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Carton X.,LPO UBO | Poulin F.J.,University of Waterloo | Pavec M.,Actimar
Geophysical and Astrophysical Fluid Dynamics | Year: 2011

The linear baroclinic and parametric instabilities of boundary currents with piecewise-constant potential vorticity are studied in a two-layer quasi-geostrophic model. The growth rates of both the exponential modes and of the optimal perturbations are calculated for the baroclinic instability of steady coastal currents. We show that the growth rates of the exponential modes are maximal for a vertically symmetric flow. Furthermore, the vertical asymmetries induced by different layer thicknesses, the presence of a barotropic potential vorticity or bottom topography, all act to dampen the growth rates and favor growth at shorter wavelengths. It is shown that this behavior can be predicted from the conditions for vertical resonance of Rossby waves on the two potential vorticity fronts. Also, the baroclinic instability of the optimal perturbations has larger growth rates at shorter wavelengths and shorter time scales. As well, the presence of a sloping bottom of moderate amplitude favors the growth of these optimal perturbations. Finally, we compute the growth rates of parametric instability of oscillatory coastal flows. We show that subharmonic resonance is the most unstable mode of growth. In addition, a second region of parametric instability is found (for the first time) away from marginality of exponential-mode baroclinic instability. It is shown that the functional dependency of the growth rates of parametric instability, for optimal excitation, are similar to that of the optimal perturbations of baroclinic instability. To explain this a mechanism for parametric instability, involving the rapid growth of short-wave optimal perturbations, is proposed. © 2011 Taylor & Francis.

PubMed | Institute for Radiological Protection and Nuclear Safety, ACTIMAR and French Research Institute for Exploitation of the Sea
Type: | Journal: Journal of environmental radioactivity | Year: 2015

The Fukushima nuclear accident resulted in the largest ever accidental release of artificial radionuclides in coastal waters. This accident has shown the importance of marine assessment capabilities for emergency response and the need to develop tools for adequately predicting the evolution and potential impact of radioactive releases to the marine environment. The French Institute for Radiological Protection and Nuclear Safety (IRSN) equips its emergency response centre with operational tools to assist experts and decision makers in the event of accidental atmospheric releases and contamination of the terrestrial environment. The on-going project aims to develop tools for the management of marine contamination events in French coastal areas. This should allow us to evaluate and anticipate post-accident conditions, including potential contamination sites, contamination levels and potential consequences. In order to achieve this goal, two complementary tools are developed: site-specific marine data sheets and a dedicated simulation tool (STERNE, Simulation du Transport et du transfert dElments Radioactifs dans lenvironNEment marin). Marine data sheets are used to summarize the marine environment characteristics of the various sites considered, and to identify vulnerable areas requiring implementation of population protection measures, such as aquaculture areas, beaches or industrial water intakes, as well as areas of major ecological interest. Local climatological data (dominant sea currents as a function of meteorological or tidal conditions) serving as the basis for an initial environmental sampling strategy is provided whenever possible, along with a list of possible local contacts for operational management purposes. The STERNE simulation tool is designed to predict radionuclide dispersion and contamination in seawater and marine species by incorporating spatio-temporal data. 3D hydrodynamic forecasts are used as input data. Direct discharge points or atmospheric deposition source terms can be taken into account. STERNE calculates Eulerian radionuclide dispersion using advection and diffusion equations established offline from hydrodynamic calculations. A radioecological model based on dynamic transfer equations is implemented to evaluate activity concentrations in aquatic organisms. Essential radioecological parameters (concentration factors and single or multicomponent biological half-lives) have been compiled for main radionuclides and generic marine species (fish, molluscs, crustaceans and algae). Dispersion and transfer calculations are performed simultaneously on a 3D grid. Results can be plotted on maps, with possible tracking of spatio-temporal evolution. Post-processing and visualization can then be performed.

Muller H.,ACTIMAR | Muller H.,CNRS Physics Laboratory | Muller H.,French Research Institute for Exploitation of the Sea | Blanke B.,CNRS Physics Laboratory | And 2 more authors.
Journal of Geophysical Research: Oceans | Year: 2010

This paper describes the surface Lagrangian residual circulation (LRC) over 2.5 day intervals in the Iroise Sea, west of France, and evaluates, for operational purposes, the influence of the different physical mechanisms that govern it. The method consists of the calculation of water displacements with a diagnostic Lagrangian tool that computes the trajectories of numerical particles in a given velocity field. The LRC is inferred from trajectories integrated over five M2 tidal cycles. The analysis is applied to both gridded genuine current measurements and ocean model outputs: the sea surface currents are derived from high-frequency (HF) radar measurements and from MARS, a 3-D regional ocean model used here in idealized configurations. To substantiate the analysis, the Lagrangian residual currents are also compared to genuine movements of drifters released in the Iroise Sea in 2005 and 2007. The LRC is mapped for typical scenarios identified from the Lagrangian analysis of HF radar surface currents measured in winter and summer, under weak (<7 m/s) and strong (>10 m/s) wind conditions, and in neap tide and spring tide seasons. Idealized numerical simulations that switch on and off each individual physical process are used to isolate in the LRC the patterns induced by the atmospheric forcing, tides, and density-driven currents. © 2010 by the American Geophysical Union.

Fontaine E.,French Institute of Petroleum | Orsero P.,University Technologique Of Compiegne | Ledoux A.,ZI Athelia I Voie Ariane | Nerzic R.,Actimar | And 2 more authors.
Structural Safety | Year: 2013

The present study is an attempt to re-assess the level of reliability of the mooring system of an existing Floating Production Storage and Offloading (FPSO) unit in West Africa. The study made use of field data for the environment including wind, waves and current together with simultaneous measurements of the FPSO offset and of the mooring line tensions. Three different approaches to predict the extreme response are compared. More specifically, the traditional design method is compared with Response Based Design (RBD) and First Order Reliability Method (FORM) analysis associated with Response Surface Models (RSM) of the moored FPSO. The results of this case study allow assessing the level of conservatism that is currently embedded in classical design rules. © 2013 Elsevier Ltd.

Maisondieu C.,French Research Institute for Exploitation of the Sea | Breivik O.,Norwegian Meteorological Institute | Allen A.A.,KNM Tordenskjold | Pavec M.,U.S. Coast Guard | And 2 more authors.
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2010

Over the past decades, various operational drift forecast models were developed for trajectory prediction of objects lost at sea for search and rescue operations. Most of these models are now based on a stochastic, Monte Carlo definition of the object's initial position and its time-evolving search area through computation of an ensemble of equally probable trajectories (Breivik [1]). Uncertainties in environmental forcing, mainly surface currents and wind, as well as the uncertainties inherent in the simplified computation of leeway speed and direction relative to the wind are also accounted for through this ensemble-based approach. Accuracy of the drift forecast obviously depends to a large extent on the quality of the environmental forecast data provided by numerical weather prediction models and ocean models, but it also depends on the level of uncertainty associated with the estimation of the drift properties (leeway) of the objects themselves. The present work mostly focuses on this second aspect of the problem. Drift properties of objects can be described by means of their downwind and crosswind leeway coefficients, according to the definition of leeway as stated by Allen [2, 3]. Assessment of the leeway coefficients is based on a direct method, which requires measurements acquired during field tests. Such field experiments basically entail deploying one or more objects at sea and simultaneously recording the environmental parameters (namely wind speed and motion of the object relative to the ambient water masses, i.e., its leeway) as well as the object's position while adrift for periods ranging from several hours to several days. Using this method, a large database providing leeway coefficients for more than sixty object classes ranging from medical waste to a person-in-water to small fishing vessels was compiled over the years by the United States Coast Guard (Allen [2]). More recently additional trials were conducted, which allowed evaluation of new objects, including 20-ft shipping containers. We present in this paper the methods and analysis procedures for field determination of leeway coefficients of typical searchand- rescue objects. As an example we present the case study of a 20-ft container and discuss results obtained from a drift forecast model assessing sensitivity of such a model to the quality of environmental data as well as uncertainty levels of some reference parameters. Copyright © 2010 by ASME.

L'Hegaret P.,French National Center for Scientific Research | Duarte R.,ACTIMAR | Carton X.,French National Center for Scientific Research | Vic C.,French National Center for Scientific Research | And 3 more authors.
Ocean Science | Year: 2015

The Arabian Sea and Sea of Oman circulation and water masses, subject to monsoon forcing, reveal a strong seasonal variability and intense mesoscale features. We describe and analyze this variability and these features, using both meteorological data (from ECMWF reanalyses), in situ observations (from the ARGO float program and the GDEM - Generalized Digital Environmental mode - climatology), satellite altimetry (from AVISO) and a regional simulation with a primitive equation model (HYCOM - the Hybrid Coordinate Ocean Model). The model and observations display comparable variability, and the model is then used to analyze the three-dimensional structure of eddies and water masses with higher temporal and spatial resolutions than the available observations. The mesoscale features are highly seasonal, with the formation of coastal currents, destabilizing into eddies, or the radiation of Rossby waves from the Indian coast. The mesoscale eddies have a deep dynamical influence and strongly drive the water masses at depth. In particular, in the Sea of Oman, the Persian Gulf Water presents several offshore ejection sites and a complex recirculation, depending on the mesoscale eddies. The associated mechanisms range from coastal ejection via dipoles, alongshore pulses due to a cyclonic eddy, to the formation of lee eddies downstream of Ra's Al Hamra. This water mass is also captured inside the eddies via several mechanisms, keeping high thermohaline characteristics in the Arabian Sea. The variations of the outflow characteristics near the Strait of Hormuz are compared with variations downstream. © Author(s) 2015.

Berger H.,French National Center for Scientific Research | Treguier A.M.,French National Center for Scientific Research | Perenne N.,Actimar | Talandier C.,French National Center for Scientific Research
Climate Dynamics | Year: 2014

In this study, we analyse the seasonal variability of the sea surface salinity (SSS) for two coastal regions of the Gulf of Guinea from 1995 to 2006 using a high resolution model (1/12°) embedded in a Tropical Atlantic (1/4°) model. Compared with observations and climatologies, our model demonstrates a good capability to reproduce the seasonal and spatial variations of the SSS and mixed layer depth. Sensitivity experiments are carried out to assess the respective impacts of precipitations and river discharge on the spatial structure and seasonal variations of the SSS in the eastern part of the Gulf of Guinea. In the Bight of Biafra, both precipitations and river runoffs are necessary to observe permanent low SSS values but the river discharge has the strongest impact on the seasonal variations of the SSS. South of the equator, the Congo river discharge alone is sufficient to explain most of the SSS structure and its seasonal variability. However, mixed layer budgets for salinity reveal the necessity to take into account the horizontal and vertical dynamics to explain the seasonal evolution of the salinity in the mixed layer. Indeed evaporation, precipitations and runoffs represent a relatively small contribution to the budgets locally at intraseasonal to seasonal time scales. Horizontal advection always contribute to spread the low salinity coastal waters offshore and thus decrease the salinity in the eastern Gulf of Guinea. For the Bight of Biafra and the Congo plume region, the strong seasonal increase of the SSS observed from May/June to August/September, when the trade winds intensify, results from a decreasing offshore spread of freshwater associated with an intensification of the salt input from the subsurface. In the Congo plume region, the subsurface salt comes mainly from advection due to a strong upwelling but for the Bight of Biafra, entrainment and vertical mixing also play a role. The seasonal evolution of horizontal advection in the Bight of Biafra is mainly driven by eddy correlations between salinity and velocities, but it is not the case in the Congo plume. © 2014, Springer-Verlag Berlin Heidelberg.

Vic C.,French National Center for Scientific Research | Berger H.,Actimar | Treguier A.-M.,French National Center for Scientific Research | Couvelard X.,French National Center for Scientific Research
Journal of Physical Oceanography | Year: 2014

The Congo River has the second largest rate of flow in the world and is mainly responsible for the broad tongue of low-salinity water that is observed in the Gulf of Guinea. Despite their importance, near-equatorial river plumes have not been studied as thoroughly as midlatitude plumes and their dynamics remain unclear. Using both theory and idealized numerical experiments that reproduce the major characteristics of the region, the authors have investigated the dynamics of the Congo River plume and examine its sensitivity to different forcing mechanisms. It is found that near-equatorial plumes are more likely to be surface trapped than midlatitude plumes, and the importance of the β effect in describing the strong offshore extent of the lowsalinity tongue during most of the year is demonstrated. It is shown that the buoyant plume constrained by the geomorphology is subject to the β pulling of nonlinear structures and wavelike equatorial dynamics. The wind is found to strengthen the intrinsic buoyancy-driven dynamics and impede the development of the coastal southward current, in coherence with observations. © 2014 American Meteorological Society.

Sicot G.,Actimar | Lennon M.,Actimar | Corman D.,Agence des Aires Marines Protegees | Gauthiez F.,Agence des Aires Marines Protegees
International Geoscience and Remote Sensing Symposium (IGARSS) | Year: 2015

The inversion of the semi-analytical radiative tranfer model proposed by Lee allows to estimate the water column paramters and depth. The solution generally used to deal with the sea bottom reflectance at the inversion stage is to use a spectral library containing relevant reflectance spectra of the studied area. In this paper, we will evaluate the influence of that spectral library. For this purpose, the simplest possible spectral library will be used: a spectral library including only one flat spectrum. The observations made using that spectral library will show results that re-evaluate the purpose of the spectral library itself at the inversion stage, leading us to propose a method to estimate the sea bottom spectral reflectance. © 2015 IEEE.

Helzel T.,Helzel Messtechnik GmbH | Petersen L.,Helzel Messtechnik GmbH | Mariette V.,Actimar | Pavec M.,Actimar
Journal of Coastal Research | Year: 2011

The WERA system (WavE RAdar) is a shore based remote sensing system to monitor ocean surface currents, waves and wind direction. The outstanding temporal resolution of this systems allows to monitor very dynamic processes on the ocean surface, such as small scale Eddy currents, wind induced currents and wave fields. Due to the shore based installation the availability of the data is very high even under harsh environmental conditions. Validation studies from various installations demonstrate the accuracy of these systems. The typical correlation of the surface current velocity measured with the radar compared with an ADCP measurement is better than 0.9. The correlation of the wave height measurements compared with wave buoys come close to these values. Statistics from permanent installations demonstrate the extreme high data availability (reliability) of this shore based instrument, during the last three years the system provided 98.7% data for all pixels of the ocean surface near Brest (France) within an area of 40 × 40 km. This high accuracy and outstanding reliability makes this ocean radar a perfect tool for coastal zone management. Combining these accurate real-time data with numerical ocean models allows to generate very reliable forecasts of the trajectories of drifting objects like persons, containers or oil spill. This makes these instruments a valuable tool for hazard management.

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