Article

On the use of long-term observation of water level and temperature along the shore for a better understanding of the dynamics: example of Toulon area, France

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Abstract

A dense network of instruments has been deployed within harbors along the Mediterranean coast, in the Toulon Metropole area, between the Hyères islands and the Sanary Bay in the framework of the observation network HTM-NET. Each station is equipped with two piezometric sensors, the first immersed and the second emerged, which allows the calculation of the water level. Both piezometric sensors are also equipped with a temperature sensor. Water level and temperature data are analyzed and discussed, also considering meteorological data provided by Météo-France stations. The tide gauges provide information about tide harmonic components, extreme water level, and seiching. Moreover, significant differences are observed between sheltered zones in enclosed bays and offshore zones, such as between the back of the Bay of Toulon and at the Port-Cros Island. Differences in water level up to 0.10 m are indeed observed under windy conditions, of the same order as the tidal range (order of 0.20 m) or the annual level variability due to the volumetric expansion (order of 0.10 m). Water level variations, up to about 1 m, are found to be mainly due to atmospheric effects, with a more or less isostatic behavior according to the weather events. In addition, seiching with an amplitude of few centimeters is observed within the Little Bay of Toulon, for east wind conditions. The near-surface water temperature is measured at the submerged piezometer location (depth of immersion range 0.10–1.80 m according to the station and to the water level). The analysis of the temperature associated with the weather conditions allows to detail a strong variability of the upwelling intensity under Mistral wind conditions in summer, leading to more or less pronounced temperature drops according to the shore configuration. The Bay of Toulon is more prone to the generation of upwellings than the neighboring bays. During winter, water exchanges between the Little Bay of Toulon and offshore are also clearly observed during windy conditions. The HTM-NET long-term observation network thus provides useful insights to increase our knowledge of the hydrodynamics and mass fluxes, and therefore enhances our modeling capacity and risk assessment at the scale of a bay.

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... A ces origines globales peuvent s'ajouter des causes plus locales, liées au vent et/ou aux états de mer, et associées à la configuration des sites. Une analyse détaillée de l'aire toulonnaise (REY et al., 2020) à partir de données du réseau HTM-NET (https://htmnet.mio.osupytheas.fr/) avait montré que les niveaux d'eau mesurés à cette échelle dans des zones abritées de la houle restaient sensiblement les mêmes, les différences de niveaux en présence de vent entre la Petite rade de Toulon (en fond de baie) et de Port Cros (au « large »), ne dépassant pas 0.10 m, d'un ordre inférieur aux principales causes de variations de niveau, la marée (environ 0.20 m) et l'effet barométrique inverse (environ 0.50 m par régime dépressionnaire). ...
... Le réseau HTM-NET, dont la période de mesure a été réduite à 2 min en 2019, et pour lequel de nouvelles stations ont été installées entre le Cap Couronne (ainsi que l'étang de Berre) à l'ouest et Menton à l'est, permet d'apporter des informations complémentaires dans la Rade (stations Les stations dans la zone étudiée ici sont indiquées sur la figure 1. Elles mesurent la température de l'eau, la pression atmosphérique et la pression dans l'eau. Le niveau d'eau est déduit des données de pression (REY et al, 2020). Les données météorologiques (notamment le vent) utilisées pour l'analyse sont fournies par Météo-France. ...
... This may explain the longer lags (54-66 h) reported at Agde. In this respect, the thermo-loggers located within semi-enclosed marinas or artificial coves (Rey et al., 2020) may not be the best proxies of intermittent wind-driven events as some may miss or underestimate the thermal responses that are best captured in the close vicinity of active cells. This also means that if up/downwelling cells are effectively co-located with the temperature monitoring location, then they should return high correlation and short lags. ...
... The wind-driven cooling at Port-Cros may be often diminished due to the Northern Current warm waters that overcome and/or push westward the upwelled cold waters during and after an event, leading to faster relaxation. To better understand how the geostrophic limitation modulates wind-forced processes, a multisensor analysis combining data from the pressure-sensors scattered along the coast (Rey et al., 2020) and satellite altimetry could help assessing the spatial variability of the cross-shore pressure gradient. ...
Article
Intermittent wind-driven coastal upwelling and downwelling are ubiquitous processes that drive a large part of the high frequency variability of coastal hydrography, with potential implications for ecosystems and socio-economic activities. Little synoptic information exists however on these processes, especially in regions characterized by rapidly changing atmospheric forcing and complex shorelines. Combining multi-annual hourly observations of nearshore temperatures with a long-term archive of Sea Surface Winds (SSW from ERA5 reanalysis), we investigate the statistical occurrence of wind-driven upwelling and downwelling events and their associated thermal responses along the northwestern Mediterranean coastlines. A Wind-based Upwelling and Downwelling Index (WUDI) is calculated at 20 km spatial resolution and validated against time-series of surface and subsurface in-situ temperatures at 11 coastal locations. We find that the WUDI index allows monitoring robustly all year round both up- and downwelling events that effectively cause coastal cooling/warming. On average, significant thermal responses to favorable winds appear after short delays (spanning 6–54 h for upwelling, 12–66 h for downwelling, depending on the site considered) with intensities 5 to 10 times stronger in stratified as compared to non-stratified conditions. Maximum near-surface cooling (subsurface warming, respectively) recorded after the most extreme events can reach up to -12.5 °C (+11 °C, respectively) during the period of seasonal stratification. A climatological database of wind-driven events that can be associated with typical thermal responses is constructed for the Northwestern Mediterranean shorelines over the last four decades. It shows that up/downwelling events are favored along certain portions of coastline, called “cells” to seasonality. We demonstrate that shorelines ranging from 4.0°E to 6.2°E are dominated by wind-driven upwelling, while shorelines ranging from 3.0°E to 3.5°E are dominated by wind-driven downwelling. Furthermore, it reveals previously overlooked cells, such as around Fréjus/Cannes and Livorno/Piombino for upwelling and near Albengua for downwelling, which are however activated about 2–3 times less frequently than the prominent cells in the Gulf of Lion. Despite differential responses, these wind-driven events are more frequent during winter-spring than during summer-autumn: for both upwelling and downwelling, the mean occurrences at the most active cells are 11 days per month in winter-spring compared to 8 days per month in summer-autumn. While the main upwelling (resp. downwelling) events are generated by the prevailing northwesterlies (resp. easterlies), both winds also force the opposite process depending on the shoreline orientation and small changes in wind direction.
... It is a urbanized zone, with several socioeconomic activities such as fishery and tourism, and the largest military port in France. Also, this highly anthropized area includes a preserved area of the National Park of Port-Cros (Rey et al. 2020). The geographic configuration of the region presents a steep continental shelf, going from tens of meter in the bay to several hundreds of meters offshore, and is composed by a semienclosed bay, the Giens Peninsula (GIENS), and several capes and islands. ...
... N1 represents an intense NC due to the influence of a strong easterly wind (during 11.5% of the time). Westerly winds occur more often than easterly ones [as observed in the climatology study by Rey et al. (2020)]. These five conditions have been previously reported and analyzed by several studies in the region, such as Guihou et al. (2013), Berta et al. (2018), Carret et al. (2019), and Molcard et al. (2021). ...
Article
To assess the contribution of wind drag and Stokes drift on the near-surface circulation, a methodology to isolate the geostrophic surface current from High-Frequency radar data is developed. The methodology performs a joint analysis utilizing wind field and in situ surface currents along with an unsupervised neuronal network. The isolation method seems robust in the light of comparisons with satellite altimeter data, presenting a similar time variability and providing more spatial detail of the currents in the coastal region. Results show that the wind-induced current is around 2.1% the wind speed and deflected from the wind direction between [18°, 23°], whereas classical literature suggests higher values. The wave-induced currents can represent more than 13% of the ageostrophic current component as function of the wind speed, suggesting that the Stokes drift needs to be analyzed as an independent term when studying surface sea currents in the coastal zones. The methodology and results presented here could be extended worldwide, as complementary information to improve satellite-derived surface currents in the coastal regions by including the local physical processes recorded by High-Frequency radar systems. The assessment of the wave and wind-induced currents have important applications on Lagrangian transport studies.
... These intrusions may be NC intrusions as in the Hyeres Bay upstream and an assessment of this hypothesis would be part of a future work. The tides contribute only for 3-4 cm on average of the free surface elevation, with a maximum of 20 cm in our zone, as confirmed by local observations (Rey et al., 2019). Therefore, we can say that the tide has a minor impact compared to other forcings. ...
... They are known to strongly impact the biogeochemistry of surrounding waters (Ross et al., 2016). To our knowledge, there is no evidence in the scientific literature for these upwellings in the area near Toulon, but recent observational data is confirming such phenomena (Rey et al., 2019). Some upwelling episodes could be identified in our simulations, but a specific study on these phenomena will have to be carried out, focusing on vertical movements, at very short time and space scale. ...
Thesis
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... These intrusions may be NC intrusions as in the Hyeres Bay upstream and an assessment of this hypothesis would be part of a future work. The tides contribute only for 3-4 cm on average of the free surface elevation, with a maximum of 20 cm in our zone, as confirmed by local observations (Rey et al., 2019). Therefore, we can say that the tide has a minor impact compared to other forcings. ...
... They are known to strongly impact the biogeochemistry of surrounding waters (Ross et al., 2016). To our knowledge, there is no evidence in the scientific literature for these upwellings in the area near Toulon, but recent observational data is confirming such phenomena (Rey et al., 2019). Some upwelling episodes could be identified in our simulations, but a specific study on these phenomena will have to be carried out, focusing on vertical movements, at very short time and space scale. ...
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Mediterranean semi-enclosed bays are often exposed to high levels of contaminants originating from anthro-pogenic activities in the bay. To assess their fate and impact on the environment, it is essential to investigate coastal circulation regimes which may play an important role in the dispersion of contaminants across the bay and beyond. In this study, a high resolution coupled hydrodynamic-passive tracer model was combined with ADCP observations, to identify major circulation patterns and associated dissolved contaminant dispersion pathways in the contaminated semi-enclosed bay of Toulon (South of France, NW Mediterranean Sea). Two dominant circulation patterns and two derived ones could be identified, driven by winds (Mistral and easterly winds) and offshore water intrusions. Medium to strong Mistral events (> 6 m s-1) with a WNW direction cause a bi-layer pattern with surface waters flowing out of the bay and marine waters entering at depth. Less frequently, west Mistral winds of medium to strong strength (> 6 m s-1) may generate an anticyclonic circulation. During easterly wind conditions (> 6 m s-1), an inward flow can be observed which is sometimes reinforced by offshore water intrusions, probably from the local boundary current, the Northern Current (NC). Furthermore, dissolved contaminant dispersion pathways were simulated under typical wind forcing conditions with three point sources of copper (Cu) that were identified based on surface Cu observations. While most of the WNW Mistral wind events transport dissolved copper plumes across and out of the bay, contaminant dispersion can remain confined to the bay under certain west mistral conditions. Conversely, during easterly wind events, contaminants are exiting the bay as a narrow vein along the Saint-Mandrier peninsula, before probably converging on the NC offshore. Accordingly, this study demonstrates the important impact of hydrodynamic-driven processes on the dispersion of contaminants within a semi-enclosed bay.
... Two stations are deployed in the N-W and the S-E parts of the lagoon, corresponding to the axis of the prevailing winds in the area. A similar experimental setup is used in the Berre lagoon thanks to the HTM-NET network (Rey et al. (2020)). Two stations are deployed in the N-W and S-E parts of the lagoon since February 2019. ...
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... For SPM measurements, water samples (2l) were filtered and weighed. For CDOM measurements, 60 ml water samples were collected and the absorbance was measured in the laboratory using a spectrometer equipped with a double beam monochromator; the excitation wavelengths are between 220 nm and 600 nm with 0.2 nm resolution (UV 1800 Shimadzu).Depth was measured with the sounder of the boat (CLIPPER NASA Marine) and corrected for the sea level [35]. Chlorophyll concentration, Suspended Particulate Matter, Colored Dissolved Organic Matter and depth for each station are given in Table 1. ...
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Thesis
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Technological advances in the recent satellite altimeter 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.
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The present work describes an operational strategy for the development of a multiscale modeling system, based on a multiple-nesting approach and open-source numerical models. The strategy was applied and validated for the Gulf of Taranto in southern Italy, scaling large-scale oceanographic model results to high-resolution coupled wave–3-D hydrodynamics simulations for the area of Mar Grande in the Taranto Sea. The spatial and temporal high-resolution simulations were performed using the open-source TELEMAC suite, forced by wind data from the COSMO-ME database, boundary wave spectra from the RON buoy at Crotone and results from the Southern Adriatic Northern Ionian coastal Forecasting System (SANIFS) regarding sea levels and current fields. Model validation was carried out using data collected in the Mar Grande basin from a fixed monitoring station and during an oceanographic campaign in October 2014. The overall agreement between measurements and model results in terms of waves, sea levels, surface currents, circulation patterns and vertical velocity profiles is deemed to be satisfactory, and the methodology followed in the process can constitute a useful tool for both research and operational applications in the same field and as support of decisions for management and design of infrastructures.
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SANIFS (Southern Adriatic Northern Ionian coastal Forecasting System) is a coastal-ocean operational system based on the unstructured-grid finite-element three-dimensional hydrodynamic SHYFEM model and providing short-term forecasts. The operational chain is based on a downscaling approach starting from the large-scale system for the entire Mediterranean basin (MFS, Mediterranean Forecasting system), which provides initial and boundary condition fields to the nested system. The model is configured to provide hydrodynamics and active tracer forecasts both in open ocean and coastal waters of South-eastern Italy using a variable horizontal resolution from 3–4 km in the open sea to 500–50 m in the coastal areas. Given that the coastal fields are driven by a combination of both local/coastal and deep ocean forcings propagating along the shelf, the performance of SANIFS was verified both in forecast and simulation mode, first (i) at the large and shelf-coastal scales by comparing with a large scale survey CTD in the Gulf of Taranto and then (ii) at the coastal-harbour scale (Mar Grande of Taranto) by comparison with CTD, ADCP and tide gauge data. Sensitivity tests were performed on initialization conditions (mainly focused on spin-up procedures) and on surface boundary conditions by assessing the reliability of two alternative datasets at different horizontal resolution (12.5 and 6.5 km). The SANIFS forecasts at a lead-time of one day were compared with the MFS forecasts highlighting that SANIFS is able to retain the large scale dynamics of MFS. Those get correctly propagated to the shelf-coastal scale improving the forecast accuracy (+17 % for temperature and +6 % for salinity compared to MFS). Moreover the added value of SANIFS was assessed at coastal-harbour scale, which is not covered by the coarse resolution of MFS, where the SANIFS forecasted fields well reproduced the observations (temperature RMSE equal to 0.11 °C). Furthermore SANIFS simulations were compared with hourly time-series of temperature, sea level and velocity measured at the coastal-harbour scale showing a good agreement. Simulations in the Gulf of Taranto described a circulation mainly characterized by an anticyclonic gyre with the presence of cyclonic vortexes in shelf-coastal areas. A surface water inflow from open sea to Mar Grande characterizes the coastal-harbour scale.
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The present work describes an operational strategy for the development of a multiscale modelling system, based on a multiple–nesting approach and open–source numerical models. The strategy was applied and validated for the Gulf of Taranto in South Italy, scaling large–scale oceanographic model results to high–resolution coupled wave – 3D hydrodynamics simulations for the area of Mar Grande in Taranto Sea. The spatial and temporal high – resolution simulations were performed using the open–source TELEMAC suite, forced by wind data from the COSMO–ME database, boundary wave spectra from the RON Buoy at Crotone, and results from the Southern Adriatic Northern Ionian coastal Forecasting System (SANIFS) regarding sea levels and current fields. Model validation was carried out using data collected in the Mar Grande basin from a fixed monitoring station and during an oceanographic campaign in October 2014. The overall agreement between measurements and model results in terms of waves, sea levels, surface currents, circulation patterns and vertical velocity profiles is deemed to be satisfactory, and the methodology followed in the process can constitute a useful tool for both research and operational applications on the same field and and as support of decisions for management and design of infrastructures.
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Sea-level variability in the Mediterranean Sea was investigated by means of in-situ (tide-gauge) and satellite altimetry data over a period spanning two decades (from 1993 to 2012). The paper details the sea-level variations during this time period retrieved from the two data sets. Mean sea-level (MSL) estimates obtained from tide-gauge data showed root mean square differences (RMSDs) in the order of 40–50 % of the variance of the MSL signal estimated from satellite altimetry data, with a dependency on the number and quality of the in-situ data considered. Considering the individual time-series, the results showed that coastal tide-gauge and satellite sea-level signals are comparable, with RMSDs that range between 2.5 and 5 cm and correlation coefficients up to the order of 0.8. A coherence analysis and power spectra comparison showed that two signals have a very similar energetic content at semi-annual temporal scales and below, while a phase drift was observed at higher frequencies. Positive sea-level linear trends for the analysis period were estimated for both the mean sea-level and the coastal stations. From 1993 to 2012, the mean sea-level trend (\(2.44\pm 0.5\) mm year\(^{-1}\)) was found to be affected by the positive anomalies of 2010 and 2011, which were observed in all the cases analysed and were mainly distributed in the eastern part of the basin. Ensemble empirical mode decomposition showed that these events were related to the processes that have dominant periodicities of \(\sim\)10 years, and positive residual sea-level trend were generally observed in both data-sets. In terms of mean sea-level trends, a significant positive sea-level trend (\(>\)95 %) in the Mediterranean Sea was found on the basis of at least 15 years of data.
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The sensitivity of the dynamics of the Mediterranean Sea to atmospheric pressure and free surface elevation formulation using NEMO (Nucleus for European Modelling of the Ocean) was evaluated. Four different experiments were carried out in the Mediterranean Sea using filtered or explicit free surface numerical schemes and accounting for the effect of atmospheric pressure in addition to wind and buoyancy fluxes. Model results were evaluated by coherency and power spectrum analysis with tide gauge data. We found that atmospheric pressure plays an important role for periods shorter than 100 days. The free surface formulation is important to obtain the correct ocean response for periods shorter than 30 days. At frequencies higher than 15 days−1 the Mediterranean basin's response to atmospheric pressure was not coherent and the performance of the model strongly depended on the specific area considered. A large amplitude seasonal oscillation observed in the experiments using a filtered free surface was not evident in the corresponding explicit free surface formulation case which was due to a phase shift between mass fluxes in the Gibraltar Strait and at the surface. The configuration with time splitting and atmospheric pressure always performed best; the differences were enhanced at very high frequencies.
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The sensitivity of the dynamics of the Mediterranean Sea to atmospheric pressure and free surface elevation formulation using NEMO (Nucleus for European Modelling of the Ocean) was evaluated. Four different experiments were carried out in the Mediterranean Sea using filtered or explicit free surface numerical schemes and accounting for the effect of atmospheric pressure in addition to wind and buoyancy fluxes. Model results were evaluated by coherency and power spectrum analysis with tide gauge data. We found that atmospheric pressure plays an important role for periods shorter than 100 days. The free surface formulation is important to obtain the correct ocean response for periods shorter than 30 days. At frequencies higher than 15 days−1 the Mediterranean basin's response to atmospheric pressure was not coherent and the performance of the model strongly depended on the specific area considered. A large-amplitude seasonal oscillation observed in the experiments using a filtered free surface was not evident in the corresponding explicit free surface formulation case, which was due to a phase shift between mass fluxes in the Gibraltar Strait and at the surface. The configuration with time splitting and atmospheric pressure always performed best; the differences were enhanced at very high frequencies.
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This paper describes the historical sea level data that we have rescued from a tide gauge, especially devised originally for geodesy. This gauge was installed in Marseille in 1884 with the primary objective of defining the origin of the height system in France. Hourly values for 1885–1988 have been digitized from the original tidal charts. They are supplemented by hourly values from an older tide gauge record (1849–1851) that was rediscovered during a survey in 2009. Both recovered data sets have been critically edited for errors and their reliability assessed. The hourly values are thoroughly analysed for the first time after their original recording. A consistent high-frequency time series is reported, increasing notably the length of one of the few European sea level records in the Mediterranean Sea spanning more than one hundred years. Changes in sea levels are examined, and previous results revisited with the extended time series. The rate of relative sea level change for the period 1849–2012 is estimated to have been \(1.08\pm 0.04\) mm/year at Marseille, a value that is slightly lower but in close agreement with the longest time series of Brest over the common period ( \(1.26\pm 0.04\) mm/year). The data from a permanent global positioning system station installed on the roof of the solid tide gauge building suggests a remarkable stability of the ground ( \(-0.04\pm 0.25\) mm/year) since 1998, confirming the choice made by our predecessor geodesists in the nineteenth century regarding this site selection.
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This paper addresses the problems of overlapping sea level time series measured using different technologies and sometimes from different locations inside a harbour. The renovation of the Spanish REDMAR (RED de MAReó-grafos) sea level network is taken here as an example of the difficulties encountered: up to seventeen old tide gauge stations have been replaced by radar tide gauges all around the Spanish coast, in order to fulfil the new international requirements on tsunami detection. Overlapping periods between old and new stations have allowed the comparison of records in different frequency ranges and the determination of the impact of this change of instrumentation on the long-term sea level products such as tides, surges and mean sea levels. The differences encountered are generally within the values expected , taking into account the characteristics of the different sensors, the different sampling strategies and sometimes the different locations inside the harbours. However, our analysis has also revealed in some cases the presence of significant scale errors that, overlapping with datum differences and uncertainties , as well as with hardware problems in many new radar gauges, may hinder the generation of coherent and continuous sea level time series. Comparisons with nearby stations have been combined with comparisons with altimetry time series close to each station in order to better determine the sources of error and to guarantee the precise relationships between the sea level time series from the old and the new tide gauges.
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A hydrodynamic model of the Bay of Toulon has been developed for use as a post-accident radionuclide dispersion simulation tool. Located in a Mediterranean urban area, the Bay of Toulon is separated into two basins by a 1.4-km long seawall. The Little Bay is semi-enclosed and connected to the Large Bay by a fairway channel. This channel is the site of significant water mass exchange as a result of both wind-driven currents and bathymetry. It is therefore a focal point for marine contamination. As part of the model calibration and validation process, the first step consisted of studying the water mass exchange between the two basins. An Acoustic Doppler Current Profiler was moored in the channel for 1 year. The present study analyses in situ data to determine the current intensity and direction, and also to better understand the vertical current profile, which is highly correlated with meteorological forcing. Comparisons of modelgenerated and measured data are presented, and various atmospheric forcing datasets are used to enhance computed results. It appears that accurate meteorological forcing data is needed to enhance the accuracy of the hydrodynamic model. This channel is an important location for water mass renewal in the Bay of Toulon, and model results are used to quantify these exchanges. The mean calculated annual water exchange time is approximately 3.4 days. However, this duration is strongly wind dependent and shortens during windy winter months. It ranges from 1.5 days during strong wind periods to 7.5 days during calm weather. Residence time values calculated through tracer dispersion modelling after release at the back of the Little Bay are found to be comparable to the mean exchange time values, especially for windy conditions.
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In the framework of climate change, the increase in ocean heat wave frequency is expected to impact marine life. Large-scale positive temperature anomalies already occurred in the northwestern Mediterranean Sea in 1999, 2003 and 2006. These anomalies were associated with mass mortality events of macrobenthic species in coastal areas (0-40 m in depth). The anomalies were particularly severe in 1999 and 2003 when thousands of kilometres of coasts and about 30 species were affected. The aim of this study was to develop a methodology to assess the current risk of mass mortality associated with temperature increase along NW Mediterranean continental coasts. A 3D regional ocean model was used to obtain the temperature conditions for the period 2001-2010, for which the model outputs were validated by comparing them with in situ observations in affected areas. The model was globally satisfactory, although extremes were underestimated and required correction. Combined with information on the thermo-tolerance of a key species (the red gorgonian P. clavata) as well as its spatial distribution, the modelled temperature conditions were then used to assess the risk of mass mortality associated with thermal stress for the first time. Most of the known areas of observed mass mortality were found using the model, although the degree of risk in certain areas was underestimated. Using climatic IPCC scenarios, the methodology could be applied to explore the impacts of expected climate change in the NW Mediterranean. This is a key issue for the development of sound management and conservation plans to protect Mediterranean marine biodiversity in the face of climate change.
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The Northern current is the main circulation feature of the North-Western Mediterranean Sea. While the large-scale to mesoscale variability of the northern current (NC) is well known and widely documented for the Ligurian region, off Nice or along the Gulf of Lions shelf, few is known about the current instabilities and its associated mesoscale dynamics in the intermediate area, off Toulon. Here, we took advantage of an oceanographic cruise of opportunity, the start of a HF radar monitoring programme in the Toulon area and the availability of regular satellite sea surface temperature and chlorophyll a data, to evaluate the realism of a NEMO-based regional high-resolution model and the added value brought by HF radar. The combined analysis of a 1/64° configuration, named GLAZUR64, and of all data sets revealed the occurrence of an anticyclonic coastal trapped eddy, generated inside a NC meander and passing the Toulon area during the field campaign. We show that this anticyclonic eddy is advected downstream along the French Riviera up to the study region and disturbs the Northern current flow. This study aims to show the importance of combining observations and modelling when dealing with mesoscale processes, as well as the importance of high-resolution modelling.
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A new generation of integrated sea surface temperature (SST) data products are being provided by the Global Ocean Data Assimilation Experiment (GODAE) High-Resolution SST Pilot Project (GHRSST-PP). These combine in near-real time various SST data products from several different satellite sensors and in situ observations and maintain the fine spatial and temporal resolution needed by SST inputs to operational models. The practical realization of such an approach is complicated by the characteristic differences that exist between measurements of SST obtained from subsurface in-water sensors, and satellite microwave and satellite infrared radiometer systems. Furthermore, diurnal variability of SST within a 24-h period, manifested as both warm-layer and cool-skin deviations, introduces additional uncertainty for direct intercomparison between data sources and the implementation of data-merging strategies. The GHRSST-PP has developed and now operates an internationally distributed system that provides operational feeds of regional and global coverage high-resolution SST data products (better than 10 km and ∼6 h). A suite of online satellite SST diagnostic systems are also available within the project. All GHRSST-PP products have a standard format, include uncertainty estimates for each measurement, and are served to the international user community free of charge through a variety of data transport mechanisms and access points. They are being used for a number of operational applications. The approach will also be extended back to 1981 by a dedicated reanalysis project. This paper provides a summary, overview of the GHRSST-PP structure, activities, and data products. For a complete discussion, and access to data products and services see the information online at www.ghrsst-pp.org.
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Heat exchanges between a medium-sized Mediterranean lagoon and the atmosphere are investigated at various time scales using long-term observations including 9 months of eddy covariance measurements. Turbulent heat fluxes are assessed using both eddy covariance and aerodynamic methods with a relative accuracy estimated at 15%. At time scales ranging from the diurnal to the seasonal cycle, heat exchanges appear to be dominated by evaporation, and most of the heat release of the lagoon occurs on time scales between 2 and 20 days. Seasonal variations are mainly due to the effects of net radiation and air temperature, while daily and short-term fluctuations of the sensible heat flux are influenced by air temperature and wind. In the short term, low relative humidity is more efficient than high wind speeds to produce strong peaks of evaporation (latent heat flux more than 350 W m-2). Episodes of Tramontane, a regional orography-induced wind, are shown to be responsible for 84% of the cooling of the lagoon surface (for cooling of more than 0.5°C) and for 70% of the heat loss due to evaporation. Short-term heat transfers between the lagoon and the atmosphere are largely punctuated by the two regional winds. The Tramontane generates pulses of evaporation lasting 1 to 4 days, while Marin episodes result in very low or negative latent heat fluxes and negative sensible heat fluxes corresponding to heat transfer from the atmosphere to the lagoon.
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A field survey was organized on the French Mediterranean coasts to investigate the effects of the tsunami induced by the 21 May 2003 Boumerdès-Zemmouri (Algeria) earthquake ( M<sub>w</sub> =6.9). The results show that eight harbours were affected by important sea level disturbances that caused material loss. Unfortunately, the low sampling rate of the French tide gage records (10 min) does not allow for a proper evaluation of the tsunami wave amplitudes since these amplitudes were probably underestimated in the harbours where these sensors are installed. The survey brings to light regional and local contrasts among the harbours' hydrological responses to the tsunami. To better understand these contrasts, a numerical simulation of the sea level elevations induced by the tsunami was conducted. The simulation showed a certain correlation between the field results and the wave amplification along the coast; however it underestimated the observed phenomena. Another simulation was then conducted using high resolution bathymetric grids (space step of 3 m) centred more specifically on 3 neighbouring harbours, however, again the simulation results did not match the amplitudes recorded through the observations. In order to better understand the wave amplification mechanisms inside each grid, a Gaussian signal was virtually broadcasted from the source to the harbours. Virtual sensors identified the periods which are stimulated – or not – by the arrival of the signal in each grid. Comparing these periods with those previously recorded emphasizes the proper period of each waterbody. This paper evaluates the limitations of such a study, focusing specifically on (1) the importance of having accurate and precise data about the source (the lack of information about the signal amplitude leads to an underestimation of the tsunami, thus reproducing only a fourth to a third of the observed phenomenon), (2) the need for networked tide gages with high resolution records and short sampling rates, and (3) the importance of conducting field studies immediately after a tsunami occurs.
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In the framework of the EU-funded MFSTEP project, autonomous drifting profilers were deployed throughout the Mediterranean Sea to collect temperature and salinity profile data and to measure subsurface currents. The realization of this profiler program in the Mediterranean, referred to as MEDARGO, is described and assessed using data collected between June 2004 and March 2006 (including more than 1500 profiles). Recommendations are provided for the permanent future implementation of MEDARGO in support of operational oceanography in the Mediterranean Sea. More than twenty drifting profilers were deployed from research vessels and ships-of-opportunity in most areas of the Mediterranean. They were all programmed to execute 5-day cycles with drift at a neutral parking depth of 350 m and CTD profiles from either 700 or 2000 m up to the surface. They stayed at the sea surface for about 6 h to be localised by, and transmit the data to, the Argos satellite system. The temperature and salinity data obtained with pumped Sea-Bird CTD instruments were processed and made available to the scientific community and to operational users in near-real time using standard ARGO protocols, and were assimilated into Mediterranean numerical forecasting models. In general, the cycling and sampling characteristics chosen for the MEDARGO profilers were found to be adequate for the Mediterranean. However, it is strongly advised to use GPS and global cellular phone telemetry or the future Argos bi-directional satellite system in order to avoid data compression and losses, for the continuation of the Mediterranean drifting profiler program.
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The predominant M2 tide in the Mediterranean Sea is examined using the linearized Laplace tidal equations including the astronomical gravitational potential, the Atlantic tide entering through the Strait of Gibraltar, the Earth tide, the effects of the Earth deformation due to the ocean tide load and bottom friction. The model solution is based on a decomposition of the primitive variables in terms of basis functions associated to the Helmholtz-Proudman potentials. Allowing for errors in the model and the observations and selecting the magnitude of the error in the model (32%), a robust estimate of the basin response yields a rms difference of 2.5 cm with the observations. -from Authors
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The densely populated cities of continental Mediterranean France, which are prone to erosion, are facing a potentially multi-hazard threat, due to a rise in sea-level that is expected to increase by between 0.07 and 0.12 m during the 21st century. The aim of this study is the superimposition of two widely used empirical indexes – the Coastal Sensitivity Index and the Social Vulnerability Index. In this research, the CSI is based on the following 6 parameters: geomorphology, coastal slope, sea-level rise, shoreline changes, mean tidal range and significant wave height, while the SVI used is constructed from 9 parameters: population < 14 years old, population over 75 years old, women, single parent families, families with more than two children, tenants, average density (inhabitants/km2), unemployed population, population with no education and foreigners. The research was initially conducted on the French Mediterranean coast, where environmental inequality was observed, and led to the selection of 3 areas of interest for a further investigation in finer scale (municipality/département/coastal district scale). It was noted that in certain cases the socio-environmental vulnerability of a municipality (as a whole) differed from the one presented in its coastal district. Thus, the socio-environmental vulnerability of a place is related to the study's scale, and the interest lies in the recognition of the most vulnerable coastal districts of cities, in conjunction with coastal sensitivity, in order to prioritize the efforts for coastal management.
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This paper presents a Coastal Risk Index (CRI-MED) developed to assess coastal risks and vulnerabilities associated with the physical and socio-economic impacts of climate change in all Mediterranean coastal zones. CRI-MED is a spatial risk index, which combines variables (multiple data layers) representing different aspects of risk in such a way that coastal areas of relatively higher risk emerge from the integration of the variables. It creates an interface between theoretical concepts of risk and the decision-making process relating to disaster risk reduction. Based on a GIS application, CRI-MED provides relative hazard, exposure, vulnerability and risk maps of the Mediterranean region that allow researchers and policy-makers to identify coastal areas most at risk from coastal erosion and coastal flooding, the so-called “hot-spots”. Through the application of CRI-MED on 21 Mediterranean countries, coastal hot-spots are found to be predominantly located in the south-eastern Mediterranean region. Countries with the highest percentage of extremely high risk values are Syria (30.5%), Lebanon (22.1%), Egypt (20.7%), and Palestine (13.7%). The CRI-MED method is intended as a scientific tool which produces easily understandable outcomes, to support international organizations and national governments to enhance and mainstream decision-making based on information that is accessible and useful. The definition of coastal hot-spots aims to support the prioritization of policies and resources for adaptation and Integrated Coastal Zone Management (ICZM). In particular, the resulting risk maps enable identification of suitable and less suitable areas for urban settlements, infrastructures and economic activities.
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The north-western Mediterranean Sea is a key location for the thermohaline circulation of the basin. The area is characterized by intense air-sea exchanges favoured by the succession of strong northerly and north-westerly wind situations (mistral and tramontane) in autumn and winter. Such meteorological conditions lead to significant evaporation and ocean heat loss that are well known as the main triggering factor for the Dense Water Formation (DWF) and winter deep convection episodes.During the HyMeX second field campaign (SOP2, 1 February to 15 March 2013), several platforms were deployed in the area in order to document the DWF and the ocean deep convection, as the air-sea interface conditions.This study investigates the role of the ocean-atmosphere coupling on DWF during winter 2012-2013. The coupled system, based on the NEMO-WMED36 ocean model (1/36° resolution) and the AROME-WMED atmospheric model (2.5 km-resolution), was run during two months covering the SOP2 and is compared to an ocean-only simulation forced by AROME-WMED real-time forecasts and to observations collected in the north-western Mediterranean area during the HyMeX SOP2.The comparison shows small differences in terms of net heat, water and momentum fluxes. On average, DWF is slightly sensitive to air-sea coupling. However fine-scale ocean processes, such as shelf DWF and export or eddies and fronts at the rim of the convective patch are significantly modified. The wind-current interactions constitute an efficient coupled process at fine scale, acting as a turbulence propagating vectors, producing large mixing and convection at the rim of the convective patch.
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General hydrodynamical features in this region of the northwestern Mediterranean Sea have already been described, mainly from the analysis of 30 one-year current meter records (Taupier-Letage and Millot, 1986). Mesoscale currents measured in the vicinity of the Ligurian current in the North and of the Western Corsican current in the South displayed marked differences. The mesoscale activity, as defined by the variance over 20-d periods, was quasi permanent in the South while it markedly increased during the winter season in the northern and central zones; in the latter one it reached maximum values at depths ranging from 100 m to at least 1100 m. These seasonal variations of the mesoscale activity led us to define a quiet and a stormy periods. This information is complemented in the present paper by results from auto- and cross-spectral analysis during the 2 periods. Features foreseen in the former paper, such as the vertical structure of the mesoscale currents, the dependence of their intensity on both space and season and the generation of deep winter-time mesoscale phenomena by the Liguro-Provencal current are supported by these computations. They also show that energy densities are relatively large in the central zone at the lowest frequencies, a feature which can be due to phenomena having different time scales and/or different phase speeds. -from Author
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A large number of several-month current time series is now available in the western Mediterranean Sea, and harmonic and spectral analyses provide spatially coherent information about the major tidal (M2, S2, N2 and K1) currents in the region. When they are significant, these currents are generally barotropic, they mainly rotate clockwise and their ellipses have orientations that are strongly dependent on the local bathymetry. Data on velocities of the semi-diurnal components and phase variations are presented for the region. -after Authors
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Air-sea exchanges play an important role during intense weather events over the Mediterranean Sea, especially in supplying heat and moisture for heavy precipitation events that often affect the area. Observations collected during the first HyMeX Special Observation Period (SOP1) over the Western Mediterranean area in autumn 2012 provide an unprecedented dataset for assessing the capabilities of numerical weather prediction systems to represent the air-sea interface and marine boundary layer during the heavy precipitation season. A HyMeX-dedicated version of AROME covering the whole western Mediterranean basin, named AROME-WMED, was evaluated through comparisons against moored buoys, drifting buoys and ship measurements deployed during the HyMeX campaign. A generally good agreement is found for near-surface meteorological parameters, whereas significant discrepancies are observed during strong air-sea exchange periods. The two main reasons are (i) SST is kept constant during the model runs; and (ii) sensible heat flux is overestimated in strong wind regimes by the AROME turbulent flux parameterisation.Air-sea exchanges during SOP1 were characterised thanks to AROME-WMED short-range (1-24 h) forecasts. It shows some areas of strong air-sea fluxes in the Gulf of Lion, and the Balearic, Ligurian and Tyrrhenian Seas. The Gulf of Lion is the area showing the highest variability of air-sea fluxes, due to dominant strong regional winds (Mistral/Tramontane). Whereas some heavy precipitation events occur without significant air-sea fluxes, all strong air-sea exchange events include, or occur only one or two days before, heavy precipitation events. A detailed analysis of an Intense Observation Period (IOP) dedicated to a heavy precipitation event (IOP13, from 12 to 15 October) illustrates how both dynamic (wind) and thermodynamic (temperature and humidity gradient effect) contributions influence air-sea flux evolution.
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Satellite altimetry, measuring sea surface heights (SSHs), has unique capabilities to provide information about the ocean dynamics. In this paper, the skill of the original full rate (10/20 Hz) measurements, relative to conventional 1-Hz data, is evaluated in the context of coastal studies in the Northwestern Mediterranean Sea. The performance and the question of the measurement noise are quantified through a comparison with different tide gauge sea level time series. By applying a specific processing, closer than 30 km to the land, the number of valid data is higher for the 10/20-Hz than for the 1-Hz observations: + 4.5% for T/P, + 10.3 for Jason-1 and + 13% for Jason-2. By filtering higher sampling rate measurements (using a 30-km cut-off low-pass Lanczos filter), we can obtain the same level of sea level accuracy as we would using the classical 1-Hz altimeter data. The gain in near-shore data results in a better observation of the Liguro-Provençal-Catalan Current. The seasonal evolution of the currents derived from 20-Hz data is globally consistent with patterns derived from the corresponding 1-Hz observations. But the use of higher frequency altimeter measurements allows us to observe the variability of the regional flow closer to the coast (~ 10-15 km from land).
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[1] This is a study of heat fluxes and heat budget of the Mediterranean Sea using the European Centre for Medium-Range Weather Forecasts (ECMWF) 45 year reanalysis data set ERA-40. The simple use of the ERA-40 surface flux components fails to close the budget and, in particular, the shortwave radiation flux is found to be underestimated with respect to observed data by about 10%. The heat flux terms are recomputed and corrected in order to close the heat and freshwater budgets of the Mediterranean basin over the period 1958 to 2001, thus producing a corrected ERA-40 surface flux data set. Various satellite and in situ observational data are used to construct spatially varying corrections to the ERA-40 products needed to compute the air-sea fluxes. The corrected interannual and climatological net surface heat and freshwater fluxes are −7 W/m2 and −0.64 m/yr, respectively, which are regarded as satisfactorily closing the Mediterranean heat and water budgets. It is also argued that there is an important contribution from large heat losses associated with a few severe winters over the Mediterranean Sea. This is shown to be related to wind regime anomalies, which strongly affect the latent heat of evaporation that is mainly responsible for the interannual modulation of the total heat flux. Furthermore, the surface total heat flux anomaly time series is compared with the North Atlantic Oscillation (NAO) index, and the result is a positive correlation with ocean warming for positive NAO index periods and ocean cooling associated with negative index periods.
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The Mediterranean is experiencing a number of immediate coastal problems which are triggering efforts to improve short-term coastal management. This paper shows that coastal management also needs to address long-term problems and, in particular, the likelihood of climate change. Regional scale studies suggest that the Mediterranean is particularly vulnerable to increased flooding by storm surges as sea levels rise—a 1-m rise in sea level would cause at least a six-fold increase in the number of people experiencing such flooding in a typical year, without considering population growth. Protection is quite feasible, however, this would place a greater burden on those Mediterranean countries in the south than those in the north. All coastal wetlands appear threatened. Case studies of coastal cities (Venice and Alexandria), deltas (Nile, Po, Rhone and Ebro), and islands (Cyprus) support the need to consider climate change in coastal planning. However, the critical issues vary from site to site and from setting to setting. In deltaic areas and low-lying coastal plains climate change, particularly sea-level rise, is already considered as an important issue, but elsewhere this is not the case. Therefore, there is a need for coastal management plans to explicitly address long-term issues, including climate change, and integrate this planning with short-term issues. This is entirely consistent with existing guidelines.1 Given the large uncertainty concerning the future, planning for climate change will involve identifying and implementing low-cost proactive measures, such as appropriate land use planning or improved design standards incorporated within renewal cycles, as well as identifying sectors or activities which may be compromised by likely climate change. In the latter case, any necessary investment can be seen as a prudent ‘insurance policy’.
Article
The effect of atmospheric pressure and surface wind vector on sea level variability in the Aegean Sea is investigated. Daily values of sea level, atmospheric pressure, and surface wind covering the period 1984-1987 from five stations are analyzed. Principal component (PC) analysis is used to extract the spatially coherent signal from the sea level and stmospheric pressure time series, while rotary principal component analysis is used to extract the coherent part of surface wind. The first PC modes are subsequently used in multiple regression analysis in the frequency domain in order to examine the dependence of sea level fluctuations on each of the parameters. The response of sea level to atmospheric pressure is found to be frequency dependent. The response is overisostatic for lower frequencies and becomes underisostatic for higher frequencies approaching isostasy only at the limit of analysis (0.5 cycles/day). The contribution of the first principal wind component is found to be very small.
Article
Sixteen years of satellite radar altimeter data are analyzed to investigate the sea-level variation (SLV) of the Mediterranean Sea. The time evolution of the overall mean sea level of the Mediterranean Sea follows its own regional dynamics. The geographical distribution of the seasonal signal (annual and semi-annual) indicates that the major features of the Mediterranean Sea circulation are driving the highest seasonal variability, and that an eastward propagation exists between the western and eastern basins. While in previous studies the trend of SLV has been modeled as linear, in this study with a longer record of observations we found that a quadratic acceleration term is statistically significant for practically the whole basin, especially in those regions where the trend provides a significant contribution to the SLV. The inclusion of the quadratic acceleration term accounts better for the Mediterranean SLV trend, as the residual low frequency SLV in wintertime is highly correlated with NAO at zero time lag in almost the whole basin. The residual high-frequency signal variability, on the other hand, can be explained by mesoscale phenomena, such as eddies and gyres. Our comprehensive analysis of the Mediterranean SLV provides source observations for monitoring and understanding of both regular and transient phenomena.
Article
The tidal propagation in the Mediterranean Sea is described through a high-resolution, two-dimensional hydrodynamic model forced by the equilibrium tide and the incoming tide at the Strait of Gibraltar. The four most significant constituents, M2, S2, K1, and O1, are included in the model. Good agreement with a set of 63 coastal gauges is achieved. The significance of the equilibrium tide and the forcing at the open boundary is investigated. The incoming wave from the Strait of Gibraltar is important in tuning the tides in the whole of the Mediterranean. For the north Aegean Sea the solution without the forcing at Gibraltar results in a doubling of the amplitudes of the semidiurnal tides. The estimated energy dissipation due to bottom friction is 8.8×108 W. The major area of dissipation is the Gulf of Gabes. Energy fluxes through the major straits are calculated.
Article
The 'barometer factor,' or ratio of daily mean sea level change to atmospheric pressure change, is reported for 17 Australian tide stations. The barometer factor is shown to be appreciably less than the theoretical value at four stations on the east Australian coast and appreciably greater than the theoretical value at two stations on the west Australian coast. Barometer factors and phase shifts as functions of frequency derived by spectrum analysis of 18-month records are presented for four of the stations. The effect of wind stress on sea level is discussed, and it is concluded that the effect is too small to account for the observed abnormal values of barometer factor. The observations suggest the presence of traveling continental shelf waves on both the east and west coasts.
Article
In the framework of the GMES (Global Monitoring for Environment and Security) MyOcean projects, funded by the European Commission, different remotely-sensed Sea Surface Temperature (SST) supercollated (L3S) and interpolated (L4) data are produced and distributed in near-real time by the Consiglio Nazionale delle Ricerche – Istituto di Scienze dell'Atmosfera e del Clima – Gruppo di Oceanografia da Satellite (CNR). These SST products are based on the night-time images collected by the infrared sensors mounted on different satellite platforms and cover the whole Southern European Seas (i.e. the Mediterranean Sea, including the eastern Atlantic Ocean, and the Black Sea). The CNR processing chains include several modules, from the data extraction and preliminary quality control, to cloudy pixel removal and satellite images collating/ merging up to L3S. A two-step algorithm finally allows to interpolate SST data at High (HR 1/16°) and Ultra-High (UHR 1/100°) spatial resolution. The basic design and the main algorithms used in MyOcean processing chains are described hereafter. A validation of the HR super-collating procedure developed during MyOcean is presented. The Mediterranean HR and UHR L3S and L4 operational products have then been validated vs in situ SST measurements from drifting buoys, covering two years of data (2010–2011). The validation includes a comparison between the UHR CNR L4 and the G1SST L4 developed by Chao et al. (2009).
Chapter
The overall functioning of the Mediterranean Sea, which transforms Atlantic Water (AW) into Mediterranean Waters (MWs), has been comprehended for awhile, and the process of dense water formation, which leads AW to sink offshore in specific northern zones of the Western and the Eastern Basins, has been studied in the world ocean. However, the circulation of the various waters from/to the basins openings to/from the zones of sinking is still debated in the Western Basin, while asimilar debate is only being initiated in the Eastern Basin. The differences between the circulation schemas published up to now can be large and they have already been commented upon in papers published recently. To provide acoherent introductory chapter, only the authors’ analysis is presented hereafter. Overall, and due to the Coriolis effect, all waters (AW and MWs) that circulate at basin scale tend to follow, in the counterclockwise sense, the isobaths at their own level. Hence they tend to describe, in both the Western and the Eastern Basins, quasi permanent gyres afew 10skm thick and afew 1000skm long along the continental slope. This simple schema is complicated by the fact that the southern parts of both gyres described by AW are markedly unstable, the AW inflow being hence identified with the so-called Algerian Current and Libyo-Egyptian Current and generating specific systems over the whole depth. Indeed, these currents (100–200 m deep) meander and generate, afew times per year, anticyclonic eddies that can reach diameters of 100–200 km (and even more), propagate downstream (i.e. eastward) at speeds up to afew km/day, and sometimes extend down to the bottom (2–3000 m). Hence, these eddies follow the deeper isobaths, separate from their parent current where these isobaths diverge from the upper continental slope, and drift for years (up to 3at least) in the central part of the basins, possibly coming back shoreward where they interact with their parent current, sometimes in adramatic way. These eddies entrain AW and MWs from the peripheral part of the basins towards their central part, together with eddies induced by the wind in the Eastern Basin only and having similar characteristics. In both basins, the northern parts of the gyres display specific features linking them to the zones of AW sinking, hence being identified (more easily in the Western Basin than in the Eastern one due to the topography) with the so-called Northern Currents. In addition to these circulation features, and because the various openings are only afew 100sm deep, the deeper MWs must be uplifted before outflowing. This is achieved permanently and more or less everywhere through continuous mixing with less dense waters and uplifting by newly formed denser waters. This is also achieved when and where intense mixing with other waters (cascading from sills or sinking) and sucking upward straits occur. Mediterranean Sea–Western and Eastern basins–Water masses– General circulation–Mesoscale
Article
Current meters were moored for a year both in and beyond the Cassis Bay to study the dispersion of muds discharged at a depth of ∼300 m, through a pipe, a few km from the coast and at the head of a submarine canyon. The east–west orientation of the coastline and a marked orography constrain the major winds to easterlies or westerlies. Empirical orthogonal function analysis shows that both wind regimes create a significant down-wind circulation, with the latter inducing also intense upwelling.Characteristics of the Northern Current, which flows along the slope, have been defined using a fine-grid hydrological survey. A narrow continental shelf allows the Northern Current to reach often the coastal zone in the study area; thus, the overall water circulation in the embayment is westward. In winter, this current develops steep mesoscale meanders and penetrates easily through the canyon, into the bay. Hence, although the coastal circulation near Marseilles is partly down-wind, it is dependent mainly on the Northern Current (on both general and meso-scales). The mesoscale variability cannot be predicted without monitoring the Northern Current itself.
Article
The long-term sea level change during 1992–2000 is investigated in the Mediterranean Sea from satellite altimetry data of the Topex–Poseidon, ERS-1 and ERS-2 missions and from tide gauge data. Tide gauge data observations during 1996–2000 agree better with Topex–Poseidon that with ERS-2 data. A relative trend between Topex/Poseidon (T/P) and ERS-2 data is observed and reduced by fitting the ERS-2 data to the T/P data. During the first eight years of the T/P mission the average linear sea level change over the entire Mediterranean Sea is 2.2 mm/year, in the western Mediterranean Sea the change is small (0.4 mm/year), while it is higher in the eastern Mediterranean and in the Ionian Sea, 9.3 mm/year and −11.9 mm/year, respectively. The high correlation of sea level height and sea surface temperature variations indicates the observed sea level change is of thermal origin primarily at seasonal scales. At low frequencies the atmospheric pressure and wind field variations play also a role.
Article
Mean sea level variability around the European coasts is explored on the basis of regional sea level indices derived through Empirical Orthogonal Function Analysis (EOF) of tide-gauge records. The regional indices are cross-correlated amongst themselves and against the major regional and climatic indices. The analysis is done for the whole year as well as seasonally. The effect of coherent atmospheric pressure signals is explored by comparing the results of the analysis before and after the data are corrected for the atmospheric pressure effects. The North Atlantic Oscillation Index (NAO) and the Mediterranean Oscillation Index are the major regional indices which are found to be significantly correlated with sea level variability around Europe. Their correlation is positive for the Northern European coast and negative for the Mediterranean coasts. The NAO influence causes an anticorrelation between northern and southern European sea level. This is stronger in winter and weakens significantly or disappears completely during the summer. When the NAO influence was removed from the regional mean sea level indices the cross-correlation between the various regions was reduced. However, residual spatial coherency indicated that probably there are other mechanisms causing spatial coherency. No statistically significant correlation with the Southern Oscillation Index (SOI) was found.
Article
An Acoustic Doppler Current Profiler (ADCP) was moored at the deep-sea site of the ANTARES neutrino telescope near Toulon, France, thus providing a unique opportunity to compare high-resolution acoustic and optical observations between 70 and 170 m above the sea bed at 2475 m. The ADCP measured downward vertical currents of magnitudes up to 0.03 m s-1 in late winter and early spring 2006. In the same period, observations were made of enhanced levels of acoustic reflection, interpreted as suspended particles including zooplankton, by a factor of about 10 and of horizontal currents reaching 0.35 m s-1. These observations coincided with high light levels detected by the telescope, interpreted as increased bioluminescence. During winter 2006 deep dense-water formation occurred in the Ligurian subbasin, thus providing a possible explanation for these observations. However, the 10-20 days quasi-periodic episodes of high levels of acoustic reflection, light and large vertical currents continuing into the summer are not direct evidence of this process. It is hypothesized that the main process allowing for suspended material to be moved vertically later in the year is local advection, linked with topographic boundary current instabilities along the rim of the 'Northern Current'.
Article
The Thau Lagoon, a lagoon that stretches over 7 500 ha, used to be connected to the sea via "graus" (channels) that are now closed. During the 17th century, the canals were dug that provide access to the Mediterranean Sea at Sète. Their width and depth, modified several times, reach 60 m and 9 m, respectively. It is important to understand how these modifications changed the hydrology of the Thau Lagoon.... Thau, étang de 7500 hectares, communiquait autrefois avec la mer par des graus, fermés actuellement. Au XVII siècle, ont été creusés les canaux de Sète qui le relient à la Méditerranée. Leur largeur et leur profondeur, plusieurs fois modifiées atteignent respectivement 60 m et 9 m. Ces modifications ont entraîné, dans l'hydrologie de l'étang de Thau, des changements qu'il importait de connaître. Au terme de cette étude, il apparaît que l'étang subit des influences diverses: climat et apports marins donnant à tous les phénomènes observés une certaine variabilité qui est la caractéristique même des nappes lagunaires. Néanmoins, sous ces variations diverses, dans le lieu et dans le temps, des facteurs hydrologiques essentiels: température et salinité, se dégagent des traits généraux qui ont été exprimés à la fin de chacun des précédents chapitres mais qu'il est utile de rappeler. 1° Les facteurs météorologiques ont un rôle marqué sur J'hydrologie de l'étang, Les principaux sont: a) la température de l'air qui, minimale en février, maximale en juillet. a une valeur moyenne de 13° 70 : b) les précipitations dont le volume, très variable d'une année à l'autre, est en moyenne de 568 mm par an, le nombre de jours pluvieux étant généralement faible (60 jours par an, en moyenne) ; c) les vents: la tramontane, le grec, le levant, le labech, l'argade et surtout le mistral qui est le plus violent et entraîne surtout en été une évaporation intense. 2° La marée est. à Sète, d'un type mixte caractérisé par une inégalité diurne variant au cours d'un mois, En mer, son amplitude est de 14 cm, Dans l'étang, elle peut atteindre 1 à 4 cm. La durée du flux et du reflux est variable mais généralement le flot est plus court que le jusant (en moyenne 5 h 35 pour le premier et 6 h 41 pour le second). A chaque marée, soit deux fois par jour, Je volume d'eau échangé entre la mer et l'étang est compris entre 750 000 et 3 750 000 m3. 3° Le vent et la pression atmosphérique influent sur les variations du niveau de la mer et favorisent les échanges entre la mer et l'étang. Le mistral provoque un abaissement des niveaux de la mer et de l'étang. qui se stabilisent entre 30 et 40 cm. Les vents d'est et les basses pressions entraînent une élévation de ces niveaux qui dans ces conditions sont parfois supérieurs à 80 cm. Le niveau de la mer est généralement inférieur de quelques centimètres à celui de l'étang, qu'il détermine. 4° Les courants, a) Les courants des canaux de Sète ont, au cours du flux et du reflux, une vitesse généralement proportionnelle à l'amplitude de la marée qui les provoque, La vitesse maximum peut dépasser 50 cm/s, mais elle est le plus souvent comprise entre 30 et 50 cm/s, b) Au cours du flux, l'étang des Eaux Blanches est parcouru par des courants de même direction générale (SE-NO) que ceux qui intéressent la partie des canaux de Sète comprise entre le pont Sadi-Carnot et l'étang: leur vitesse varie entre 0.20 m et 0,10 m à la seconde, Dans la partie orientale du Grand Etang, le courant de flot s'infléchit vers l'ouest et sa vitesse diminue: elle est généralement inférieure à 0,10 m/s. c) Pendant plusieurs heures parfois, des courants de sens inverse s'établissent en surface et sur le fond dans les canaux de Sète: ils s'accompagnent d'une diminution de vitesse. 5° En été, les températures minimales des eaux des canaux de Sète correspondent généralement aux salinités maximales. En hiver, les maximum de température et salinité sont concomitants, ainsi que les minimum, pour une même couche. Ces variations sont en relation directe avec les courants de marée qui sont à l'origine de fluctuations journalières dont le rôle dans l'hydrologie de l'étang est prépondérant. L'amplitude de ces variations peut dépasser pour la température 3° et pour la salinité 3 0/00 6° Dans une même journée, par le jeu du flux et du reflux, des eaux de natures différentes s'affrontent dans l'étang des Eaux Blanches: ce sont, soit des eaux d'origine marine récente, soit des eaux d'origine limnique, soit des eaux de mélange. Tantôt elles se remplacent, tantôt par suite des différences de densité, les couches superficielles glissent simplement sur celles du fond en prenant des directions opposées. 7° Dans un cycle hydrologique annuel. deux périodes principales ont été mises en évidence. a) Une période hivernale de novembre à mars au cours de laquelle la température des eaux de l'étang est in férieure à 13°. Les Eaux Blanches sont moins froides que le Grand Etang avec des différences le plus souvent inférieures à 1n mais pouvant atteindre 1°46. Les eaux du fond ont une température inférieure à celle des eaux superficielles avec des différences ne dépassant pas, généralement, 0° 50. Les écarts thermiques entre surface et fond sont plus accentués clans les Eaux Blanches que dans le Grand Etang (écart maximum observé pour le premier: 0° 97, et pour le second: 0° 31) , En effet, les eaux du fond présentent des caractéristiques qu'elles tiennent de leur origine marine et qui les différencient plus nettement des eaux limniques de surface. Ces différences nettes dans les Eaux Blanches, tendent à s'atténuer dans le Grand Etang. Les rives peu profondes sont plus froides que le centre (différences de l'ordre de quelques dixièmes de degré) car les couches d'eau qui sont directement en contact avec l'air froid se refroidissent plus rapidement sous faible épaisseur. Une courte période de transition située généralement en avril et caractérisée par une isothermie entre la mer, l'air et l'étang sépare les périodes hivernale et estivale. La température est alors voisine de 13° b) Une période estivale de mai à septembre au cours de laquelle la température des eaux de l'étang est supérieure à 13° . Les Eaux Blanches sont moins chaudes que le Grand Etang avec des différences dépassant parfois 2°. Les eaux du fond ont une température inférieure de 1° à 2° à celles de la surface. Les écarts thermiques entre surface et fond sont plus accentués dans les Eaux Blanches que dans le Grand Etang (écart maximum observé pour le premier: 3°85, et pour le second: 1°07). Ces différences, déjà observées en hiver, sont plus nettes en cette saison. Les rives peu profondes sont plus chaudes que le centre (les différences sont comprises entre quelques dixièmes de degré et plusieurs degrés) car l'insolation exerce une influence d'autant plus considérable que la couche d'eau est plus mince. 8° La température moyenne de Thau est généralement maximale en juillet (au plus 25°) et minimale en février (moins de 0° dans les cas extrêmes). Elle suit les fluctuations de celle de l'air et ne s'en écarte localement que sous l'influence marine et dans de faibles limites. En dehors de l'évolution normale, consécutive aux changements de saisons, des variations plus rapides peuvent se produire: elles sont dues, le plus souvent: aux vents et parmi ceux-ci plus particulièrement au mistral qui agit sur la température de l'air; en toutes saisons, il tend à refroidir les points de l'étang les moins abrités, le long du cordon littoral; aux pluies qui par J'intermédiaire des cours d'eau sont aussi la cause de variations localisées à leur embouchure. 9° La salinité moyenne de Thau est généralement plus élevée en été et en automne qu'au printemps et en hiver. Elle suit les fluctuations de la zone marine côtière qui est fortement influencée en surface par le courant du Rhône et les fleuves tributaires du golfe d'Aigues-Mortes. La salinité de Thau varie entre 31 et 39 0/00. Elle est presque toujours inférieure à celle de la mer (34 à 38 0/00). Les cours d'eau qui se jettent dans l'étang de Thau, auxquels il faut ajouter la Bise, sont à l'origine de ces différences et l'on constate dans ce cas que les Eaux Blanches sont plus salées que le Grand Etang. Il arrive cependant accidentellement que la salinité de Thau soit supérieure à celle de la mer par suite de la dilution brutale des eaux marines côtières, qui peut intervenir en toutes saisons mais ne dure généralement que quelques jours, ou par suite de l'évaporation intense qui se produit en Pté dans l'étang. Les Eaux Blanches sont alors moins salées que le Grand Etang. 10° On observe des différences de salinité entre la surface et le fond, celui-ci étant généralement plus salé. Pour les Eaux Blanches, la moyenne de nos mesures donne 35,04 en surface et 35,73 sur le fond. 11° L'étang des Eaux Blanches est plus salé que le Grand Etang. On observe aussi des variations d'un point à un autre: les stations voisines des cours d'eau ont une salinité moyenne moindre; la côte nord est, pour cette raison, moins salée que celle du cordon littoral. Sur un plan général l'étang de Thau présente une salinité qui diffère peu de celle de la Mer Méditerranée, toutefois les fluctuations de sa température, plus étendues à mesure que l'influence marine s'estompe, lui confèrent des caractères limniques. Il constitue une étape vers Je milieu lagunaire dont quelques-uns de ses points, éloignés de la mer. ont déjà nettement L'aspect bien différent des autres étangs du littoral méditerranéen français, à l'exception de l'étang de Berre, il reste cependant un milieu submarin caractéristique. (OCR non contrôlé)
The computation of Rossby radii of deformation for the Mediterranean Sea
  • F Grilli
  • N Pinardi