Postglacial variability in near‐bottom current speed on the continental shelf off south‐west Spitsbergen          下载免费PDF全文

BEATA STERNAL  WITOLD SZCZUCISKI  MATTHIAS FORWICK  MAREK ZAJĄCZKOWSKI  STANISŁAW LORENC  JOANNA PRZYTARSKA 《第四纪科学杂志》2014,29(8):767-777

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Flow processes and sedimentation in unidirectionally migrating deep‐water channels: From a three‐dimensional seismic perspective          下载免费PDF全文

Chenglin Gong  Yingmin Wang  Ronald J. Steel  Jeff Peakall  Xiaoming Zhao  Qiliang Sun 《Sedimentology》2016,63(3):645-661

Three‐dimensional seismic data were used to infer how bottom currents control unidirectional channel migration. Bottom currents flowing towards the steep bank would deflect the upper part of sediment gravity flows at an orientation of 1° to 11° to the steep bank, yielding a helical flow circulation consisting of a faster near‐surface flow towards the steep bank and a slower basal return flow towards the gentle bank. This helical flow model is evidenced by the occurrence of bigger, muddier (suggested by low‐amplitude seismic reflections) lateral accretion deposits and gentle channel wall with downlap terminations on the gentle bank and by smaller, sandier (indicated by high‐amplitude seismic reflectors) channel fills and steep channel walls with truncation terminations on the steep bank. This helical flow circulation promotes asymmetrical depositional patterns with dipping accretion sets restricted to the gentle bank, which restricts the development of sinuosity and yields unidirectional channel migration. These results aid in obtaining a complete picture of flow processes and sedimentation in submarine channels.  相似文献   

Contained density currents with high volume of release   总被引：1，自引：0，他引：1       下载免费PDF全文

Quentin Theiler  Mário J. Franca 《Sedimentology》2016,63(6):1820-1842

Contained density currents with high volume of release reflect against the boundaries of the reception environment commonly leading to oscillatory flow. These flows exist in sediment clarifiers, compromising their operations, and deposited signatures of contained turbidity currents are found as part of the infill of sedimentary basins; for operation of the former and interpretation of the latter it is essential to understand the dynamic processes of these flows. Six lock‐exchange experiments with different initial densities were made in a horizontal flume, where the volume of the saline mixture in the lock was equivalent to the volume of the ambient fluid. A further two tests, with a repeated initial density, were made: one with high volume of release and very long duration; and another with low volume of release. Firstly, the movement of the current is discussed, including the oscillations within the experimental tank involving the density current and an upper layer counter‐current. It is shown that the cyclic behaviour is self‐similar with the reduced gravity of the initial density in the lock. Secondly, entrainment and water mixing processes are characterized. The time evolution of mixing is characterized qualitatively by analysing the background potential energy of the density distributions to show that mixing occurs even in the earlier stages of the current, and mainly within the first cycle of the oscillation. Quantified analysis reveals that, in currents with high volume of release, entrainment discharge is one order of magnitude higher, mainly due to the larger interface between the ambient fluid and the current. A model for the evolution of the mixing process is proposed for density currents with high volume of release. Finally, the dynamics of the head of the current is analysed. The entrainment in the head, when compared to the entrainment in the remainder body of the flows, is less important for the configuration with a larger lock.  相似文献   

Tide,Wind, and River Forcing of the Surface Currents in the Fraser River Plume     

M. Halverson  R. Pawlowicz 《大气与海洋》2016,54(2):131-152

A long-term record of surface currents from a high-frequency radar system, along with near-surface hydrographic transects, moored current meter records, and satellite imagery, are analyzed to determine the relative importance of river discharge, wind, and tides in driving the surface flow in the Fraser River plume. The observations show a great deal of oceanographic and instrumental variability. However, averaged quantities yielded robust results. The effect of river flow, which determines buoyancy and inertia near the river mouth, was found by taking a long-term average. The resulting flow field was dominated by a jet with two asymmetric gyres; the anticyclonic gyre to the north had flow speeds consistent with geostrophy. The mean flow speed near the river mouth was 14.3?cm?s^–1, while the flow further afield was 5?cm?s^–1 or less. Wind stress and surface currents were highly coherent in the subtidal frequency band. Northwesterly winds drive a surface flow to the southeast at speeds of nearly 30?cm?s^–1. Southeasterly winds drive a surface flow to the northwest at speeds reaching 20?cm?s^–1; however, there is more spatial variability in speed and direction relative to the northwesterly wind case. A harmonic analysis was used to extract the tidally driven flows. Ellipse parameters for the major tidal constituents varied considerably in both alignment and aspect ratio over the radar domain, in direct contrast to a barotropic model which predicted rectilinear flow along the Strait of Georgia. This is a result of water filling and draining the shallow mud flats north of the Fraser's main channel. The M₂ velocities at the surface were also weaker than their barotropic counterparts. However, the shallow water constituent MK₃ was enhanced at the surface and nearly as strong as the mean flow, implying that non-linear interactions are important to surface dynamics.  相似文献   

A Loop Current experiment: Field and remote measurements     

《Dynamics of Atmospheres and Oceans》2016

An overview of a new comprehensive observational study of the Loop Current (LC) in the eastern Gulf of Mexico that encompassed full-depth and near-bottom moorings, pressure-equipped inverted echo sounders (PIES) and remote sensing is presented. The study array was designed to encompass the LC from the Campeche Bank to the west Florida escarpment. This overview centers about principal findings as they pertain to mesoscale dynamics. Two companion papers provide in-depth analyses. Three LC anticyclonic eddy separation events were observed with good 3D spatial coverage over the 2½ year extent of the field study; the three separations exhibited similar processes after the LC had extended into the eastern Gulf. Large scale (∼300 km wavelength, 40–60 day periods) southward propagating meanders developed on the eastern side of the LC over deep (∼3000 m) water that were the result of baroclinic instability between the upper layer meandering jet and lower layer cyclones and anticyclones. The lower layer was only highly energetic during relatively short (∼2–3 months) intervals just prior to or during eddy detachments because of baroclinic instability. The steepening of the meanders lead to a pinch-off of LC eddies. The deep lower-layer eddies, constrained by the closed topography of the southeastern Gulf, propagated westward across the detachment zone and appear to assist in achieving separation. Small scale (∼50–100 km, periods ∼10 days) frontal eddies, observed on the western side of the LC along the Campeche Bank slope, decay over the deep water of the northern part of an extended LC, and have little influence on lower layer eddies, the east side meanders and the eddy detachment processes.  相似文献   

黄土高原一次超级单体短时强降水中尺度分析   总被引：2，自引：2，他引：0  

王楠  李萍云  井宇  赵强 《气象科学》2016,36(6):742-751

2012年盛夏陕西绥德县出现短时强降水,4 h雨量超过100 mm。利用NCEP 1°×1°再分析资料进行诊断分析发现,在陕北东北部对流层中下层有一中α尺度气旋存在,且随高度向东北倾斜,垂直方向上形成次级环流,使强降水区的上升运动加剧。分析地面观测、卫星资料发现,西路冷空气引导干侵入加强层结不稳定性,东路冷空气楔形抬升作用,使得不稳定能量释放;同时,在东西两路干冷空气的夹击作用下,绥德县内形成中气旋,暖湿空气在当地得以聚集上升;2个中β尺度对流云团在东西路风场作用下合并后迅速增强。多普勒雷达资料显示,2个中γ尺度的超级单体回波在绥德先后发展形成列车效应;2个超级单体中的中气旋特征各有不同,是回波结构演变的主要原因。  相似文献   

Dynamics and role of the Durban cyclonic eddy in the KwaZulu-Natal Bight ecosystem     

LA Guastella  MJ Roberts 《African Journal of Marine Science》2016,38(1):S23-S42

The semi-permanent Durban Eddy is a mesoscale, lee-trapped, cold-core cyclonic circulation that occurs off the east coast of South Africa between Durban in the north and Sezela, some 70 km to the south. When present, strong north-eastward countercurrents reaching 100 cm s^–1 are found inshore. It is hypothesised that the cyclone is driven by the strong south-westward flowing Agulhas Current offshore of the regressing shelf edge near Durban. Analysis of ADCP data and satellite imagery shows the eddy to be present off Durban approximately 55% of the time, with an average lifespan of 8.6 days, and inter-eddy periods of 4 to 8 days. After spin-up the eddy breaks loose from its lee position and propagates downstream on the inshore boundary of the Agulhas Current. The eddy is highly variable in occurrence, strength and downstream propagation speeds. There is no detectable seasonal cycle in eddy occurrence, with the Natal Pulse causing more variability than any seasonal signal. A thermistor array deployed in the eddy centre, together with ship CTD data, indicates upward doming of the thermal structure in the eddy core associated with cooler water and nutrients being moved higher in the water column, stimulating primary production. Together with the use of satellite imagery, our findings indicate a second mechanism of upwelling, viz. divergent upwelling in the northern limb of the eddy. Satellite-tracked surface drifters released in the eddy demonstrated the potential for nutrient-rich eddy water to be transported northwards along the inshore regions of the KwaZulu-Natal (KZN) Bight, thus contributing to the functioning of the bight ecosystem, as well as southwards along the KZN and Transkei coasts – both by the eddy migrating downstream and by eddy water being recirculated into the inshore boundary of the Agulhas Current itself.  相似文献   

Natural ring structures on the Baikal ice cover: Analysis of experimental data and mathematical modeling     

N.G. Granin  I.B. Mizandrontsev  V.V. Kozlov  E.A. Tsvetova  R.Yu. Gnatovskii  V.V. Blinov  I.A. Aslamov  K.M. Kucher  V.G. Ivanov  A.A. Zhdanov 《Russian Geology and Geophysics》2018,59(11):1514-1525

Hydrophysical studies and mathematical modeling of ring structures during ice cover on Lake Baikal have shown that their existence at the stage of ice cover degradation is due to anticyclonic currents. Such currents can be generated as a result of local upwelling, which we associate with the rise of methane hydrates from the top layer of bottom sediments and their dissociation. Analysis of satellite images shows that the radii of ice rings range from 1300 to 2400 m, which is close to the baroclinic Rossby radius. The measured ice thicknesses in the area of the rings are in agreement with model calculations. Deep water renewal in Lake Baikal can also be associated with the rise of hydrates.  相似文献   

Seasonal cycle of surface circulation in the coastal North Indian Ocean     

S R Shetye  S Satheesh Chandra Shenoi 《Journal of Earth System Science》1988,97(1):53-62

Monthly-mean winds and currents have been used to identify the driving mechanisms of seasonal coastal circulation in the North Indian Ocean. The main conclusions are: (i) the surface circulation off Arabia is typical of a wind-driven system with similar patterns of longshore current and wind stress; (ii) circulation off the west coast of India is consistent with the dynamics of a wind-driven eastern boundary current only during the southwest monsoon. During the northeast monsoon it is possible that the influence of the interior flow is important. (iii) There are at least three mechanisms that influence the surface circulation off the east coast of India: wind-stress, influence of fresh-water run off and contribution of the interior flow. It is difficult at present to assess the relative importance of these three processes.  相似文献   

陕西洛川不同时代黄土中的碳酸钙的沉积速率及其在地质年代测定中的意义          下载免费PDF全文

盛学斌  王克鲁 《地震地质》1989,11(1):91-98

选择地处西北半干旱区、又具有代表性的黑木沟标准剖面,逐一测试其土层中的碳酸钙与干容重,进而由相关参数推算各土层的碳酸钙的沉积速率(V,下同),这是笔者的尝试。结果表明,该剖面中的碳酸钙的沉积速率,均有自下而上递增的趋势,各层总平均速率为0.20g/cm2·103a  相似文献