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Alexander V. Soloviev and Peter Schlüssel

Abstract

Heat and gas transport in molecular sublayers at the air-sea interface is governed by similar laws. A model of renewal type based on the physics of molecular sublayers allows the derivation of a parameterization of the temperature difference across the cool skin of the ocean and of the coefficient of the direct air-sea gas transfer. The surface Richardson number controls the transition from convective instability to wind-induced instability (“rollers” on breaking wavelets) and the Keulegan number controls the transition from the regime of rollers to long-wave breaking. A critical value of the surface Richardson number and of a nondimensional constant can be evaluated by comparing the parameterizations of the cool skin with field data. The critical value of the Keulegan number is determined from the wind speed at which long-wave breaking appears. The parameterizations have been compared with cool skin data obtained from campaigns in the tropical and subtropical Atlantic Ocean, while the gas transfer data are compiled from several experiments in the global ocean. The cool skin data have been used for an adjustment of the parameterization of the direct gas transfer. The parameterization does not include effects of bubble and droplet production in whitecaps, which can be important at high wind speed conditions.

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Vladimir N. Kudryavtsev and Alexander V. Soloviev

Abstract

Measurements made in the Equatorial Atlantic during the 35th cruise of the R/V Akademic Vernadsky using a free-rising profiler and drifters revealed a near-surface slippery layer of the ocean arising due to daytime solar heating. The solar heating warms and stabilizes the surface layer of the ocean. This suppresses turbulent exchange and limits the penetration depth of the wind-induced turbulent mixing. The heated near-surface layer is then slipping over the underlying water practically without friction. At daytime warming of 1°C the resistance coefficient in the upper 5-m ocean, C u = (U *U s)2 became smaller by a factor of 25–30 as compared with the case of neutral stratification. The effect of slipping results in forming a daytime near-surface current. At low wind speed the velocity of this current was observed to achieve 19 cm s−1. A simple one-dimensional integral model reproduces the main diurnal variation of the temperature and the current velocity in the near-surface layer of the ocean.

For daytime the experimental data suggest the existence of a self-regulating state of the diurnal thermocline, which predicts linear temperature and velocity profiles and an equilibrium value of the bulk Richardson number. This provides simple relations coupling the temperature and velocity differences and the thickness of thermocline. An estimation of the upper velocity limit of the daytime near-surface current is equal to 29 cm s−1.

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Alexander V. Babanin and Yury P. Soloviev

Abstract

The variability of frequency spectra of waves is considered; for example, the dependencies of integral and spectral parameters of waves on wave-development factors and the interrelationships of the parameters are examined. Also studied is the transformation of the frequency spectrum shape in the course of its development, as well as the transition from the spectrum of developing waves to the spectrum of fully developed waves. Data were obtained in situ with common methods during a long-term program in the Black Sea.

The variability of the spectrum of developing waves, as a function of the stage of wave development, is described on the basis of field data using estimates of parameters for the spectrum form of the JONSWAP type. A novel approximation of the equilibrium interval level dependence on the dimensionless peak frequency m is obtained, which includes periods of stable and changeable behavior of the spectrum level. Transformation of the spectrum of wind-generated waves related to the development of the wave field in terms of the JONSWAP spectrum form is obtained. Continuous transition to the Pierson–Moskowitz spectrum is described. An approximation of the dependence of the enhancement coeffiecient γ on wave development stage m is suggested, which describes continuous variation of the coefficient γ for all wave ages. Self-similarity of the spectrum of developing waves is not observed.

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John A. Kluge, Alexander V. Soloviev, Cayla W. Dean, Geoffrey K. Morrison, and Brian K. Haus

Abstract

A magnetic signature is created by secondary magnetic field fluctuations caused by the phenomenon of seawater moving in Earth’s magnetic field. A laboratory experiment was conducted at the Surge Structure Atmosphere Interaction (SUSTAIN) facility to measure the magnetic signature of surface waves using a differential method: a pair of magnetometers, separated horizontally by one-half wavelength, were placed at several locations on the outer tank walls. This technique significantly reduced the extraneous magnetic distortions that were detected simultaneously by both sensors and additionally doubled the magnetic signal of surface waves. Accelerometer measurements and local gradients were used to identify magnetic noise produced from tank vibrations. Wave parameters of 4-m-long waves with a 0.56-Hz frequency and a 0.1-m amplitude were used in this experiment. Freshwater and saltwater experiments were completed to determine the magnetic difference generated by the difference in conductivity. Tests with an empty tank were conducted to identify the noise of the facility. When the magnetic signal was put through spectral analysis, it showed the primary peak at the wave frequency (0.56 Hz) and less pronounced higher-frequency harmonics, which are caused by the nonlinearity of shallow water surface waves. The magnetic noise induced by the wavemaker and related vibrations peaked around 0.3 Hz, which was removed using filtering techniques. These results indicate that the magnetic signature produced by surface waves was an order of magnitude larger than in traditional model predictions. The discrepancy may be due to the magnetic permeability difference between water and air that is not considered in the traditional model.

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