摘要:

Previous laboratory experiments have shown that salt fingers are easily disrupted by turbulence. However, in the interior of the ocean, turbulence is both patchy and intermittent, so that there may be sufficient time for the fingers to form between turbulent events. The growth of salt fingers after disruption by turbulence was examined in an experiment in which an oceanic turbulent event was simulated by a grid falling through fingers set up in a laboratory tank. Both horizontal conductivity and temperature profiles showed that after the grid turbulence had decayed the fingers grew rapidly, and that the time scale for growth was of the same order as thee‐folding time for the fastest‐growing long salt finger modes that have been described previously by several authors. This time scale is consistent with the observation that the peak in the temperature gradient spectrum was close to the predicted wave number for fastest‐growing fingers. On the other hand, visualization of the fingers suggests that the period of exponential growth of the finger amplitudes was short, of the order of onee‐folding period. Finally, the results of these experiments suggest that with the typical intermittency of turbulence in the interior of the ocean, salt fingers should have sufficient time to grow between

ISSN:0148-0227    

DOI:10.1029/90JC02313

年代:1991

数据来源: WILEY

摘要:

Laboratory experiments have been conducted to examine the effects of rotation on the structure and stability of hydrothermal plumes. It is shown that the plume forces a baroclinic vortex, consisting of an anticyclonic eddy at the neutral buoyancy level and cyclonic circulation in the ambient fluid around the rising plume. These features agree qualitatively with Speer's (1989) model. However, the experiments show that this state is unstable, leading to the breakup of the plume and the unsteady generation of baroclinic vortices (hetons) that propagate away from the source. Two nearby plumes interact repulsively for separations ≳NZs/ƒ, and initially coalesce for smaller separations. HereNis the buoyancy frequency, ƒ is the Coriolis frequency, andZsis the height of the spreading level. It is suggested that oceanic hydrothermal plumes may shed isolated baroclinic vortices, which are capable of retaining anomalous properties (temperature, salinity,3He, etc.) over large distan

ISSN:0148-0227    

DOI:10.1029/90JC02643

年代:1991

数据来源: WILEY

摘要:

A series of laboratory experiments in a rotating differentially heated annulus have been conducted to determine the effect of azimuthal thermal forcing at the lower boundary, of the kind representative of continent/ocean thermal contrasts, on the forcing of standing waves and the modulation of free baroclinic waves. An azimuthal wave number 2 thermal forcing at the lower boundary induces a wave number 2 in the flow field, which interacts with transient baroclinie waves. The induced wave number 2 in the flow field is a climatologically standing wave, with its phase dependent on the intensity of forcing. An analysis of the velocity data reveals that the trough of the time‐averaged stream field is located at the “continent/ocean” boundary, on the downstream side of the “continent,” and that the variance of the transient waves peaks approximately in the middle of the

ISSN:0148-0227    

DOI:10.1029/91JC00494

年代:1991

数据来源: WILEY

摘要:

We report on measurements of low grazing angle radar sea spikes in a laboratory wave tank. Using anXband radar with 10‐cm range resolution and multiple‐range bin capability, the sea spikes were observed to have a horizontal/vertical polarization ratio much larger than unity. This is similar to observations in earlier field experiments, but is in contrast to a ratio of unity recently observed for moderate grazing angles (Jessup et al., 1990). Our experiment was designed to validate the low grazing field results and to establish design criteria for spatial‐temporal measurement instrumentation; single‐point measurements currently used as surface truth cannot provide water surface profiles needed for modeling of radar scatter from deterministic surface features. Surface scattering features were generated using a paddle‐generated wave, combined with wind waves for a series of wind speeds. The surface feature responsible for the sea spike echo was visually identified as a short wave crest, occurring on the windward side of the paddle wave, driven to extreme steepness by the passage of the faster paddle wave crest. Long decorrelation times observed in the field are also seen in these studies, of the order of 500 ms for horizontal polarization and less than 5 ms for vertical. We suggest that rather than Bragg scatter, the occurrence of similar ubiquitous features on the ocean are responsible for horizontally polarized low grazing angle sea scatter at theXband for low to moderate winds, and perhaps at other frequencie

ISSN:0148-0227    

DOI:10.1029/91JC00705

年代:1991

数据来源: WILEY

摘要:

The problem of a gravity current along a coast is considered in the laboratory. The flow originates from a continuous pointsource and, when facing downstream, has the shore to its right. The shore is the vertical wall of a 13‐m‐diameter rotating basin. The density of the current, r1, is slightly less than that of the surrounding fluid. This buoyant flow is shown to depend upon three parameters; namely, the Burger and Ekman numbers and the aspect ratio of the current at its source. Disregarding friction and taking the flow as hydrostatic so that the current aspect ratio does not need to be considered, the stability characteristics of the current are shown to depend mostly upon the Burger number. To study its role in the flow, we performed several experiments by varying the Burger numberBubetween 0.15 and 0.82. For large Burger numbers the current is stable, but as it decreases, increasing numbers of anticyclonic disturbances develop and grow along the axis of the current. Measurements of the normalized distance from the source, and times for these disturbances to develop, are given as functions of the Burger number. The dependence of the normalized separation distance of disturbances upon the Burger number, in cases of multiple instabilities, is also discussed. Finally, these laboratory results are compared with the Algerian Current, where both anticyclonic and cyclonic eddies have been observed. The experimental current simulates the Algerian Current when the Burger number is 0

ISSN:0148-0227    

DOI:10.1029/91JC00998

年代:1991

数据来源: WILEY

摘要:

Turbulent eddy motions are produced, and characterized, by inertial‐vortex forces. Growth of the energy, or Obukhov, scaleL0is constrained by buoyancy and Coriolis forces at the stratified fossil OzmidovLR0and rotational fossil HopfingerLΩ0length scales, and constrained at all scales by viscous, buoyancy, and Coriolis forces at sizes proportional to the stratified fossil KolmogorovLKFand rotational fossil KolmogorovLKƒlength scales, respectively. Universal constants of proportionality between turbulence wavelengths and these fossil turbulence length scales at their respective buoyant‐inertial, Coriolis‐inertial, buoyant‐inertial‐viscous, and Coriolis‐inertial‐viscous transitions are inferred from available laboratory, numerical, and field studies. It is important to distinguish between turbulence, which entrains, and these nonturbulent, or fossil turbulence, internal waves, Coriolis‐inertial eddies, and viscous flow regimes, which are nonlinear remnants of turbulence but do not entrain and have other properties different from turbulence. Hydrodynamic phase diagrams based on the universal constants of transition may be used to classify microstructure and mesostructure as actively turbulent, active‐fossil, fossil, or nonturbulent. One advantage of making these distinctions is to avoid undersampling errors from data sets consisting only of fossil turbulence microstructure and mesostructure. This may improve the reliability of dissipation methods used for estimating average heat, mass, and momentum fluxes in the ocean and atmosphere from microstructure and

ISSN:0148-0227    

DOI:10.1029/91JC00186

年代:1991

数据来源: WILEY

摘要:

Results are presented from an experimental investigation into the generation and growth of a turbulent mixed layer above and below an oscillating horizontal grid in a rotating stratified fluid. The (square) grid was located at middepth at one end of a long rectangular container, and was confined on three of its four sides by the vertical walls of the container. Flow visualization data are presented to show (1) the rapid initial vertical growth of the mixed layer above and below the grid, (2) the collapse of the mixed layer and the generation of an intrusive flow into the initially undisturbed interior of the fluid, (3) the deflection of the intrusive flow toward the right side wall, and (4) the formation of a boundary current, which circulates around the container before returning mixed fluid to the source region at the grid. Examples of stable and unstable boundary currents are illustrated. Quantitative data are presented to show that the thickness of the mixed layer increases with either decreasing buoyancy frequency, and/or increasing grid frequency, throughout all phases of its development. The influence of background rotation upon the vertical growth of the mixed layer is shown to be negligible during the rapid initial vertical expansion of the layer, though this is not the case in the longer‐term behavior. For this latter phase, the layer thickness increases with increasing rotation rate of the system, under otherwise identical conditions. Measurements of the initial equilibrium thicknesshcof the layer show that it scales with the parameter (K/N)½, whereKis the grid action, andNis the buoyancy frequency of the stratified fluid. Values of the dimensionless parameter [hc/(K/N)½] show no significant dependence upon the ratioN/2Ω of the buoyancy and inertial frequencies, for a range of values of the latter between 0.4 and 29.7. This behavior is shown to be compatible with the result (Hopfinger, 1987) that shear‐free turbulence is dominated by background rotation only if the Rossby number based on the rms velocity and integrated length scale is less th

ISSN:0148-0227    

DOI:10.1029/91JC00188

年代:1991

数据来源: WILEY

摘要:

The effects of boundary mixing on a continuous gravity current (a plume) propagating along an incline were investigated using a laboratory experiment. Boundary mixing was generated by oscillating the incline along its plane. It was observed that when the background turbulence is strong, such typical characteristics of the gravity current as the “raised‐head” structure and the sharp front disappear. Quantitative measurements on the frontal velocity, density structure, and the thickness of the gravity current were made, and their dependencies on the intensity of turbulence at the boundary are pres

ISSN:0148-0227    

DOI:10.1029/90JC02488

年代:1991

数据来源: WILEY

摘要:

In addition to its relationship to evaporative cooling and the atmosphere's radiative balance, anomalies of freshwater flux at the ocean surface have a potential impact on the stratification of the tropical mixed layer. Under light wind conditions when the thermocline is deep, excess rainfall can cause a low‐salinity layer to form, inhibiting vertical convection and mixing. It has previously been hypothesized that under conditions where the heat entering the mixed layer is balanced by cooling at the base of the mixed layer, such as occurs in upwelling regions, this effect could raise sea surface temperatures (Miller, 1976). Conversely, excess evaporation in such a region could destabilize the mixed layer, enhancing convection and lowering sea surface temperature. Here we report on a numerical experiment to test these ideas. We employ a model of the seasonal tropical Atlantic circulation which includes full representation of salinity and heat transport. Two simulations are compared. In one the surface boundary conditions include complete hydrological forcing by river discharge, rainfall, and evaporation. In the other simulation the vertical flux of water at the surface is set to zero. We find that seasonal changes in net freshwater flux at the surface do alter sea surface temperature by ≈±0.1°C. This change is too small to significantly alter the exchange of heat across the surface of the ocean. However, the effect is larger in areas strongly affected by river discharge and at the base of the mixed

ISSN:0148-0227    

DOI:10.1029/91JC01256

年代:1991

数据来源: WILEY

摘要:

The annual and interannual variations of the major zonal currents of the central tropical Pacific for the period 1970–1987 are described by analyzing two data sets: sea level data from a few selected island stations and ship‐of‐opportunity expendable bathythermograph (XBT) observations. The annual cycles of geostrophic surface current indices (dynamic height or sea level difference between the ridge and the trough defining the current) in the North Equatorial Current (NEC) and North Equatorial Countercurrent (NECC) are found to be in phase (maxima in November), to be strong, and to explain about 50% of the total variance. The annual cycle of volume transport relative to 400 dbar in the NECC is also strong, but NEC volume transport has a weak annual cycle. The discrepancy between the NEC surface current and the transport is due to the variation of the surface current being larger than that of the deep currents. The annual cycle of the transport of the South Equatorial Current (SEC) is 6 months out of phase with that of the NEC and NECC. Phases of the annual cycles of the sea level and dynamic height indices of flow agree well in general, but there are significant differences between the annual amplitudes of the two types of measurements. These differences arise from a variety of sources: dynamic height reference level assumptions, longitudinal spacing of sea level gauges, meridional resolution of sea level array, and different signal to noise ratios for the two sets of indices. The NECC shows marked flow maxima during the peaks of the strong Niños of 1972 and 1982, which were followed by near disappearance of the current the following year; these fluctuations are clearly present in both the surface current indices and the volume transport. In 1976 and 1986–1987 (moderate Niños) the same pattern is not marked. The El Niño signal in NEC transport is weak. There was a strong rise of equatorial sea level during the early phases of the 1972 and 1982 Niños followed by a fall the next year. This resulted in a decrease of the transport of the SEC in the first year followed by an increase to a maximum during the second year; the maximum coincides with a minimum in NEC

ISSN:0148-0227    

DOI:10.1029/91JC00781

年代:1991

数据来源: WILEY