摘要:

Circulation near a submarine canyon is analyzed with a numerical model. Previous theoretical work indicated that stratification controlled the interaction of coastal flow with canyons, specifically, the ratio of canyon width to the internal radius of deformation. A wide canyon was thought to merely steer the flow, while a narrow canyon would create substantial cross‐shelf exchange. Four cases are analyzed considering two directions of alongshore flow and two choices of initial stratification. The weakly stratified case has an internal radius about equal to the canyon width, while the strongly stratified case has one about 3 times the canyon width. The direction of the alongshore flow is shown in this study to be the more important of the two factors. In particular, right‐bounded flow (flow with the coast on the right, looking downstream in the northern hemisphere) leads to shallow downwelling in the canyon and weak exchange across the shelf break, while left‐bounded flow creates upwelling at the head of the canyon and strong exchange between the ocean and shelf. In left‐bounded flow (upwelling), dense water is pumped onto the shelf, even for strong stratification. However, the stratification limits the vertical extent of the topographic influence so that the alongshore flow above the canyon is only weakly affected in the strongly stratified case. With any level of stratification, the surface temperature (density) is not modified at all by the flow interaction with the submarine canyon. The important dynamics involve pressure gradients and Coriolis acceleration and how they interact with the bathymetric gradients but not advection of momentum. Advection of density is clearly important in the upwelling cases. Finally, continued upwelling onto the shelf acts as a drag mechanism and retards the alongshore coast

ISSN:0148-0227    

DOI:10.1029/95JC02901

年代:1996

数据来源: WILEY

摘要:

Geochemical Ocean Sections Study (GEOSECS) Indian Ocean Expedition data show that silica and3He in the Red Sea Deep Water are linearly correlated, suggesting a common hydrothermal source. Silica sections for the Red Sea from the GEOSECS and Mer Rouge (MEROU) expeditions show a strong vertical silica maximum centered at 500 to 600 m depth. The MEROU sections, which have higher horizontal resolution than the GEOSECS sections, suggest that the primary silica source lies in the south at that depth. The interpretation is that the large‐scale silica and3He distributions are due to the dispersal in the Red Sea Deep Water of warm or hot hydrothermal vent waters associated with the seafloor spreading center of the southern Red Sea. The helium‐ and silica‐rich hot brines of the central Red Sea probably cannot provide the large fluxes of silica or3He required to maintain the inventory of either of these deep water properties in steady state. The hypothesis of a common hydrothermal source for silica and3He can most readily be tested by measurements of the germanium‐silicon ratio in Red Sea Dee

ISSN:0148-0227    

DOI:10.1029/95JC02695

年代:1996

数据来源: WILEY

摘要:

The simplest member of a family of ocean models with inhomogeneous layers is used to study the physics of an active mixed layer overlying a quiescent denser layer. The dynamical variables of the system are the depth‐averaged horizontal velocityuand buoyancy ϑ fields and the layer thicknessh. The potential vorticityqis not conserved in these models, unless ϑ is constant. An integral of motion quadratic in the deviation from a steady and symmetric basic state is derived and shown not to be positive definite; that is, there is no formal stability theorem (except in the case of uniform flow). This is due to a term proportional to the square of the distance between the isolines ofqand ν, which is a measure of the nonconservation ofq. A reference state without mean currents must have ΘH2= constant, where Θ andHare the buoyancy and layer thickness, respectively, in that state. Linear waves superimposed on this basic state are Poincaré and Rossby waves, analogous to those of a homogeneous layer problem but with a different topography. In addition, there is a force‐compensating mode (FCM), which corresponds to changes in buoyancy and layer thickness that represent a vanishing contribution to the depth‐averaged pressure force. The phase speed of gravity waves (in the absence of rotation effects) is . Rossby waves are driven by the gradient of ƒ/c2(where ƒ is the Coriolis parameter) rather than by the gradient of ƒ/H, as they are in the homogeneous layer case. The quadratic integrals of motion are used to classify the types of instability. First, this is done in an a priori sense, looking for properties of an unstable basic flow that constrain the structure of growing perturbations. Second, this is done in an a posteriori sense, through the evaluation of the rate change of selected parts of those integrals of motion, as a function of the perturbation fields. It is stressed that for this method to give unambiguous results it is necessary that the integrals of motion be quadratic, to lowest order, in the perturbation. Unlike the case of the classical shallow water equations, a uniform flow in a channel may be unstable, if the buoyancy increases toward its right (left) in the northern (southern) hemisphere. In the ƒ plane this necessary instability condition corresponds to mean flow in the direction opposite to that of the propagation of Rossby waves. A growing perturbation is very well represented by the combination of a Rossby wave and an FCM (from the system without currents), which interact due to the presence of t

ISSN:0148-0227    

DOI:10.1029/95JC02899

年代:1996

数据来源: WILEY

摘要:

When small‐amplitude surface gravity waves progress in deep water, all the fluid particles are observed to orbit in circles within the depth of wave influence. Each fluid particle therefore has angular momentum with respect to the center of its orbit, and the angular momentum vectors are directed parallel to the crests and troughs or perpendicular to the wave number. The angular momentum per unit volume is calculated for each fluid particle, and then, by vertical integration, the angular momentum per unit horizontal area is computed. The total energy and the magnitude of the angular momentum, both per unit area or both per unit volume, are found to be proportional, the factor of proportionality being the wave frequency; specifically, angular momentum magnitude equals energy divided by frequency. Wave action, which has become an increasingly popular quantity for interpreting surface gravity wave problems and is defined as wave energy divided by frequency, is the same thing as the magnitude of the angular momentum. A general equation for the conservation of angular momentum along wave rays is given, and it contains on the right‐hand side two torque terms; one changes the magnitude and the other changes the direction of the angular momentum. In applying this equation to the wave‐current refraction problem, there is only one torque, which changes the direction of the angular momentum, and it is explicitly determined as a function of the horizontal shear in the current. The magnitude of the angular momentum, and therefore also the wave action, will be conserved along the rays if there are no torques that could alter it, as in pure wave‐current refraction. Our conservation equation for angular momentum is easily adapted to describing wave generation and dissipation by including the appropriate torques on the right side, and this may prove to be helpful for calculations of wave ev

ISSN:0148-0227    

DOI:10.1029/95JC02923

年代:1996

数据来源: WILEY

摘要:

The nonlinear transformation of wave spectra in shallow water is considered, in particular the role of wave breaking and the energy transfer among spectral components due to triad interactions. Energy dissipation due to wave breaking is formulated in a spectral form, both for energy‐density models and complex‐amplitude models. The spectral breaking function distributes the total rate of random‐wave energy dissipation in proportion to the local spectral level, based on experimental results obtained for single‐peaked spectra that breaking does not appear to alter the spectral shape significantly. The spectral breaking term is incorporated in a set of coupled evolution equations for complex Fourier amplitudes, based on ideal‐fluid Boussinesq equations for wave motion. The model is used to predict the surface elevations from given complex Fourier amplitudes obtained from measured time records in laboratory experiments at the upwave boundary. The model is also used, together with the assumption of random, independent initial phases, to calculate the evolution of the energy spectrum of random waves. The results show encouraging agreement with observed surface elevations as well a

ISSN:0148-0227    

DOI:10.1029/95JC03219

年代:1996

数据来源: WILEY

摘要:

Recent measurements under wave‐breaking conditions in the ocean, lakes, and tanks reveal a layer immediately below the surface in which dissipation decays as depth to the power −2 to −4 and downwind velocities are approximately linear with depth. This behavior is consistent with predictions of a conventional, one‐dimensional, level 2.5 turbulence closure model, in which the influence of breaking waves is parameterized as a surface source of turbulent kinetic energy. The model provides an analytic solution which describes the near‐surface power law behavior and the deeper transition to the “law of the wall.” The mixing length imposed in the model increases linearly away from a minimum value, the roughness length, at the surface. The surface roughness emerges as an important scaling factor in the wave‐enhanced layer but is the major unknown in the formulation. Measurements in the wave‐affected layer are still rare, but one exceptional set, both in terms of its accuracy and proximity to the surface, is that collected byCheung and Street[1988] in the Stanford wind tunnel. Their velocity profiles first confirm the accuracy of the model, and, second, allow estimation, via a best fit procedure, of roughness lengths at five different wind speeds. Conclusions are tentative but indicate that the roughness length increases with wind speed and appears to take a value of approximately one sixth the dominant surface wavelength. A more traditional wall‐layer model, which ignores the flux of turbulent kinetic energy, will also accurately reproduce the measured velocity profiles. In this case, enhanced surface turbulence is forced on the model by the assumption of a large surface roughness, three times that required by the full model. However, the wall‐layer model cannot predict the enhanced dissipa

ISSN:0148-0227    

DOI:10.1029/95JC03220

年代:1996

数据来源: WILEY

摘要:

This paper describes an interactive Eulerian‐Lagrangian model of the turbulent transport of evaporating droplets. Ak‐ε (wherekis turbulent kinetic energy and ε is its rate of dissipation) turbulence closure model is used to accurately simulate stable, near‐neutral, and unstable boundary layers within the large air‐sea interaction tunnel at the Institut de Mécanique Statistique de la Turbulence (IMST), Luminy, France. These results are then used with the Lagrangian model described in part 1 [Edson and Fairall, 1994]. The coupled model is shown to give excellent agreement with droplet dispersion measurements made during the 1988 Couche Limite Unidimensionelle Stationnaire d'Embrums (CLUSE, a French acronym that translates to one‐dimensional stationary droplet boundary layer) campaign. Additionally, this paper describes how the coupled model can now be used to investigate the interaction between the evaporating droplets and the turbulent fields of temperature and humidity. The investigation shows that although the influence of the droplets is small under the conditions simulated at IMST, the potential for substantial modification of the surface energy budget exists for high‐wind conditions

ISSN:0148-0227    

DOI:10.1029/95JC03280

年代:1996

数据来源: WILEY

摘要:

To obtain bulk surface flux estimates approaching the ±10 W m−2accuracy desired for the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (COARE) program, bulk water temperature data from ships and buoys must be corrected for cool‐skin and diurnal warm‐layer effects. In this paper we describe two simple scaling models to estimate these corrections. The cool‐skin model is based on the standardSaunders[1967] treatment, including the effects of solar radiation absorption, modified to include both shear‐driven and convectively driven turbulence through their relative contributions to the near‐surface turbulent kinetic energy dissipation rate. Shear and convective effects are comparable at a wind speed of about 2.5 m s−1. For the R/VMoana WaveCOARE data collected in the tropical western Pacific, the model gives an average cool skin of 0.30 K at night and an average local noon value of 0.18 K. The warm‐layer model is based on a single‐layer scaling version of a model byPrice et al.[1986]. In this model, once solar heating of the ocean exceeds the combined cooling by turbulent scalar heat transfer and net longwave radiation, then the main body of the mixed layer is cut off from its source of turbulence. Thereafter, surface inputs of heat and momentum are confined to a depthDTthat is determined by the subsequent integrals of the heat and momentum. The model assumes linear profiles of temperature‐induced and surface‐stress‐induced current in this “warm layer.” The model is shown to describe the peak afternoon warming and diurnal cycle of the warming quite accurately, on average, with a choice of a critical Richardson number of 0.65. For a clear day with a 10‐m wind speed of 1 m s−1, the peak afternoon warming is about 3.8 K with a warm‐layer depth of 0.7 m, decreasing to about 0.2 K and 19 m at a wind speed of 7 m s−1. For an average over 70 days sampled during COARE, the cool skin increases the average atmospheric heat input to the ocean by about 11 W m−2; the warm layer decreases it by about 4 W m

ISSN:0148-0227    

DOI:10.1029/95JC03190

年代:1996

数据来源: WILEY

摘要:

Deep‐drogued drifters are in use to measure the near‐surface geostrophic currents. An attempt is made to study the slippage of these drifters due to wind and Ekman currents. The results are based on a data set from the unstratified North Sea obtained in winter 1991–1992, where the currents were decomposed into Ekman currents and barotropic currents. The influence of these Ekman currents on the drift performance of drifters drogued below the mixed layer in the barotropic current is determined by using quadratic drag laws. In 90% of all cases (1540 data points) the combined effect of wind drag and Ekman currents on buoy and 100‐m tether produces a slippage of less than 2 cm/s. Drifters drogued within the mixed layer show less slippage due to the reduced drag on the tether, but they are primarily designed to measure the actual near‐surface currents, which are strongly dependent on the wind conditions. It is concluded that deep‐drogued drifters are a reliable device to study weakly baroclinic geostroph

ISSN:0148-0227    

DOI:10.1029/95JC02686

年代:1996

数据来源: WILEY

摘要:

We describe a nuclear detector system for measuring low activities of223Ra and224Ra in natural waters based on an original design of Giffin et al. (1963). Samples are obtained by adsorbing223Ra and224Ra onto a column of MnO2coated fiber (Mn fiber). The short‐lived Rn daughters of223Ra and224Ra which recoil from the Mn fiber are swept into a scintillation detector where alpha decays of Rn and Po occur. Signals from the detector are sent to a delayed coincidence circuit, which discriminates decays of the224Ra daughters,220Rn and216Po, from decays of the223Ra daughters,219Rn and215Po. The system is calibrated using232Th and227Ac standards with daughters in equilibrium adsorbed on Mn fiber. Results of samples from Tampa Bay, Florida, and the Atchafalaya and Mississippi Rivers mixing zones are reported. The method is extendible to measurements of227Ac,231Pa,228Th, and228R

ISSN:0148-0227    

DOI:10.1029/95JC03139

年代:1996

数据来源: WILEY