The authors characterize impacts on heat in the ocean climate system from transient ocean mesoscale eddies. Their tool is a suite of centennial-scale 1990 radiatively forced numerical climate simulations from three GFDL coupled models comprising the Climate Model, version 2.0–Ocean (CM2-O), model suite. CM2-O models differ in their ocean resolution: CM2.6 uses a 0.1° ocean grid, CM2.5 uses an intermediate grid with 0.25° spacing, and CM2-1deg uses a nominal 1.0° grid.Analysis of the ocean heat budget reveals that mesoscale eddies act to transport heat upward in a manner that partially compensates (or offsets) for the downward heat transport from the time-mean currents. Stronger vertical eddy heat transport in CM2.6 relative to CM2.5 accounts for the significantly smaller temperature drift in CM2.6. The mesoscale eddy parameterization used in CM2-1deg also imparts an upward heat transport, yet it differs systematically from that found in CM2.6. This analysis points to the fundamental role that ocea...

Impacts on Ocean Heat from Transient Mesoscale Eddies in a Hierarchy of Climate Models

Microwave radar is a well-established tool for all-weather monitoring of film slicks which appear in radar imagery of the surface of water bodies as areas of reduced backscatter due to suppression of short wind waves. Information about slicks obtained with single-band/one-polarized radar seems to be insufficient for film characterization; hence, new capabilities of multi-polarization radars for monitoring of film slicks have been actively discussed in the literature. In this paper the results of new experiments on remote sensing of film slicks using dual co-polarized radars—a satellite TerraSAR-X and a ship-based X-/C-/S-band radar—are presented. Radar backscattering is assumed to contain Bragg and non-Bragg components (BC and NBC, respectively). BC is due to backscattering from resonant cm-scale wind waves, while NBC is supposed to be associated with wave breaking. Each of the components can be eliminated from the total radar backscatter measured at two co-polarizations, and contrasts of Bragg and non-Bragg components in slicks can be analyzed separately. New data on a damping ratio (contrast) characterizing reduction of radar returns in slicks are obtained for the two components of radar backscatter in various radar bands. The contrast values for Bragg and non-Bragg components are comparable to each other and demonstrate similar dependence on radar wave number; BC and NBC contrasts grow monotonically for the cases of upwind and downwind observations and weakly decrease with wave number for the cross-wind direction. Reduction of BC in slicks can be explained by enhanced viscous damping of cm-scale Bragg waves due to an elastic film. Physical mechanisms of NBC reduction in slicks are discussed. It is hypothesized that strong breaking (e.g., white-capping) weakly contributes to the NBC contrast because of “cleaning” of the water surface due to turbulent surfactant mixing associated with wave crest overturning. An effective mechanism of NBC reduction due to film can be associated with modification of micro-breaking wave features, such as parasitic ripples, bulge, and toe, in slicks.

https://www.mdpi.com/2072-4292/10/7/1097/pdf

Remote Sensing of Organic Films on the Water Surface Using Dual Co-Polarized Ship-Based X-/C-/S-Band Radar and TerraSAR-X

The devastating tsunami triggered by the Tōhoku-Oki earthquake of 11 March 2011 caused a crisis at the Fukushima Daiichi nuclear power station where it overtopped the seawall defences. On retreating, the tsunami carried loose debris and wreckage seaward and marshalled buoyant material into extensive plumes. Widespread concern over the fate of these and numerous other Tōhoku tsunami depositions prompted attempts to simulate debris dispersion throughout the wider Pacific. However, the effects of locally perturbed wind and wave fields, active Langmuir circulation and current-induced attrition determine a complex and poorly understood morphology for large floating agglomerations. Here we show that the early post-tsunami evolution of marine-debris plumes near Fukushima Daiichi was also shaped by near-surface wind modifications that took place above relatively calm (lower surface roughness) waters covered by surface films derived from oil and other contaminants. High-spatial-resolution satellite tracking reveals faster-than-expected floating-debris motions and invigorated plume evolution within these regions, while numerical modelling of turbulent air flow over the low-drag, film-covered surface predicts typically metre-per-second wind strengthening at centimetric heights, sufficient to explain the observed debris-speed increases. Wind restructuring probably stimulates the dispersion of flotsam from both biological and anthropogenic sources throughout a global ocean of highly variable surface roughness. The tsunami triggered by the 2011 Tōhoku-Oki earthquake carried debris seaward. Satellite data and numerical simulations suggest that small-scale wind modifications over a smooth film-covered sea surface affected debris motions.

Dynamics and early post-tsunami evolution of floating marine debris near Fukushima Daiichi

We investigate the dissipation of linear, two-dimensional, interfacial waves in a setting comprising three fluids (an upper fluid of semi-infinite depth, a middle fluid-layer of finite thickness, and a lower fluid of semi-infinite depth) separated by two distinct interfaces, which we consider to be elastic. We derive analytic expressions for the dissipation rate of capillary-gravity waves in such a system, in both the barotropic and baroclinic modes of propagation. Using the dissipation rate model formulated herein, we conduct parametric studies of barotropic gravity waves in an air–oil–water system. We consider six different wavenumbers within the range of 0.0165 m−1 (corresponding to ocean swell) to 44.5 m−1 (corresponding to a typical laboratory gravity wave) and investigate the effects of three major mechanisms of loss of energy, which are the dissipation due to the (i) dynamics in the upper fluid (air), (ii) elastic interfaces, and (iii) viscous middle fluid (oil) layer of finite thickness. For waves with wavenumbers of 0.0165 m−1 and 0.04 m−1, the dominant mechanism for the energy loss is that due to the dynamics in air. For waves with wavenumbers of 1 m−1 and 4 m−1, the oil layer acts to increase the dissipation rates significantly but only when its thickness is beyond a threshold value. For waves with wavenumbers of 36.2 m−1 and 44.5 m−1, the elastic interfaces cause significant increases in the dissipation rates, when their elasticities change from a value of 0.01 N/m to 0.0225 N/m. The three-fluid model developed herein is applicable to capillary-gravity waves propagating in a generic fluid system with arbitrary values for the densities, viscosities, interfacial elasticities, and with an arbitrary value for the middle fluid-layer thickness within an upper limit. This model is useful in predicting the dissipation rates of waves on the ocean surface, which is (in general) covered with biofilms and oil layers of thicknesses ranging from a few μm to a few mm, and in predicting the dissipation rates of waves such as swell, for which the dynamics in the upper fluid (air) are important.

A three-fluid model for the dissipation of interfacial capillary-gravity waves

Investigation of microwave scattering mechanisms is extremely important for developing methods for ocean remote sensing. Recent studies have shown that a common two-scale scattering model accounting for resonance (Bragg) scattering has some drawbacks, in particular it often overestimates the vertical-to-horizontal polarization radar return ratio and underestimates the radar Doppler shifts if the latter are assumed as associated with quasi linear resonance surface waves. It is supposed nowadays that radar backscattering at moderate incidence angles is determined not only by resonance Bragg mechanism but also contains non polarized (non Bragg) component which is associated supposedly with wave breaking but which is still insufficiently studied. Better understanding of the scattering mechanisms can be achieved when studying variations of radar return due to long wind waves. In this paper, results of experiments from an Oceanographic Platform on the Black Sea using dual co-polarized X-band scatterometers working at moderate incidence are presented and variations of Bragg and non-Bragg components (BC and NBC, respectively) and radar Doppler shifts are analysed. It is established that BC and NBC are non-uniformly distributed over profile of dominant (decametre-scale) wind waves (DWW). Variations of BC are characterized by some “background” return weakly modulated with the dominant wind wave periods, while NBC is determined mostly by rare and strong spikes occurred near the crests of the most intense individual waves in groups of DWW. We hypothesize that the spikes are due to intensification of nonlinear structures on the profile of short, decimetre-scale wind waves when the latter are amplified by intense DWW. Bragg scattering in slicks under the experimental conditions was suppressed stronger than NBC and spikes dominated in total radar return. It is obtained that radar Doppler shifts at HH-polarization are larger than at VV-polarization, particularly in slicks, the same relation is for NBC and BC Doppler shifts, thus indicating different scattering mechanisms for these components.

Modulation of Dual-Polarized X-Band Radar Backscatter Due to Long Wind Waves

/pdf/remote-sensing-methods-for-measuring-the-thickness-of-oil-3bmuoeko2l.pdf

Remote sensing methods for measuring the thickness of oil/oil product films on the sea surface

Traditionally spiral slick structure is interpreted as a manifestation of marine eddy, which length scale is taken equal to the scale of spiral. This approach is based on the assumption that wind has no effect on the kinematics of forming slick bands which, according to our estimation, is incorrect in real sea conditions. This can lead to misinterpretation of remote sensing data in the field of definition of the characteristics of marine eddies, particularly, in radar images. The system of equations for the description of kinematics of a Lagrangian particle (element of surface active substance) in stationary fields of axisymmetric eddy with non-zero radial velocity component and homogeneous wind was obtained. It was shown that the center of the spiral is not collocated with the center of the eddy, and the distance between them can achieve the scale of eddy core. It was shown that the displacement of the spiral center is quasi perpendicular to the wind direction in case of small radial velocity component compared to the tangential one. It was shown analytically that there is a threshold wind velocity which corresponds to the breakdown of the spiral structure. Simulation based on the discrete-time approximation of particle trajectories, as well as radar observation of marine eddies, demonstrates the possibility of appearance of a “focus” and a “saddle” in the characteristic shape of slick bands. The perspectives of correct retrieval of length scales and character velocities of observed sub mesoscale marine eddies are discussed.

Manifestation of sub mesoscale marine eddies in the structure of surface slick bands

Marine slicks are one of the most common features on the sea surface and a significant part of the slicks is a result of accidental or deliberate oil spills. The shape of oil slicks is their important characteristic that can be used to identify the nature of slick signatures in radar or optical images of the sea surface and possibly to describe them quantitatively. Nowadays, however, there is a lack of systematic experiments with slicks, and the very physical mechanisms of slick spreading are still not well understood. This paper presents results of controlled experiments with spills of surfactants, and a possible physical mechanism of slick asymmetry is discussed. Experiments with artificial film slicks were carried out in different environmental conditions: from an Oceanographic Platform on the Black Sea, and from a vessel on the Gorky Water Reservoir. Slick shape and its evolution were studied using photographic methods, and satellite radar imagery. In the satellite experiments surfactants were poured on the surface at certain time intervals before the satellite overpass. It is obtained that film spreading is not axial symmetric, and the spills are stretched along the wind, a long-to-short slick axis ratio weakly depends on spreading time and grows with wind speed. A physical mechanism of slick deformation due to mean surface currents induced by wind waves is proposed. Namely, drift currents induced by oblique propagating surface waves increase in film slicks due to enhanced wave damping and these currents result in reduced spreading rate in the cross wind direction. Theoretical analysis of slick spreading accounting for the effect of surface waves is presented, and theoretical estimates are shown to be consistent with experiment.

Spreading of oil films on the sea surface: radar/optical observations and physical mechanisms

Surfactant films on the sea surface can appear due to pollutions, river and collector drains, as well as biological processes. Film slicks can indicate different processes in the upper ocean and in atmosphere. In particular, slick signatures in SAR-imagery of the sea surface at low and moderate wind speeds are often associated with marine currents. Other factors such as wind and physical characteristics of films can significantly influence the dynamics of slick structures. A perspective approach aimed at measuring surface currents is developed. Based on the approach an impact of wind on the kinematics of artificial slick bands is determined. Simulation of slick band propagation from the localized source of surfactant in the field of wind and eddy-shape current is performed. As a result of simulation the shape of surface slick structure, which is close to the observed on SAR image of water surface, is obtained. It is shown that the possibility of spiral bands formation due to presence of marine submesoscale eddies is determined by near-surface wind. Moreover, it is declared that a traditional estimation of scales of marine eddies based on the scales of spiral slick structures is not basically correct.

Slick bands kinematics due to marine current and wind: study and simulation

/pdf/on-the-estimation-of-the-contribution-of-near-surface-wind-55zojc8a3y.pdf

Ivan A. Kapustin

Papers

Remote sensing methods for measuring the thickness of oil/oil product films on the sea surface

Manifestation of sub mesoscale marine eddies in the structure of surface slick bands

Spreading of oil films on the sea surface: radar/optical observations and physical mechanisms

Slick bands kinematics due to marine current and wind: study and simulation

On the estimation of the contribution of near-surface wind to the kinematics of slicks on the sea surface under conditions of finite wave fetch