Regional climate model simulation of the AMMA Special Observing Period #3 and the pre-Helene easterly wave     

Leonard M. Druyan  Matthew Fulakeza  Patrick Lonergan  Erik Noble 《Meteorology and Atmospheric Physics》2009,105(3-4):191-210

The study examines results of dynamic downscaling of two global analyses: the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis II and the Global Forecast System final analysis (FNL). Downscaling to a 0.5° grid over West Africa and the adjacent Atlantic Ocean is accomplished by each of two regional models, the Regional Model, version 3 (RM3) of the Center for Climate Systems Research and the Weather, Research and Forecasting model (WRF). Simulations are for September 2006, the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period #3 (SOP-3). The aim of this study is to exploit the increased spatial detail in the simulations and representations of climate fields by the regional models to analyze meteorological systems within the SOP-3 area of interest and time frame. In particular, the paper focuses on the regional models’ representations of the structure and movement of a prominent easterly wave during September 10–13th, the precursor of Tropical Storm/Hurricane Helene. It describes the RM3 simulated structure of the developing storm in terms of circulation, precipitation, vertical motion, cumulus heating rates, and cross-sections of wind and geopotential height anomalies. Simulated cumulus heating rates within the wave’s main precipitation area imply a lowering of the bases of active cumulus in the transition from the African continent to the Atlantic, indicating that the ocean environment promotes greater upward latent heat flux that in turn intensifies overlying storms. RM3 circulation, precipitation patterns, and storm trajectory are reasonably consistent with observational evidence. Experiments show that precipitation rates near 6°N over the eastern North Atlantic are sensitive to vertical thermal stability, such that they are enhanced by warmer in situ sea-surface temperatures (SSTs) and diminished by colder SSTs. However, prescribing colder SST causes increases in precipitation north of 9°N within areas of large scale upward vertical motion where rainfall rates are less sensitive to in situ SSTs. The evaluation of WRF indicates that its storm propagation is too fast over West Africa, where associated WRF precipitation rates are exaggerated, but its performance is improved over the Atlantic.  相似文献   

Variability and comparison of hyporheic water temperatures and seepage fluxes in a small Atlantic salmon stream   总被引：2，自引：0，他引：2  

Alexander MD  Caissie D 《Ground water》2003,41(1):72-82

Ground water discharge is often a significant factor in the quality of fish spawning and rearing habitat and for highly biologically productive streams. In the present study, water temperatures (stream and hyporheic) and seepage fluxes were used to characterize shallow ground water discharge and recharge within thestreambed of Catamaran Brook, a small Atlantic salmon (Salmo salar) stream in central New Brunswick, Canada. Three study sites were instrumented using a total of 10 temperature sensors and 18 seepage meters. Highly variable mean seepage fluxes, ranging from 1.7 x 10(-4) to 2.5 cm3 m(-2) sec(-1), and mean hyporheic water temperatures, ranging from 10.5 degrees to 18.0 degrees C, at depths of 20 to 30 cm in the streambed were dependent on streambed location (left versus right stream bank and site location) and time during the summer sampling season. Temperature data were usefulfor determining if an area of the streambed was under discharge (positive flux), recharge (negative flux), or parallel flow (no flux) conditions and seepage meters were used to directly measure the quantity of water flux. Hyporheic water temperature measurements and specific conductance measurements of the seepage meter sample water, mean values ranging from 68.8 to 157.9 microS/cm, provided additional data for determining flux sources. Three stream banks were consistently under discharge conditions, while the other three stream banks showed reversal from discharge to recharge conditions over the sampling season. Results indicate that the majority of the water collected in the seepage meters was composed of surface water. The data obtained suggests that even though a positive seepage flux is often interpreted as ground water discharge, this discharging water may be of stream water origin that has recently entered the hyporheic zone.The measurement of seepage flux in conjunction with hyporheic water temperature or other indicators of water origin should be considered when attempting to quantify the magnitude of exchange and the source of hyporheic water.  相似文献   

An Alpine Lacustrine Record of Early Holocene North American Monsoon Dynamics from Dry Lake, Southern California (USA)     

Broxton W. Bird  Matthew E. Kirby 《Journal of Paleolimnology》2006,35(1):179-192

Dry Lake (2763 m), located in the San Bernardino Mountains of southern California, USA, provides a high-resolution climate record from the coastal southwest depicting early Holocene terrestrial climate. 27 AMS ¹⁴C dates and multi-proxy analyses, including magnetic susceptibility, total organic mater, microfossil counts, and grain size, suggest the early Holocene was significantly wetter then present, due to an enhanced North American Monsoon (NAM). Elevated insolation at 9000 cal year B.P., raised summer sea surface temperatures in the Gulf of California and the eastern tropical Pacific, as well as land surface temperatures, extending the NAM into southern California. The data also provide evidence of the 8.2 ka event, which registers as a 300-year cool period characterized by reduced monsoonal precipitation, depressed basin productivity, and increased erosion. We suggest this event is the most likely period for the early to middle Holocene (9000–5000 cal year B.P.) glacial advance in the San Bernardino Mountains proposed by Owen et al. (2003, Geology 31: 729–732).  相似文献   

Subduction factory processes beneath the Guguan cross-chain, Mariana Arc: no role for sediments, are serpentinites important?   总被引：1，自引：0，他引：1  

Robert J. Stern  Ed Kohut  Sherman H. Bloomer  Matthew Leybourne  Matthew Fouch  Jeff Vervoort 《Contributions to Mineralogy and Petrology》2006,151(2):202-221

We need to understand chemical recycling at convergent margins and how chemical interactions between subducted slab and the overlying mantle wedge affect mantle evolution and magmagenesis. This requires distinguishing contributions from recycled individual subducted components as well as those contributed by the mantle. We do this by examining magmatic products generated at different depths above a subduction zone, in an intra-oceanic arc setting. The Guguan cross-chain in the intra-oceanic Mariana arc overlies subducted Jurassic Pacific plate lithosphere at depths of ~125--230 km and erupts mostly basalt. Basalts from rear-arc volcanoes are more primitive than those from the magmatic front, in spite of being derived by lower degrees of melting of less-depleted mantle. Rear-arc magmas also show higher temperatures and pressures of equilibration. Coexisting mineral compositions become more MORB- or OIB-like with increasing height above the subduction zone. Trace element and isotopic variations indicate that the subduction component in cross-chain lavas diminishes with increasing depth to the subduction zone, except for water contents. There is little support for the idea that melting beneath the Mariana Trough back-arc basin depleted the source region of arc magmas, but melting to form rear-arc volcanoes may have depleted the source of magmatic front volcanoes. Enrichments in rear-arc lavas were not caused by sediment melting; the data instead favor an OIB-type mantle that has been modestly affected by subduction zone fluids. Our most important conclusion is that sediment fluids or melts are not responsible for the K--h relationship and other cross-chain chemical and isotopic variations. We speculate that an increasing role for supercritical fluids released from serpentinites interacting with modestly enriched mantle might be responsible for cross-chain geochemical and isotopic variations. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.  相似文献   

El Niño- or La Niña-like climate change?     

Matthew Collins 《Climate Dynamics》2005,24(1):89-104

The potential for the mean climate of the tropical Pacific to shift to more El Niño-like conditions as a result of human induced climate change is subject to a considerable degree of uncertainty. The complexity of the feedback processes, the wide range of responses of different atmosphere–ocean global circulation models (AOGCMs) and difficulties with model simulation of present day El Niño southern oscillation (ENSO), all complicate the picture. By examining the components of the climate-change response that projects onto the model pattern of ENSO variability in 20 AOGCMs submitted to the coupled model inter-comparison project (CMIP), it is shown that large-scale coupled atmosphere–ocean feedbacks associated with the present day ENSO also operate on longer climate-change time scales. By linking the realism of the simulation of present day ENSO variability in the models to their patterns of future mean El Niño-like or La Niña-like climate change, it is found that those models that have the largest ENSO-like climate change also have the poorest simulation of ENSO variability. The most likely scenario (p=0.59) in a model-skill-weighted histogram of CMIP models is for no trend towards either mean El Niño-like or La Niña-like conditions. However, there remains a small probability (p=0.16) for a change to El Niño-like conditions of the order of one standard El Niño per century in the 1% per year CO₂ increase scenario.  相似文献   

Volcanic stratigraphy of large-volume silicic pyroclastic eruptions during Oligocene Afro-Arabian flood volcanism in Yemen   总被引：1，自引：0，他引：1  

Ingrid Ukstins Peate  Joel A. Baker  Mohamed Al-Kadasi  Abdulkarim Al-Subbary  Kim B. Knight  Peter Riisager  Matthew F. Thirlwall  David W. Peate  Paul R. Renne  Martin A. Menzies 《Bulletin of Volcanology》2005,68(2):135-156

A new stratigraphy for bimodal Oligocene flood volcanism that forms the volcanic plateau of northern Yemen is presented based on detailed field observations, petrography and geochemical correlations. The >1 km thick volcanic pile is divided into three phases of volcanism: a main basaltic stage (31 to 29.7 Ma), a main silicic stage (29.7 to 29.5 Ma), and a stage of upper bimodal volcanism (29.5 to 27.7 Ma). Eight large-volume silicic pyroclastic eruptive units are traceable throughout northern Yemen, and some units can be correlated with silicic eruptive units in the Ethiopian Traps and to tephra layers in the Indian Ocean. The silicic units comprise pyroclastic density current and fall deposits and a caldera-collapse breccia, and they display textures that unequivocally identify them as primary pyroclastic deposits: basal vitrophyres, eutaxitic fabrics, glass shards, vitroclastic ash matrices and accretionary lapilli. Individual pyroclastic eruptions have preserved on-land volumes of up to ∼850 km³. The largest units have associated co-ignimbrite plume ash fall deposits with dispersal areas >1×10⁷ km² and estimated maximum total volumes of up to 5,000 km³, which provide accurate and precisely dated marker horizons that can be used to link litho-, bio- and magnetostratigraphy studies. There is a marked change in eruption style of silicic units with time, from initial large-volume explosive pyroclastic eruptions producing ignimbrites and near-globally distributed tuffs, to smaller volume (<50 km³) mixed effusive-explosive eruptions emplacing silicic lavas intercalated with tuffs and ignimbrites. Although eruption volumes decrease by an order of magnitude from the first stage to the last, eruption intervals within each phase remain broadly similar. These changes may reflect the initiation of continental rifting and the transition from pre-break-up thick, stable crust supporting large-volume magma chambers, to syn-rift actively thinning crust hosting small-volume magma chambers.Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

Strontium in coral aragonite: 3. Sr coordination and geochemistry in relation to skeletal architecture     

Nicola Allison  Adrian A. Finch  Matthew Newville 《Geochimica et cosmochimica acta》2005,69(15):3801-3811

Use of the coral Sr palaeothermometer assumes that the Sr in coral skeletons is substituted randomly for Ca in the aragonite structure. The presence of Sr in additional phases e.g., strontianite, or the non random distribution of Sr across metal sites in aragonite, would complicate the Sr/Ca-sea surface temperature relationship. We have used Sr K-edge microEXAFS (extended X-ray absorption fine structure) to determine the structural state of Sr across selected microvolumes of four coral skeletons (Porites lobata, Acropora palmata, Pavona clavus, and Montastrea annularis). We used a 5 × 3 μm beam to analyse specific areas of the coral skeletal architecture, i.e., centres of calcification, fasciculi, and dissepiments. All EXAFS analyses refine, within error, to an ideally substituted Sr in aragonite, and we found no evidence of strontianite or partly ordered structural states. Anisotropy in the first shell responses results from the fact that the analysed microvolumes are not necessarily averaged for the responses of all crystal orientations in the aragonite. Although secondary ion mass spectrometry confirmed that Sr/Ca composition can vary substantially between skeletal components, we find no evidence for any contrast in Sr structural state. Sr heterogeneity may result from kinetic effects, reflecting complex disequilibrium processes during crystal precipitation, or biological effects, resulting from variations in the composition of the calcifying fluid which are biologically mediated.  相似文献   

Mercury content of the Springfield coal, Indiana and Kentucky     

James C. Hower  Maria Mastalerz  Agnieszka Drobniak  Jeffrey C. Quick  Cortland F. Eble  Matthew J. Zimmerer 《International Journal of Coal Geology》2005,63(3-4):205-227

With pending regulation of mercury emissions in United States power plants, its control at every step of the combustion process is important. An understanding of the amount of mercury in coal at the mine is the first step in this process. The Springfield coal (Middle Pennsylvanian) is one of the most important coal resources in the Illinois Basin. In Indiana and western Kentucky, Hg contents range from 0.02 to 0.55 ppm. The variation within small areas is comparable to the variation on a basin basis. Considerable variation also exists within the coal column, ranging from 0.04 to 0.224 ppm at one Kentucky site. Larger variations likely exist, since that site does not represent the highest whole-seam Hg nor was the collection of samples done with optimization of trace element variations in mind. Estimates of Hg capture by currently installed pollution control equipment range from 9–53% capture by cold-side electrostatic precipitators (ESP) and 47–81% Hg capture for ESP + flue-gas desulfurization (FGD). The high Cl content of many Illinois basin coals and the installation of Selective Catalytic Reduction of NO_x enhances the oxidation of Hg species, improving the ability of ESPs and FGDs to capture Hg.  相似文献   

Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe     

Harangi  Szabolcs; Downes  Hilary; Thirlwall  Matthew; Gmeling  Katalin 《Journal of Petrology》2007,48(12):2261-2287

We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension  相似文献   

Atacamite in the oxide zone of copper deposits in northern Chile: involvement of deep formation waters?     

Eion M. Cameron  Matthew I. Leybourne  Carlos Palacios 《Mineralium Deposita》2007,42(3):205-218

Atacamite, a copper hydroxychloride, is an important constituent of supergene oxide zones of copper deposits in northern Chile, whereas in similar deposits elsewhere, it is rare. In Chile, it has generally been assumed to be a primary constituent of the supergene zones. There are two difficulties with this supposition. The first is that atacamite requires saline water for its formation, whereas supergene oxidation was caused by percolating, oxygenated meteoric water, mainly rainwater. The second is that atacamite dissolves rapidly or undergoes phase change when exposed to fresh water. Supergene enrichment of copper deposits in northern Chile extended over a long period, 44 to 9 Ma, being terminated by the onset of hyperaridity. During this period, there was at least intermittent rainfall, exposing previously formed atacamite to dissolution or phase change. Furthermore, atacamite-bearing oxide zones in several deposits are directly overlain by thick Miocene alluvial gravels; the stream waters that transported these gravels would have permeated the oxide zones. In some deposits, atacamite-bearing assemblages occur both in the oxide zones and in contiguous gravels. We suggest that atacamite-bearing oxide assemblages are more likely to have been a replacement of preexisting oxide phases after the onset of hyperaridity at about 9 Ma. A hyperarid climate made possible evaporation and concentration of chloride in meteoric waters. In this paper, we discuss another source of saline waters to modify oxide zones. Dewatering of the Domeyko Basin expelled brines along faults, some of which had earlier guided the location of porphyry deposits. At the Spence porphyry copper deposit, saline waters, which δD vs δ ¹⁸O isotope analyses identify as basinal brines, are presently rising through the deposit, then flowing away along the base of the covering gravels. Compositions of these waters lie within the stability fields of atacamite and brochantite, the two minerals that comprise the oxide zone. Evidence is presented for other porphyry deposits, Radomiro Tomic and Gaby Sur, that basinal brines may have been involved in the late formation of atacamite.  相似文献