Lateral Density Gradients Research Articles

Present-Day deformation across the basin and range province, western united states

The distribution of deformation within the Basin and Range province was determined from 1992, 1996, and 1998 surveys of a dense, 800-kilometer-aperture, Global Positioning System network. Internal deformation generally follows the pattern of Holocene fault distribution and is concentrated near the western extremity of the province, with lesser amounts focused near the eastern boundary. Little net deformation occurs across the central 500 kilometers of the network in western Utah and eastern Nevada. Concentration of deformation adjacent to the rigid Sierra Nevada block indicates that external plate-driving forces play an important role in driving deformation, modulating the extensional stress field generated by internal buoyancy forces that are due to lateral density gradients and topography near the province boundaries.

Soil mineral nitrogen availability to young maize plants as related to root length density distribution and fertilizer application method

Minirhizotron observations from 4 experiments in 1992 and 1993 with maize on a sandy soil in the Wageningen Rhizolab, Netherlands, showed strong vertical and lateral root density gradients during the first 9 weeks after emergence. Root length density (Lrv), as determined in core samples 9 weeks after emergence, was positively related (P

Salt Intrusion and Secondary Flow in a Shallow, Well-mixed Estuary

Observations of an axial convergence front formed within the Conwy estuary, North Wales, U.K., during the flood tide were indicative of density-driven secondary circulations. It has been proposed that these were the result of lateral density gradients established by the interaction of the lateral shear of the longitudinal currents with the axial salinity gradients through a differential-advection mechanism. In the present study, the evolution of axial salinity gradients during a tidal cycle along the length of a shallow well-mixed estuary was examined using six recording instruments deployed over a 25-day period. The evolution of channel geometry and of the lateral shear of axial current speed was examined using simple models in conjunction with the observations of near-bed pressure and current speed. It is hypothesized that the complex interaction between these three parameters (channel geometry, lateral shear of axial current and axial salinity gradient) results in the observed evolution of secondary flow throughout the tide, which itself causes a redistribution of the density field.

A mathematical model for the density-augmented spreading of a particle-laden discharge in a turbulent shallow-water flow

A two-dimensional horizontal dispersion equation is derived for the many ways that buoyancy modifies the spreading of a mono-disperse discharge of particles in a shallow-water flow. The particle-induced lateral density gradient gives rise to a secondary transverse flow which tends to carry particles outwards. Stratification with the consequent reduced mixing, changes the local vertical profile of the original flow and gives rise to a Burgers nonlinearity (slowing down of sinking particles or a speeding up of buoyant particles). Stratification also modifies the non-local horizontal distribution of the current, with an inflow towards particle-laden regions where the drag is reduced. A simple eddy-diffusivity turbulence model is used which permits explicit evaluation of most of the linear and nonlinear coefficients. Nonlinear effects are shown to be significant for particle volume concentrations of only 10 parts per million.

Dynamical properties of a buoyancy‐driven coastal current

The outflow of buoyant waters from major estuaries affects the dynamics of inner continental shelves profoundly as lateral density gradients force an alongshore current. Often the Coriolis force causes the outflow to remain trapped near the coast. We observed one such current, the Delaware Coastal Current, on the inner shelf near the Delaware Estuary on the eastern seaboard of the United States. The spatial variability along the shelf, however, suggests at least two dynamically distinct regions that we term source and plume regions. In the source region we find fronts, a current whose width scales well with the internal deformation radius, and a ratio of relative to planetary vorticity that reaches unity, that is, the Rossby number is O(1). As nonlinear inertial forces in the across‐shelf momentum balance are weak, we suggest that such forces contribute to the along‐shelf momentum balance only. Farther downstream in the plume region, we find much reduced lateral density gradients, a current much wider than the deformation radius, and relative vorticities that are much smaller than the planetary vorticity. From our observations we compute nondimensional dynamical parameters, with which we discuss our observations. The Burger, Rossby, and Ekman numbers for the Delaware Coastal Current suggest that most models of buoyancy‐driven coastal currents do not apply to this coastal flow.

Mechanisms driving groundwater flow near salt domes

Groundwater flow near salt domes is complex because groundwater is subject to a variety of driving forces including the release of geopressured fluids, large lateral density gradients, and regional hydraulic head gradients. The complexity of this environment is born out by recent geochemical and geophysical observations that indicate the occurrence of upward groundwater flow near some salt domes. In order to evaluate the relative importance of different mechanisms driving groundwater flow near salt domes, we have developed a numerical model that couples groundwater flow, heat transport, and transport of dissolved salt, and accounts for salt diapirism. Our calculations indicate that upward groundwater flow can occur as the result of thermal convection when the regional background salinity is greater than 15 weight percent, a value typical of many areas of the south Louisiana salt dome province. For lower background salinities, dissolution causes salt‐laden groundwater near the dome to sink, leading to depressed isotherms. While the release of geopressured fluids is difficult to quantify, it remains a likely mechanism for driving upward groundwater flow near some salt domes.

Richardson number profiles in laboratory experiments applied to shallow seas

Abstract A unified analysis is given of the critical conditions for the onset of stratification due to either a vertical or a horizontal buoyancy flux, with tidal or wind stirring. The critical conditions for the onset of stratification with a horizontal buoyancy flux are found to be of the form of ratios of the tidal slope, or wind setup, to the equivalent surface slope due to the lateral density gradient. These ratios, which are easily determined from sea data, indicate that the profiles of critical flux Richardson Number, averaged over the stirring cycle, are similar to those inferred from the laboratory experiments of Hopfinger and Linden (1982) in which there is zero mean shear turbulence with a stabilising buoyancy flux, and also that the efficiency for the conversion of kinetic energy to potential energy for tidal stirring is similar to that for wind stirring. The observed much greater efficiency for wind stirring, compared with tidal stirring with a vertical buoyancy flux, is also consistent with ...

Convection in a rotating cavity: modelling ocean circulation

Large-scale oceanic flows, ranging from large estuaries to basin-scale abyssal circulation, can be driven by imposed lateral density gradients and are significantly influenced by the Earth's rotation. Some essential features of such flows have been incorporated into a laboratory model by differentially heating and cooling the vertical endwalls of a shallow rectangular cavity, rotating about a vertical axis through its centre. In this paper, the flow observed after this system had been allowed to develop to a steady mean state is described. Measurements were made of the velocity and temperature fields, as well as the variability in temperature field associated with eddy motions. The temperature data were also used to calculate the potential vorticity distribution within the cavity. These results indicate that the heated and cooled end-walls acted not only as continuous sources of buoyancy, but also of anomalous potential vorticity. It is shown that the potential vorticity served as an indicator and tracer of the laboratory flow. The structure and location of boundary currents were largely determined by nonlinear potential vorticity dynamics which resulted in a mean circulation consisting of two basin-scale counter-rotating gyres. This flow differed markedly from the initial boundary currents set up by Kelvin waves.

The formation of longitudinal fronts in a coastal plain estuary

Longitudinal estuarine fronts, aligned parallel to the axis of the estuary, are characteristic of the York River. Their time scale for genesis and decay is of the order of hours; they are usually located at the inner edge of the shoals or over the main channel. Field measurements have shown marked intratidal differences in density and current speeds across this estuary. These can be correlated with changes in depth and the strength of the advective processes. Differential advection between the channel and shoal regions, when acting upon a constant longitudinal density gradient, is shown to be of sufficient magnitude to generate strong lateral density gradients, and thus fronts, at particular times within the tidal cycle.

Geostrophic Adjustment: A Mechanism for Frontogenesis

Abstract A simple two-dimensional model is used to determine the geostrophically adjusted state of an initially motionless fluid with lateral density gradient. It is an extension of the much studied two-layer models to the case when the initial stratification is continuous, and allows a more detailed examination of the density and current fields. In addition, the model has demonstrated that geostrophic adjustment with its accompanying transverse circulation can produce regions of convergence and thus steepen the initial density gradient. In cases when the initial density transition is sharp enough, fronts, or density discontinuities, are formed at these convergence regions, along which the surface fluid particles drawn in from both sides of the fronts move into the interior. The density jump across the fronts is the greatest at the top and bottom surface and diminishes toward the interior where differential vertical motion during the adjustment can significantly level the isopycnals. One observed frontal ...

Optimum expulsion of brine from sea ice

A simple model is constructed to exploit the concept of drainage “trees” as observed during the process of expulsion for desalination of sea ice. Linear analysis is used together with a consistent approximation procedure for solution of the system to determine critical minimum Rayleigh numbers for the onset of instability. It is found that the system functions optimally when there is both a finite angle of tilt of the drainage channel as well as no tilt at all. The determination of the nonzero angle depends critically on the lateral mean density gradient, and numerical values for critical angles of inclination to the vertical obtained agree quantitatively with those reported.

Spectral interpretation of gravity anomalies due to horizontal slab like bodies with lateral variation of density

Fourier spectrum of the gravity effect due to a horizontal slab with lateral variation of density is derived. By analysing this spectrum a procedure is formulated to determine the parameters of the causative bodyviz. depths to the upper and lower surfaces, width of the horizontal portion having the lateral density variation and the lateral density gradient. Two synthetic models and a field example are discussed to establish the validity of the procedure formulated. This is much simpler than Gendzwill’s type curves method.

The spring transition in currents over the Oregon Continental Shelf

Current meter moorings maintained over the Oregon continental shelf in 1973 and 1975 clearly show the difference between winter and spring oceanographic regimes and the rapid transition between the regimes. In winter the mean alongshore current is northward at all depths and strongest near shore; there is no mean vertical shear, no mean offshore density gradients, and mean sea level is high. In spring the mean alongshore current is weak near the bottom and strongly southward at the surface with a maximum over the mid‐shelf; the strong vertical shear is balanced by strong offshore density gradients, and mean sea level is low. In both 1973 and 1975 the transition between these regimes occurs within about a week during a strong southward wind event. In many respects the transition event does not differ significantly from previous southward wind events, but the lateral density gradients established during the transition event exceed those of earlier events. The transition appears to be the result of a large cumulative offshore Ekman transport caused by local wind stress rather than by propagation of effects generated elsewhere. The lateral density gradients and the vertical shear established during the transition event subsequently persist, even during northward wind events with moderate onshore Ekman transport.

The baroclinic instability of a mid-ocean circulation

The instability of a flow modelling a linear oceanic thermocline is investigated with a view towards understanding the intermediate-scale fluctuations observed by Crease (1962) and others. Robinson's (1962) analysis of the stability of a thermal ocean circulation is extended to include amplifying quasi-geostrophic perturbations. North-south wavelengths of several hundred kilometers are predicted and agree with previous measurements. For the special case of perturbations independent of longitude, a uniformly valid solution is obtained for oceans of large latitudinal extent. Growth rates are of the order of one year, and the north-south wavelength is a function of latitude. Although the basic state chosen is different from its atmospheric counterpart examined by Eady (1949) and Green (I960), both are baroclinically unstable and have perturbations with similar properties. At maximum instability, both have perturbations which are constant in the direction parallel to the basic lateral density gradient, and this result is true both on an ƒ-plane and ?-plane. DOI: 10.1111/j.2153-3490.1967.tb01483.x

The baroclinic instability of a mid-ocean circulation

The instability of a flow modelling a linear oceanic thermocline is investigated with a view towards understanding the intermediate-scale fluctuations observed by Crease (1962) and others. Robinson's (1962) analysis of the stability of a thermal ocean circulation is extended to include amplifying quasi-geostrophic perturbations. North-south wavelengths of several hundred kilometers are predicted and agree with previous measurements. For the special case of perturbations independent of longitude, a uniformly valid solution is obtained for oceans of large latitudinal extent. Growth rates are of the order of one year, and the north-south wavelength is a function of latitude. Although the basic state chosen is different from its atmospheric counterpart examined by Eady (1949) and Green (I960), both are baroclinically unstable and have perturbations with similar properties. At maximum instability, both have perturbations which are constant in the direction parallel to the basic lateral density gradient, and this result is true both on an ƒ-plane and ?-plane. DOI: 10.1111/j.2153-3490.1967.tb01483.x