all seasons but summer, the most important
source of variability is ENSO (El Ni?o Southern Oscillation), although ENSO shows a great contribution also in summer. The
ENSO impact on the frequency of extreme precipitation events is also important in all seasons, being generally even more significant
than the influence on seasonal rainfall totals. Climate change associated with increasing emission of greenhouse gases shows
potential to impact seasonal amounts of precipitation in South America, but there is still great uncertainty associated with
the projected changes, since there is not much agreement among the models’ outputs for most regions in the continent, with
the exception of southeastern South America and southern Andes. Climate change can also impact the natural variability modes
of seasonal precipitation associated with ENSO. 相似文献
142.
Nina Stark Hendrik Hanff Christian Svenson Verner B. Ernstsen Alice Lefebvre Christian Winter Achim Kopf 《Geo-Marine Letters》2011,31(4):249-258
In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal
inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification
pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below
(quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the
top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared
to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based
on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated
from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion
of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport
and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase
in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers
can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface
layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such
data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes
in highly dynamic coastal environments. 相似文献
143.
Abstract Tepetates, altered indurated and sterile volcanic tuffs, are located among the neo‐volcanic belt in central Mexico. They are divided into three classes according to the erosion level: outcrop tepetates, discontinuous tepetates (in process of erosion), and underlying tepetates (related to fragile areas with high erosion risk). For local communities and peasants, the existence of tepetates is a heavy constraint, with the impossibility to cultivate the land as it is. In order to know the localization and extension of tepetates, we experimented with various classification techniques of a Landsat Thematic Mapper satellite image. Masking techniques were used, assisted both by a DEM and photo interpretation, in addition to radiometric segmentation to obtain a map of tepetates in a study area. The resulting map showed the location of outcrop tepetates well, in agreement with existing soil maps, but underestimated underlying tepetates and high‐risk erosion localities, due mainly to the heterogeneity of the classes at the scale used (30 m pixel). 相似文献
144.
145.
Richard G. Edgar Alice C. Quillen Jaehong Park 《Monthly notices of the Royal Astronomical Society》2007,381(3):1280-1286
We consider the minimum mass planet, as a function of radius, that is capable of opening a gap in an α-accretion disc. We estimate that a half-Jupiter mass planet can open a gap in a disc with accretion rate for viscosity parameter α= 0.01 , and solar mass and luminosity. The minimum mass is approximately proportional to . This estimate can be used to rule out the presence of massive planets in gapless accretion discs. We identify two radii at which an inwardly migrating planet may become able to open a gap and so slow its migration; the radius at which the heating from viscous dissipation is similar to that from stellar radiation in a flared disc, and the radius at which the disc becomes optically thin in a self-shadowed disc. In the inner portions of the disc, we find that the minimum planet mass required to open a gap is only weakly dependent on radius. If a migrating planet is unable to open a gap by the time it reaches either of the transition radii, then it is likely to be lost on to the star. If a gap-opening planet cuts off disc accretion allowing the formation of a central hole or clearing in the disc then we would estimate that the clearing radius would approximately be proportional to the stellar mass. 相似文献
146.
147.
Alice A. Horton Claus Svendsen Richard J. Williams David J. Spurgeon Elma Lahive 《Marine pollution bulletin》2017,114(1):218-226
Sewage effluent input and population were chosen as predictors of microplastic presence in sediments at four sites in the River Thames basin (UK). Large microplastic particles (1 mm–4 mm) were extracted using a stepwise approach to include visual extraction, flotation and identification using Raman spectroscopy. Microplastics were found at all four sites. One site had significantly higher numbers of microplastics than other sites, average 66 particles 100 g? 1, 91% of which were fragments. This site was downstream of a storm drain outfall receiving urban runoff; many of the fragments at this site were determined to be derived of thermoplastic road-surface marking paints. At the remaining three sites, fibres were the dominant particle type. The most common polymers identified included polypropylene, polyester and polyarylsulphone. This study describes two major new findings: presence of microplastic particles in a UK freshwater system and identification of road marking paints as a source of microplastics.
Capsule
This study is the first to quantify microplastics of any size in river sediments in the UK and links their presence to terrestrial sources including sewage and road marking paints. 相似文献148.
John M. Senko Alice C. Dohnalkova Kenneth M. Kemner 《Geochimica et cosmochimica acta》2007,71(19):4644-4654
Microbially mediated in situ reduction of soluble U(VI) to insoluble U(IV) (as UO2) has been proposed as a means of preventing the migration of that radionuclide with groundwater, but preventing the oxidative resolubilization of U has proven difficult. We hypothesized that relatively slow rates of U(VI) bioreduction would yield larger UO2 precipitates that would be more resistant to oxidation than those produced by rapid U(VI) bioreduction. We manipulated U(VI) bioreduction rates by varying the density of Shewanella putrefaciens CN32 added to U(VI) containing solutions with lactate as an electron donor. Characterization of biogenic UO2 particles by extended X-ray absorption fine-structure spectroscopy and transmission electron microscopy revealed that UO2 nanoparticles formed by relatively slow rates of U(VI) reduction were larger and more highly aggregated than those formed by relatively rapid U(VI) reduction. UO2 particles formed at various rates were incubated under a variety of abiotically and biologically oxidizing conditions. In all cases, UO2 that was formed by relatively slow U(VI) reduction was oxidized at a slower rate and to a lesser extent than UO2 formed by relatively rapid U(VI) bioreduction, suggesting that the stability of UO2 in situ may be enhanced by stimulation of relatively slow rates of U(VI) reduction. 相似文献
149.
Extra-tropical cyclonic/anticyclonic activity in North-Eastern Pacific and air temperature extremes in Western North America 总被引:1,自引:1,他引:1
Synoptic extra-tropical cyclone and anticyclone trajectories have been constructed from mean daily sea level pressure (SLP) data using a new automated scheme. Frequency, intensity and trajectory characteristics of these transients have been summarized to form indices describing wintertime cyclonic and anticyclonic activity over the North-Eastern Pacific (east of 170°W) during 1950–2001. During this period, the strength of anticyclones gradually diminished and their frequency became more variable, while cyclones intensified in a discrete shift with deeper lows and further southerly trajectories occurring since the mid-1970s. These changes in synoptic transients translate into anomalously low seasonal mean SLP in the Aleutian Low, a low-level circulation anomaly consistent with the positive phase of the North Pacific Decadal Oscillation, with positive sea surface temperature (SST) anomalies along the west coast of North America and negative in the central North Pacific Ocean. A link between cyclonic/anticyclonic activity and tropical SST anomalies also exists, but this link only becomes significant after the mid-1970s, a period that coincides with more southerly cyclone trajectories. Southward excursions of mid-latitude cyclones during El Niño/positive NPO winters accomplish the northward advection of tropical air and discourage the southward penetration of polar air masses associated with transient anticyclones. Naturally, these changes in cyclonic/anticyclonic activity directly impact surface air temperatures, especially at night. We document these profound impacts on observed wintertime minimum temperatures over Western North America. 相似文献
150.
Analysis of tidal marsh vegetation patterns in two Georgia estuaries using aerial photography and GIS 总被引:1,自引:0,他引:1
Aerial photographs and GIS analysis were used to map the distribution of tidal marsh vegetation along the salinity gradients
of the estuaries of the Altamaha and Satilla Rivers in coastal Georgia. Vegetation maps were constructed from 1993 U.S. Geological
Survey Digital Orthophoto Quarter Quads, 1∶77,000-scale color infrared photographs taken in 1974 and 1∶24,000-scale black
and white photographs taken in 1953, Changes between years were identified using a GIS overlay analysis. Four vegetation classifications
were identified and groundtruthed with field surveys: salt marsh (areas containing primarilySpartina alterniflora), brackish marsh (Spartina cynosuroides andS. alterniflora), Juncus (Juncus roemerianus), and fresh marsh (Zizania aquatica, Zizaniopsis miliacae, and others). There was no evidence for an upstream shift in marsh vegetation along the longitudinal axis of either estuary
over the time frame of this analysis, which implies there has not been a long-term increase in salinity. Although the inland
extent of each marsh zone was further upstream in the Satilla than the Altamaha, they corresponded to similar average high
tide salinities in each estuary: areas classified as salt marsh occurred from the mouth up to where average high tide salinity
in the water was approximately 15 psu;Juncus ranged from 21 to 1 psu; brackish marsh ranged from 15 to 1 psu; and fresh marsh was upstream of 1 psu. Approximately 63%
of the 6,786 ha of tidal marsh vegetation mapped in the Altamaha and 75% of the 10,220 ha mapped in the Satilla remained the
same in all 3 yr.Juncus was the dominant classification in the intermediate regions of both estuaries, and shifts between areas classified asJuncus and either brackish or salt marsh constituted the primary vegetation change between 1953 and 1993 (87% of the changes observed
in the Altamaha and 95% of those in the Satilla). This analysis suggests that the broad distribution of tidal marsh vegetation
along these two estuaries is driven by salinity, but that at the local scale these are dynamic systems with a larger number
of factors affecting the frequently changing borders of vegetation patches. 相似文献