Topographic and stratification effects on shelf edge flows     

Yakun Guo  Peter A. Davies  Alessandra Cavalletti  Pieter Jacobs 《Dynamics of Atmospheres and Oceans》2000,31(1-4)

Results are presented from two sets of laboratory model experiments on the effects of an isolated seamount upon the flow of an intermediate-water slope current along a continental shelf. The experimental results for initial ambient conditions of respectively two-layer and linearly stratified fluids show that the structure of such a boundary current depends primarily on the values of the appropriate set of dimensionless dynamical parameters (namely the Burger (Bu), Ekman (Ek) and Rossby (Ro) numbers), as well as the dimensionless lateral separation of the seamount and shelf and the proportional height of the seamount relative to the distance from the bottom at which the intermediate-water flows. Comparisons of the present results with those from a previous two-layer fluid study with no obstacle present reveals that the presence of the obstacle does not alter significantly the stability of the current even when situated close to the shelf. However, for such configurations, the density, velocity and vorticity fields in the local zone of interaction between the current and the obstacle are distorted significantly by the presence of the obstacle, provided that the summit of the obstacle penetrates the level of current flow. Measurements of density, velocity and vorticity fields show no significant dependence of the flow interaction upon the detailed bathymetry of the shelf-slope. For stable intermediate-water slope currents, the nature of the interaction with the obstacle is determined primarily by (i) the lateral separation of the obstacle and the shelf edge and (ii) the Ro of the flow. For sufficiently low values of the former and high values of the latter, the interaction results in a splitting of the incident flow around the obstacle, with cyclonic and anticyclonic eddy pairs being generated in the lee. Geostrophic equilibrium is seen to be maintained in the current, even in the near wake of the obstacle. For cases in which the summit of the seamount is below the initially-undisturbed intermediate water level, no Taylor column-like division of the slope current occurs and no significant distortion of the current structure (velocity and density) occurs for the parameter ranges investigated. For linearly stratified cases, measurements show that no significant local elevation or depression of the density interfaces is observed in the interaction zone. The distributions of the local buoyancy frequencies calculated from the density profiles reveal that the minimum value of the frequency upstream of the obstacle is smaller than that downstream, indicating that the flow interactions generate local mixing downstream, with consequent erosion of the density interfaces.  相似文献   

In other worlds: on the politics of research in a transforming South Africa   总被引：1，自引：1，他引：0  

Gustav Visser 《Area》2000,32(2):231-235

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Wisconsinan and Holocene Climate History from an Ice Core at Taylor Dome, Western Ross Embayment, Antarctica   总被引：4，自引：0，他引：4  

Eric J. Steig  David L. Morse  Edwin D. Waddington  Minze Stuiver  Pieter M. Grootes  Paul A. Mayewski  Mark S. Twickler  & Sallie I. Whitlow 《Geografiska Annaler: Series A, Physical Geography》2000,82(2-3):213-235

Geochemical data and geophysical measurements from a 554-m ice-core from Taylor Dome, East Antarctica, provide the basis for climate reconstruction in the western Ross Embayment through the entire Wisconsinan and Holocene. In comparison with ice cores from central East and West Antarctica, Taylor Dome shows greater variance of temperature, snow accumulation, and aerosol concentrations, reflecting significant variability in atmospheric circulation and air mass moisture content. Extreme aridity during the last glacial maximum at Taylor Dome reflects both colder temperatures and a shift in atmospheric circulation patterns associated with the advance of the Ross Sea ice sheet and accounts for regional alpine glacier retreats and high lake levels in the Dry Valleys. Inferred relationships between spatial accumulation gradients and ice sheet configuration indicate that advance of the Ross Sea ice sheet began in late marine isotope stage 5 or early stage 4. Precise dating of the Taylor Dome core achieved by trace-gas correlation with central Greenland ice cores shows that abrupt deglacial warming at Taylor Dome was near-synchronous with the ∼14.6 ka warming in central Greenland and lags the general warming trend in other Antarctic ice cores by at least 3000 years. Deglacial warming was following by a warm interval and transient cooling between 14.6 and 11.7 ka, synchronous with the Bølling/Allerød warming and Younger Dryas cooling events in central Greenland, and out of phase with the Antarctic Cold Reversal recorded in the Byrd (West Antarctica) ice core. Rapid climate changes during marine isotope stages 4 and 3 at Taylor Dome are similar in character to, and may be in phase with, the Northern Hemisphere stadial–interstadial (Dansgaard–Oeschger) events. Results from Taylor Dome illustrate the importance of obtaining ice cores from multiple Antarctic sites, to provide wide spatial coverage of past climate and ice dynamics.  相似文献   

Competitive interactions among beam trawlers exploiting local patches of flatfish in the North Sea   总被引：5，自引：4，他引：1  

Rijnsdorp  A. D.; van Mourik Broekman  P. L.; Visser  E. G. 《ICES Journal of Marine Science》2000,57(4):894-902

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Micro-scale distribution of beam trawl effort in the southern North Sea between 1993 and 1996 in relation to the trawling frequency of the sea bed and the impact on benthic organisms   总被引：10，自引：5，他引：5  

Rijnsdorp  A. D.; Buys  A. M.; Storbeck  F.; Visser  E. G. 《ICES Journal of Marine Science》1998,55(3):403-419

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Change in low flows due to catchment management dynamics—Application of a comparative modelling approach     

Tesfay G. Gebremicael  Yasir A. Mohamed  Pieter van der Zaag  Khalid Hassaballah  Eyasu Y. Hagos 《水文研究》2020,34(9):2101-2116

Understanding the natural low flow of a catchment is critical for effective water management policy in semi-arid and arid lands. The Geba catchment in Ethiopia, forming the headwaters of Tekeze-Atbara basin was known for its severe land degradation before the recent large scale Soil and Water conservation (SWC) programs. Such interventions can modify the hydrological processes by changing the partitioning of the incoming rainfall on the land surface. However, the literature lacks studies to quantify the hydrological impacts of these interventions in the semi-arid catchments of the Nile basin. Statistical test and Indicators of Hydrological Alteration (IHA) were used to identify the trends of streamflow in two comparatives adjacent (one treated with intensive SWC intervention and control with fewer interventions) catchments. A distributed hydrological model was developed to understand the differences in hydrological processes of the two catchments. The statistical and IHA tools showed that the low flow in the treated catchment has significantly increased while considerably decreased in the control catchment. Comparative analysis confirmed that the low flow in the catchment with intensive SWC works was greater than that of the control by >30% while the direct runoff was lower by >120%. This implies a large proportion of the rainfall in the treated catchment is infiltrated and recharge aquifers which subsequently contribute to streamflow during the dry season. The proportion of soil storage was more than double compared to the control catchment. Moreover, hydrological response comparison from pre- and post-intervention showed that a drastic reduction in direct runoff (>84%) has improved the low flow by >55%. This strongly suggests that the ongoing intensive SWC works have significantly improved the low flows while it contributed to the reduction of total streamflow in the catchment.  相似文献   

Time-varying storm surges on Lorentz’s Wadden Sea networks     

Koen R. G. Reef  Giordano Lipari  Pieter C. Roos  Suzanne J. M. H. Hulscher 《Ocean Dynamics》2018,68(8):1051-1065

We present an idealized network model for storm surges in the Wadden Sea, specifically including a time-dependent wind forcing (wind speed and direction). This extends the classical work by H.A. Lorentz who only considered the equilibrium response to a steady wind forcing. The solutions obtained in the frequency domain for the linearized shallow-water equations in a channel are combined in an algebraic system for the network. The velocity scale that is used for the linearized friction coefficient is determined iteratively. The hindcast of the storm surge of 5 December 2013 produces credible time-varying results. The effects of storm and basin parameters on the peak surge elevation are the subject of a sensitivity analysis. The formulation in the frequency domain reveals which modes in the external forcing lead to the largest surge response at coastal stations. There appears to be a minimum storm duration, of about 3–4 h, that is required for a surge to attain its maximum elevation. The influence of the water levels at the North Sea inlets on the Wadden Sea surges decreases towards the shore. In contrast, the wind shearing generates its largest response near the shore, where the fetch length is at its maximum.  相似文献   

New insights from low-temperature thermochronology into the tectonic and geomorphologic evolution of the south-eastern Brazilian highlands and passive margin     

Gerben Van Ranst  Antônio Carlos Pedrosa-Soares  Tiago Novo  Pieter Vermeesch  Johan De Grave 《地学前缘(英文版)》2020,11(1):303-324

The South Atlantic passive margin along the south-eastern Brazilian highlands exhibits a complex landscape,including a northern inselberg area and a southern elevated plateau,separated by the Doce River valley.This landscape is set on the Proterozoic to early Paleozoic rocks of the region that once was the hot core of the Aracuai orogen,in Ediacaran to Ordovician times.Due to the break-up of Gondwana and consequently the opening of the South Atlantic during the Early Cretaceous,those rocks of the Araquai orogen became the basement of a portion of the South Atlantic passive margin and related southeastern Brazilian highlands.Our goal is to provide a new set of constraints on the thermo-tectonic history of this portion of the south-eastern Brazilian margin and related surface processes,and to provide a hypothesis on the geodynamic context since break-up.To this end,we combine the apatite fission track(AFT)and apatite(U-Th)/He(AHe)methods as input for inverse thermal history modelling.All our AFT and AHe central ages are Late Cretaceous to early Paleogene.The AFT ages vary between 62 Ma and90 Ma,with mean track lengths between 12.2μm and 13.6μm.AHe ages are found to be equivalent to AFT ages within uncertainty,albeit with the former exhibiting a lesser degree of confidence.We relate this Late Cretaceous-Paleocene basement cooling to uplift with accelerated denudation at this time.Spatial variation of the denudation time can be linked to differential reactivation of the Precambrian structural network and differential erosion due to a complex interplay with the drainage system.We argue that posterior large-scale sedimentation in the offshore basins may be a result of flexural isostasy combined with an expansion of the drainage network.We put forward the combined compression of the Mid-Atlantic ridge and the Peruvian phase of the Andean orogeny,potentially augmented through the thermal weakening of the lower crust by the Trindade thermal anomaly,as a probable cause for the uplift.  相似文献   

Open‐foldable domes with high‐tension textile membranes: The GREGOR dome     

R.H. Hammerschlag  J.N. Kommers  S. Visser  F.C.M. Bettonvil  A.G.M. van Schie  S.J. van Leverink  G. Sliepen  A.P.L. Jgers  W. Schmidt  R. Volkmer 《Astronomische Nachrichten》2012,333(9):830-839

Double layers of high‐tensioned textile membranes were applied to the completely open‐foldable dome for the GREGOR telescope for the first time. Simultaneous climate measurements inside and outside the dome have proven the thermalinsulating capability of this double‐layer construction. The GREGOR dome is the result of the continuation of the ESO research on open‐foldable domes with textile structures, followed by the research for the DOT dome with high‐tensioned textile membranes. It cleared the way to extreme stability required for astronomical practice on high mountain sites with heavy storms and ice formation. The storm Delta with 245 km/h 1‐minute mean maximum at the location of the GREGOR caused no problems, nor did other storms afterwards. Opening and closing experiences up to wind speeds of 90 km/h were without problems. New technical developments were implemented and tested at the GREGOR dome, opening the way for application to much larger domes up to the 30 m diameter‐class range (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献