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Over the last 3 decades, satellite altimetry has observed sea surface height variations, providing a regular monitoring of the surface ocean circulation. Altimetry measurements have an intrinsic signal-to-noise ratio that limits the spatial scales of the currents that can be captured. However, the recent progress made on both altimetry sensors and data processing allows us to observe smaller geophysical signals, offering new perspectives in coastal areas where these structures are important. In this methodological study, we assess the ability of three altimeter missions with three different technologies to capture the Northern Current (northwestern Mediterranean Sea) and its variability, namely Jason-2 (Ku-band low-resolution-mode altimeter, launched in 2008), SARAL/AltiKa (Ka-band low-resolution-mode altimeter, launched in 2013) and Sentinel-3A (synthetic aperture radar altimeter, launched in 2016). Therefore, we use a high-resolution regional model as a reference. We focus along the French coast of Provence, where we first show that the model is very close to the observations of high-frequency radars and gliders in terms of surface current estimates. In the model, the Northern Current is observed 15–20 km from the coast on average, with a mean core velocity of 0.39 m s−1. Its signature in terms of sea level consists of a drop whose mean value at 6.14∘ E is 6.9 cm, extending over 20 km. These variations show a clear seasonal pattern, but high-frequency signals are also present most of the time. In comparison, in 1 Hz altimetry data, the mean sea level drop associated with the Northern Current is overestimated by 3.0 cm for Jason-2, but this overestimation is significantly less with SARAL/AltiKa and Sentinel-3A (0.3 and 1.4 cm respectively). In terms of corresponding sea level variability, Jason-2 and SARAL altimetry estimates are larger than the model reference (+1.3 and +1 cm respectively), whereas Sentinel-3A shows closer values (−0.4 cm). When we derive geostrophic surface currents from the satellite sea level variations without any data filtering, in comparison to the model, the standard deviations of the velocity values are also very different from one mission to the other (3.7 times too large for Jason-2 but 2.4 and 2.9 times too large for SARAL and Sentinel-3A respectively). When low-pass filtering altimetry sea level data with different cutoff wavelengths, the best agreement between the model and the altimetry distributions of velocity values are obtained with a 60, 30 and 40–50 km cutoff wavelength for Jason-2, SARAL and Sentinel-3A data respectively. This study shows that using a high-resolution model as a reference for altimetry data allows us not only to illustrate how the advances in the performances of altimeters and in the data processing improve the observation of coastal currents but also to quantify the corresponding gain.

International audience; Echosounders are widely used to quantify fish behavior, fish stocks, and zooplankton biomass. Acoustic Doppler Current Profilers have also been used to accurately measure currents in all of the world’s major water bodies over the last 30 years. The present work evaluates the performance of a combined echosounder/ADCP system, the Nortek Signature100, for simultaneous biomass assessment and current profile data analysis. Due to its combined current profiling and scientific echosounding capabilities, the system is seeing increased usage in biomass flux applications, particularly in Antarctic krill research. However, capabilities of the system are still being studied and the present work aims to expand characterization of its performance. To that effect, a four month deployment was carried out by the French National Center for Scientific Research (CNRS) in the Mediterranean Sea with an up-looking Signature100 mounted atop the ALBATROSS mooring line. The line was at a total water depth of 2420 m and its top was approximately 370 m below the surface. Data show significant variations in scattering conditions between daytime and nighttime due to diel vertical migration (DVM), often unrelated to horizontal velocity fluctuations, highlighting not only the multiple frequency band capabilities of the system (up to 7 bands), but also the strength of the combined echosounder and current profiling functions. Echoview, a commercial software package for hydroacoustic data processing, was used to further explore the spatial and temporal patterns of the organisms observed in the echosounder data. A semi-automated technique was implemented to efficiently and objectively clean (e.g. remove interference generated by passing ship traffic), classify (e.g. based on relative frequency response or morphology), and characterize the narrow bandwidth and pulse compressed echosounder data by generating outputs that can contribute to the management and monitoring of aquatic resources.

International audience; Twenty-year coastal altimetry data set (X-TRACK) is used, for the first time, to gain insight into the long-term inter-annual variations of the surface circulation in the Sicily Channel. Firstly, a spectral along with a time/space diagram analysis are applied to the monthly means of the X-TRACK geostrophic currents over the period 1993-2013. They reveal a regionally coherent current patterns from track to track with a marked inter-annual variability that is unequally shared between the Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhenian Current outflow northeast of Sicily. Secondly, an empirical altimetry-based transport-like technique is proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks and coastlines over the study area. A set of hydrographic measurements is used to validate the method. The inferred altimetry transports give a well-balanced mean eastward Atlantic Waters baroclinic flow of 0.4 Sv and standard deviations of 0.2 Sv on a yearly basis throughout the Sicily Channel and toward the Ionian Sea, which is fairly coherent with those found in the literature. Furthermore, the analysis allows to quantify the intrusions of Atlantic Waters over the Tunisian Shelf (0.12 +/- 0.1 Sv) and highlights two main modes of variability of the main surface waters path over the Sicily Channel through the Bifurcation Atlantic Tunisian Current and Atlantic Ionian Stream systems. Some physical mechanisms are finally discussed with regards to changes in the observed currents and transports.

International audience; Technological advances in the recent satellite al-timeter missions of Jason-2, SARAL/AltiKa and CryoSat-2 have improved their signal-to-noise ratio, allowing us to observe finer-scale ocean processes with along-track data. Here, we analyse the noise levels and observable ocean scales in the northwestern Mediterranean Sea, using spectral analyses of along-track sea surface height from the three missions. Jason-2 has a higher mean noise level with strong seasonal variations, with higher noise in winter due to the rougher sea state. SARAL/AltiKa has the lowest noise, again with strong seasonal variations. CryoSat-2 is in synthetic aperture radar (SAR) mode in the Mediterranean Sea but with lower-resolution ocean corrections; its statistical noise level is moderate with little seasonal variation. These noise levels impact on the ocean scales we can observe. In winter, when the mixed layers are deepest and the submesoscale is energetic, all of the altimeter missions can observe wavelengths down to 40–50 km (individual feature diameters of 20–25 km). In summer when the submesoscales are weaker, SARAL can detect ocean scales down to 35 km wavelength, whereas the higher noise from Jason-2 and CryoSat-2 blocks the observation of scales less than 50–55 km wavelength. This statistical analysis is completed by individual case studies, where filtered along-track altimeter data are compared with co-located glider and high-frequency (HF) radar data. The glider comparisons work well for larger ocean structures, but observations of the smaller, rapidly moving dynamics are difficult to co-locate in space and time (gliders cover 200 km in a few days, altimetry in 30 s). HF radar surface currents at Toulon measure the meandering Northern Current, and their good temporal sampling shows promising results in comparison to co-located SARAL altimetric currents. Techniques to separate the geostrophic component from the wind-driven ageostrophic flow need further development in this coastal band.

International audience; A Mediterranean Ocean Observing System for the Environment (MOOSE) has been set up as an interactive, distributed, and integrated observatory system of the North West Mediterranean Sea in order to detect and identify long-term environmental anomalies. In this framework, the Mediterranean Institute of Oceanography (MIO) operates two couple of High Frequency Surface Wave Radars (HFSWR). HFSWRs provide synoptic observations of sea surface currents with high temporal (0.25-1 h) and spatial (1-3 km) resolution and long range (30-100 km). They can therefore be used to study the variability of the current as well as eddy dynamics. One site, based on the WERA (Helzel Messtechnik) technology and installed near Toulon, is operational and monitored in real-time since more than 3 years. It is composed of two WERA systems featuring respectively a non-linear receiving array and a full bistatic configuration, both working in Direction Finding mode. The second site, intended to extend further East the coverage of the Observatory, was set up in 2014 near Nice with two SeaSonde (Codar) systems. We present here an overview of the HFSWR network, the surface current mapping facility offered by the system, and recent observational results and applications.

International audience; In the framework of the French MOOSE project (Mediterranean Ocean Observing System on Environment), the Mediterranean Institute of Oceanography is operating HF radars on the North Western Mediterranean coast. The surface circulation in this region is characterized by a large-scale flow (Northern Current) and by a broad range of other scales of variability induced by meteorological and tidal forcing. The ability of HF radars is to provide synoptic observation as sea surface current map every hour and over long distances. One site is already operational nearby Toulon for more than two years and a second one is in deployment around Nice. This paper gives an overview of the radars network, of the surface current mapping facility offered by the system, and of recent observation results and applications.

The scope of this paper is the general features of the marine and atmospheric circulations in the Mediterranean. It aims at providing keys to managers and other scientists to better understand the physical phenomena that impact the Hyères archipelago, in order to improve the environmental management if needed, and face potential risks., Cet article a pour but de présenter les grandes lignes du fonctionnement de la circulation marine et atmosphérique de la Méditerranée, afin de mieux comprendre les phénomènes physiques qui affectent les îles d'Hyères, d'optimiser la gestion de leur environnement et le cas échéant des risques potentiels. Mots-clés : Méditerranée, îles d'Hyères, Port-Cros, circulation des masses d'eau, courant, vent, état de mer, circulation atmosphérique, moyenne échelle. Abstract. The Hyères Archipelago in the marine and atmospheric circulation systems. The scope of this paper is the general features of the marine and atmospheric circulations in the Mediterranean. It aims at providing keys to managers and other scientists to better understand the physical phenomena that impact the Hyères archipelago, in order to improve the environmental management if needed, and face potential risks.

Primary production, the basic component of the food web and a sink for dissolved inorganic carbon, is a major unknown in Arctic seas, particularly ice algal production, for which detailed and comprehensive studies are often limited in space and time. We present here a simple ice alga model and its coupling with a regional 3D ice-ocean model of the Hudson Bay system (HBS), including Hudson Strait and Foxe Basin, as a first attempt to estimate ice algal production and its potential contribution to the pelagic ecosystem on a regional scale. The ice algal growth rate is forced by sub-ice light and nutrient availability, whereas grazing and ice melt control biomass loss from the underside of the ice. The simulation shows the primary role of sea-ice dynamics on the distribution and production of ice algae with a high spatio-temporal variability in response to the great variability of ice conditions in different parts of the HBS. In addition to favourable light and nutrient conditions, there must be a sufficient time lag between the onset of sufficient light and ice melt to ensure significant ice algal production. This suggests that, in the context of enhanced warming in Arctic and sub-Arctic regions, earlier melt could be more damaging for ice algal production than later freezing. The model also includes a particulate organic matter (POM) variable, fed by ice melting losses to the water column, and shows a large redistribution of the POM produced by the ice ecosystem on a regional scale.

Nous examinons des résultats récents issus d'observations et de simulations numériques de la circulation générale et du climat de l'estuaire moyen et du golfe du Saint-Laurent. Nous focalisons cette étude sur l'intensité de la circulation estuarienne. L'approche méthodologique est présentée avec une emphase sur l'intégration, à l'aide de simulations numériques, des processus clés tels que le mélange turbulent associé aux marées à la tête du chenal laurentien et les échanges atmosphère-océan dans l'ensemble de l'estuaire maritime et du golfe. Les simulations numériques permettent de suivre l'évolution de la circulation et du climat sur des périodes de quelques heures à plusieurs années. Nous examinons d'une part la variabilité tidale de la circulation à la tête du chenal Laurentien et dans l'estuaire moyen, illustrant l'importance des processus qui gouvernent largement la circulation estuarienne du Saint-Laurent. Ces principaux processus sont les instabilités dynamiques associées à l'étirement de marée et aux sauts hydrauliques internes aux abords des seuils. D'autre part, la contrainte du vent est dominante pour propulser les eaux douces vers l'océan Atlantique dans l'estuaire maritime et le golfe. À l'aide d'expériences de sensibilité, les effets sur la circulation de variations dans l'apport d'eau douce et dans l'intensité des vents sont isolés. Les résultats montrent que notre modèle du Saint-Laurent répond aux forçages externes à l'instar d'un estuaire classique à deux couches lors des périodes de forte stratification (printemps et été), c'est-à-dire lorsque l'apport en eau douce entraîne une intensification de la circulation de la couche intermédiaire froide du golfe vers le continent jusqu'à une profondeur d'environ 150m. D'autre part, lors des périodes de plus faible stratification (automne et hiver), la contrainte du vent gouverne la circulation des couches d'eau profondes (plus de 150m) vers le continent. Un excès dans l'apport d'eau douce inhibe partiellement leur ventilation, comme dans une mer marginale nordique. Ces résultats suggèrent qu'une diminution éventuelle de l'apport en eau douce du fleuve Saint-Laurent, associée au réchauffement climatique ou au développement, pourrait diminuer l'intensité de la circulation estuarienne au printemps et à l'été. Durant l'hiver, une diminution du débit pourrait favoriser l'apport en eaux profondes du chenal Laurentien.

Twenty-year coastal altimetry data set (X-TRACK) is used, for the first time, to gain insight into the long-term inter-annual variations of the surface circulation in the Sicily Channel. Firstly, a spectral along with a time/space diagram analysis are applied to the monthly means of the X-TRACK geostrophic currents over the period 1993-2013. They reveal a regionally coherent current patterns from track to track with a marked inter-annual variability that is unequally shared between the Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhenian Current outflow northeast of Sicily. Secondly, an empirical altimetry-based transport-like technique is proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks and coastlines over the study area. A set of hydrographic measurements is used to validate the method. The inferred altimetry transports give a well-balanced mean eastward Atlantic Waters baroclinic flow of 0.4 Sv and standard deviations of 0.2 Sv on a yearly basis throughout the Sicily Channel and toward the Ionian Sea, which is fairly coherent with those found in the literature. Furthermore, the analysis allows to quantify the intrusions of Atlantic Waters over the Tunisian Shelf (0.12 +/- 0.1 Sv) and highlights two main modes of variability of the main surface waters path over the Sicily Channel through the Bifurcation Atlantic Tunisian Current and Atlantic Ionian Stream systems. Some physical mechanisms are finally discussed with regards to changes in the observed currents and transports.