摘要:

Simultaneous observations of the Gulf Stream surface and subsurface (15°C at 200 m) fronts have shown their relative positions to be quite variable. Cross‐frontal Ekman advection of the surface front by the local wind stress is a possible major source of this variability. We investigate this possibility and show that on the average and for a particular set of observations, only a small fraction of the observed variability in the Gulf Stream surface‐subsurface frontal separation is due to advection of the surface front by the local wind stress. As an alternative, it is suggested that most of the observed variability in frontal separation may be due to unstable meandering of the Gulf Stream surface f

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03519

年代:1988

数据来源: WILEY

摘要:

A simple numerical model is developed that successfully simulates observations of small‐scale, laboratory, density currents flowing down inclines of constant slope. The model results suggest that laboratory determinations of the bulk Richardson number have been biased by molecular processes but that determinations of the entrainment coefficient are probably applicable to large‐scale currents, and even to turbidity currents in which the gravitational driving force is provided by suspended sediment. The entrainment coefficient is a function of the gradient Richardson numberRiabove the velocity maximum in both density and turbidity currents;Riis itself a strong function of the bottom slope but not of the sediment settling velocity or the roughness of the bottom boundary. Therefore for a given bottom slope the entrainment coefficient is uniquely defined. The bulk Richardson numberRi0, on the other hand, is additionally a function of the drag coefficient, which depends on both the Reynolds number and the bottom roughness. However,Ri0is not a strong function of the sediment settling velocity as long as the current is energetic enough to maintain sediment in suspens

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03528

年代:1988

数据来源: WILEY

摘要:

A simple numerical model predicts that properties of a turbidity current can vary significantly in the vertical direction, particularly when more than one sediment size is in suspension. Each sediment size decreases in concentration with increasing distance from the bottom boundary at a rate which depends on its settling velocity, the current velocity, the entrainment coefficient, and the bottom slope. However, it is shown that the details of the vertical distribution of sediment load do not necessarily greatly influence either the vertical structure of the hydraulic properties of the flow (velocity, stress) or the bulk properties such as the entrainment coefficient, at least when only one sediment size is in suspension. The ability of a turbidity current to sustain itself is strongly dependent on the sediment load and an autosuspension criterion is obtained by taking into account the vertical structure of the currents. The model turbidity currents often cannot maintain their sediment in suspension even when the bottom stress is greater than the critical stress of deposition for low concentration flows. When this happens, the concentration near the bottom increases without limit as the turbidity current evolves with time. In real turbidity currents, grain‐grain interactions, which become important at high concentrations, may at times cause the current to quickly deposit its near‐bottom sediment load. This is suggested as a possible mechanism for the formation of the poorly sorted sediments frequently found in turbidi

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03543

年代:1988

数据来源: WILEY

摘要:

A particle‐in‐cell method, which has been successfully applied to problems in plasma physics, is adapted to the problem of oceanic geostrophic fronts. The thrust of the present article is the development, exposition, and testing of the method by means of synthetic examples. The advantage of this approach is its direct applicability to a class of oceanic problems which, for the most part, have remained unsolved. The method is computationally straightforward and efficient. It is also robust (the overall solution does not depend on the details of the initial distribution of material points) and numerically accurate (invariants are well conserve

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03554

年代:1988

数据来源: WILEY

摘要:

A nearly homogeneous water mass, the Norwegian Sea Deep Water, is found below 2000‐m depth in the Norwegian and Lofoten basins of the Norwegian Sea. Recent observations indicate that this water is a mixture of relatively cold and fresh Greenland Sea Deep Water with warmer, saltier Eurasian Basin Deep Water from the Arctic Ocean. We have found this mixture along the western and southern periphery of the Greenland Sea, near the level where the pressure‐compensated densities of the parent water masses are equal. The along‐isopycnal mixing produces a remarkably uniform water mass, which can be traced only a short distance away from its entry into the Norwegian Sea through gaps in the mid‐ocean ridge north of Jan Mayen Island. Direct measurements of flow through these gaps confirm motion in the proper sense to accomplish this con

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03563

年代:1988

数据来源: WILEY

摘要:

A perturbation analysis of the Lagrangian equations of motion is used to examine the diffusion induced by a random field of internal waves. At second order there is a random field of shearing motion in the horizontal plane. We calculate the single‐particle, two‐particle, and patch diffusivities that result from this motion. Patch and two‐particle diffusivity both increase as scale raised to the power of 4/3 for an internal wave field in which the spectrum of horizontal velocity varies a

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03570

年代:1988

数据来源: WILEY

摘要:

The numbers and apparent sizes of icebergs in the South Atlantic Ocean in midwinter were measured by radar and visually from F. S.Polarsternduring the 1986 Winter Weddell Sea Project cruise. Results show that in a heavy sea (sea state 7–8), icebergs have to be at least 115 m in diameter to be detected at all and that detectability falls off severely for all bergs at ranges exceeding 8 n. mi. (15 km); that most bergs had diameters of less than 1 km with a preferred size of 400–500 m; and that a high density of icebergs in the latitude band 53°–56°S at longitude 19°–30°W contrasted with a virtual absence of bergs in the same latitude band at longitude 1°–9°E. The latter effect is ascribed to melt and wave–induced deterioration causing the disappearance of this iceberg population between the two s

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03583

年代:1988

数据来源: WILEY

摘要:

When surface gravity waves break, they generate an acoustic signal which can be used to derive information about their properties and distribution. Observations of sound formed in this way have been obtained at 1, 10, and 40 m below the surface in the fetch limited environment of Georgia Strait, British Columbia. Interpretation of the results is aided by a model of randomly distributed sound sources on the ocean surface. It is shown that the signal variability is governed by the number density, or mean spacing of the breakers, by the tendency for breakers to repeat at half the wave frequency, and by the group structure of the wave field. Calculated wave field properties reveal several features which can be related to changes in the wind, including variations in wave period, mean breaker spacing, and the average acoustic strength of individual breakers. Bubble clouds caused by the breaking waves can also be detected, and there is evidence that they contribute to the trapping of sound in a shallow layer close to the surface.

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03591

年代:1988

数据来源: WILEY

摘要:

The energy transfer equation, which describes the evolution of the surface wave spectrum, is solved numerically for different water depths, with an empirical expression for the bottom dissipation and explicit calculations of the resonant four‐wave interactions. The resulting growth curves for total energy, peak frequency, etc., are explained from the balance between the source terms at the various stages of growth. The equilibrium which is reached between bottom friction and the resonant interactions on the forward face of the spectrum turns out to be important. The calculated results are briefly compared with recent shallow water wave experiments (TMA, Lake Marken) and with the Coastal Engineering Research Centre growth curves. Finally the results are compared with the SWIM stud

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03601

年代:1988

数据来源: WILEY

摘要:

The steady motion of an iceberg under wind action is studied in the presence of a mixed layer. Analytical solutions are obtained when the iceberg depth is less than or much larger than the mixed layer depth. A few numerical examples are given. Our results show that winds of medium or low strength have a limited effect on the motion of deep icebergs extending below the thermocline. Furthermore, the larger the horizontal scale of the iceberg the smaller the wind drift. Hence for winds of less than 10 m s−1the trajectory of deep icebergs more than 1 km long is shown to be representative of geostrophic currents. The effect of a possible “lift” force is also esti

ISSN:0148-0227    

DOI:10.1029/JC093iC04p03608

年代:1988

数据来源: WILEY