Center Militaire dOceanographie

Brest, France

Center Militaire dOceanographie

Brest, France
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Gervaise C.,ENSTA Bretagne | Stephan Y.,Center Militaire dOceanographie | Simard Y.,University of Rimoursky
ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings | Year: 2010

A modern method for discrete underwater channel characterization is the passive tomography concept which takes advantage of the generated signals by the natural sources (opportunity sources). The main difficulty which appears in this field is due to the lack of any information about the received signals. In this paper, we present a method for underwater channel characterization, supposing that the received signal is a sum of shifted chirps, having the same chirp rate and start frequency. Hence, using the fractional Fourier transform, it is possible to express the received signal as a sum of sinusoids whose frequencies are directly related to the times of arrival. In order to distinguish the closed arrivals, we apply a high resolution spectral estimation method, providing also the time of arrivals. ©2010 IEEE.

Pairaud I.L.,CNRS Laboratory for Aerology | Pairaud I.L.,French Research Institute for Exploitation of the Sea | Auclair F.,CNRS Laboratory for Aerology | Marsaleix P.,CNRS Laboratory for Aerology | And 2 more authors.
Continental Shelf Research | Year: 2010

A regional baroclinic model forced with several tidal constituents at different frequencies is used to investigate the internal tide of the Bay of Biscay. The regional ocean model is free surface, sigma-coordinate and it is implemented in order to accurately take into account the barotropic forcing, the strong bathymetry gradients and the temperature and salinity stratifications. In a previous paper, the barotropic component of the tides was studied in details and the boundary conditions of the three-dimensional model were extracted from the atlases. In the present paper, we focus on the baroclinic component of the tides and the simulations are validated against observations from the MINT94 experiment. The observed currents and stratification are accurately reproduced by the model. The internal tide pattern is consistent with the descriptions found in the literature. Combining wavelet and principal component analysis we extract the patterns of generation and propagation of the internal tide at the semi-diurnal and quarter-diurnal frequencies. Secondary internal wave generation areas are identified over the plain. The vertical displacements of isopycnal surfaces for the M4 internal tide are found to be locally half those induced by the semi-diurnal internal tide. A sensitivity study shows the impact of using a three-dimensional initial stratification over the direction of propagation and wavelengths of the internal tides. © 2009 Elsevier Ltd. All rights reserved.

Pichon A.,Center Militaire dOceanographie | Morel Y.,Center Militaire dOceanographie | Baraille R.,Center Militaire dOceanographie | Quaresma L.S.,Instituto Hidrografico
Journal of Marine Systems | Year: 2013

The paper addresses three-dimensional dynamics and interactions between internal waves in the Bay of Biscay, observed during an oceanographic survey conducted in summer 2006. These interactions are modelled and compared with observations. The effect of the internal tide (IT) beam structure in the deep ocean, on the interfacial IT along the seasonal thermocline, is analysed.To determine the regions of strong three-dimensional interactions and explain the observed IT structures, numerical simulations are used. Model results and observations show evidence of IT following analytical ray paths, but also interactions between beams. IT currents higher than 0.20m.s-1, are measured at 3000m near the bottom of the French continental slope, and are clearly modelled. In addition, in the mid-bay of Biscay, both the model and the data highlight a strong internal tidal current from 0 to a depth of 1500m, generated by three-dimensional interactions.Large interfacial ITs, associated with short wavelengths, are shown to be generated by deep ocean ITs when the latter reach the surface and impinge on the seasonal thermocline. It is indeed demonstrated that:. •In the model and in the observations, interfacial ITs are intensified where the beam of the deep ocean IT reaches the seasonal thermocline.•The vertical variations of currents, measured in the surface layers, are related to short wavelengths observed in the seasonal thermocline, and also to the velocity modelled along the beam of the deep ocean IT.•These short wavelengths are generated in the middle of the Bay of Biscay where the deep IT impinges on the seasonal thermocline.Drifting thermistor chains were launched to follow the evolution of the seasonal thermocline. Along their trajectories, internal tide events occur at locations where the modelled surface tidal current has maximum values, and during spring baroclinic tides. The model clearly reproduces these events, but with a delay of one or two days compared to the observations. This delay is most likely due to a mixed layer too deep in the model. © 2011 Elsevier B.V.

Xie X.H.,CAS South China Sea Institute of Oceanology | Xie X.H.,University Pierre and Marie Curie | Cuypers Y.,University Pierre and Marie Curie | Bouruet-Aubertot P.,University Pierre and Marie Curie | And 3 more authors.
Journal of Geophysical Research: Oceans | Year: 2015

High-frequency mooring data were collected near the northern shelf edge of the Bay of Biscay to investigate the generation and propagation of internal tides and internal solitary waves (ISWs). During spring tide, strong nonlinear internal tides and large amplitude ISWs are observed every semidiurnal tidal period. While onshore propagation was expected since the mooring is located shoreward of the maximum internal tidal generation location, both onshore and seaward traveling internal tides are identified. Within a tidal period at spring tide, three ISW packets are observed. Like internal tides, different ISW packets have opposite (seaward and shoreward) propagating direction. Based on realistic hydrostatic HYCOM simulations, it is suggested that advection by the barotropic tide affects wave generation and propagation significantly and is essential for the seaward traveling internal tides to appear shoreward of their generation location. A two-layer idealized nonhydrostatic model derived by Gerkema (1996) further confirms the effect of advection on the generation and propagation of internal tides. Moreover, the two-layer model reproduces one seaward propagating ISW packet and one shoreward propagating ISW packet, indicating that the offshore and onshore traveling ISWs are excited by nonlinear steepening of the seaward and shoreward traveling internal tides, respectively. © 2015. American Geophysical Union. All Rights Reserved.

Xie X.H.,University Pierre and Marie Curie | Cuypers Y.,University Pierre and Marie Curie | Bouruet-Aubertot P.,University Pierre and Marie Curie | Ferron B.,French National Center for Scientific Research | And 3 more authors.
Geophysical Research Letters | Year: 2013

Microstructure and fine-scale measurements collected in the central Bay of Biscay during the MOUTON experiment are analyzed to investigate the dynamics of internal waves and associated mixing. Large-amplitude internal tides (ITs) that excite internal solitary waves (ISWs) in the thermocline are observed. ITs are dominated by modes 3 and 4, while ISWs projected on mode 1 that is trapped in the thermocline. Therein, ITs generate a persistent narrow shear band, which is strongly correlated with the enhanced dissipation rate in the thermocline. This strong dissipation rate is further reinforced in the presence of ISWs. Dissipation rates during the period without ISWs largely agree with the MacKinnon-Gregg scaling proposed for internal wavefields dominated by a low-frequency mode, while they show poor agreement with the Gregg-Henyey parameterization valid for internal wavefields close to the Garrett-Munk model. The agreement with the MacKinnon-Gregg scaling is consistent with the fact that turbulent mixing here is driven by the low-frequency internal tidal shear. © 2013 American Geophysical Union. All Rights Reserved.

Leckler F.,French Research Institute for Exploitation of the Sea | Ardhuin F.,French Research Institute for Exploitation of the Sea | Filipot J.-F.,Center Militaire dOceanographie | Mironov A.,French Research Institute for Exploitation of the Sea
Ocean Modelling | Year: 2013

Whitecaps are the main sink of wave energy and their occurrence has been related to the steepness of the waves. Recent parameterizations of the wave dissipation in numerical models are based on this property, but wave models have seldom been verified in terms of whitecap properties. Here we analyze and adjust the breaking statistics used in two recent wave dissipation parameterizations implemented in the spectral wave model WAVEWATCH III® and now used operationaly at NOAA/NCEP. For dominant breaking waves, the reduction of breaking probabilities with wave age is well reproduced. Across the spectrum, the parameterizations produce a reasonable distribution of breaking fronts for wave frequencies up to three times the dominant frequency, but fail to reproduce the observed reduction in breaking front lengths for the shorter waves. Converted to whitecap coverage, the breaking parameterizations agree reasonably well with the classical empirical fits of whitecap coverage against wind speed and the global whitecap coverage estimated from space-borne radiometry. © 2013 Elsevier Ltd.

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