Gravity Waves Modelling Model Outputs Analysis Coastal Altimetry 3D Circulations Sediment transport, Ecology Data Assimilation
 
Contacts:
Claude Estournel
Coastal circulation associated to the wind curl
Along slope circulation & dense water formation over the shelf
Circulation in a small semi-enclosed bay
River plume

 
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The Fetch Experiment:
 
Observation and modelisation of the winter coastal oceanic circulation in the Gulf of Lions under wind conditions influenced by the continental orography (FETCH experiment):
                    The Gulf of Lion area is characterised by strong Northwesterly winds, Tramontane and Mistral. In winter, the turbulent mixing induced by wind tends to homogeneize the water column. Over the shelf, especially in the western part (relatively less sensitive to buoyancy effects due to the Rhone river outputs), the barotropic component of the circulation can prevail. The depth-averaged current is influenced by the bottom slope and the wind curl. Eddy circulations over the shelf were observed during the FETCH experiment (Gulf of Lion, winter 1998). Simulations showed that the spatial distribution of winds (wind curl) were responsible for theses particular oceanic features.
 


Atalante & Thetys, research vessels involved in FETCH and Moogli experiments. Copyright Laurence Eymard
      Hydrological and currentmeter observations were collected on the continental shelf and slope of the Gulf of Lion during the FETCH experiment (13 March to 15 April 1998). Results from the first part of the cruise, characterized by strong northern winds, are presented. The hydrological structures evidence well-mixed water masses on the eastern and western ends of the shelf. In the central part, the situation is more complex, with the influence of the Rhône river's freshwater plume in the first 40 m of the water column and, closer from the bottom, with the confrontation of downwelled coastal cold water and upwelled warmer and saltier slope water. Current measurements show the path of the cyclonic circulation along the slope, which is part of the general circulation of the western Mediterranean, and suggest the presence of large and temporary eddies on the shelf. This oceanic circulation is simulated with a free surface three-dimensional model using realistic forcing. The model outputs are in agreement with the main hydrological and circulation patterns. The results further indicate that coastal eddies are generated by the mesoscale structure of the wind field.
 
More details in: Estournel C., Durrieu de Madron X., Marsaleix P., Auclair F., Julliand C. and R. Vehil, 2003 Observation and modelisation of the winter coastal oceanic circulation in the Gulf of Lions under wind conditions influenced by the continental orography (FETCH experiment). Journal of Geophysical Research 108, C3, pages 7-1 to 7-18. doi:10.1029/2001JC000825
Modeling the deep convection in the northwestern Mediterranean Sea using an eddy-permitting and an eddy-resolving model: Case study of winter 1986-1987

    In the Northwestern Mediterranean sea, winter 1986-87 was particularly cold, inducing a strong open-ocean convection event. In order to investigate the impact of numerical models spatial resolution on the convection representation and the effects of deep convection on the Northwestern Mediterranean circulation, we perform two numerical three-dimensional simulations (eddy-permitting vs. eddy-resolving). Models are forced at the surface by the ERA40 atmospheric fluxes, with a simple heat flux correction to better mimic the observed value. We examine the characteristics of the deep convection (mixed layer, water masses characteristics , convection zone and mesoscale structures), and perform temporal analysis of this event in terms of kinetic energy, buoyancy equilibrium and deep water (DW) evolution. The convection characteristics are represented similarly on a global scale by both models and are in good agreement with observations, except for the size of the convection region. However, the eddy-resolving model reproduces better the mesoscale structures, whose role in the DW formation, mixing and transport is shown to be essential. The boundary circulation and the overturning are enhanced during the convection event. 66% of the DW spreading is due to the bleeding effect into the Catalan sea during the convection event, whereas 33% is due to the mesoscale structures southwestward advection after the event. 60% of the restratification with respect of the water column initial structure occurs before July 1987 and is due to light water advection. Afterwards, restratification is due to the mixing, and is not complete before next year convection.

Herrmann, M. J., S. Somot, F. Sevault, C. Estournel, and M. Deque, 2008. Modeling the deep convection in the northwestern Mediterranean Sea using an eddy-permitting and an eddy-resolving model: Case study of winter 1986-1987. Journal of Geophysical Research http://dx.doi.org/10.1029/2006JC003991

Impact of storms and dense water cascading on shelf-slope exchanges in the Gulf of Lion (NW Mediterranean)

      In situ observations of ocean temperature, salinity, density and current collected from November 2003 to May 2004 in the Gulf of Lion were combined with numerical modeling in order to better understand the mechanisms and forcing conditions that control shelf-slope exchanges during autumn and winter times. Outputs from a 3D coastal circulation model revealed that marine storms (and related processes) and dense water cascading were the two major mechanisms controlling shelf-slope exchanges. Marine storms induced accumulation of sea water along the coast, generated a strong cyclonic circulation on the shelf, and caused downwelling in submarine canyons that facilitated export of shelf water. During fall, because of strong water column stratification at that time, the depth of export remained shallow. In winter, the destratification together with the density increase of shelf water due to the cooling effect of strong and cold northerly winds - enabled shelf water to plunge down the slope. The results of this study thus highlighted the importance of marine storms for shelf slope exchanges, particularly during winter mixed conditions when they reinforced the cascading of dense water.

Ulses, C., C. Estournel, J. Bonnin, X. Durrieu de Madron, and P. Marsaleix 2008. Impact of storms and dense water cascading on shelf-slope exchanges in the Gulf of Lion (NW Mediterranean). Journal of Geophysical Research http://dx.doi.org/10.1029/2006JC003795

Dense water formation in the Gulf of Lions shelf: impact of atmospheric interannual variability and climate change
Herrmann M., Estournel C.,Déqué M., Marsaleix P., Sevault F., Somot S. (CSR 2008)

   Dense water formed over the continental shelf and cascading down the slope is responsible for shelf-slope exchanges in many parts of the world ocean, and transports large amounts of sediment and organic matter into the deep ocean. Here we perform numerical modeling experiments to investigate the impact of atmospheric interannual variability and climate change on dense water formation over the Gulf of Lions shelf, in the Northwestern Mediterranean Sea. Results obtained for a 140 years eddy-permitting simulation (1960-2100) performed over the whole Mediterranean sea under IPCC A2 scenario forcings are used to force a regional eddy resolving model of the Northwestern Mediterranean Sea. For the years selected in the present period, the quantity of dense water formed over and exported from the shelf is well correlated with atmospheric conditions, and dense water cascading is in agreement with available observations. During years colder than the average, most of the dense water formed over the shelf sinks into the deep ocean by cascading. During warmer years, dense water is mainly consumed by mixing with lighter surrounding water, and only a small quantity escapes the shelf, flowing along the coast without sinking. For the years selected in the future period, dense water formation over the shelf is strongly reduced, due to the stronger stratification of the water column. Most of the dense water formed is consumed over the shelf by mixing. A very small part escapes the shelf, flowing mainly in the surface layer: cascading practically disappears. The extrapolation of the results obtained for the selected years to the whole present and future periods suggests that volumes of dense water annually formed on the shelf, exported and cascading from the shelf are reduced by respectively 50%, 90% and 90% between the 20th century and the end of the 21st century. Uncertainties regarding our results are evaluated: the uncertainty due to the choice of the atmospheric forcing model is the most important, however, a decrease of cascading of at least 60% for the end of the 21st century compared to the present climate is obtained for every atmospheric model examined.
doi:10.1016/j.csr.2008.03.003
Dense water formation:

Relevance of ERA40 dynamical downscaling for modeling deep convection in the Mediterranean Sea

Herrmann, M. J., and S. Somot, 2008. Relevance of ERA40 dynamical downscaling for modeling deep convection in the Mediterranean Sea, Geophysical. Research. Letters http://dx.doi.org/10.1029/2007GL032442

      A strong open-ocean convection event was observed in the Northwestern Mediterranean sea during the 1986-87 winter. This period was used as a case study to evaluate the impact of the spatial resolution of atmospheric forcing on deep convection modeling. Twin numerical experiments were performed with an oceanic model forced by atmospheric forcing sets with different resolutions. A low resolution atmospheric forcing extracted from the ERA40 reanalysis was compared with a high resolution forcing produced by a dynamical downscaling of ERA40. A high resolution climate model spectrally driven by ERA40 fields for the large scales provided the dynamical downscaling dataset covering the 1958-2001 period. The oceanic simulation performed under low resolution meteorological forcing did not reproduce the observed convection. The simulation performed under high resolution forcing correctly reproduced the convection event. This was principally due to the enhancement of spatial and temporal meteorological extremes under the high resolution forcing.



Three-dimensional modeling of the Gulf of Lion's hydrodynamics (northwest Mediterranean) during Jannuary 1999 (MOOGLI3 Experiment) and late winter 1999: Western Mediterranean Intermediate Water's (WIW's) formation and its cascading over the shelf break
    The Gulf of Lion's hydrodynamics are investigated with a numerical simulation of the 1998-1999 winter, performed with a three-dimensional (3-D) free-surface model and real forcings. The model initial state derives from the large-scale outputs of the MOM (1/8°) using a specific initialization method. The model results are validated with an oceanographic data set collected in the gulf during the MOOGLI3 cruise (11-21 January 1999). The modeled outputs agree well with the main hydrological and circulation patterns observed during MOOGLI3. The study focuses on a strong and peculiar event of dense water (Western Mediterranean Intermediate Water (WIW)) formation on the continental shelf and its cascading over the shelf break. This latter was detected by a steep temperature decrease in the time series of a mooring line in the nearby Lacaze-Duthiers canyon which is well reproduced in the simulation. Modeled dense plumes appear during the MOOGLI3 period but more especially during late winter. Shapiro's theory of dense plume propagating on a sloping bottom is applied and provides information of the plume driving mechanisms. In both cases of plume studied, friction effects and Ekman drainage by the ambient current explain only a part of the plume motion. The complementary forcing is probably the steep local bathymetry irregularities. This 3-D simulation permits also to evaluate the total amount of WIW formed during the 1998-1999 winter over the Gulf of Lion's shelf to 500 km3.

More details in: Dufau-Julliand C., Marsaleix P., Petrenko A. and Dekeyser I., 2004. Three-dimensional modeling of the Gulf of Lion's hydrodynamics (northwest Mediterranean) during Jannuary 1999 (MOOGLI3 Experiment) and late winter 1999: Western Mediterranean Intermediate Water's (WIW's) formation and its cascading over the shelf break, Journal of Geophysical Research, 109, C11002, doi:10.1029/2003JC002019

Dense water formation and cascading in the Gulf of Thermaikos (North Aegean) from observations and modelling
Estournel C. , Zervakis V., Marsaleix P., Papadopoulos A., Auclair F., Perivoliotis L., Tragou E. (CSR 2005)

     Observations of dense water formation on the shelf of the Gulf of Thermaikos (North Aegean) are presented, based mainly on continuous monitoring of temperature and currents, during the winter of 2001-2002, at an instrumented mooring and a CTD survey carried out in early February 2002. A 2.5-month realistic simulation, corresponding to the period of observation, was performed to investigate the processes of dense water formation and cascading. The simulation is first compared to the main characteristics of the dense water, time variation of bottom temperature and spatial distribution of the dense water on the shelf. Subsequently, the simulation is used (a) to show that the formation of dense water takes place within the semi-enclosed Thessaloniki Bay and (b) to explain the intermittence of cascading out of the bay in relation to wind variations. The pathways of the dense water through the shelf are investigated with an emphasis on the role of the bottom slope and friction in the Ekman layer. The export of dense water towards the open sea occurs primarily along the slope bounding the western coast. doi:10.1016/j.csr.2005.08.014
 Along slope circulation
  The Liguro-Provencal Current (LPC) is a main feature of the general circulation of the Gulf of Lion (NW Mediterranean). This cyclonic regional circulation has a seasonal variability that is partly related to the offshore dense water formation induced by the strong atmospheric forcing. The LPC can develop small scale circulations, depending of the background stratification or external forcings, that can influence the shelf circulation.
  The LPC circulation has been studied using the 3D primitive equations Symphonie. Validations of model outputs were possible thanks to CTD and ADCP data or satellite images.
Contact: Claire Dufau-Julliand


Dense water formation over the shelf
  Intense, cold and dry, Tramontane and Mistral winds are likely to produce strong cooling of surface waters in winter. The continental shelf of the Gulf of Lion, wide and shallow, is especially sensitive to this meteorological constraint. Dense water can be formed over the shelf and then go down, following the slope. Dense water plume motions are rather complex because of the combined effects of gravity, Coriolis or turbulence in bottom boundary layer.
Contact: Claire Dufau-Julliand

The Gulf of Fos:
    The Gulf of Fos is a small semi-enclosed bay to the North of the Gulf of Lion and to the East of the Rhone river. Actually the proximity of the river mouth is such that intrusion of fresh water often occur, especially under South wind conditions. The bathymetry and coastline geometry of the Gulf of Fos are complex and affect residence times of water masses. High resolution simulations allowed to understand the circulation schemes in the two principal areas of the Gulf separated by the "They de la Gracieuse"
 
Circulation in a semi-enclosed bay under influence
of strong freshwater input

A high horizontal resolution 3D hydrodynamic model was applied to a semi-enclosed bay (Gulf of Fos, western Mediterranean Sea) in order to describe the specific circulation patterns driven by winds and to infer the scales of residence times. Freshwater inputs to this bay come from the Rhône river and navigation channels. Idealised simulations under typical wind forcing conditions were performed and are described in this paper. They revealed several features of the exchanges of water masses in the Gulf of Fos, in particular the intrusion of the Rhône river plume generated by south winds. During northern wind conditions, surface waters flushed out of the system. To compensate for this outflow, bottom currents transporting marine waters took the opposite direction. Residence times in the different areas of the gulf were also investigated. A realistic simulation was performed under actual conditions of Rhône river discharges, meteorological forcing and impact of the surrounding general circulation. Model outputs, mainly salinity fields, were compared to 10-day observations acquired during a multidisciplinary cruise that took place in May 2001. Model results and observations were in good agreement and showed a massive inflow of the Rhône river plume waters into the Gulf of Fos during a south wind event. Sensitivity studies showed that the salinity field was strongly dependent on the wind direction and on the local freshwater discharges.
 		 
 
More details in:
 Ulses C., Grenz C., Marsaleix P., Schaaff E., Estournel C., Meulé S., and Pinazo C., 2005 Circulation in a semi enclosed bay under the influence of strong fresh water input, Journal of Marine Systems, 56, 113-132 doi:10.1016/j.jmarsys.2005.02.001 		 

The Rhône river plume:
 

RV Tethys at Port Saint Louis du Rhône


The Rhone river is a major source of fresh water (actually the biggest river discharge of the Mediterranean Sea). Its mouth is located to the East of the Gulf of Lion continental shelf. In macrotidal cases, tidal currents in shallow areas such estuaries tend to mix the plume with oceanic waters. Because of the weakness of tidal currents in the Gulf of Lion, the case of the Rhone river is quite original since the plume can export itself a few dozen kilometers away from the mouth without loosing its high salinity contrast. 3D simulations allowed us to study the influence of wind on plume shape. Model outputs were validated thanks to various in situ measurements as well as remote sensing observations of the plume.
 
Numerical simulation and remote sensing:

   The plume of the Rhone (western Mediterranean) is studied with the help of a three-dimensional ocean model using primitive equations. Particular attention is paid to the wind forcing. Three simulations of realistic cases, using wind and river discharge measurements, are described. Satellite observations of the plume make it possible to assess the quality of the numerical simulation in each case. The analysis of circulation enables an evaluation of the characteristic time scales of deformations in the plume during changes in wind conditions and also highlights the high sensitivity of the plume to the surrounding circulation, dominated by coastal currents associated with upwelling and downwelling motions induced by the northwest and southeast winds.

More details in:
Estournel C., Kondrachoff V., Marsaleix P., Vehil R., 1997
The plume of the Rhône : numerical simulation and remote sensing, Continental Shelf Research, 17,899-924. doi:10.1016/S0278-4343(96)00064-7


The Rhone river plume in unsteady conditions:

   Surface current maps obtained in the plume of the Rhome river by two VHF radars gave rise to comparisons with simulations produced by a primitive-equation model. Two situations were investigated: one in steady conditions, the other with a variable wind. The model gives a good estimation of current direction and velocity not only in the plume itself but also in the surrounding water. The model reveals the mechanisms involved in the plume dilution and more generally in the renewal of surface water near the coast when the wind changes direction. The sensitivity of the model results to the upper boundary condition for turbulent kinetic energy is also shown....

More details in:
Estournel C., Broche P., Marsaleix P., Devenon J.L., Auclair F. and Vehil R, 2001
The Rhone river plume in unsteady conditions : numerical and experimental results. Estuarine, Coastal and Shelf Science. 53, 25-38. doi:10.1006/ecss.2000.0685

 
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