CNRS Mediterranean Institute of Oceanography (MIO)

Marseille, France

CNRS Mediterranean Institute of Oceanography (MIO)

Marseille, France
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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.

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.

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.

Tamburini C.,Aix - Marseille University | Tamburini C.,CNRS Mediterranean Institute of Oceanography (MIO) | Boutrif M.,Aix - Marseille University | Boutrif M.,CNRS Mediterranean Institute of Oceanography (MIO) | And 4 more authors.
Environmental Microbiology | Year: 2013

Effects of hydrostatic pressure on pure cultures of prokaryotes have been studied extensively but impacts at the community level in the ocean are less well defined. Here we consider hydrostatic pressure effects on natural communities containing both unadapted (piezosensitive) prokaryotes originating from surface water and adapted (including piezophilic) prokaryotes from the deep sea. Results from experiments mimicking pressure changes experienced by particle-associated prokaryotes during their descent through the water column show that rates of degradation of organic matter (OM) by surface-originating microorganisms decrease with sinking. Analysis of a much larger data set shows that, under stratified conditions, deep-sea communities adapt to in situ conditions of high pressure, low temperature and low OM. Measurements made using decompressed samples and atmospheric pressure thus underestimate in situ activity. Exceptions leading to overestimates can be attributed to deep mixing events, large influxes of surface particles, or provision of excessive OM during experimentation. The sediment-water interface, where sinking particles accumulate, will be populated by a mixture of piezosensitive, piezotolerant and piezophilic prokaryotes, with piezophilic activity prevailing deeper within sediment. A schematic representation of how pressure shapes prokaryotic communities in the ocean is provided, allowing a reasonably accurate interpretation of the available activity measurements. © 2013 Society for Applied Microbiology and Blackwell Publishing Ltd.

Boudouresque C.F.,CNRS Mediterranean Institute of Oceanography (MIO) | Verlaque M.,CNRS Mediterranean Institute of Oceanography (MIO)
Developments in Aquaculture and Fisheries Science | Year: 2013

The purple sea urchin Paracentrotus lividus is a Mediterranean and north eastern Atlantic species. It is particularly common in the subtidal, down to depths of 10-20 m and in tidal pools, mainly on solid rocks, boulders and in seagrass meadows. Densities usually range from a few to a dozen individuals per square meter, but very high densities (over 50 to 100 ind·m-2) may occur. In the field, when food resources are not limited, P. lividus is basically herbivorous, with a number of species of MPOs (Multicellular Photosynthetic Organisms) clearly 'preferred' or 'avoided'. Under conditions of limited food resources, it appears to be a very opportunistic generalist, able to exploit any kind of food resource, including drift material, sponges, hydrozoans and even small conspecifics (cannibalism) and particulate organic matter (POM). Competition between P. lividus and other herbivores (e.g., the sea urchin Arbacia lixula and the teleost Sarpa salpa) and predation by teleosts, crustaceans and mollusks may affect its abundance and behavior, and hence its effect on benthic communities. The density of P. lividus shows a general and conspicuous negative correlation with erect MPO coverage and biomass. It is a key species and structuring force, not only in overgrazed habitats with high sea urchin densities (barren grounds), but also in habitats with low sea urchin densities and a dense cover of MPOs (forests). In addition, it may control the shift between these two alternative 'stable' states. © 2013 Elsevier B.V.

Nencioli F.,Plymouth Marine Laboratory | Petrenko A.A.,CNRS Mediterranean Institute of Oceanography (MIO) | Doglioli A.M.,CNRS Mediterranean Institute of Oceanography (MIO)
Journal of Geophysical Research: Oceans | Year: 2016

Exchanges between coastal regions and the open ocean are often associated with intermittent and localized processes such as eddies, fronts, and filaments. Since these features are difficult to observe, their impact has been predominantly investigated using numerical models and remote sensing. In this study, satellite sea surface temperature maps, Lagrangian surface drifter trajectories, and ship-based surveys of currents and hydrography from the Latex10 campaign are used to quantify cross-shelf exchanges associated with a temperature front in the western Gulf of Lion. Satellite imagery and thermosalinograph sections provide the characterization of the various water masses associated with the front. Lagrangian drifter trajectories are used to identify the main transport structures and to quantify the velocity components associated with near-inertial oscillations. These are removed from the instantaneous ADCP observations with which the cross-shelf exchanges are then computed. The results indicate an average outflow of 0.074±0.013 Sv and an inflow of 0.021±0.006 Sv. Integrated over the 2 week lifetime of the front, such outflow induced a total export of ∼90±14 km3 of water, indicating that three to four of such events are sufficient to completely renew the surface waters of the Gulf of Lion. The total import was ∼25±7 km3, suggesting larger inflows at depth or in the eastern part of the gulf to maintain its volume balance. These in situ estimates represent a key term of comparison for the further development of numerical model-based and satellite-based studies of cross-shelf exchanges associated with this type of processes. © 2016. American Geophysical Union. All Rights Reserved.

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.

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.

Krupovic M.,Institute Pasteur Paris | Gonnet M.,CNRS Lab for Microbiology of Extreme Environments | Hania W.B.,CNRS Mediterranean Institute of Oceanography (MIO) | Forterre P.,Institute Pasteur Paris | And 3 more authors.
PLoS ONE | Year: 2013

Mobilome of hyperthermophilic archaea dwelling in deep-sea hydrothermal vents is poorly characterized. To gain insight into genetic diversity and dynamics of mobile genetic elements in these environments we have sequenced five new plasmids from different Thermococcus strains that have been isolated from geographically remote hydrothermal vents. The plasmids were ascribed to two subfamilies, pTN2-like and pEXT9a-like. Gene content and phylogenetic analyses illuminated a robust connection between pTN2-like plasmids and Pyrococcus abyssi virus 1 (PAV1), with roughly half of the viral genome being composed of genes that have homologues in plasmids. Unexpectedly, pEXT9a-like plasmids were found to be closely related to the previously sequenced plasmid pMETVU01 from Methanocaldococcus vulcanius M7. Our data suggests that the latter observation is most compatible with an unprecedented horizontal transfer of a pEXT9a-like plasmid from Thermococcales to Methanococcales. Gene content analysis revealed that thermococcal plasmids encode Hfq-like proteins and toxin-antitoxin (TA) systems of two different families, VapBC and RelBE. Notably, although abundant in archaeal genomes, to our knowledge, TA and hfq-like genes have not been previously found in archaeal plasmids or viruses. Finally, the plasmids described here might prove to be useful in developing new genetic tools for hyperthermophiles. © 2013 Krupovic et al.

Escudier R.,CSIC - Mediterranean Institute for Advanced Studies | Escudier R.,CNRS Laboratory for Glaciology and Environmental Geophysics | Bouffard J.,Aix - Marseille University | Bouffard J.,CNRS Mediterranean Institute of Oceanography (MIO) | And 3 more authors.
Geophysical Research Letters | Year: 2013

We present an innovative approach to the generation of remotely sensed high-resolution sea surface topography that improves coastal and mesoscale dynamic characterization. This new method is applied for the period 2002-2010 in the northwestern Mediterranean Sea, an area marked by a small Rossby radius. The spectral content of the new mapped data is closer to that of the along-track signal and displays higher levels of energy in the mesoscale bandwidth with the probability distribution of the new velocity fields 30% closer to drifter estimations. The fields yield levels of eddy kinetic energy 25% higher than standard altimetry products, especially over regions regularly impacted by mesoscale instabilities. Moreover, qualitative and quantitative comparisons with drifters, glider, and satellite sea surface temperature observations further confirm that the new altimetry product provides, in many cases, a better representation of mesoscale features (more than 25% improvement in correlation with glider data during an experiment). © 2013 American Geophysical Union. All Rights Reserved.

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