On the seasonality of eddies in the Western Mediterranean Sea: answers with altimetry and modeling.

Trabajo presentado en la EGU General Assemby 2013, celebrada del 7 al 12 de abril de 2013 en Viena (Austria)Eighteen years of weekly SLA merged maps in the Western Mediterranean are analyzed using the new method proposed by Chelton et al. (2011) to identify and track mesoscale eddies. The method has been adapted to take into account the specificity of the Mediterranean basin. Results are similar to the global ocean results with a radius smaller due to a smaller Rossby radius. The areas of intense rotational speed and amplitude of eddies are similar to the areas of intense eddy kinetic energy calculated from altimetry sea level anomalies. Eddies propagation speed shows a wide range of values without a clear preferred direction. Nevertheless, eddies seems to propagate following the main currents. Temporal analysis of the number of eddies per day is made focusing on the annual and semiannual variability. This annual and semi-annual cycle is analyzed using a regional model of the Mediterranean Sea and studying the interaction with atmospheric forcingsPeer reviewe

Monitoring open-ocean deep convection from space

0094-8276WOS:000263174700002Deep convection (DC) is a key-process of the oceanic circulation, costly to monitor in situ and under the influence of climate change. Our study is a first step toward monitoring DC from space: we investigate the feasibility of observing its variability using improved satellite altimetry. An oceanic simulation of the Mediterranean circulation was performed for the 1999-2007 period. DC interannual variability is realistically modelled, and the sea surface elevation (SSE) is in agreement with altimetry data. Numerical results show a strong correlation between the annual DC characteristics and the winter SSE. From that, we propose a method to monitor DC interannual variability and long term evolution using altimetry data. Our method, applied to the longest available altimetry series, represents correctly the interannual variability of DC in the Northwestern Mediterranean between 1994 and 2007. Citation: Herrmann, M., J. Bouffard, and K. Beranger ( 2009), Monitoring open-ocean deep convection from space, Geophys. Res. Lett., 36, L03606, doi:10.1029/2008GL036422

Modeling the deep eddy field in the southwestern Mediterranean: The life cycle of Sardinian eddies

International audienc

Shelf Impact on Buoyant Coastal Current Instabilities

International audienceThe impact of shelf slope on the linear stability of buoyant coastal currents and on the nonlinear formation of coastal meanders and eddies is investigated. The authors consider a simplified two-layer stratification in cylindrical geometry where a buoyant surface current flows along the coast above a denser water, with a flat bottom or steep shelves. Simulations were performed using the Nucleus for European Modelling of the Ocean (NEMO) ocean global circulation model. The initial state of these simulations was defined according to laboratory experiments performed in the same configuration. Comparisons between laboratory and numerical results highlight the role of momentum diffusion and of the initial perturbations amplitude. The authors' results confirm that the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the current) is the relevant parameter to quantify the shelf impact on the linear and nonlinear dynamics of the surface current. When the evolution of the buoyant coastal current is controlled by the baroclinic instability, the increase of To yields a selection of smaller unstable wavelengths and a decrease of the unstable growth rates. For finite values of To, a complete stabilization of the surface current can be reached. The typical radius of the first eddies generated by the coastal current is set by the linear stage of the baroclinic instability. However, secondary nonlinear processes may lead to larger or smaller structures. The authors exhibit a new dynamical sequence, leading to the formation of submesoscale cyclonic eddies over a steep shelf by splitting of mesoscale eddies. These cyclonic eddies trap and transport water masses and may play an important role in the cross-shelf exchanges

Ocean response in numerical mesoscale modelling during high-wind events over the Gulf of Lions

International audienceThe near-sea surface meteorological conditions associated with strong wind events constitute a strong forcing on the ocean mixed layer. The Gulf of Lions is one of the most windy region of the Mediterranean basin, with frequent Mistral and Tramontane events. These northerly and north-westerly low-level flows, generally induced by a cyclogenesis in the Ligurian basin, transport cold continental air over sea and induce strong momentum and heat exchanges at the air-sea interface. The local continental shelf circulation with sometimes transient coastal upwellings is also sensitive to these intense meteorological events. A preliminary study addresses the question of the sea surface scheme used in mesoscale atmospheric numerical modelling to represent the ocean mixed layer response under these severe wind events. Several slab ocean models have been used coupled with the Weather Research and Forecasting (WRF) model at 21 and 7-km resolution and applied on two Mistral/Tramontane cases. We mainly focused on the slab models performances to represent the ocean mixed layer response under Mistral and Tramontane situations at mesoscale, i. e. local and fast cooling and deepening, and finally we investigated the feedbacks of an interactive ocean mixed layer on the atmospheric simulation. In a second experimental set, the downscaling of the NCEP reanalyses over the full Mediterranean basin has been done with the WRF model between August 1998 and July 1999. The atmospheric fields obtained are then used to drive the regional NEMO-MED12 ocean model with a 1/12° resolution in a perpetual mode. The benefit of increasing the space and time resolutions of the atmospheric forcing (20 to nearly 7 km; daily to 3-hourly) is estimated by a comparison of the ocean model performances to represent the general Mediterranean circulation as the characteristics of the mixed layer, of the deep convection and of the upwellings between the sensitivity experiments, and by a comparison of our experiments to observations and climatologies. A special focus on the local 3D circulation in the Gulf of Lions under high-wind events in these simulations will be presented during the conference

Ocean response to strong precipitation events in the Gulf of Lions (northwestern Mediterranean Sea): a sensitivity study

International audienceThe Mediterranean Sea is a region of intense air-sea interactions, with in particular strong evaporation over sea which drives the thermohaline circulation. The Mediterranean region is also prone to strong precipitation events characterized by low spatial extent, short duration, and high temporal variability. The impacts of intense offshore precipitation over sea, in the Gulf of Lions which is a spot for winter deep convection, are investigated using four sensitivity simulations performed at mesoscale resolution with the eddy-resolving regional ocean model NEMO-MED12. We use various atmospheric fields to force NEMO-MED12, downscaled from reanalyses with the non-hydrostatic mesoscale Weather Research and Forecasting model but differing in space resolutions (20 and 6.7 km) or in time frequencies (daily and three-hourly). This numerical study evidences that immediate, intense, and rapid freshening occurs under strong precipitation events. The strong salinity anomaly induced extends horizontally (~50 km) as vertically (down to 50 m) and persists several days after strong precipitation events. The change in the space resolution of the atmospheric forcing modifies the precipitating patterns and intensity, as well as the shape and the dynamics of the low-salinity layer formed are changed. With higher forcing frequency, shorter and heavier precipitation falls in the ocean in the center of the Gulf of Lions, and due to a stronger vertical shear and mixing, the low-salinity anomaly propagates deeper

Modelling of the Mediterranean circulation using atmospheric fields from the WRF model at different space-time resolutions

International audienceIn the framework of the MORCE-MED project, a two-way ocean-atmosphere coupling is developed between the Weather Research and Forecasting (WRF) atmospheric model and the NEMO ocean model over the Mediterranean basin. The future ocean-atmosphere coupled system is part of the future regional numerical platform including also the modelling of the continental superficial layers, atmospheric chemistry and marine biogeochemistry. Forced by global reanalyses or by the global climatic numerical system outputs, the whole regional coupled model aims to study the impacts of the climate change over the Mediterranean basin. Before applying the full two-way interactive coupling between the ocean and atmospheric regional models for long-term simulations over the Mediterranean, the forcing mode is considered through a sensitivity study. The downscaling of the NCEP reanalyses over the full Mediterranean basin with a 20-km resolution has been done with the WRF model between August 1998 and July 1999. The daily atmospheric fields obtained are then used to drive the NEMO model (with a 6-8 km resolution) over the Mediterranean Sea in a perpetual mode during a spinup of 8 years. Then, three experiments are done for a period of 4 years. The first experiment (or control experiment) is the continuity of the spinup. In the second experiment, a higher temporal resolution is used and the frequency of the forcing is 3 hours, which allows a good representation of the diurnal cycle and of the extreme air-sea exchanges that occur with a short duration during severe meteorological events. In the third experiment, a finer spatial resolution of the forcing is applied over the Gulf of Lions area in order to approach the ocean model resolution and to well represent the channelling of the Mistral and Tramontane. The benefit of increasing the space-time resolutions of the atmospheric forcing is estimated by a comparison of the ocean model performances to represent the general Mediterranean circulation as the characteristics of the mixed layer, of the deep convection and of the upwellings between the control experiment and the two sensitivity experiments, and by a comparison of our experiments to observations and climatologies

Ocean response in numerical mesoscale modelling during high-wind events over the Gulf of Lions

International audienceThe near-sea surface meteorological conditions associated with strong wind events constitute a strong forcing on the ocean mixed layer. The Gulf of Lions is one of the most windy region of the Mediterranean basin, with frequent Mistral and Tramontane events. These northerly and north-westerly low-level flows, generally induced by a cyclogenesis in the Ligurian basin, transport cold continental air over sea and induce strong momentum and heat exchanges at the air-sea interface. The local continental shelf circulation with sometimes transient coastal upwellings is also sensitive to these intense meteorological events. A preliminary study addresses the question of the sea surface scheme used in mesoscale atmospheric numerical modelling to represent the ocean mixed layer response under these severe wind events. Several slab ocean models have been used coupled with the Weather Research and Forecasting (WRF) model at 21 and 7-km resolution and applied on two Mistral/Tramontane cases. We mainly focused on the slab models performances to represent the ocean mixed layer response under Mistral and Tramontane situations at mesoscale, i. e. local and fast cooling and deepening, and finally we investigated the feedbacks of an interactive ocean mixed layer on the atmospheric simulation. In a second experimental set, the downscaling of the NCEP reanalyses over the full Mediterranean basin has been done with the WRF model between August 1998 and July 1999. The atmospheric fields obtained are then used to drive the regional NEMO-MED12 ocean model with a 1/12° resolution in a perpetual mode. The benefit of increasing the space and time resolutions of the atmospheric forcing (20 to nearly 7 km; daily to 3-hourly) is estimated by a comparison of the ocean model performances to represent the general Mediterranean circulation as the characteristics of the mixed layer, of the deep convection and of the upwellings between the sensitivity experiments, and by a comparison of our experiments to observations and climatologies. A special focus on the local 3D circulation in the Gulf of Lions under high-wind events in these simulations will be presented during the conference