A numerical study of island wake generated by an elliptical tidal flow     

Philippe Estrade  Jason H. Middleton 《Continental Shelf Research》2010

An idealized numerical study of the influence of a tidal flow around an island has been undertaken with ROMS. The study focusses on coastal island wakes which are mainly controlled by elliptical tidal current flows on shallow shelves. This model is typical of some isolated continental shelf islands. The model is forced by a semi-diurnal barotropic inertia gravity wave imposed on the four open boundaries of a rectangular domain and its propagation results in an elliptical tidal flow within the domain in which the circular island lies. The influence of the surrounding island bathymetry and of the ellipse shape has been studied both in two and three dimensions. In the island vicinity, the residual circulation patterns over a tidal period show alongshore flow divergence along the major axis and convergence along the minor axis. A thin tidal ellipse (i.e. with a large ratio between major and minor axes) leads to strong eddy activity periods in the lee of the island during the flood and ebb phases, with eddy dissipation phases in between. By contrast, an almost round ellipse (axis ratio nearly 1) leads to vorticity filaments which continuously progress around the island without eddy shedding. The presence of a topographic slope in the vicinity of the island strengthens the eddy activity. This study suggests that the tidal current rotation favors the development of the eddy rotating in the same direction and weakens the development of the second eddy. In three dimensions with a surrounding bathymetry, an intense upwelling occurs in a large area in the lee of the island and the vertical velocities are stronger with thinner ellipses. With a flat bottom the vertical motions are almost fully generated by convergence and divergence of the secondary flow. With a varying bottom topography, the vertical motions come from a combination of this mechanism with convergence and divergence of the depth averaged flow.  相似文献   

High-energy X-ray spectra of Seyferts and Unification schemes for active galactic nuclei     

Matthew Middleton  Chris Done  Nick Schurch 《Monthly notices of the Royal Astronomical Society》2008,383(4):1501-1505

The Unified Model of active galactic nuclei (AGN) predicts that the sole difference between type 1 and 2 Seyfert galaxies nuclei is the viewing angle with respect to an obscuring structure around the nucleus. High-energy photons above 20 keV are not affected by this absorption if the column is Compton thin, so their 30–100 keV spectra should be the same. However, the observed spectra at high energies appear to show a systematic difference, with type 1 Seyfert galaxies having Γ∼ 2.1 whereas type 2 Seyfert galaxies are harder with Γ∼ 1.9. We estimate the mass and the accretion rate of Seyferts detected in these high-energy samples, and show that they span a wide range in   L / L _Edd  . Both black hole binary systems and AGN show a correlation between spectral softness and Eddington fraction, so these samples are probably heterogeneous, spanning a range of intrinsic spectral indices which are hidden in individual objects by poor signal-to-noise ratio. However, the mean Eddington fraction for the type 1 Seyfert galaxies is higher than for the type 2 Seyfert galaxies, so the samples are consistent with this being the origin of the softer spectra seen in type 1 Seyfert galaxies. We stress that high-energy spectra alone are not necessarily a clean test of Unification schemes, but that the intrinsic nuclear properties should also change with   L / L _Edd  .  相似文献   

Glacial geomorphology in Utsjoki, Finnish Lapland proposes Younger Dryas fault-instability     

Raimo Sutinen  Matti Piekkari  Maarit Middleton   《Global and Planetary Change》2009,69(1-2):16-28

Northern Fennoscandia has experienced high-magnitude postglacial fault (PGF) events, yet the role of seismic tremors in subglacial deformations and meltwater discharge has remained obscure. We studied glacial geomorphology in Utsjoki, Finnish Lapland, an area characterized by the Utsjoki drumlin field fanning out north and northeast to the Younger Dryas End Moraines (YDEMs) in northern Norway. Paleolandslides were common on fells (i.e. mountains shaped by Pleistocene glaciations) and were formed in nunatak position evidencing fault-instability in app. 11,900 calibrated (cal) BP. An anastomosing network of eskers was found throughout Utsjoki, and was probably generated through short-lived sliding bed stages during the discharge of subglacial lake(s). The formation of networks is different from time-transgressive evolution of single-ridged eskers in arborescent (treelike) systems. The most probable triggering mechanism for the meltwater outburst(s) was an earthquake tremor(s) associated with fault-instability during the late and post-Younger Dryas (YD). The alignment of the esker network was inconsistent with parallel-to-iceflow streamlining and the eskers erode or superimpose drumlins. Hence the esker network post-dates the streamlining. In some cases, hummocky (Pulju) moraine was observed to coexist with esker network and peculiar ‘kettle’ and ‘liquefaction’ features. We propose that glacio-seismotectonic events contributed not only to landslides but were also the primary force behind subglacial evolution of esker networks and possibly even hummocky moraine.  相似文献   

The CTIP Model with SuperDARN Electric Potential Patterns as Input: First Comparison of Modelled Ion Densities with Radio Tomography Observations     

E. L. Whittick  S. E. Pryse  H. R. Middleton  A. R. Lotinga  A. D. Aylward 《Earth, Moon, and Planets》2009,104(1-4):45-49

In a recent development of the Coupled Thermosphere Ionosphere Plasmasphere (CTIP) model, electric potential patterns from the SuperDARN radar network are used as the input to drive the high-latitude ionospheric convection. Spatial images of the ionised atmosphere under conditions of IMF Bz negative given by the Aberystwyth University tomography experiments in Scandinavia and the UK are compared with model simulations for assessment of the model development. Results are presented showing broad agreement between the observations and the model.  相似文献   

The influence of habitat structure on juvenile fish in a New Zealand estuary     

Darren M. Parsons  Mark A. Morrison  Simon F. Thrush  Crispin Middleton  Matt Smith  Keren T. Spong  Dane Buckthought 《Marine Ecology》2013,34(4):492-500

Estuarine and sheltered coastal habitats that contain physical structure are potentially important nurseries for juvenile fish. Many of these structured habitats, however, are potentially vulnerable to stressors such as elevated turbidity. Quantifying the benefits that structured habitats provide to juvenile fish may therefore be an important step in the management process. We investigated the value of structured habitat for juvenile fishes in northeastern New Zealand, using artificial seagrass units (ASUs) with varying blade density. ASUs were predominantly settled by juvenile snapper (Pagrus auratus) and spotty (Notolabrus celidotus). The density of both snapper and spotty was greatest on ASUs with the highest blade density. For snapper, a gradient in abundance was present (with higher abundance closer to the harbour mouth), suggesting either a gradient in the supply of recruits or a potential recruitment shadow effect. The size distribution of juvenile snapper (12–70 mm fork length) was very similar on both sampling trips, despite the 2‐month interim period, suggesting an ontogenetic habitat shift dependent on size. The condition of juvenile snapper from ASUs with the highest blade density was also greater than the other ASU treatments. Overall, these results provide new empirical evidence that habitats with physical structure within shallow estuarine systems are important to early stage juvenile fishes such as snapper, and indicate that the location‐specific context of that habitat is also likely important.  相似文献   

The oceanography of the Sydney region     

Jason H. Middleton  Deborah Cox  Peter Tate 《Marine pollution bulletin》1996,33(7-12):124-131

An understanding of the oceanographic processes affecting effluent plume behaviour formed an important basis for the broad range of activities which were carried out as part of the Sydney Deepwater Outfalls Environmental Monitoring Programme. The main processes responsible for coastal circulation in the Sydney region include the East Australia Current, coastal trapped waves, winds, internal waves and outflows from major estuaries. As each of these processes is dynamically distinct, they occur on differing time and space scales, and the net current strength at any location can vary as differing processes wax and wane in their relative or absolute strength. Each of these processes and their effects on the transport and dispersion of sewage effluent discharged from the Sydney Deepwater Outfalls are briefly described.  相似文献   

Assessment of Rock-Eval Tmax Data in Bambra-2 Well of Barrow Sub-basin, North West Shelf of Australia： A Case Study of Contaminated Rock-Eval Data     

HeSheng AlexKaiko MikeMiddleton JiangChunqing 《中国地质大学学报(英文版)》2003,14(2):144-150

The contamination of cuttings and side-wall core (SWC) samples in the Bambra-2 well by drilling-mud additives and natural hydrocarbons may cause Ruek-Eval Tmax (℃) data to be suspect, and affect its utility in the assessment of thermal maturity. The Rock-Eval results of 284 cuttings samples, 31 side-wall core samples and conventional core samples from the Jurassic-Cretaceous sedimentary sequences in the Bambra-2 well are presented in this paper. Significantly lower Tmax values from cuttings samples compared with Tmax values from conventional core samples and solvent extracted SWC samples, from the deeper and higher maturity interval, are thought to have been caused by contamination by diesel and other drilling-mud additives. The cuttings samples in the Barrow Group of Cretaceous may be contaminated by natural hydrocarbons, resulting their Tmax values to be 2-10℃ lower than a regularly increased Tmax trend from core samples. This study indicates that more reliable Rock-Eval Tmax data are obtained from the conventional core samples and solvent extracted SWC samples. This study also indicates that the Tmax values from some SWC samples were also affected by free hydrocarbons, due to the use of diesel as a mud additive as well.  相似文献   

The changing frequency of dust storms through time   总被引：19，自引：0，他引：19  

A. S. Goudie  N. J. Middleton 《Climatic change》1992,20(3):197-225

Dust storms are major, but under-studied actors in the world's drylands. Not only are they an important manifestation of desertification and land degradation, but they also have a whole suite of important environmental impacts, including possible rainfall suppression (Maley, 1982), fertilization of offshore areas, and disturbance to satellite communications. It is therefore important to ascertain whether their frequency and extent is changing. An increasing dust-storm incidence could be both a manifestation of and a contributor to global change.By analysing long-term meteorological records for a large number of areas (the Great Plains of the USA, the USSR, Morocco, The Arabian Gulf, Australia, the Sahel-Sudan zone of Africa, China, Mongolia and Mexico) certain conclusions can be drawn. The first of these is that there is no one global pattern of dust-storm frequency trend. Some stations (e.g. in the Sahel) show a clear upward trend of great severity, others show a downward trend (e.g. Mexico City), while others show a more cyclical pattern. In many cases it is evident that essentially natural processes (precipitation totals, snow cover, wind strength) determine the frequency of dust events in any one year. It has also been possible to show the importance of runs of drought years (e.g. in the High Plains in the 1930s, and in the Sahel zone of Africa in the 1970s and 1980s). Elsewhere, however, various human activities have been significant in determining dust-storm frequency variations: the introduction of centre-pivot irrigation in the High Plains, the abstraction of water from the Owens and Mono basins in California, the disruption of surfaces by construction activity and vehicle use (e.g. in Ulan Bator, Mongolia), and the deliberate stabilisation of susceptible surfaces (e.g. the Lake Texcoco scheme in Mexico).  相似文献   

Cosmic ray produced Be and Al in Antarctic rocks: exposure and erosion history     

K. Nishiizumi  C.P. Kohl  J.R. Arnold  J. Klein  D. Fink  R. Middleton 《Earth and Planetary Science Letters》1991,104(2-4)

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Beryllium-10 in continental sediments     

Louis Brown  I. Selwyn Sacks  Fouad Tera  J. Klein  R. Middleton 《Earth and Planetary Science Letters》1981,55(3):370-376

The concentration of¹⁰Be has been measured in 10 samples taken from a transect of surface sediments beginning in the Atchafalaya River and extending across the Bay 136 km into the Gulf of Mexico. If corrected for a lower retentivity of sand for Be, they have a concentration that is constant within 13%. This concentration is about an order of magnitude smaller than that of deep ocean sediments. For comparison, measurements of¹⁰Be in rainwater, in a sample of soil and in a deep ocean core were made.  相似文献