The Benthic Boundary Layer: geochemical and oceanographic data from the GEOSTAR-2 observatory

Geochemical and oceanographic data, acquired throughout 6 months by the GEOSTAR-2 benthic observatory in southern Tyrrhenian Sea, evidenced ocean-lithosphere interactions in the 1900-m deep Benthic Boundary Layer (BBL), distinguishing two water masses with different origin and, possibly, benthic residence time. Gas concentration, helium isotopic ratios, radioactivity, temperature, salinity and vertical component of the current converged towards the indication of a BBL characterised by a colder and fresher western water (WW), which is episodically displaced by the cascading of the warmer and saltier Eastern Overflow Water (EOW). The benthic WW has higher concentration of geochemical tracers diffusing from the seafloor sediments. The data set shows the potential of long-term, continuous and multiparametric monitoring in providing unique information which cannot be acquired by traditional, short-term or single-sensor investigations

Time variability on hydrology and biogeochemistry induced by mesoscale eddies in the Algerian Basin: a one year high resolution and multiplatform experiment

In the framework of the ELISA project (1997-1998, MAST-3/MTP/MATER program) 1-year high frequency time series were acquired at fixed points in the Algerian Basin, an open sea area of the Mediterranean sea (3000m). This multidisciplinary and multiplatform experiment was dedicated to study the algerian eddies and their influence on general circulation and on biological phenomena. During 3 main cruises, 2 specific  Anticyclonic Eddies (AEs) tracked by satellites images during 4 years were mainly studied by mean of vertical sections, performed inside AEs and in the surrounding smaller structures such as sub-mesoscale filaments and small-scale shear eddies. These sections helped describing the hydrological and biogeocheminal structures of eddies according to the seasons and the surrounding dynamics, knowing their historical circulation in and/or out of the Algerian Current. With benefit of this first stage of the study, when an AE flowed throughout a 9-mooring network (lines 50 km-spaced), including 50 currentmeters down to the bottom, it is possible to comprehend its signature at fine spacio-temporal scale. The presented analysis will focus on the temporal signal induced by these eddies recorded on 4 autonomous CTD/Fluorometers, located in the central mooring of the network, in the upper layer. The temporal analysis will apply recent analysis methods like the Empirical Mode Decomposition that works with gappy records, and help filtering the data as well. This study will show how much the seasonal signal and the (sub)mesoscale-induced time variability can be comparable in this area. It will also explain how eddies history can play a role in the observed signature at a given time

Timbalès (Température et Imagerie. Mesures à profondeur Bulk par Avance en Ligne d’une Escadrille à Submeso-échelle)

National audienceNotre projet concerne le développement d’un jeu de dix drones de surface motorisés et coordonnés pour mesurer les variations spatiales de la température de surface de la mer, et fournir pour la première fois des champs de température superficielle de la mer à dix mètres de résolution sur une zone de 50 m x 50 m avec une précision de 0.002°C, et à 10 cm de profondeur, ce qui est sans précédent, au mieux de notre connaissance.On espère ainsi visualiser des variations spatiales de sub-submesoéchelle correspondant soit à des circulations de Langmuir sous l’effet de convergence-divergence du courant de surface généré par le vent, soit à des tourbillons en température, soit à des schémas spatiaux de température de surface correspondant à des risées, à des plumes ou thermiques (wind gustiness), ou à des phénomènes de circulation océanique de surface.Le projet est initié, un prototype a déjà été réalisé, un second prototype est en cours de réalisation, et la demande que nous soumettons ici à LEFE-IMAGO porte sur l’extension du projet à une flotte de dix drones.Référence : Puigserver Cécile, Denis Bourras, Luneau Christopher, Jean-Luc Fuda, Hubert Branger, et al.. Observation des variations spatiales de la SST à sub meso-échelle : quelle stratégie de mesure adopter ? AEI 2019, Atelier d’Expérimentation et d’Instrumentation de l’INSU, Jul 2019, Lille, France. ⟨hal-02373301

Characterization of turbulence from a fine-scale parameterization and microstructure measurements in the Mediterranean Sea during the BOUM experiment

International audienceOne of the main purposes of the BOUM experiment was to find evidence of the possible impact of submesoscale dynamics on biogeochemical cycles. To this aim physical as well as biogeochemical data were collected along a zonal transect through the western and eastern basins of the Mediterranean Sea. Along this transect 3-day fixed point stations were performed within anticyclonic eddies during which microstructure measurements of the temperature gradient were collected over the top 100 m of the water column. We focus here on the characterization of turbulent mixing. The analysis of microstructure measurements revealed a high level of turbulent kinetic energy (TKE) dissipation rate in the seasonal pycnocline and a moderate level below with mean values of the order of 10−6 W kg−1 and 10−8 W kg−1, respectively. The Gregg Henyey (Gregg, 1989) fine-scale parameterization of TKE dissipation rate produced by internal wave breaking, and adapted here following Polzin et al. (1995) to take into account the strain to shear ratio, was first compared to these direct measurements with favorable results. The parameterization was then applied to the whole data set. Within the eddies, a significant increase of dissipation at the top and base of eddies associated with strong near-inertial waves is observed. Vertical turbulent diffusivity is increased both in these regions and in the weakly stratified eddy core. The stations collected along the East–West transect provide an overview of parameterized TKE dissipation rates and vertical turbulent diffusivity over a latitudinal section of the Mediterranean Sea. Strong TKE dissipation rates are found within the first 500 m and up to 1500 m above the bottom. Close to the bottom where the stratification is weak, the inferred vertical turbulent diffusivity can reach Kz≃10−3 m2 s−1 and may therefore have a strong impact on the upward diffusive transport of deep waters masses

Characterization of turbulence and validation of fine-scale parametrization in the Mediterranean Sea during BOUM experiment

International audienceOne main purpose of BOUM experiment was to give evidence of the possible impact of submesoscale dynamics on biogeochemical cycles. To this aim physical as well as biogeochemical data were collected along a zonal transect through the western and eastern basins. Along this transect 3 day fixed point stations were performed within anticyclonic eddies during which microstructure measurements were collected over the first 100 m. We focus here on the characterization of turbulent mixing induced by internal wave breaking. The analysis of microstructure measurements revealed a high level of turbulence in the seasonal pycnocline and a moderate level below with energy dissipation mean values of the order of 10-6 W kg-1 and 10-8 W kg-1, respectively. Fine-scale parameterizations developed to mimic energy dissipation produced by internal wavebreaking were then tested against these direct measurements. Once validated a parameterization has been applied to infer energy dissipation and mixing over the whole data set, thus providing an overview over a latitudinal section of the Mediterranean sea. The results evidence a significant increase of dissipation at the top and base of eddies associated with strong near inertial waves. Vertical turbulent diffusivity is increased both in these regions and in the weakly stratified eddy core

Large-scale circulation over and around the Northern Kerguelen Plateau

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The East Caledonian Current: A Case Example for the Intercomparison between AltiKa and In Situ Measurements in a Boundary Current

This paper presents an assessment of SARAL/AltiKa satellite altimeter for the monitoring of a tropical western boundary current in the south-western Pacific Ocean: the East Caledonian Current. We compare surface geostrophic current estimates obtained from two versions of AltiKa along-track sea level height (AVISO 1 Hz and PEACHI 40 Hz) with two kinds of dedicated in situ datasets harvested along the satellite ground tracks: one deep-ocean current-meter mooring deployed in the core of the boundary current and five glider transects. It is concluded that the AltiKa-derived current successfully captures the velocity of the boundary current, with a standard error of 11 cm/s with respect to the in situ data. It also appears important to reference AltiKa sea level anomaly to the latest mean dynamic topography available in our area. Doing so, Ka-band altimetry provides a satisfactory representation of the western boundary current. Thereby, it usefully contributes to observing its variability in such a remote and under-observed ocean region. However, the rather long repeat period of SARAL (35 days) in comparison to the high frequency variability seen in the flow velocity of the boundary current calls for a combined use of SARAL with the other satellite altimetry missions

Observation of the spatial structures of the SST at sub-mesoscale: data analysis and instrument design

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Internal tides and vertical mixing over the Kerguelen Plateau

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Spatio-temporal variability of SSS and SST in the western Mediterranean basin with the TRANSMED data

International audienceInitiated by the CIESM and upgraded to the operational status within the HyMeX/MISTRALS program, the TRANSMEDinitiative aimed at developing an autonomous and low-cost thermosalinometer system to record SSS (Sea Surface Salinity) andSST (Sea Surface Temperature) while underway. The seven-year time series recorded from February 2012 till present betweenFrance (Marseille) and Algeria yield an unprecedented insight in the basin-wide seasonal evolution of the Western Basin, up tomesoscale features, and is used to validate the Copernicus Marine Service global operational system in the area