Sharp Density Fronts Research Articles

Pseudo-schlieren CT measurement of three-dimensional flow phenomena on shock waves and vortices discharged from open ends

1A pseudo-schlieren technique is applied to the interferometric computed tomography (CT) measurement of three-dimensional (3-D) shock waves discharged from a square open end and a pair of circular open ends in a shock tube experiment. The experiment is performed for incident shock Mach numbers of 2.0 and 2.2 in nitrogen gas under supersonic post shock flow conditions at the open end. The 3-D density-gradient distributions are evaluated from the CT data of the 3-D density distributions, and are depicted in gray-scale CT images of the gradient magnitude and in pseudo-color CT images of the gradient component. The resultant pseudo-schlieren CT images clearly illustrate the 3-D flow features of shock waves, contact surfaces, and the other sharp density fronts. Their image characteristics and meaning in gas dynamics are discussed in comparison with the pseudo-color images of the density. We demonstrate that the pseudo-schlieren CT technique is a useful tool for studying 3-D problems in shock dynamics.

Using the SKAGEX dataset for evaluation of ocean model skills

Numerical ocean models are now being applied in numerous oceanographic studies. However, the qualities of the model results are often uncertain and there is a great need for standards and procedures for evaluation of the skills of numerical general circulation models. In this paper measurements from repeated hydrographical sections across Skagerrak taken in 1990, the SKAGEX dataset, are used to evaluate the skills of two σ-coordinate ocean models and to study the sensitivity of these models to model parameters. A methodology for quantification of model skills based on observations from repeated hydrographical sections in general is suggested. Area averages of absolute differences are for Skagerrak completely dominated by the discrepancies in the upper few meters of the ocean and may not be used to assess models' abilities to reproduce the fields in the larger and deeper part of the ocean. Therefore, discrepancies between average values in time from the observed fields and time averaged values from model outputs are related to the natural variability of the fields. The numbers produced with the suggested measure are relative numbers that will be specific for each section and for each series of observation. Ideally we would therefore like to see the measures computed for a number of sections for various models and choices of model parameters in order to assess model skills. The value of the SKAGEX dataset as a tool for model improvements is demonstrated. Evidence to support the importance of applying non-oscillatory, gradient preserving advection schemes in areas with sharp density fronts is given. The method is used to identify that the forcing/initial values/boundary values for the temperature field are inferior to the corresponding values for the salinity field. With the present coarse resolution, 11 layers in the vertical, it is shown that it is far from obvious that the quality of the model results improve when replacing simple Richardson number formulations for vertical mixing processes with higher order turbulence closure in the Skagerrak area.

On horizontal and vertical transport of Ford water

Abstract Three transects of shelf-origin water (Ford water) at the slope water — Gulf Stream boundary between 69 °30′W and 72 °10′W (August 1–5, 1992) were run with a towed scanning profiler equipped with conductivity-temperature-depth probe. The sections from the surface to 200 m depth with horizontal resolution of about 1 km demonstrate simultaneous propagation of the surface and subsurface Ford water connected by a thin and narrow (several km wide) transition area. The thermohaline structure and sharp density fronts at the former water boundaries are maintained downstream. Transport is estimated at 460 ∗ 10 3 m 3 / s (with 33% of shelf water, whose salinity is taken to be 32%.) that is 4.6 times greater than a previous estimate. It is suggested that evidence is found of intermittent vertical transport of freshened water from the surface band to the subsurface one. The subsurface band separates in two features in the transition area from predominent thermoclinicity of the Gulf Stream front to its noticeable baroclinicity owing to the increasing slope of the isopycnal ≅ 26σ t , the lower feature being subducted along the property front to a local depth of the isopycnal bounding the feature from below. Thermohaline intrusion of the freshened water in the Gulf Stream thermocline in the same transition area provides cross-frontal transport of the water, a trace of which is found in the Gulf Stream water at a depth of ≅ 190 m. The association of formation of the intrusion, whose axis is inclined to isopycnal surfaces, with thermohaline parameters of the front and ageostrophic circulation at the site of its cyclonic curvature due to a spin-off eddy is discussed.

Offshore transport of dense shelf water in the presence of a submarine canyon

The formation and offshore transport of dense water over a uniformly sloping shelf crosscut by a submarine canyon is examined using a three‐dimensional primitive‐equation numerical model. A constant negative buoyancy flux is applied in a limited region adjacent to a straight coast to represent brine rejection from ice production in an idealized coastal polynya. A sharp density front forms at the edge of the forcing region, with surface and bottom intensified jets along the front. The flow around the head of the submarine canyon triggers a frontal instability that initially grows only on one side of the canyon. The unstable waves on the other side of the canyon are blocked by a localized barotropic flow that develops near the canyon head. Unstable waves also grow where the forcing region intersects the coast. The frontal waves grow rapidly (with O(1 day) e‐folding timescales) and form eddies with horizontal scales of O(15 km) which extract the densest water from the forcing region and carry it offshore, directly across isobaths. In this way the eddies limit the maximum water density that appears in the model despite continued negative buoyancy forcing. Some dense water descends into the canyon, forming a bottom‐trapped plume that transports the dense water offshore ahead of the eddies. The plume moves relatively slowly (i.e., small Froude number), with little turbulent entrainment, so the advancement and structure of the plume nose can be described successfully as a simple gravity current with an advective‐diffusive heat balance. Eddies may slump into the canyon from the side, altering both the density anomaly and speed of the canyon plume, suggesting that canyon plumes are likely to be highly variable in both space and time.

Mesoscale eddies, jets, and fronts off Point Arena, California, July 1986

A broad (about 40 km wide) cold filament observed off Point Arena in advanced very high resolution radiometer images at the end of June 1986 evolved into a narrow (10–20 km wide) cold filament embedded in a large‐scale (about 200 km alongshore) cool anomaly by mid July. The cool anomaly was part of a much larger‐scale anomaly, stretching from north of Cape Mendocino to south of San Francisco, in which several cold filaments were embedded. The filament off Point Arena, surveyed by ship from July 7 to 19, 1986, extended offshore around the edge of a cyclonic center. The horizontal temperature gradient was sharpest (up to 1.5°C/2.2 km) across the southern edge of the filament, and a salinity front was located within the cold filament; temperature and salinity fronts were not coincident. The surface salinity was high (33.2 psu (practical salinity unit)) in the cyclonic center and decreased across the cold filament to about 32.6 psu. The entire cool anomaly was denser than surrounding water, and there was a sharp density front (1 sigma‐t unit/10 km) at its northern edge. At the inner (cyclonic) edge of the filament the temperature and salinity variations were density compensating. The cold filament was advected offshore by a jet which was symmetric about its axis, located about 10 km north of the cold filament core. The surface manifestation of the cold filament was highly variable in space and time: the location of the filament core changed by about 5 km in 14 hours along one transect; the velocity field was less variable. Several narrow (10 km wide) water mass anomalies were evident, including a salinity subduction zone in the northern segment of the cold filament. Thermohaline intrusions 50–100 m thick may have been caused by complex frontal interactions upstream of the anomaly and offshore in a region of jet confluence. From a second survey (July 27 to August 5, 1986) the offshore jet had reoriented to alongshore, possibly related to an observed increase in the poleward flow over the continental slope. Prior to the second survey there were changes in the nearshore wind stress and the large‐scale wind stress curl.

Gravity-driven flows in a turbulent fluid

The formation and destruction of a gravity current in a turbulent fluid is examined in laboratory experiments. The gravity current is produced by lock exchange and the fluid is kept turbulent by bubbling air from the base of the tank. When the lock is released the buoyancy forces associated with the reduced gravity g′ between the fluid on the two sides of the lock drives a counterflow, with the dense fluid slumping underneath the less-dense fluid, and a gravity current is formed. The current has a sharp density front at its leading edge, and a stable density stratification is established behind the front. The turbulence, characterized by a longitudinal turbulent diffusion coefficient K, tends to mix this stable stratification. Once the fluid is vertically mixed the gravity current front is destroyed, and the density varies smoothly with horizontal distance over a zone whose length increases with time owing to the continuing longitudinal turbulent diffusion and buoyancy driving. It is found that the gravity current propagates over a distance L1 before it is destroyed, where L1/H ≈ 0.08(g′H)½ H/K, and H is the fluid depth. At this point turbulent dissipation balances the buoyancy driving and frontogenesis is inhibited. The turbulent dispersion coefficient is found to increase with the buoyancy driving with K∞ Ri½, where Ri = g′H/q2 and q is the r.m.s. turbulence velocity fluctuations. It is also shown that when the turbulence level is reduced nonlinearities in the horizontal density gradient can sharpen up to form a front. The implications of these frontogenetical processes to the sea-breeze front and fronts in shallow seas is discussed.

Depth-Dependent Studies of Tidally Induced Residual Currents on the Sides of Georges Bank

Using a depth-dependent tidal model, the tidally induced residual currents on the northern and southern sections of Georges Bank are computed and the effects of various physical parameters on the current are examined. Because of significant on-bank Stokes' drift, the rigid-lid approximation is not appropriate for the shallow region of the southern section. Also, in the same shallow region, the curvilinear structure of isobaths significantly increases the one-/off-bank residual flow in the lower/upper portion of the water column, and thus enhances the upwelling near the shallow end of the section. Although the amplitudes of the residual current and the center of gyres in the cross-bank circulation vary significantly with the values and forms of the vertical eddy viscosity and the stratification in the water column, the overall residual circulation is generally insensitive to these physical parameters. The only exception is that, for a sharp density front or a large horizontal variation in the vertical eddy viscosity, an additional tidally forced cross-bank Lagrangian gyre can be induced in the middle portion of the northern section. In comparison to the wintertime tidally induced residual currents, the summertime cross-isobath residual gyres are generally stronger and extend over larger areas, especially toward the shallow portion of the bank. In stratified cases, the cross-bank Eulerian residual current near the frontal region of the northern section can be very significant (up to ≈ 7 cm s−1). The computed along-bank residual currents are generally consistent with the observations. However, the strong on-bank residual current (≈0.7 cm s−1) at 75 m of Station A on the southern section, which is estimated from the observed low-frequency modulation of residual current, cannot be reproduced by the model.

Ageostrophic instability of ocean currents

We investigate the stability of gravity currents, in a rotating system, that are infinitely long and uniform in the direction of flow and for which the current depth vanishes on both sides of the flow. Thus, owing to the role of the Earth's rotation in restraining horizontal motions, the currents are bounded on both sides by free streamlines, or sharp density fronts. A model is used in which only one layer of fluid is dynamically important, with a second layer being infinitely deep and passive. The analysis includes the influence of vanishing layer depth and large inertial effects near the edges of the current, and shows that such currents are always unstable to linearized perturbations (except possibly in very special cases), even when there is no extremum (or gradient) in the potential vorticity profile. Hence the established Rayleigh condition for instability in quasi-geostrophic models, where inertial effects are assumed to be vanishingly small relative to Coriolis effects, does not apply. The instability does not depend upon the vorticity profile but instead relies upon a coupling of the two free streamlines. The waves permit the release of both kinetic and potential energy from the mean flow. They can have rapid growth rates, the e-folding time for waves on a current with zero potential vorticity, for example, being close to one-half of a rotation period. Though they are not discussed here, there are other unstable solutions to this same model when the potential vorticity varies monotonically across the stream, verifying that flows involving a sharp density front are much more likely to be unstable than flows with a small ratio of inertial to Coriolis forces.Experiments with a current of buoyant fluid at the free surface of a lower layer are described, and the observations are compared with the computed mode of maximum growth rate for a flow with a uniform potential vorticity. The current is observed to be always unstable, but, contrary to the predicted behaviour of the one-layer coupled mode, the dominant length scale of growing disturbances is independent of current width. On the other hand, the structure of the observed disturbances does vary: when the current is sufficiently narrow compared with the Rossby deformation radius (and the lower layer is deep) disturbances have the structure predicted by our one-layer model. The flow then breaks up into a chain of anticyclonic eddies. When the current is wide, unstable waves appear to grow independently on each edge of the current and, at large amplitude, form both anticyclonic and cyclonic eddies in the two-layer fluid. This behaviour is attributed to another unstable mode.

Splitting of the subtropical gyre in the western North Pacific

Examined here is a hypothetical idea of the splitting of the subtropical gyre in the western North Pacific on the basis of two independent sources of data,i.e., the long-term mean geopotential-anomaly data compiled by the Japanese Oceanographic Data Center and the synoptic hydrographic (STD) data taken by the Hakuho Maru in the source region of the Kuroshio and the Subtropical Countercurrent in the period February and March 1974. Both of the synoptic and the long-term mean dynamic-topographic maps reveal three major ridges, which indicate that the western subtropical gyre is split into three subgyres. Each subgyre is made up of the pair of currents, the Kuroshio and the Kuroshio Countercurrent, the Subtropical Countercurrent and a westward flow lying just south of the Countercurrent (18°N–21°N), and the northern part of the North Fquatorial Current and an eastward flow at around 18°N. The subgyres are more or less composed of a train of anticyclonic eddies with meridional scales of between 300 and 600 km, so that the volume transport of the subgyres varies by a factor of two or more from section to section. The upper-water characteristics also support the splitting of the subtropical gyre; the water characteristics are fairly uniform within each subgyre, but markedly different between them. The northern rim of each subgyre appears as a sharp density front accompanied by an eastward flow. The bifurcations of the sharp density fronts across the western boundary current indicate that the major part of the surface waters in the North Equatorial Countercurrent is not brought into the Kuroshio. The western boundary current appears as a continuous feature of high speed, but the waters transported change discontinuously at some places.