ISSN:0148-0227    

DOI:10.1029/JC083iC06p02873

年代:1978

数据来源: WILEY

摘要:

Evidence is presented to show that Kelvin‐Helmholtz instability dominates the structure in the mixing region near the sea surface when there is a net flux of heat into the sea and in conditions soon after the onset of win

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02875

年代:1978

数据来源: WILEY

摘要:

It is now accepted that double‐diffusive processes play a part in the mixing of heat and salt in the ocean and that relevant information about them can be obtained from laboratory experiments. There is still a considerable gap, however, between the laboratory work and the conditions under which the phenomena occur in the ocean. Most of the laboratory experiments have been one dimensional, whereas in the ocean, strong double‐diffusive layering is often associated with large horizontal gradients ofTandS, especially where water with one set ofT‐Sproperties intrudes at its own density level into an environment with different properties. The main purpose of the present paper is to report a series of laboratory experiments which are related to the latter case. The sugar‐salt system and shadowgraph photography have been used to explore various situations of increasing complexity. The double‐diffusive effects are compared with the behavior of a simple source of salt solution, which intrudes horizontally as a thin layer into a salinity gradient at its own density level. In strong contrast to this a source of sugar in the same salinity gradient produces vigorous vertical convection near the source, followed by spreading at several levels. The extending fluid moves out at an angle to the horizontal, rising or falling across isopycnals in a manner which can be related to the composition of the environment and source fluids and the double‐diffusive transports across the boundaries of the intrusions. When the ambient fluid is set up with opposing gradients, so that potential energy is stored in one of the components, there is a more rapid spreading both horizontally and vertically, and strong shears are produced. The laboratory results are compared with published oceanic data to suggest explanations of some existing observations and to predict what might be measurable in

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02887

年代:1978

数据来源: WILEY

摘要:

Stepped fine structure of temperature and salinity observed in the Tyrrhenian Sea at a depth of between 600 and 1400 m is an established feature which has been studied over a number of years. At these depths the temperature and the salinity both decrease with depth, so it is tempting to relate this fine structure to salt finger convection. Recent observations with an optical device have shown that the fingers do not exist continuously across the interfaces between the steps but are found only in certain regions inside the interface. This discontinuous finger structure had not been observed previously in the laboratory, and this paper describes some experiments in which the breakup of salt fingers in an interface is documented. The conditions under which the fingers break up are investigated, and some implications of these new observations to the interpretation of oceanic fine structure are discussed.

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02902

年代:1978

数据来源: WILEY

摘要:

Recent theoretical work suggests that salt fingers have sufficiently rapid growth rates in much of the thermocline to produce significant vertical fluxes of heat and mass, even in an active internal wave field. Using laboratory determinations of thepower law for the salt flux we have estimated the salt flux at depth for several CTD stations in the central water of the North Atlantic by assuming the fingers to be intermittently active on the high‐gradient sheets in the thermocline. The flux is found to be comparable to the surface input of salt due to evaporation. This implies a release of energy by the falling out of salt, comparable to the rate of work done by the wind on the mean circulation, and supports the model of Stern (1969b). Salt fingers transport heat and mass at different rates, and thus the usual concept of an ‘eddy diffusivity’ is invalid when salt fingers make a major contribution to the vertical mixing proc

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02913

年代:1978

数据来源: WILEY

摘要:

As part of a field study of the relation between fine scale and large‐scale variations of water properties in the western North Atlantic, the waters in the vicinity of Bermuda were investigated in detail. Previous work in the area had revealed regions of intense temperature fine structure confined to the sides of the island. Generally quieter levels of activity elsewhere in the midocean have suggested that significant mixing might only occur at the solid and fluid boundaries of the ocean. During the course of our investigation, two Gulf Stream rings were found in the vicinity of the island. The exchange of water between them caused three regions of strong alongshore flow. In these three areas we find elevated levels of temperature fine structure in the upper 800 m as measured by the variance in the temperature gradient normalized by the square of the mean temperature gradient over the interval. The normalized temperature variances on small scales (0.2–1 m) are most energetic in patches tightly bound to the island sides, whereas the fine structure on larger scales (5–25 m) is also energetic away from the island in a region of outflow. Velocity profiles show that vertical scales shorten as one approaches the island, and the energy increases in the counterclockwise component. There is no correlation evident between the shear measurements of the internal wave field and the intensity of the fine structure. Possible mechanisms for the production of fine structure are explored within the context of these observa

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02921

年代:1978

数据来源: WILEY

摘要:

Measurements of velocity shear microstructure were taken March 3–7, 1975, adjacent to the island of Santa Maria in the Azores. From the data it is possible to estimate the energy dissipation over 5‐m vertical intervals. Measurements within 5 km of the island show regions of high dissipation (up to 10−3W/m3) as much as 45 m thick at the base of the mixed layer. The maximum dissipations 80 km from the island are not in the seasonal thermocline but rather at depths exceeding 250 m, where the values averaged over 5 m reach 10−4W/m3or more. The vertical thickness scales of the dissipative patches are of the order of 10 m. Statistical analysis shows that 80% of the estimates of ∈ are 10−5W/m3or less; the other 20% of the water column is responsible for 85% of the total energy

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02939

年代:1978

数据来源: WILEY

摘要:

The generation of spray under high wind speed conditions has been investigated in a laboratory wind, water wave research facility. An electrostatic capacitance wire probe system was used to measure spray. Mean energy transfers were determined by a standard boundary layer integral technique. For a given wind velocity the spray concentration decreases logarithmically with increased distance from the mean water surface. The spray concentration and distribution results of this study are consistent with field and other laboratory studies (with one exception). While wave breaking, whitecapping, and bubble bursting during spray formation contribute to increased interface roughness, calculations based on the laboratory data show that the latent and sensible heat transfers from the spray drops make a negligible contribution to the energy transfer observed. The marked increase in measured energy transport under spray conditions is due almost entirely to increased transfer from the roughened interface (there is a linear correlation between the latent and total heat transfers and the root‐mean‐square water surface displacement).However, extrapolation of the laboratory results suggests strongly that spray drops make a significant contribution to the energy transfer in field situations with wind, temperature, and humidity conditions equivalent to those in the laboratory because of greatly increased droplet boundary layer thicknesses in the fi

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02959

年代:1978

数据来源: WILEY

摘要:

The conventional definition of group speedU=dω/dkis the speed of a wave group composed of collinear wave components. When there is dispersion and refraction, the wave components cannot remain parallel. ThenUis found to be the apparent speed of the group in the direction of movement of the wavelets within the group. The velocity of a group composed of nonparallel wave components is found to depend upon the directional distribution of the wave components, and it is here termed the geometric group velocity. The geometric group speedG=Ucos ϕ, where the angle ϕ is the difference between the direction of the wave group and the direction of the wavelets. For a wave group in which the bandwidth and the directional spread of the wave components are both small the wavelet velocity is nearly equal to the phase velocity of a wave component. The path of a wave packet is found to be given by Snell's law with the geometric group velocity, while at each point along the path the wavelets have a direction defined by Snell's law with the wavelet velocity. The findings are illustrated by using examples of gravity water waves and are confirmed by field observatio

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02970

年代:1978

数据来源: WILEY

摘要:

The analysis of the natural distribution of deuterium and oxygen 18 in moisture inside the turbulent boundary layer developed above a water surface makes possible the investigation of the mechanism of evaporation. The distribution of isotopes in water vapor allows the calculation of the relative contributions of molecular and turbulent transfer to the total mass transport (Merlivat and Coantic, 1975). We have applied this method to assess the influence of the conditions at the liquid surface on the evaporation flux. In particular, the influence of wind waves and superposed artificially generated waves was studied. Experiments have been carried out at the Institut de Mécanique Statistique de la Turbulence air‐water tunnel, specially designed for the simulation of ocean‐atmosphere energy exchanges. Here the wind velocities could be varied from 0.7 to 6 m/s. Waves were generated by a motor‐driven paddle. The wave frequency was 1 Hz, and the maximum height 12 cm, while the surface roughness Reynolds numberResvaried from 0.02 to 2. Mean wind velocity profiles and isotopic profiles were measured. Experimental drag coefficients are described quite well by Charnock's relationZ0=u*2/ag., wherea= 81.1. No specific difference is observed when waves are artificially generated. Agreement between experiment and theoretical isotopic distribution is checked for six models proposed to describe evaporation processes. Quite good agreement is found with Brutsaert's model (Brutsaert, 1975a,b) for a smooth surface ifRes1. Again, no specific effect due to the presence of artificially generated waves is observed. The above observations allow, then, the calculation of the drag and bulk evaporation coefficients,CDandCq, as a function of the surface roughness Reynolds number: these coefficients, as well as their ratio, when calculated for a height ofz= 10 m, vary from 0.99 × 10−3to 1.24 × 10−3and from 1.31 to 0.84, respectively, whenResincreases from 0.02 to 10, corresponding to a mean wind speed range extending from 2 to 13 m/s. These evaporation bulk transfer coefficients are given for near‐neutral stability conditions of the atmospheric layer. In the case of moderate instability, frequently observed over the sea, we would expect, and we observe, that the transfer coefficients are ge

ISSN:0148-0227    

DOI:10.1029/JC083iC06p02977

年代:1978

数据来源: WILEY