Patrick Marsaleix

The plume of the Rhone: numerical simulation and remote sensing

Abstract 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.

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.

Space-time structure and dynamics of the forecast error in a coastal circulation model of the Gulf of Lions.

Abstract The probability density function (pdf) of forecast errors due to several possible error sources is investigated in a coastal ocean model driven by the atmosphere and a larger-scale ocean solution using an Ensemble (Monte Carlo) technique. An original method to generate dynamically adjusted perturbation of the slope current is proposed. The model is a high-resolution 3D primitive equation model resolving topographic interactions, river runoff and wind forcing. The Monte Carlo approach deals with model and observation errors in a natural way. It is particularly well-adapted to coastal non-linear studies. Indeed higher-order moments are implicitly retained in the covariance equation. Statistical assumptions are made on the uncertainties related to the various forcings (wind stress, open boundary conditions, etc.), to the initial state and to other model parameters, and randomly perturbed forecasts are carried out in accordance with the a priori error pdf. The evolution of these errors is then traced in space and time and the a posteriori error pdf can be explored. Third- and fourth-order moments of the pdf are computed to evaluate the normal or Gaussian behaviour of the distribution. The calculation of Central Empirical Orthogonal Functions (Ceofs) of the forecast Ensemble covariances eventually leads to a physical description of the model forecast error subspace in model state space. The time evolution of the projection of the Reference forecast onto the first Ceofs clearly shows the existence of specific model regimes associated to particular forcing conditions. The Ceofs basis is also an interesting candidate to define the Reduced Control Subspace for assimilation and in particular to explore transitions in model state space. We applied the above methodology to study the penetration of the Liguro-Provencal Catalan Current over the shelf of the Gulf of Lions in north-western Mediterranean together with the discharge of the Rhone river. This region is indeed well-known for its intense topographic and atmospheric forcings.

Post-processing Altimeter Data Towards Coastal Applications and Integration into Coastal Models

Altimetry missions in the last 16 years (TOPEX/Poseidon, ERS-1/2, GFO, Jason-1 and ENVISAT) and the recently-launched Jason-2 mission have resulted in great advances in deep ocean research and operational oceanography. However, oceanographic applications using satellite altimeter data have become very challenging over regions extending from near-shore to the continental shelf and slope (Cipollini et al. 2008). In these regions, intrinsic difficulties in the corrections (e.g., the high frequency ocean response to tidal and atmospheric loading, the mean sea level, etc.) and issues of land contamination in the radar altimeter and radiometer footprints result in systematic flagging and rejection of these data. Forthcoming altimeter missions (SARAL/AltiKa, SWOT, Sentinel-3, etc.) are designed to be better-suited for use in the coastal ocean. However, a number of studies have dealt with the problem of re-analysing, improving and exploiting the existing archive to monitor coastal dynamics. The early encouraging results (Vignudelli et al. 2005; Bouffard et al. 2008, Birol et al. submitted J Mar Syst 2009) support the need for continued research in coastal altimetry, with the opportunity of providing input and recommendations for future missions.

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.

On the wind-driven coastal upwelling in the Gulf of Lions

Abstract Satellite imagery has revealed the existence of discrete and relatively cool zones of surface upwell water along the Mediterranean coast of the Gulf of Lions off the South of France. The imagery relates to a period of time during which the character of the local surface wind field corresponds to the onset and subsequent persistence of a Mistral-Tramontane event, this consisting of strong northerly and northwesterly winds. Using a multi-level three-dimensional numerical model of wind-driven coastal upwelling dynamics in which turbulence closure is achieved at the level of the turbulence energy equation, experiments have been performed with a parameter setting appropriate to the onset of a Mistral-Tramontane event in the Gulf of Lions. Good agreement is obtained between the predicted zones of coastal upwelling (characterized by local reductions in sea-surface temperature) and the results of the satellite thermal imagery. In process-orientated evaluations, an account is given of the depth-structure of the momentum and thermal fields supporting the coastal upwelling.

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.

On coastal ocean embedded modeling.

The initialization and forcing of embedded coastal ocean models is shown to raise several serious difficulties, however, surprisingly enough, very little attention is paid by coastal modelers to resulting dynamical discrepancies introduced over both short and long time scales. A modular and “Knowledge accumulating” approach for coastal ocean embedded modeling is proposed based on a variational approach. The extrapolation situations mostly associated to bathymetry constraints are discussed and are shown to be of great importance for any embedded coastal model. A well adapted extrapolation scheme is proposed. Two variational constraints are more particularly shown to lead to important improvements of the downscaling: the mass balance is satisfied in a dynamically coherent way based on Greens theorem and an optimal scheme is presented to extrapolate both the temperature and the salinity in the deepest regions where no data is available.