Laurent Mortier

Impact of the spatial distribution of the atmospheric forcing on water mass formation in the Mediterranean Sea

The impact of the atmospheric forcing on the winter ocean convection in the Mediterranean Sea was studied with a high-resolution ocean general circulation model. The major areas of focus are the Levantine basin, the Aegean-Cretan Sea, the Adriatic Sea, and the Gulf of Lion. Two companion simulations differing by the horizontal resolution of the atmospheric forcing were compared. The first simulation (MED16-ERA40) was forced by air-sea fields from ERA40, which is the ECMWF reanalysis. The second simulation (MED16-ECMWF) was forced by the ECMWF-analyzed surface fields that have a horizontal resolution twice as high as those of ERA40. The analysis of the standard deviations of the atmospheric fields shows that increasing the resolution of the atmospheric forcing leads in all regions to a better channeling of the winds by mountains and to the generation of atmospheric mesoscale patterns. Comparing the companion ocean simulation results with available observations in the Adriatic Sea and in the Gulf of Lion shows that MED16-ECMWF is more realistic than MED16-ERA40. In the eastern Mediterranean, although deep water formation occurs in the two experiments, the depth reached by the convection is deeper in MED16-ECMWF. In the Gulf of Lion, deep water formation occurs only in MED16-ECMWF. This larger sensitivity of the western Mediterranean convection to the forcing resolution is investigated by running a set of sensitivity experiments to analyze the impact of different time-space resolutions of the forcing on the intense winter convection event in winter 1998-1999. The sensitivity to the forcing appears to be mainly related to the effect of wind channeling by the land orography, which can only be reproduced in atmospheric models of sufficient resolution. Thus, well-positioned patterns of enhanced wind stress and ocean surface heat loss are able to maintain a vigorous gyre circulation favoring efficient preconditioning of the area at the beginning of winter and to drive realistic buoyancy loss and mixing responsible for strong convection at the end of winter.

Observations of open‐ocean deep convection in the northwestern Mediterranean Sea: Seasonal and interannual variability of mixing and deep water masses for the 2007‐2013 Period

We present here a unique oceanographic and meteorological data set focus on the deep convection processes. Our results are essentially based on in situ data (mooring, research vessel, glider, and profiling float) collected from a multiplatform and integrated monitoring system (MOOSE: Mediterranean Ocean Observing System on Environment), which monitored continuously the northwestern Mediterranean Sea since 2007, and in particular high-frequency potential temperature, salinity, and current measurements from the mooring LION located within the convection region. From 2009 to 2013, the mixed layer depth reaches the seabed, at a depth of 2330m, in February. Then, the violent vertical mixing of the whole water column lasts between 9 and 12 days setting up the characteristics of the newly formed deep water. Each deep convection winter formed a new warmer and saltier “vintage” of deep water. These sudden inputs of salt and heat in the deep ocean are responsible for trends in salinity (3.3 ± 0.2 × 10−3/yr) and potential temperature (3.2 ± 0.5 × 10−3 C/yr) observed from 2009 to 2013 for the 600–2300 m layer. For the first time, the overlapping of the three “phases” of deep convection can be observed, with secondary vertical mixing events (2–4 days) after the beginning of the restratification phase, and the restratification/spreading phase still active at the beginning of the following deep convection event.

Spreading of Levantine Intermediate Waters by submesoscale coherent vortices in the northwestern Mediterranean Sea as observed with gliders

Since 2007, gliders have been regularly deployed in the northwestern Mediterranean Sea, a crucial region regarding the thermohaline circulation of the Mediterranean Sea. It revealed for the first time very warm (10.48C) and saline (10.1) submesoscale anticyclones at intermediate depth characterized by a small radius (

Modeling nitrate fluxes in an open coastal environment (Gulf of Lions): Transport versus biogeochemical processes

5 km), high Rossby (

A Sensitivity Study of the General Circulation of the Western Mediterranean Sea. Part I: The Response to Density Forcing through the Straits

0.3), and Burger (

Model intercomparison in the Mediterranean: MEDMEX simulations of the seasonal cycle

0.7) numbers. They are likely order of 10 to be formed each year, have a life time order a year and certainly contribute significantly to the spreading of the Levantine Intermediate Waters (LIW) toward the whole subbasin, thus potentially impacting wintertime vertical mixing through hydrographical and dynamical preconditioning. They could be mainly formed by the combined action of turbulent mixing and flow detachment of the northward flow of LIW at the northwestern headland of Sardinia. Upwelling conditions along the western coast of Sardinia associated with a southward geostrophic flow within the upper layers seem to play a key role in their formation process.

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

A 13-compartment model of primary production and degradation of dissolved organic matter has been coupled with a general circulation model in an open coastal environment (Gulf of Lions, Mediterranean) so as to quantify exchanges with the open sea. The biogeochemical model had been previously calibrated with a one-dimensional vertical version on a 1-year data set, and this simulation provides boundary conditions for the three-dimensional model. After a 1-year spin-up simulation, quasi-equilibrium is obtained for all the compartments, and the results of an additional annual simulation are compared with coastal zone color scanner images and other data collected in the area: the simulation of chlorophyll a, nutrient concentrations, and primary production is quite satisfactory, although the spring bloom starts slightly earlier than usual. Total gross community production is estimated at 76 g C m−2 y−1, and New Gross Community Production at 45 g Carbon m−2 y−1 (f = 0.37). The model was used to understand the nitrate annual cycle in the coastal zone of the Gulf of Lions. Nitrate input from the Rhone river, from the sediment, and from marine advection are compared in terms of potential fertilization. Model results indicate that, regarding the open sea, the margin acts most of the time as a sink for nitrate. However, during winter, when phytoplankton growth is reduced and cascading of dense waters is active along the shelf, the margin is shown to export nitrate toward the open sea. These results may qualitatively apply to other open Mediterranean margins, although the intensity of the fluxes would be modulated by local features.

Observing mixed layer depth, nitrate and chlorophyll concentrations in the northwestern Mediterranean: A combined satellite and NO3 profiling floats experiment

Abstract In this paper, the influence of the density gradients at the straits of Gibraltar and Sicily on the large-scale circulation of the Western Mediterranean Sea is investigated through a 3D numerical model. The topography constraint is analyzed by comparing an experiment with a flat bottom with an experiment with a realistic bathymetry. The results show the ability of the density gradients at the straits to force a cyclonic surface circulation in the whole basin, particularly in the northern basin where the transport in the Liguro–Provencal current is about 40% of the observed one. These experiments point out the key role played by the strait of Sicily and its topography. There the eastward surface flow separates into two branches: one enters the Eastern Mediterranean Sea, while the other flows along the Italian coasts to feed the cyclonic circulation of the northern basin. In the Alboran Sea, which is a priori the region where the dynamics is the most strongly induced by the transport through the St...

Finescale Vertical Structure of the Upwelling System off Southern Peru as Observed from Glider Data

The simulation of the seasonal cycle in the Mediterranean by several primitive equation models is presented. All models were forced with the same atmospheric data, which consists in either a monthly averaged wind-stress with sea surface relaxation towards monthly mean sea surface temperature and salinity fields, or by daily variable European Centre for Medium Range Weather Forecast (ECMWF) reanalysed wind-stress and heat fluxes. In both situations models used the same grid resolution. Results of the modelling show that the model behaviour is similar when the most sensitive parameter, vertical diffusion, is calibrated properly. It is shown that an unrealistic climatic drift must be expected when using monthly averaged forcing functions. When using daily forcings, drifts are modified and more variability observed, but when performing an EOF analysis of the sea surface temperature, it is shown that the basic cycle, represented similarly by the models, consists of the seasonal cycle which accounts for more than 90% of its variability.

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

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.