Claude Estournel

Dense shelf water cascading in the northwestern Mediterranean during the cold winter 2005: Quantification of the export through the Gulf of Lion and the Catalan margin

Dense shelf water cascading in the northwestern Mediterranean Sea during winter 2005, which was shown to cause large erosion in the canyons and to influence deep benthic ecosystem, was investigated using numerical modeling validated with temperature and current observations. Intense dense water formation took place on the Gulf of Lion and Catalan shelves. Dense shelf water was transferred to the deep basin through three pathways. The Cap de Creus canyon in the western Gulf of Lion already identified as a huge pathway was shown to export about 1000 km3 of dense water during two months. The Palamos and Blanes canyons located on the Catalan margin were shown to be important pathways for water formed locally and for water transiting from the Gulf of Lion. After the cascading period, dense shelf water was transported mostly toward the Balearic Sea.

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

Numerical 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 NPPs 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.

A numerical study of the formation of the Rhône River plume

A three-dimensional primitive equation ocean model is applied to the Rhone River plume (northwestern Mediterranean). The plume dynamics are analyzed through the vorticity balance of the depth-averaged current. This balance highlights the importance of the JEBAR and nonlinear terms. The topography induces the freshwaters to enter the Gulf of Fos and progress eastwards. The momentum advection is responsible for the large offshore extension of the plume. Northwestern winds (characteristic of the regional climate) may detach the plume from the coast. The wind stress magnitude required for this detachment is commensurate with Garvines dimensional analysis. A satisfactory comparison is made between the numerical experiments and satellite observations for weak and strong wind conditions. Finally, a sensitivity test of the model to the vertical coordinate reveals that a double-sigma system preserves the salinity gradients and current intensity in the frontal zones of the plume better than a conventional sigma coordinate.

Influence of Geostrophic Wind on Atmospheric Nocturnal Cooling

Abstract A dynamic radiative model was used to study the influence of the geostrophic wind on the nocturnal cooling processes. For weak wind conditions, an important difference appears between the top levels of the inversion and turbulent layers. The dimensionless vertical profile of turbulent heat flux presents an important curvature at the beginning of the night; afterwards this profile varies little during the night.

High resolution modeling of dense water formation in the north-western Mediterranean during winter 2012-2013: Processes and budget

The evolution of the stratification of the north-western Mediterranean between summer 2012 and the end of winter 2013 was simulated and compared with different sets of observations. A summer cruise and profiler observations were used to improve the initial conditions of the simulation. This improvement was crucial to simulate winter convection. Variations of some parameters involved in air - sea exchanges (wind, coefficient of transfer used in the latent heat flux formulation, and constant additive heat flux) showed that the characteristics of water masses and the volume of dense water formed during convection cannot be simply related to the time-integrated buoyancy budget over the autumn - winter period. The volume of dense water formed in winter was estimated to be about 50,000 km 3 with a density anomaly larger than 29.113 kg m -3 . The effect of advection and air/sea fluxes on the heat and salt budget of the convection zone was quantified during the preconditioning phase and the mixing period. Destratification of the surface layer in autumn occurs through an interaction of surface and Ekman buoyancy fluxes associated with displacements of the North Balearic front bounding the convection zone to the south. During winter convection, advection stratifies the convection zone: from December to March, the absolute value of advection represents 58 % of the effect of surface buoyancy fluxes.

Estimating dense water volume and its evolution for the year 2012–2013 in the Northwestern Mediterranean Sea: An observing system simulation experiment approach

The Northwestern Mediterranean (NWMed) Sea includes one of the best observed ocean deep convection sites in the World. An observing system simulation experiment (OSSE) is developed to provide a methodology for estimating observing network errors. It is applied to quantify dense water volumes in the NWMed during 2012–2013 with their observation error from MOOSE network. Results from the OSSE show low spatiotemporal sampling errors, which confirms MOOSE network ability to measure dense waters. However, results are highly sensitive to instrumental stability. The dense water volume is then estimated in observations from four ship cruises between summers 2012 and 2013. A large seasonal cycle is found, maximal in spring 2013 and dominated by the area west of 6.5°E. The dense water volume ( σ0>29.11 kg/m3) is stable between summer 2012 ( 13.3±0.6 × 1013 m3) and winter 2013 ( 13.7±1.3 × 1013 m3). It increases dramatically in spring 2013 ( 17.7±0.9 × 1013 m3) due to an intense convective event, and it finally decreases rapidly in summer 2013 ( 15.1±0.6 × 1013 m3) due to restratification and spreading. We estimate an open-sea dense water formation (DWF) rate of 1.4±0.3 Sv between summer 2012 and spring 2013 over the studied area, extrapolated to 2.3±0.5 Sv over the whole NWMed Sea and for the optimal timing. This is to our knowledge the highest measured DWF rate, suggesting winter 2013 was exceptionally convective. The observed restratification rate between spring and summer 2013 is −0.8±0.4 Sv. This study provides robust quantifications of deep convection during an exceptional event that will allow to evaluate numerical simulations.

The penetration of the Northern Current over the Gulf of Lions (Mediterranean) as a downscaling problem

Abstract Coastal shelf models suffer from the difficulty of specifying both the initial field and the external circulation at the open boundary. Only basin scale models can have the right space-time variability to routinely provide such information. De facto, such models have however a coarser resolution than the coastal model and the downscaling of the circulation to the high resolution grid is biased. Indeed the models are not necessarily based on the same physics and the interactions of the general circulation with the topography, in particular over the shelf break region, are generally not correctly represented on the large-scale grid. Studying the response of the Northern Current to the atmospheric forcing in the region of Marseille (France) and its interactions with the continental shelf break, we evaluate in this paper the spurious consequences at short and medium range that can result from a crude interpolation of basin scale model outputs to initialize and force along its open boundaries a high resolution coastal model. We show how an analysis of these fields based on a 3D variational initialization technique can improve the results.

A new parameterization of eddy diffusivities for nocturnal boundary-layer modeling

Exchange coefficients and mixing lengths under stable stratification have been studied through measurements of mean wind velocity and temperature in the nocturnal boundary layer. For values of the gradient Richardson number lower than 0.15, our measurements fit well the relation of Delage (1974). Beyond Ri = 0.15, the decrease of mixing length is much slower. So a new parameterization of turbulent exchanges is suggested. When introduced in a model of the nocturnal boundary layer, it results in a thickening of the turbulent and inversion layers.

Impact of climate change on the northwestern Mediterranean Sea pelagic planktonic ecosystem and associated carbon cycle

The northwestern Mediterranean Sea (NWMS) is biologically one of the most productive Mediterranean regions. NWMS pelagic planktonic ecosystem is strongly influenced by hydrodynamics, in particular by deep convection that could significantly weaken under the influence of climate change. Here we investigate the response of this ecosystem and associated carbon cycle to the long-term evolution of oceanic and atmospheric circulations. For that we developed a tridimensional coupled physical-biogeochemical model and performed two groups of annual simulations under the climate conditions of respectively the 20th and the end of 21st centuries. Our results suggest that the evolution of oceanic and atmospheric circulations does not modify the NWMS pelagic planktonic ecosystem and associated carbon cycle at a first order. However, differences mainly induced by the deep convection weakening and the surface warming are obtained at a second order. The spring bloom occurs 1 month earlier. Resulting from the decrease in nutrients availability, the bottom up control of phytoplankton development and bacteria growth by the nitrogen and phosphorus availability strengthens and the microbial loop intensifies as the small-sized plankton biomass increases. Carbon net fixation and deep export do not change significantly. The choice of the biogeochemical initial and boundary conditions does not change the representation of the ecosystem seasonal cycle, but the associated uncertainty range can be one order of magnitude larger than the predicted interannual and long-term variabilities. The uncertainty range of long-term trends associated with the physical forcing (hydrological, atmospheric, hydrodynamical, and socioeconomic) is much smaller (<10%).

Effects of Sahel dust layers upon nocturnal cooling of the atmosphere (ECLATS experiment)

Abstract A study of the effect of a desert aerosol layer on the nocturnal cooling of the atmosphere is presented. The experimental data were obtained during the ECLATS experiment which was run in the Sahel region of the Niger in November 1980. This study uses measurements of thermodynamic and radiative parameters, aerosol size distribution, and a radiative model. The results show that the presence of a dust layer at night increases the downward infrared flux at the surface (and then modifies the energy budget) and increases the radiative cooling rate of the atmosphere, slightly in the layers near the ground (∼0.15 K h−1) and in a more important way at the top of the haze layer.