Michel Crépon

A Three-Dimensional Numerical Study of Deep-Water Formation in the Northwestern Mediterranean Sea

Abstract Deep-water formation (DWF) in the northwestern Mediterranean Sea and the subsequent horizontal circulation are investigated in a rectangular basin with a three-dimensional primitive equation model. The basin is forced by constant climatological heat and salt fluxes. Convective motion is parameterized by a simple nonpenetrative convective adjustment process plus Richardson number–dependent vertical eddy viscosity and diffusivity. A homogeneous column of dense water is progressively formed in the forcing area. Meanders of 40-km wavelength develop at the periphery of the column. These features agree with observations. Energy studies show that the meanders are generated mainly through a baroclinic instability process. These meanders, and the associated cells of vertical motion, contribute to the process of DWF. They generate vertical advection, while the associated horizontal advection tends to restratify the surface water of the column, and thus to inhibit very deep convection. Just before the end o...

A neural network approach for modeling nonlinear transfer functions: Application for wind retrieval from spaceborne scatterometer data

The present paper shows that a wide class of complex transfer functions encountered in geophysics can be efficiently modeled using neural networks. Neural networks can approximate numerical and nonnumerical transfer functions. They provide an optimum basis of nonlinear functions allowing a uniform approximation of any continuous function. Neural networks can also realize classification tasks. It is shown that the classifier mode is related to Bayes discriminant functions, which give the minimum error risk classification. This mode is useful for extracting information from an unknown process. These properties are applied to the ERS1 simulated scatterometer data. Compared to other methods, neural network solutions are the most skillful.

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.

Large-Scale Preconditioning of Deep-Water Formation in the Northwestern Mediterranean Sea

Abstract The large-scale processes preconditioning the winter deep-water formation in the northwestern Mediterranean Sea are investigated with a primitive equation numerical model where convection is parameterized by a non-penetrative convective adjustment algorithm. The ocean is forced by momentum and buoyancy fluxes that have the gross features of mean winter forcing found in the MEDOC area. The wind-driven barotropic circulation appears to be a major ingredient of the preconditioning phase of deep-water formation. After three months, the ocean response is dominated by a strong barotropic cyclonic vortex located under the forcing area, which fits the Sverdrup balance away from the northern coast. In the vortex center, the whole water column remains trapped under the forcing area all winter. This trapping enables the thermohaline forcing to drive deep-water formation efficiently. Sensitivity studies show that, β effect and bottom topography play a paramount role and confirm that deep convection occurs on...

A Sensitivity Study of the General Circulation of the Western Mediterranean Sea. Part II: The Response to Atmospheric Forcing

Abstract This paper investigates the influence of sea surface thermohaline fluxes and wind stress on the circulation of the Western Mediterranean Sea using a high-resolution 3D primitive equation model. An 18-year experiment was forced with the daily output of a fine grid mesh numerical weather prediction model. The major characteristics of the circulation are well reproduced. The basin surface circulation is cyclonic over all of the basin. The two anticyclonic Alboran gyres are present. The instabilities of the Algerian Current generate large anticyclonic eddies that invade the whole Algerian Basin. The Liguro–Provencal–Catalan Current is well marked. Deep water convection down to the bottom only occurs during the first 3 years, then winter intermediate water is produced. The north–south gradient of the atmospheric thermohaline fluxes induces a northward surface transport of water from the Algerian Basin into the Liguro–Provencal Basin. This pattern can be associated with the Balearic front. Sensitivity ...

The effect of thermohaline forcing variability on deep water formation in the western Mediterranean Sea: a high-resolution three-dimensional numerical study

Abstract Deep water formation in the northwestern Mediterranean Sea is investigated with a high-resolution three-dimensional primitive equation numerical model. A rectangular basin is forced by various heat and salt fluxes. A 1000-m thick patch of dense water is formed within the forcing area, which is surrounded by a cyclonic vortex. Meanders develop at the periphery of the patch and then tend to occupy the whole patch area. These features agree with observations. Sensitivity studies of the space and time variability of the forcing are presented. It is found that these variabilities are of great importance in deep water formation. Deep water formation appears to depend on a convective process parameterized by a simple non-penetrative convective adjustment and on vertical motion induced by baroclinic instability. The two processes are strongly linked. The convective process triggers the depth of the convection while the baroclinic adjustment process is responsible for the volume of dense water formed.

Effects of Coastline Geometry on Upwellings

Abstract The shallow water equations applied to a two-layer ocean are solved in order to study the cited of capes on upwellings. First, analytical solutions of the linearized version of these equations are given for academic right angle corners. Two cases must be envisaged depending on whether or not the wind, favorable to upwelling, is parallel to one coast or to the other. Then, the nonlinear version of the equations is solved by using a numerical model dealing with the. finite elements method. Capes of various shapes are studied. Numerical results can be interpreted in the light of analytical solutions. The major observed phenomena are a difference in the interface elevation between the upwind and downwind coasts, and a generation of currents flowing in opposite direction to the wind in the bottom layer. Both effects are due to the generation of Kelvin waves by the variability of the coast.

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

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...

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

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.

A strait outflow circulation process study : The case of the Alboran Sea

Abstract A three-dimensional primitive equation model is used to investigate the physical processes governing the exchanges of water between the Mediterranean Sea and the Atlantic Ocean at the level of the Strait of Gibraltar. The circulation is driven by connecting two reservoirs filled with waters of different densities. The motion starts from rest and is initiated by removing the dam separating them at the initial time. The results are in agreement with observations. In the strait, the light Atlantic water flows into the Mediterranean Basin in the surface layer while the denser Mediterranean water moves toward the ocean as a deep current. After a spinup time interval of three days, the flow transport in the strait reaches a quasi-stationary value of 0.9 Sv (Sv ≡ 106 m3 s−1). As it enters the Alboran Sea the flow of Atlantic water experiences a transition regime where the surface current is transformed into a shallow buoyant jet. After several days of integration, this flow intrudes into the Alboran Bas...