Large-scale Meandering Research Articles

Lagrangian reconstructions of temperature and velocity in a model of surface ocean turbulence

The characterization of submesoscale dynamics is crucial to apprehend their impact on the global ocean properties. Direct measurements of fine structures over the world oceans, nevertheless, are at present severely limited by the spatial resolution of available satellite products. In this work we numerically investigate the possibility to reconstruct tracer fields, like surface temperature, at small scales, from low-resolution data using a Lagrangian technique based on the properties of chaotic advection. The capabilities of the method are explored in the context of a forced Surface Quasi Geostrophic (SQG) turbulent flow representing a large-scale meandering jet and smaller-scale eddies. Both qualitative and quantitative comparisons are performed between the original (high-resolution) fields and their reconstructions that use only low-resolution data. Good agreement is found for filamentary structures, even in the presence of a large-scale forcing on the tracer dynamics. The statistics of tracer gradients, which are relevant for assessing the possibility to detect fronts, are found to be accurately reproduced. Exploiting SQG theory, the reconstruction technique is also extended to obtain the velocity field in three dimensions when temperature is the tracer. The results indicate that relevant features of dynamical quantities at small scales may be adequately deduced from only low-resolution temperature data. However, the ability to reconstruct the flow is critically limited by the energetic level of submesoscales. Indeed, only structures generated by non-local mesoscale features can be well retrieved, while those associated to the local dynamics of submesoscale eddies cannot be recovered.

Transformation of a lowland river from a meandering and multi-channel pattern into an artificial canal: retracing a path of river channel changes (the Middle Obra River, W Poland)

At the beginning of the nineteenth century, a significant change in sedimentation took place along the middle course of the Obra River (W Poland): a “natural” channel bed was replaced by three artificial canals. Before that time the Obra River bed had been characterized by various river patterns (braided, meandering and disappearing in wetlands). GPR (ground-penetrating radar) surveys and the analysis of satellite and aerial images were carried out to retrace the natural river course near the village of Kopanica. The GPR, ground-truthed with core data, revealed that at least two types of the river pattern had been functioning here before the period of the hydrotechnical works: a large-scale meandering pattern (width-to-depth (w/d) ratio ranging from 15 to 40) and a relatively smaller-scale multi-channel pattern (w/d = 11–22). Possibly, the transformation was caused by forming a series of large floods followed by in-channel aggradation and avulsions. Next, the multi-channel pattern was artificially transformed into the North Obra Canal. This paper discusses possibilities of a creation of network of channels that were active after the construction of the North Obra Canal in the beginning of the nineteenth century. Such intervention would increase natural values of the presented section of the Obra valley. The presented study also shows that a detailed analysis of history and evolution of a given river course is needed before undertaking any restoration works.

Algorithm for Flow Direction Enforcement Using Subgrid-Scale Stream Location Data

Producing realistic surface water flow patterns can be difficult for hydrologic models when there is insufficient grid resolution as a result of computational constraints or when available digital elevation model (DEM) data are relatively coarse. This technical note describes an algorithm that allows for more realistic overland flow by incorporating subgrid-scale location data without increasing grid resolution. The algorithm takes location data from the National Hydrography Dataset (NHD), maps it to the hydrologic model horizontal grid coordinates, and produces a list of ordered points along the stream. Because the algorithm uses indexes to efficiently order points along the stream, large-scale meanders require special treatment, whereas small (grid) scale meanders are explicitly included in the algorithm logic. Slopes are ensured to be continuous along the stream's path as defined on the model grid, distinguishing this approach from traditional stream burning algorithms. Stream coordinates on the model grid are calculated along with corresponding elevation and slope values so that the can then be integrated into the DEM if desired. The algorithm's flow routing capabilities are demonstrated by using an integrated surface water-groundwater model, ParFlow, under rain and recession conditions. This case study is performed by using the real topography of Owens Valley, California, and NHD flowline data for the Owens River. An approach using a DEM that has undergone standard processing to fill sinks and a typical stream burning approach [similar to those often used in geographic information system (GIS) applications] fail to route flow out of the flood plain in ParFlow's overland flow model that partitions and routes topography-driven flow along adjacent cells in one of four cardinal directions. In contrast, this approach, with the river elevations and continuous slopes integrated into the DEM, routes water to the river and out of the catchment, creating more realistic surface flow patterns in the region. Although the method is applied here to address problems associated with a flat flood plain, it may also be applied to any area in which flow is discontinuous because of insufficient resolution of topography on a model grid. DOI: 10.1061/(ASCE)HE.1943-5584.0000340. (C) 2011 American Society of Civil Engineers.

Lagrangian stochastic modelling for oil spills turbulent dispersion on ocean surface

Releases of liquid petroleum hydrocarbon into the environment, especially into the ocean, are called {\em oil spills}. To correctly model dispersion on ocean surface both large and small scale processes must be considered. In fact, the motion of the centre of mass is driven by large scale meandering and spot enlarging is controlled by small scale turbulence. A Lagrangian stochastic model for turbulent dispersion of oil spills on ocean surface is here formulated: large scale meandering is analytically prescribed by the stream function proposed by Bower for meandering jet in the Gulf Stream; small scale turbulence has been modelled by a Lagrangian stochastic process for turbulent relative dispersion which is based on turbulent flow rotation. Numerical simulations are performed to highlight the effects on oil spills dispersion of differences in the velocity field intensity and those due to releases in the centre or in the boundary of a meandering jet.

Fluvial style changes during the last glacial–interglacial transition in the middle Tisza valley (Hungary)

The Weichselian Late Pleniglacial, Lateglacial and Holocene fluvial history of the middle Tisza valley in Hungary has been compared with other river systems in West and Central Europe, enabling us to define local and regional forcing factors in fluvial system change. Four Weichselian to Holocene floodplain generations, differing in palaeochannel characteristics and elevation, were defined by geomorphological analysis. Coring transects enabled the construction of the channel geometry and fluvial architecture. Pollen analysis of the fine-grained deposits has determined the vegetation development over time and, for the first time, a bio(chrono)stratigraphic framework for the changes in the fluvial system. Radiocarbon dating has provided an absolute chronology; however, the results are problematic due to the partly reworked character of the organic material in the loamy sediments. During the Late Pleniglacial, aggradation by a braided precursor system of the Tisza and local deflation and dune formation took place in a steppe or open coniferous forest landscape. A channel pattern change from braided to large-scale meandering and gradual incision occurred during the Late Pleniglacial or start of the Lateglacial, due to climate warming and climate-related boreal forest development, leading to lower stream power and lower sediment supply, although bank-full discharges were still high. Alternatively, this fluvial change might reflect the tectonically induced avulsion of the River Tisza into the area. The climatic deterioration of the Younger Dryas Stadial, frequently registered by fluvial system changes along the North Atlantic margin, is not reflected in the middle Tisza valley and meandering persisted. The Lateglacial to Holocene climatic warming resulted in the growth of deciduous forest and channel incision and a prominent terrace scarp developed. The Holocene floodplain was formed by laterally migrating smaller meandering channels reflecting lower bank-full discharges. Intra-Holocene river changes have not been observed.

CFD4NRS with a focus on experimental and CMFD investigations of bubbly flows

The paper discusses first current computational trends related to reactor safety problems and developments needed, as well as the need for new kinds of much more detailed experimental data for the validation of the new methods and codes. The numerous bubbly flow experiments conducted during the last decade at PSI are then reviewed and put in perspective with regards to the needs of the analysts. The latest experiments produced first-of-a-kind sets of experimental data from bubbly plumes and jets. Some of these data were conditionally ensemble or phase averaged to separate the effects of large-scale meandering and oscillations or coherent structures from the time-dependent small-scale effects; then the Reynolds stress terms could be properly identified. The large data sets that have been created can be further mined to extract additional information for the two-phase flow analyst. The companion analytical developments and simulations of bubbly flows with RANS and LES methods are discussed. LES methods are needed to overcome some of the basic limitations of RANS simulations.

Hydraulic Evaluation of W-Weir for River Restoration

Various structural measures have been advocated for river restoration and habitat improvement schemes. The W-weir is one such structure that can be used in mobile bed alluvial rivers to diversify habitat and provide grade control. Laboratory studies have been carried out in a large-scale meandering channel with a mobile bed to investigate their effects on flow and sediment transport processes. A W-weir placed immediately downstream of a riffle section created a strongly three-dimensional flow pattern and high-turbulence zones. Two adjacent scour holes of different depths and substrate are formed under clearwater and live bed conditions. The continuity of sediment transport along the channel was not interrupted by the structure and the upstream afflux is minimal. Overbank flow significantly influenced the action of the weir and the scour hole was shifted closer to the structure. In a relatively tight bend followed by a short crossover reach, the weir may affect bed load transport pathways in the downstream bend. Finally, the study provides insights to guide their design for restoration projects.

The Hillarys Transect (1): Seasonal and cross-shelf variability of physical and chemical water properties off Perth, Western Australia, 1996–98

A 27-month study of the water properties across the continental shelf off Perth, Western Australia (the “Hillarys Transect”), has provided the first systematic interdisciplinary climatology of the physical, chemical, optical and biological cycles across the shelf. This paper describes the main features of the seasonal and cross-shelf variability of the physical oceanography and chemistry, while companion papers discuss some of the links between the biology and physics of the region. The oceanography is dominated by the seasonally varying Leeuwin Current flowing southwards along the shelf break and outer shelf, and the northwards inshore Capes Current which is driven by the net southerly wind stress between about October and March (the austral summer). As a result of the poleward boundary current, there is no large-scale upwelling comparable with the Humboldt and Benguela Current systems. Water temperature and salinity in the shallow coastal waters are largely influenced by air–sea heat flux processes, while advection plays a more important role along the outer shelf; as a consequence, seasonal variations in the inshore temperature and salinity are much larger than those offshore. Cross-shelf exchange of water and plankton is effected by (1) large-scale meandering of the Leeuwin Current, (2) horizontal mixing as tongues of Leeuwin Current water penetrate across the shelf, (3) cascading of high-density coastal water offshore along the seabed and (4) sporadic summer upwelling onto the outer shelf (including the wake effect north of Rottnest Island). Nutrient and chlorophyll concentrations are low in comparison with other typical west coast situations. While there is some indication of a seasonal cycle, the relatively short sampling period and high patchiness have precluded definitive patterns being described and longer-term sampling may be required to resolve this. The effects of smaller-scale temperature and chlorophyll variability on satellite remote sensing measurements (both “within pixel” and “between pixel”) in these coastal waters have been quantified using the underway (horizontal) and profile (vertical) data from the surveys. The project has demonstrated the great potential of using remote sensing information for regular monitoring of the Western Australian continental shelf waters provided that adequate in situ validation measurements are also undertaken.

Destabilization of barotropic flows small-scale topography

The stability of a barotropic zonal jet aligned with zonal topography on the beta-plane is investigated. The topography is assumed to be spatially periodic, with a period much smaller than the width of the jet. The problem is examined both by linear normal-mode analysis and by direct numerical simulations.The following results are obtained. If the topography is sufficiently weak, the growth-rate of the most unstable normal mode has two maxima. The long-wave maximum occurs at wavelengths comparable to the width of the jet, and is described by Benilov's (J. Phys. Oceanogr. vol. 30, 2000, p. 733) asymptotic theory. The short-wave maximum occurs at wavelengths comparable to the scale of the topography or at a shorter one. The nonlinear evolution of the flow is, in this case, similar to that in the case of a flat bottom, i.e. the jet begins to meander and breaks up into separate vortices.For a stronger topography, long-wave disturbances are stable, as predicted by Benilov's (2000) asymptotic theory, whereas short-wave instabilities are still present. The instabilities are strongest near the lines of maximum shear. In nonlinear simulations, the flow becomes turbulent within narrow strips along these lines, and potential vorticity there homogenizes. As the strips grow wider, they begin to interact, and the subsequent evolution is again similar to that of a jet over a flat bottom: large-scale meandering and break-up of the jet into vortices. In the presence of topography, however, the vortices are ‘filled’ with short-wave turbulence, and break-up occurs later.

Validation of a Three-Dimensional Numerical Code in the Simulation of Pseudo-Natural Meandering Flows

Validation of a three-dimensional finite volume code solving the Navier–Stokes equations with the standard k-epsilon turbulence model is conducted using a high quality and high spatial resolution data set. The data set was collected from a large-scale meandering channel with a self-formed fixed bed, and comprises detailed bed profiling and laser Doppler anemometer velocity measurements. Comparisons of the computed primary and secondary velocities are made with those observed and it is found that the lateral momentum transfer is generally under predicted. At the apices this results in the predicted position of the primary velocity maximum having a bias towards the channel center, compared to the position where it has been measured. Using a simplified two zone roughness distribution whereby a separate roughness height was prescribed for the channel center and channel sides relative to a single distributed roughness height, generally led to a slightly improved longitudinal velocity distribution; the higher velocities were located nearer to the outside of the bend. Improving both the free surface calculation and scheme for discretization of the convection terms led to no appreciable difference in the computed velocity distributions. A more detailed study involving turbulence measurements and bed form height distribution should discriminate whether using distributed roughness height is a precursor to using an anisotropic turbulence representation for the accurate prediction of three-dimensional river flows.

Kinematic elements of Antarctic Intermediate Water in the western South Atlantic

The northward flowing Antarctic Intermediate Water (AAIW) is a major contributor to the large-scale meridional circulation of water masses in the Atlantic. Together with bottom and thermocline water, AAIW replaces North Atlantic Deep Water that penetrates into the South Atlantic from the North. On the northbound propagation of AAIW from its formation area in the south-western region of the Argentine Basin, the AAIW progresses through a complex spreading pattern at the base of the main thermocline. This paper presents trajectories of 75 subsurface floats, seeded at AAIW depth. The floats were acoustically tracked, covering a period from December 1992 to October 1996. Discussions of selected trajectories focus on mesoscale kinematic elements that contribute to the spreading of AAIW. In the equatorial region, intermittent westward and eastward currents were observed, suggesting a seasonal cycle of the AAIW flow direction. At tropical latitudes, just offshore the intermediate western boundary current, the southward advection of an anticyclonic eddy was observed between 5 °S and 11°S. Farther offshore, the flow lacks an advective pattern and is governed by eddy diffusion. The westward subtropical gyre return current at about 28°S shows considerable stability, with the mean kinetic energy to eddy kinetic energy ratio being around one. Farther south, the eastward deeper South Atlantic Current is dominated by large-scale meanders with particle velocities in excess of 60 cm s -1. At the Brazil–Falkland Current Confluence Zone, a cyclonic eddy near 40°S 50°W seems to act as injector of freshly mixed AAIW into the subtropical gyre. In general, much of the mixing of the various blends of AAIW is due to the activity of mesoscale eddies, which frequently reoccupy similar positions.

Mesoscale Response to a Meandering Surface Temperature Interface

Mesoscale variation of the boundary-layer (BL) front produced by a surface temperature interface depends upon the scale of meander along that interface. For a relatively large-scale meander, the circulations are quasi two-dimensional relative to the local interface boundary, and a meander signature appears in the BL structure. For a relatively small-scale meander, alongfront blending eliminates organization about individual meanders to produce a quasi two-dimensional circulation and gradients oriented perpendicular to the mean front. The fundamental atmospheric scale controlling this transition is the mesoscale deformation radius, which depends upon the warm-side BL depth. With strong large-scale geostrophic forcing, however, the resulting alongfront advection length scale increases the meander size required to approach the large-scale limit. Large-scale meanders typically create two local maxima of vertical velocity, whereas small-scale meanders develop a single maximum on the warm side of the frontal zone. At intermediate scales, variations of the vertical velocity maximum are particularly complex when large-scale geostrophic winds are relatively weak.

On the Decay of the Meanders of Eastward Currents

The separation of western boundary currents from the coast and their eastward extensions into the open ocean are characterized by the presence of quasi-stationary, large-scale meanders. These meanders result when the western boundary current overshoots the latitude of zero wind-stress curl. The wavelength and the scale of decay of those meanders have previously been estimated by considering the superposition of a westward Rossby wave and an eastward mean flow. While observations indicate that the wavelength of the meanders is in good agreement with theory, the decay scale is much shorter than that indicated by scaling arguments. The purpose of this article is to show that the rapid decay of the meanders of eastward currents can be related to the effect of the meridional shear of the current.

Behavior of steps on Si(001) as a function of vicinality.

The surface step morphology of the Si(001) surface as a function of miscut angle between 0.3\ifmmode^\circ\else\textdegree\fi{} and 5.25\ifmmode^\circ\else\textdegree\fi{} has been investigated using scanning tunneling microscopy. On samples with large average terrace widths, the kinks in the steps behave independently and the intrinsic step energy can be determined from the kink-length distribution which follows simple Boltzmann statistics. For wide terraces the large-scale meandering of the steps is governed by the long-range strain fields caused by the anisotropic surface stress tensor. Here the step separation distributions can be described by a Boltzmann distribution of segments of the step moving in this strain potential. At intermediate average terrace widths, the distribution of kink lengths is influenced by the confinement between neighboring steps, which suppresses the number of long kinks. As steps get closer together, a short-range direct step-step interaction arising from the local strain due to the rebonding of the SB step begins to influence the step distribution. The sense of this interaction is such that the rough SB step is attracted to its downhill neighbor. Images and terrace-width distributions of surfaces for which the miscut angle is great enough to measure an appreciable increase in the percentage of double atomic-height steps are used to provide a description of the evolution of the surface from one containing single-height to mostly double-height steps.

Regional Primitive Equation Studies of the Gulf Stream Meander and Ring Formation Region

Abstract Primitive equation and quasi-geostrophic eddy resolving, open ocean models are used for hindcast studies in the Gulf Stream meander and ring formation region. A feature model approach is used to initialize the models, based on one month of observations during November to December 1984. Flat bottom and topographic calculations are carried out using an initial Gulf Stream velocity profile based on the Pegasus dataset. All of the major events observed in the upper thermocline are reproduced by both numerical models. The addition of bottom topography is shown to significantly alter the character of the deep velocity fields. Large, basin scale circulations found near the bottom in both flat bottom calculations were replaced by energetic jets and eddies associated with the dominant spatial scales of the bottom topography. Use of the quasi-geostrophic model to dynamically adjust the initial conditions for the primitive equation model is shown to reduce the growth of large scale meanders on time scales o...

Major Features of the Summer Near-Surface Circulation of Western Baffin Bay, 1978 and 1979

In the summers of 1978 and 1979, an extensive physical oceanographic program was carried out in western Baffin Bay and Lancaster Sound. Data collected include satellite-tracked drifter measurements, CTD profiles and time series of subsurface currents. They indicate that the near-surface circulation of the area is dominated by the southward-flowing Baffin Current. This current, observed to extend at least as far north as Lady Ann Strait (76° N) and south to Cape Dyer (67° N), is largely confined to within 100 km of the coastline. It varies in both intensity and width with the strongest flows occurring where the current follows a cyclonic intrusion into and out of eastern Lancaster Sound; here in the core of the current the median near-surface (4 to 11 m) speeds are 75 cm/s, decreasing to 50 cm/s at 40 m depth and 25 cm/s at 250 m depth. To the east of Devon, Bylot and Baffin islands, the current is well-defined, but generally less intense with typical near-surface speeds of 30 cm/s. Important spatial variations occurred in the circulation of the area. In eastern Lancaster Sound, two distinct and alternating flow patterns were observed in the strong intrusive current in the summer of 1979. Transient large-scale meanders of the circulation were detected in the offshore portion of the Baffin Current off the east coasts of Bylot and Baffin islands. Between the eastern coastline of Bylot Island and the core of the southeasterly flowing Baffin Current, anticyclonic eddies of approximately 20 km diameter occasionally occurred.Key words: Baffin Bay, Baffin Current, circulation, Lancaster Sound, oceanography, surface currents, sub-surface currents

Spatial and temporal variability of major ocean currents and mesoscale eddies

A global picture of ocean current variability may be obtained by analyzing surface drift currents in terms of their mean and eddy kinetic energies. High values for both quantities are found in the western boundary currents and in the equatorial current system; low values are found in the interior of major gyres. However, nowhere are eddy energies less than 200 cm2 s−2, indicating that, even in the least energetic parts of the oceans, surface speeds of ~20 cm s−1 prevail. Recent experimental studies also support the widespread occurrence of mesoscale mid-oceanic eddies. Another type of eddy is abundant in the vicinity of boundary currents: examples include Gulf Stream Rings, the ‘Great Whirl’ of the Somali Current, and disturbances of the predominantly zonal equatorial flow manifested by large-scale meandering about the equator. Recent numerical models using low-viscosity and high-resolution computational grids also reveal the ubiquitous existence of mesoscale structures. The importance of eddies is that they seem to be energetic enough and sufficiently widespread so as to play some part — not yet understood — in the circulation of the world ocean. Speculative analogies to the atmosphere suggest that the mesoscale ocean eddies are ‘the storms and weather systems of the sea’. We need global statistics on their distribution, their occurrence in various oceanic regions, their dimensions, and their lifetimes. The prospect of even a single global oceanic ‘weather map’, comparable to those obtained daily for the atmosphere, is hopeless in terms ofin situ oceanographic observations. Remote sensing may provide a partial solution.

Meanders and long waves in the equatorial Atlantic

Observations from the GATE Equatorial Oceanographic Experiment are presented. They reveal large scale meandering of the westward flowing South Equatorial Current and of the eastward flowing Equatorial Undercurrent with time scales of 2–3 weeks. Meandering of the flow pattern was found to be related to corresponding displacements of the high salinity core of the undercurrent. The observations tend to support the assumption of a long wave propagating westward with a phase speed of 2.3 m s−1 and a wavelength of 3,200 km. A possible explanation may be given in terms of unstable waves caused by large scale horizontal shear in the Equatorial Current System.

Transient Gulf Stream Meandering. Part I: An Observational Experiment

Abstract The results from an observational experiment on the mesoscale space-time variability of the Gulf Stream are reported. Various techniques, including aerial surveys, ship trackings of the 15C isotherm at 200 m, drogues and moored current meters were used and are compared, to give estimates of the variability of the motion over a wide range of scales. A two-week time series of daily tracks of the Stream near 70W are used to interpolate instantaneous paths over 2° of longitude. These paths provide the first detailed information on the small-scale variability of the path indicator of the Gulf Stream northeast of Cape Hatteras. Similarly, the long time series of triweekly aerial surveys provides a detailed picture of the evolution of a large-scale meander.

On the Variation of the Kuroshio

The review on the variation of the Kuroshio extending from several hours to ten years is described here. We have had no clear understandings on the mechanisms of occurrence of large and comparatively stable meander off Kisyu-Ensyu Nada. It is known, at present, that the selected several small meanders in spring off the east coast of Kyusyu progress towards east along the south coasts of Shikoku and Kii Peninsular and stop its advance off Ensyu Nada and become large. This large meander is always accompanied, inside of it, with the ascending cold intermediate water in the Pacific. We cannot fully understand the mechanisms of occurrence and continuation of this large meander without knowledges of the deep and bottom current based on the direct measurement and the long-term variation in the atmosphere over the Pacific especially subtropical western Pacific.There exist middle and small scale meanders which progress easterly almost every time except the period of large stable meander off Kisyu-Ensyu Nada. The Kuroshio which flows along the south coast of Japan might exist in the duration only about 1/4-1/6 of total period after the World War II. Therefore the paths of large scale meander off Kisyu-Ensyu Nada and Middle and small scale meanders traveling easterly must not be considered abnormal. The very short-period fluctuations, one or half a day, of the velocity and direction of the Kuroshio were observed only by several systematic programmes with several vessels in Japan. From these simultaneous observations, comparatively large variations of velocity of the Kuroshio which may mask the amplitude of long-period variation of the Kuroshio were found. The observations of detailed pattern and direction of propagation of disturbed wave will be necessary to get the causes of these variations.