International audienc

[1] 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.

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International audienceThe winter of 2012 experienced peculiar atmospheric conditions that triggered a massive formation of dense water on the continental shelf and in the deep basin of the Gulf of Lions. Multiplatforms observations enabled a synoptic view of dense water formation and spreading at basin scale. Five months after its formation, the dense water of coastal origin created a distinct bottom layer up to a few hundreds of meters thick over the central part of the NW Mediterranean basin, which was overlaid by a layer of newly formed deep water produced by open-sea convection. These new observations highlight the role of intense episodes of both dense shelf water cascading and open-sea convection to the progressive modification of the NW Mediterranean deep waters

International audienceThis paper focuses on the energy conservation properties of a hydrostatic, Boussinesq, coastal ocean model using a classic finite difference method. It is shown that the leapfrog time-stepping scheme, combined with the sigma-coordinate formalism and the motions of the free surface, prevents the momentum advection from exactly conserving energy. Because of the leapfrog scheme, the discrete form of the kinetic energy depends on the product of velocities at odd and even time steps and thus appears to be possibly negative when high-frequency modes develop. Besides, the study of the energy balance clarifies the numerical choices made for the computation of mixing processes. The time-splitting technique used to reduce the computation costs associated to the resolution of surface waves leads to the well-known external and internal mode equations. We show that these equations do not conserve energy if the coupling of these two modes is forward in time. Even if non-linear terms are negligible, this shortcoming can be significant regarding the pressure gradient term ‘frozen' over a baroclinic time step. An alternative energy-conserving time-splitting technique is proposed in this paper. Discussion and conclusions are conducted in the light of a set of numerical experiments dedicated to surface and internal gravity waves

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 km3 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.

[1] In the northwestern Mediterranean Sea, winter 1986-1987 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 versus 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 similarly represented 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 better reproduces 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. Sixty-six percent 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. Sixty percent of the restratification with respect of the water column initial structure occurs before July 1987 and is due to light water advection. Afterward, restratification is due to the mixing and is not complete before next year convection.

Plastic floating at the ocean surface, estimated at tens to hundreds of thousands of metric tons, represents only a small fraction of the estimated several million metric tons annually discharged by rivers. Such an imbalance promoted the search for a missing plastic sink that could explain the rapid removal of river-sourced plastics from the ocean surface. On the basis of an in-depth statistical reanalysis of updated data on microplastics—a size fraction for which both ocean and river sampling rely on equal techniques—we demonstrate that current river flux assessments are overestimated by two to three orders of magnitude. Accordingly, the average residence time of microplastics at the ocean surface rises from a few days to several years, strongly reducing the theoretical need for a missing sink.