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Millot C.,CNRS Mediterranean Institute of Oceanography (MIO)
Progress in Oceanography | Year: 2014

This paper develops a relatively new concept regarding the outflow of Mediterranean Waters (MWs) through the Strait of Gibraltar. While other papers assume that this outflow is composed of only two MWs, we previously found evidence from a re-analysis of 1980s CTD profiles (profiles collected with conductivity-temperature-depth probes during GIBEX, the Gibraltar Experiment), for two other MWs. We also analysed 2003-2008 time series from two CTDs moored (HYDROCHANGES Programme) at the southern sill of Camarinal and on the shelf of Morocco, and we developed a new concept. East of the Strait, the four MWs roughly lay one above the other, but while progressing westward, the associated isopycnals tilt up southward. In the Strait, the MWs are thus juxtaposed, and they all mix with one of two Atlantic Water components (the inflow acronym is thus AWs), and the outflow is horizontally heterogeneous. West of the Strait, the outflow progressively becomes vertically heterogeneous again, hence splitting into a series of superimposed veins. We compared these CTD time series with one collected at the southern sill of Espartel (University of Malaga, INGRES projects and HYDROCHANGES Programme). Fortunately, the CTDs moored at the two sills were generally along the same streamlines so that the MWs' evolution could be monitored. We demonstrated the significance of mixing lines computed from two successive records and the possibility of linking two sets of data (such as CTD profiles) collected at different locations along the Strait. The outflow, which does not show any clear seasonal variability before the Strait, strongly mixes with the inflow within the Strait. This is due mainly to the internal tide and, because the inflow is seasonally variable, leads to an outflow that displays marked seasonal and fortnightly variabilities. Both the outflow and the inflow also display marked spatial heterogeneity and both long-term/yearly and short-term/daily temporal variabilities before they mix; thus, accurately predicting the outflow characteristics in the Atlantic Ocean appears almost impossible. Herein, we first propose a fully objective description of the AWs and MWs during two GIBEX campaigns. Where the AWs and the MWs do not markedly mix, they are defined in terms of density and temperature ranges. Where a MW mixes with one of the AWs down to the bottom, the mixing line characteristics allow for that MW to be followed from one section to one downstream and for the validation of our concept: while superimposed east of the Strait, the MWs come to be juxtaposed within the Strait before becoming superimposed again. We also analysed additional CTD time series collected by the University of Malaga on the south and north sides of the southern sill of Espartel. We demonstrate the following: (a) even though the MWs at the sill (E) and on the south side (ES) were roughly the same, the densest ones out-flowed at ES, i.e., at depths shallower than at E, (b) the MWs on the north side (EN) were very different from those at E and each mixed with different AWs, and (c) using the mixing lines computed from each time series, the data recorded at E and ES allow for the retrieval, with good accuracy, of those recorded at Camarinal (C), which is not the case for the data recorded at EN. Finally we emphasise how different the AWs' heterogeneities are from the MWs' heterogeneities. The inflow is sucked into the Mediterranean Sea, due to the water budget (E-P) deficit there, and it can be composed of any type of AW present west of the Strait at any time and any specific location. The outflow is a product of the Mediterranean Sea, which is like a machine producing a series of MWs that first circulate as alongslope density currents before entering the Strait in a specific order and at specific locations.Consequently, we attempt to schematise the AWs-MWs mixing processes and our understanding of the outflow dynamics. Notwithstanding the difficulty of the working conditions within such a narrow strait, having up to four MWs outflowing side-by-side and mixing with two AWs that have a heterogeneous and variable distribution clearly leads to spatial and temporal heterogeneities that are actually much larger than the ones that have been observed up to now from a relatively low number of CTD profiles and time series. © 2013 Elsevier Ltd. Source

Sous D.,CNRS Mediterranean Institute of Oceanography (MIO) | Sommeria J.,CNRS Laboratory of Geophysical and Industrial Flows | Boyer D.,Arizona State University
Physics of Fluids | Year: 2013

We use spin-up/spin-down laboratory experiments to study the neutrally stratified Ekman boundary layer. The experiments are performed in the 13 m diameter, 1 m deep Coriolis rotating tank of the LEGI in Grenoble, France. A global flow rotation is produced by an initial change in the tank rotation speed. It then slowly decays under the effect of Ekman friction, evolving from the turbulent state to the laminar state. It is checked that the Ekman layer itself remains in a quasi-steady state during this decay. The velocity is measured by Particle Imaging Velocimetry (PIV) at two scales: the global rotation in a horizontal plane, and the vertical profile inside the boundary layer, where the three velocity components are obtained by stereoscopic PIV. The friction law is obtained by relating the decay rate of the bulk velocity to the velocity itself. This method is justified by the fact that this bulk velocity is independent of height beyond the top of the boundary layer (a few cm), as expected from the Taylor-Proudman theorem for rotating fluids. The local measurements inside the boundary layer provide profiles of the mean velocity and Reynolds stress components, in particular the cross-isobar angle between the interior and near surface velocities. In the laminar regime, good agreement is obtained with the classical Ekman's theory, which validates the method. In the turbulent regime, the results are found consistent with the classical Atmospheric Boundary Layer (ABL) model based on the von Karman logarithmic layer. Our experiments therefore indicate that this theory, in principle valid for very large Reynolds numbers, is already relevant close to the transitional regimes. A fit of the empirical coefficients A and B appearing in this theory yields A = 3.3 and B = 3.0. Extrapolating the results to the atmospheric case gives a friction velocity u* about 12% higher than the traditional fit for the ABL. We may safely deduce that for the oceanic bottom boundary layer, corresponding to lower Reynolds numbers than the atmosphere, our result provides a correct estimate within 10%. The previous laboratory results of Caldwell et al. ["A laboratory study of the turbulent Ekman layer," Geophys. Fluid Dyn.3, 125-160 (1972)10.1080/03091927208236078] provided frictions velocities about 20% higher than in our experiments, and slightly higher cross-isobar angles. We attribute this difference to the higher vortical Rossby number Rot in those experiments, and maybe also to roughness effects. We take into account the effect of this vortical Rossby number within the framework of the Ekman layer (Rot → 0) by replacing the tank rotation rate by the fluid rotation rate. © 2013 AIP Publishing LLC. Source

Costa A.,Aix - Marseille University | Costa A.,CNRS Mediterranean Institute of Oceanography (MIO) | Osborne A.R.,Nonlinear Waves Research Corporation | Resio D.T.,University of North Florida | And 5 more authors.
Physical Review Letters | Year: 2014

We analyze shallow water wind waves in Currituck Sound, North Carolina and experimentally confirm, for the first time, the presence of soliton turbulence in ocean waves. Soliton turbulence is an exotic form of nonlinear wave motion where low frequency energy may also be viewed as a dense soliton gas, described theoretically by the soliton limit of the Korteweg-deVries equation, a completely integrable soliton system: Hence the phrase "soliton turbulence" is synonymous with "integrable soliton turbulence." For periodic-quasiperiodic boundary conditions the ergodic solutions of Korteweg-deVries are exactly solvable by finite gap theory (FGT), the basis of our data analysis. We find that large amplitude measured wave trains near the energetic peak of a storm have low frequency power spectra that behave as ∼ω-1. We use the linear Fourier transform to estimate this power law from the power spectrum and to filter densely packed soliton wave trains from the data. We apply FGT to determine the soliton spectrum and find that the low frequency ∼ω-1 region is soliton dominated. The solitons have random FGT phases, a soliton random phase approximation, which supports our interpretation of the data as soliton turbulence. From the probability density of the solitons we are able to demonstrate that the solitons are dense in time and highly non-Gaussian. © 2014 American Physical Society. Source

Touboul J.,CNRS Mediterranean Institute of Oceanography (MIO) | Touboul J.,Aix - Marseille University | Pelinovsky E.,Johannes Kepler University
European Journal of Mechanics, B/Fluids | Year: 2014

The bottom pressure distribution under solitonic waves, travelling or fully reflected at a wall is analysed here. Results given by two kind of numerical models are compared. One of the models is based on the Green-Naghdi equations, while the other one is based on the fully nonlinear potential equations. The two models differ through the way in which wave dispersion is taken into account. This approach allows us to emphasize the influence of dispersion, in the case of travelling or fully reflected waves. The Green-Naghdi model is found to predict well the bottom pressure distribution, even when the quantitative representation of the runup height is not satisfactorily described. © 2014 Elsevier Masson SAS. All rights reserved. Source

Rontani J.-F.,Aix - Marseille University | Rontani J.-F.,CNRS Mediterranean Institute of Oceanography (MIO) | Volkman J.K.,CSIRO | Prahl F.G.,Oregon State University | And 2 more authors.
Organic Geochemistry | Year: 2013

Lipid biomarkers in sediments are widely used to infer environmental conditions occurring in the geological past, but such reconstructions require careful consideration of the biotic and abiotic processes that degrade and alter lipid biomarker compositions before and after deposition. In this study, we use alkenones produced by haptophyte microalgae to explore the range of effects of these degradative processes. Alkenones are now perhaps the best studied of all biomarkers, with several hundred references on their occurrence in organisms, seawater and sediments. Much information has been obtained on their degradation from laboratory incubation studies and inferences from changes in their distribution in aquatic environments. Although alkenones are often considered as more stable than many other lipid classes, it is now clear that their distributions can be affected by processes such as prolonged oxygen exposure, aerobic bacterial degradation and thiyl radical-induced stereomutation which, in some cases, can lead to changes in the proportions of the alkenones used in the U37K' temperature proxy. The same set of chemical and biological processes act on all lipids in aquatic environments and, in cases where there is a marked difference in reactivity, this may lead to significant changes in the biomarker distributions and relative proportions of different lipid classes. © 2013 Elsevier Ltd. Source

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