Particle exchange and residence times in the North Western Mediterranean

The effects of the hydrodynamic processes on the distribution of passive drifters in the Gulf of Lions (GoL) have been investigated using a Lagrangian approach coupled with a 3D circulation model (Symphonie). We consider passive drifters, for which transport processes are determined solely by the 3D flow fields, which are in turn primarily forced by the North Mediterranean Current (NMC) and by the Rhˆone fresh-water inputs. The model reproduces 600 3D Lagrangian trajectories of particles released along the coastal area of the GoL during the winter period (January-February). The GoL has been divided into four sectors, each corresponding to a zone playing a strategic role in the hydrodynamics of the study area. The macroscopic characteristics of the transport on the shelf zone are analyzed in terms of total concentration and residence times of the cluster released in the basin. Particle distributions are strongly related to the mesoscale and sub-mesoscale hydrodynamic structures on the shelf and to the offshore circulation associated with the NMC. Two crucial areas are identified: a dispersive zone, corresponding to the central part of the continental shelf, and a wide offshore zone, representing an area of both aggregation and transition

Transfer of particulate matter from the Northwestern Mediterranean continental margin: Variability and controlling factors

International audienceLong-term observations of monthly downward particle fluxes and hourly currents and temperaturewere initiated in 1993 in two canyons of the continental margin of the Gulf of Lion. The goals of thissurvey were to estimate its contribution to the CO 2 global budget and to understand the role offorcing factors in the control of present-day particle exchange across this margin. A previousstatistical analysis of the long-term time series suggested that variability in the transfer of particulatematter to the deep ocean could be the result of the effect of the meandering of the Northern Currentand by dense water formation in winter rather than variations in the sources of matter. Numericalsimulations have been carried out to consider these hypotheses. A model is used to examine theimpact of local atmospheric forcing (wind stress, heat fluxes, precipitation–evaporation budget) onthe variability of the oceanic circulation and of mass fluxes within the canyons from December toApril, for five consecutive years between 1996 and 2001. Results show an east-west gradient of massexport on the shelf and a positive correlation between anomalies of dense water formation rates andinterannual variability of particle fluxes. However, in the eastern part of the Gulf, the simulated massexport from the shelf is not significant, even during a winter of strong convection, when the measuredparticle fluxes are at maxima. Moreover, although the model suggests that the dense water formationcould be the major hydrodynamic forcing factor, this process is not sufficient to completely explainthe space and time variations of observed particle fluxes, especially at depth

Assessment of the amount of Cesium-137 released into the Pacific Ocean after the Fukushima accident and analysis of its dispersion in Japanese coastal waters

International audienceNumerical modeling was used to provide a new estimate of the amount of 137Cs released directly into the ocean from the Fukushima Daiichi nuclear power plant (NPP) after the accident in March 2011 and to gain insights into the physical processes that led to its dispersion in the marine environment during the months following the accident. An inverse method was used to determine the time-dependent 137Cs input responsible for the concentrations observed at the NPP's two liquid discharge outlets. The method was then validated through comparisons of the simulated concentrations with concentrations measured in seawater at different points in the neighborhood of the plant. An underestimation was noticed for stations located 30 km offshore. The resulting bias in the release inventory was estimated. Finally, the maximum 137Cs activity released directly to the ocean was estimated to lie between 5.1 and 5.5 PBq (Peta Becquerel = 1015 Bq) but uncertainties remain on the amount of radionuclides released during the first few days after the accident. This estimate was compared to previous ones and differences were analyzed further. The temporal and spatial variations of the 137Cs concentration present in the coastal waters were shown to be strongly related to the wind intensity and direction. During the first month after the accident, winds blowing toward the south confined the radionuclides directly released into the ocean to a narrow coastal band. Afterwards, frequent northward wind events increased the dispersion over the whole continental shelf, leading to strongly reduced concentrations

Comportamiento del nitrógeno y fósforo en el estuario y en la pluma del río Ebro

La desembocadura del río Ebro es un estuario estratificado que cuando desemboca en el mar Mediterráneo forma una pluma cuya distribución está controlada fundamentalmente por la descarga del río y los vientos locales. Para estudiar el comportamiento de los nutrientes en el proceso de mezcla estuarina se recurrió a los gráficos de "curvas de mezcla" que aportan gran información si bien requiere una apropiada elección de los extremos (dulce y salino). Al estudiar el comportamiento del nitrógeno y fósforo se observó que amonio y PSR se comportan de diferente manera en el estuario que en la pluma, mientras que el nitrato y el nitrito tienen pautas de comportamiento similares en ambos ambientes. En el estuario se observan claras ganancias para el amonio y el fósforo que son consecuencia de la remineralización de la materia orgánica que se acumula principalmente en la zona de la interfase de la cuña salina. En cambio, en la pluma el comportamiento del amonio y del fósforo muestran pérdidas debidas probablemente a la absorción por parte del fitoplancton. Nitrito y nitrato exhiben comportamientos prácticamente conservativos tanto en el estuario como en la pluma.Romero Gil, I.; Falco Giaccaglia, SL.; González Del Rio Rams, J.; Rodilla Alamá, M.; Sierra, J.; Pérez Baliero, MDC.; Mosso, C. (2007). Comportamiento del nitrógeno y fósforo en el estuario y en la pluma del río Ebro. INGENIERIA DEL AGUA. 14(1):47-56. https://doi.org/10.4995/ia.2007.2902S4756141Agius, C. y V. Jaccarini, (1982). The effect of nitrate and phosphate enrichments on the phytoplankton from Marsaxlokk Bay, Malta (Central Mediterranean). Hydrobiologia, 87, 89-96.Aminot, A., (1983). Mesure de la salinité. En: Manuel des analyses chimiques en milieu marin, Aminot, A. y M. Chaus-sepied, (Eds.), M. Centre National pour l'Explotation des Océans, Brest, 45-62.APHA, (1995). Standard methods for the examination of water and wastewater, 19th edition. American Public Health Association, American Water Works Association, Water Environment Federation, Washington, 1108 pp.Broche, P., J.L. Devenon, P. Forget, J.C. de Maistre, J.J. Naudin y G. Cauwet, (1998). Experimental study of the Rhone plume. Part I: Physics and dynamics. Oceanologica Acta, 21 (6), 725-738.Burton, J.D., (1976). Basic properties and processes in estuarine chemistry. En: Burton, J. D. y P. S. Liss (eds), Estuarine chemistry. Academis Press, 1-36.Davies, A.M. y J.X. Xing, (1999). Sensitivity of plume dynamics to the parameterization of vertical mixing. International journal for numerical methods in fluids, 30, 357-405.Dolz Ripollés, J., M. Gómez Valentín y J. Nieto Romeral, (1997). El Ebro en el delta. Revista de Obras Públicas, No. 3.368, 7-14.Estrada, M., (1996). Primary production in the northwestern Mediterranean. Scient. Mar., 60 (Suppl 2), 50-64.Geyer, W.R. y D.M. Farmer, (1989). Tide-induced variation of the dynamics of a salt wedge estuary. Journal of Physical Oceanography, 19, 1060-1072.Ibáñez, C., D. Pont y N. Prat, (1997). Characterization of the Ebre and Rhone estuaries - A basis for defining and classifying saltwedge estuaries. Limnol. Oceanogr., 42 (1), 89-101.Kirkwood, D., A. Aminot y M. Pertillä, (1991). Report on the results of the fourth intercomparison exercise for nutrients in seawater. ICES Cooperative Research Report, 174, 83 pp.Kress N. y B. Herut, (1998). Hypernutrification in the oligotrophic eastern Mediterranean: A study in Haifa Bay (Israel). Estuar. coast. Shelf Sci., 46 (5), 645-656.Krom, M.D., N. Kress, S. Brenner y L.I. Gorden, (1991). Phosphorus limitation of primary productivity in the eastern Mediterranean sea. Limnol. Oceanogr., 36 (3), 424-432.Lohrenz, S.E., G.L. Fahnenstiel, D.G. Redalje, G.A. Lang, M. J. Dagg, T.E. Whitledge y Q. Dortch, (1999). Nutrients, irradiance, and mixing as factors regulating primary production in coastal waters impacted by the Mississippi River plume. Cont. Shelf Res., 19 (9), 1113-1141.Loder, T.C. y R.P. Reichard, (1981). The dynamics of conservative mixing in estuaries. Estuaries, 4 (1), 64-69.Lopez-Veneroni, D. y L.A. Cifuentes, (1994). Transport of dissolved organic nitrogen in Mississippi River plume and Texas-Louisiana continental shelf near-surface waters. Estuaries, 17 (4), 796-808.Magnien, R.E., R.M. Summers, y K.G. Sellner, (1992). External nutrient sources, internal nutrient pools, and phytoplankton production in Chesapeake Bay. Estuaries, 15 (4), 497{516.Marsaleix, P., C. Estournel, V. Kondrachoff y R. Vehil, (1998). A numerical study of the formation of the Rhone River plume. J. Mar. Syst., 14 (1-2), 99-115.Morris, A.W., J.I. Allen, R.J.M. Howland, y R.G. Wood, (1995). The estuary plume zone: Source or sink for land derived nutrient discharges?. Estuar. coast. Shelf Sci., 40, 387-402.Naudin, J.J., G. Cauwet, C. Fajon, L. Oriol, S. Terzic, J.L. Devenon y P. Broche, (2001). Effect of mixing on microbial communities in the Rhone River plume. J. Mar. Syst., 28, 203-227.Parsons, T.R., Y. Maita y C.M. Lalli, (1984). A manual of chemical and biological methods for seawater analysis. Pergamon Press, London, 173 pp.Pritchard, D.W., (1955). Estuarine circulation patterns. Prog. Asoc. Civ. Eng., 81, 1-11.Riley, J.P. y R. Chester, (1971). Introduction to marine chemistry. Academic Press, London, 465 pp.Ryther, J.H. y W.M. Dunstan, (1971). Nitrogen, phosphorus and eutrophication in the coastal marine environment. Science, 171- 1008-1013.Rodríguez, J., (1982). Oceanografía del mar Mediterráneo. Ediciones Pirámide, S.A., Madrid, 174 pp.Sanders, R., C. Klein y T. Jickells, (1997). Biogeochemical nutrient cycling in the Upper Great Ouse Estuary, Norfolk, U.K. Estuar. coast. Shelf Sci., 44, 543-555.Souchu P., A. Gasc, G. Cahet, A. Vaquer, Y. Collos y J.M. Deslous-Paoli, (1997). Biogeochemical composition of Mediterranean waters outside Thau Lagoon. Estuar. coast. Shelf Sci., 44 (3), 275-284.Thingstad, T.F., U. Li-Zweifel y F. Rassoulzadegan, (1998). P limitation of heterotrophic bacteria and phytoplankton in the northwest Mediterranean. Limnol. Oceanogr., 43 (1), 88-94.Treguer, P. y P. Le Corre, (1975). Manuel d'analyse des sels nutritifs dans l'eau de mer. Université de Bretagne Occidentale, Brest, 110 pp.Vollenweider, R.A., A. Rinaldi, R. Viviani y E. Todini, (1996). Assessment of the state of eutrophication in the Mediterranean sea. MAP Technical Reports Series no. 106. UNEP., Athens, 456 p

Seasonal and interannual variability of the pelagic ecosystem and of the organic carbon budget in the Rhodes Gyre (eastern Mediterranean): influence of winter mixing

The Rhodes Gyre is a cyclonic persistent feature of the general circulation of the Levantine Basin in the eastern Mediterranean Sea. Although it is located in the most oligotrophic basin of the Mediterranean Sea, it is a relatively high primary production area due to strong winter nutrient supply associated with the formation of Levantine Intermediate Water. In this study, a 3D coupled hydrodynamic–biogeochemical model (SYMPHONIE/Eco3M-S) was used to characterize the seasonal and interannual variability of the Rhodes Gyre's ecosystem and to estimate an annual organic carbon budget over the 2013–2020 period. Comparisons of model outputs with satellite data and compiled in situ data from cruises and Biogeochemical-Argo floats revealed the ability of the model to reconstruct the main seasonal and spatial biogeochemical dynamics of the Levantine Basin. The model results indicated that during the winter mixing period, phytoplankton first progressively grow sustained by nutrient supply. Then, short episodes of convection driven by heat loss and wind events, favoring nutrient injections, organic carbon export, and inducing light limitation on primary production, alternate with short episodes of phytoplankton growth. The estimate of the annual organic carbon budget indicated that the Rhodes Gyre is an autotrophic area, with a positive net community production in the upper layer (0–150 m) amounting to 31.2 ± 6.9 gCm-2yr-1. Net community production in the upper layer is almost balanced over the 7-year period by physical transfers, (1) via downward export (16.8 ± 6.2 gCm-2yr-1) and (2) through lateral transport towards the surrounding regions (14.1 ± 2.1 gCm-2yr-1). The intermediate layer (150–400 m) also appears to be a source of organic carbon for the surrounding Levantine Sea (7.5 ± 2.8 gCm-2yr-1) mostly through the subduction of Levantine Intermediate Water following winter mixing. The Rhodes Gyre shows high interannual variability with enhanced primary production, net community production, and exports during years marked by intense heat losses and deep mixed layers. However, annual primary production appears to be only partially driven by winter vertical mixing. Based on our results, we can speculate that future increase of temperature and stratification could strongly impact the carbon fluxes in this region.</p

Mississippi River and Sea Surface Height Effects on Oil Slick Migration

Millions of barrels of oil escaped into the Gulf of Mexico (GoM) after the 20 April, 2010 explosion of Deepwater Horizon (DH). Ocean circulation models were used to forecast oil slick migration in the GoM, however such models do not explicitly treat the effects of secondary eddy-slopes or Mississippi River (MR) hydrodynamics. Here we report oil front migration that appears to be driven by sea surface level (SSL) slopes, and identify a previously unreported effect of the MR plume: under conditions of relatively high river discharge and weak winds, a freshwater mound can form around the MR Delta. We performed temporal oil slick position and altimeter analysis, employing both interpolated altimetry data and along-track measurements for coastal applications. The observed freshwater mound appears to have pushed the DH oil slick seaward from the Delta coastline. We provide a physical mechanism for this novel effect of the MR, using a two-layer pressure-driven flow model. Results show how SSL variations can drive a cross-slope migration of surface oil slicks that may reach velocities of order km/day, and confirm a lag time of order 5–10 days between mound formation and slick migration, as observed form the satellite analysis. Incorporating these effects into more complex ocean models will improve forecasts of slick migration for future spills. More generally, large SSL variations at the MR mouth may also affect the dispersal of freshwater, nutrients and sediment associated with the MR plume

Origin and dynamics of mesoscale eddies in the Catalan Sea (NW Mediterranean): Insight from a numerical model study

Past observations and satellite sea surface temperature imagery indicate the presence of mesoscale anticyclonic eddies drifting along the Catalan coast. In September 2001 one of these anticyclonic eddies was surveyed over the shelf break during an océanographie cruise which permitted the 3-D description of its structure. In this work we investigate the origin and dynamics of such >Catalan eddies> using a numerical circulation model of the northwest Mediterranean at 3 km resolution driven by high-resolution atmospheric analyses and compare model eddies with the observations in the Catalan Sea. We identify two zones of eddy formation in the Gulf of Lions, in front of the city of Marseille and at the southeast of coast of Roussillon, from which anticyclonic eddies are observed to drift toward the Catalan Sea. The hydrology and dynamics of the structures observed in the simulations are characterized. Sensitivity experiments and energy analysis are performed which allow us to identify the mechanisms associated with their generation. Properties of the eddy observed during the 2001 cruise at the Catalan shelf break are found to compare well with model eddies generated at the southeast of the Roussillon coast. The model relates the origin of these eddies to the separation of the coastal current downstream from Creus Cape: flow separation is linked to intense downwelling taking place in front of the Roussillon coast when strong northwesterly winds events occur. Copyright 2009 by the American Geophysical Union.This work has been partially performed in the framework of the project "Génesis, evolución temporal, translación y efectos de remolinos de mesoescala sobre la plataforma continental Catalana (MAR1999-1010)" which has been funded by the Spanish Comission Interministerial de Ciencia y Tecnología (CYCIT) and the project MFSTEP funded by the European CommissionPeer Reviewe

On the role of wind and tides in shaping the Gironde River plume (Bay of Biscay)

International audienceWe study the Gironde River plume in the Bay of Biscay (North-East Atlantic) using a high resolution modelling approach of the estuary-mouth-shelf continuum. The Gironde River plume flows over a gently sloping shelf where semi-diurnal tides reach an amplitude of 1.3–1.5 m on average; the Gironde River discharge undergoes a significant seasonal variability with floods (>1000 m3/s) in winter and spring. From two years of simulation (2011–12), we document the behavior of the plume under varying forcing conditions. We then discuss the dynamical mechanisms at play, from the estimate of some mechanical energy budget terms and using sensitivity runs: run with no tides, run with filtered atmospheric forcing (no variability at time scales smaller than 30 days) and run with no river runoff. We first show that the plume's properties are determined by the processes occurring in a transition area at the estuary outlet between the Royan and Cordouan sections. The outflow passes through a deep and narrow channel both because of bathymetric constraints and tides. Indeed, tides generate a residual circulation made of anticyclonic and cyclonic vortices with diameters of 5–12 km. Tides also induce intense mixing that strongly modifies the stratification between the two sections. The mean conditions lead to classify the plume as a narrow outflow which is a favorable condition for a bulge to be formed according to the literature. We indeed observe the recurrent formation of a bulge with sharp salinity gradients and a narrow coastal current. The bulge develops in moderate to high discharge conditions, during the spring to neap tides transition and when winds are weak. When the intensity of tides and wind increases, the bulge breaks down and the plume spreads offshore under upwelling wind conditions or along the coast (both in northward and southward directions) under strong eastward wind conditions. Tides are the main driver at the estuary outlet (with a bottom friction term larger by two orders of magnitude than over the adjacent shelf) but winds and atmospheric pressure are the main forcing over the inner shelf. These results evidence the need to model accurately the whole estuary-shelf continuum when studying the fate of riverine waters over the shelf

Low-order pressure gradient schemes in sigma coordinate models: The seamount test revisited

International audienceThis paper revisits the classic seamount test used in numerous previous studies to evidence the sigma errors of the pressure gradient force (PGF) and their long-term effects on circulation. Two kinds of analysis are developed. We first consider the initial PGF errors. Then, the global level of erroneous kinetic energy is computed along a 180-day simulation. The long-term circulation appears to be better correlated to the initial vorticity errors than to the initial error diagnostics. The original feature of this study is to reconsider the currently admitted idea that Density-Jacobian type PGFs perform better than the primitive sigma formulation discretized in a straightforward way (hereafter Straightforward-Primitive PGF). Errors on the discrete hydrostatic pressure are actually closely related to the way the density field is initialized. If a mass conserving method is preferred to a straightforward initialization, the rectangular integral of the Straightforward-Primitive PGF is likely to be more accurate than the trapezoidal rule usually involved in Density-Jacobian PGFs. Errors on the vorticity field of the Straightforward-Primitive PGF depend on the discretization of the hydrostatic correction term. A modified version of the Straightforward-Primitive PGF is shown to be in better agreement with the concept of bottom torque consistency. The seamount tests show that this so-called Modified-Primitive PGF performs globally better than the current low-order Density-Jacobian PGFs