Pierre Brasseur

Seasonal temperature and salinity fields in the Mediterranean Sea: Climatological analyses of a historical data set

Abstract Climatological analyses of a historical data base have been carried out with the aim of reconstructing the three-dimensional temperature and salinity fields in the Mediterranean Sea. Seasonal and monthly distributions of hydrographic properties have been computed by a variational inverse method as an alternate to the standard Gandin (1969; Objective analysis of meterological fields, Israeli Program for Scientific Translation, Jerusalem) procedure. The spline solutions of the minimization problem are demonstrated to be numerically and theoretically equivalent to field estimates obtained by conventional objective analysis. The application of a finite-element technique allows analysis to be performed in the model space rather than in the observational space, which substantially improves the numerical efficiency of the procedure. The parameters of the scheme are adjusted according to the statistics of the climatological data. The results, realized as gridded data sets (horizontal resolution of 0.25°), show some trends in seasonal variability affecting the properties of water masses. As expected, the upper layer is subject to a well-defined seasonal signal affecting both the temperature and salinity fields. Error maps, reflecting the degree of uncertainty in the analyses, have been systematically produced. The present work is conceived as a basic support to more advanced studies such as diagnostic calculations, initialization of dynamical models, assimilation of hydrological data into primitive equation models, or planning of experimental surveys. New versions of the climatological fields will be released as data are added to the historical data base.

The general circulation in the Mediterranean Sea: a climatological approach

A significant amount of hydrographic data has been collected in the Mediterranean during this century, providing the basic ingredients needed to construct a climatological picture of the general circulation. A pooled hydrographic data set has been prepared, which currently contains over 100,000 station profiles (CTD, Nansen bottle, XBT and MBT) taken in the Eastern and Western Mediterranean Sea between 1900 and 1993. Climatological analyses of temperature and salinity data are performed at seasonal and monthly scales, using a variational method and a finite element numerical technique. The variational formulation had been demonstrated to be equivalent to objective analysis, and a hybridation of the statistical and variational methods allows to compute error fields associated to the climatology. The free parameters of the scheme are determined using a cross-validation algorithm to extract the best seasonal statistics from the observed data sets. Compared to earlier climatologies, the recent analyses show significant improvements with respect to the regional scales of the general circulation gyres. In addition, several features of the general circulation are described with a better accuracy. Finally, the barotropic equations (external mode) of a free-surface, primitive equation model are integrated in time to diagnostically adjust the sea-surface elevation to the thermohaline structures. Geostrophic velocities are then computed, integrating the thermal-wind equations with the sea-surface pressure as a reference. Results of fully prognostic, three-dimensional, primitive equation models are compared with these diagnostic computations in order to identify which features of the general circulation can be captured by observations.

Three-dimensional General Circulation Model of the Northern Bering Sea’s Summer Ecohydrodynamics

The main features of the northern Bering Seas summer ecohydrodynamics are investigated with the help of two three-dimensional—direct and inverse—models developed at theGeoHydrodynamics andEnvironmentResearch Laboratory of the University of Liege (GHER). Each model consists of two interacting sectorial submodels for (i) the general circulation hydrodynamics and synoptic structures, and (ii) the associated plankton ecosystem dynamics. The direct model is used to simulate, from an initial state compatible with historical, climatological and all available data pertinent to the summer season, a typical overview of the northern Bering Seas ecohydrodynamics during the summer. The inverse model is applied in a two-fold perspective: (i) the reconstruction of typical summer distributions of temperature and salinity by using more than 1500 CTD profiles measured during the months of July, August and September, in the course of the ISHTAR program; (ii) considering the observations from specific ISHTAR surveys as quasi-synoptic, the reconstruction of individual data fields in order to provide additional information to assess the variability of the system. The models predictions indicate that the summer dynamics are dominated by a few cogent semi-permanent and reproducible mechanisms which govern the main water mass transports, the upwellings, the fronts and the subsequent seasonal patterns of primary and secondary productions. The general circulation fields calculated by the direct model are considered as a standard of reference to give a coherent interpretation of—local and often instantaneous—observations, process studies and related results, in the context of the natural variability of the system. The simulated flow pattern has been validated, using the set of current measurements provided by 1985 and 1986 ISHTAR moorings. The contribution of the Anadyr Stream to the northward transport is reproduced qualitatively and quantitatively. The vertical motions—undetectable from direct experiments—are computed by the model, and represent one of the most efficient constraints on the ecohydrodynamics. For instance, the strong upwelling located along the Siberian coast—the existence of which was only presumed until recently—is now correctly estimated in position and intensity. The exceptionally high concentrations of nutrients found in the upwelled water turn this hydrodynamic structure into a catalyst element for the development of biological species in the region. The pattern of primary production shows two successive maxima: the first appears as a direct consequence of the frontal conditions associated with the Anadyr Stream, whereas the second develops further north, in the Chukchi Sea. The results display a fairly good agreement with the classical descriptions induced from observations, and suggest that the advection-growth coupling is the main physical conditioning factor for biological processes.

Space and seasonal distributions of nitrates in the Mediterranean Sea derived from a variational inverse model

A variational inverse model was applied to the processing of biogeochemical data in order to generate conceptual models useful for understanding the role of anthropogenic and physical factors in eutrophication and other environmental characteristics of the Mediterranean Sea. For this purpose, nitrate concentrations available in our data base were grouped into synoptic or quasi-synoptic subsets representative of averaged values of the measured vertical profiles in typical water layers. These data sets were then used as input for our two-dimensional variational inverse model based on a minimum principle to reconstruct mean horizontal distributions of the nitrate data fields. The latter are obtained as gridded solutions to the measured concentrations. The resulting seasonal distributions of nitrates provide an interesting general view, and indicate gaps where new data must be supplemented for further investigations. In some coastal regions where data are available, known phenomena such as nutrient loadings from rivers, upwelling regions and gyres were reproduced by this model in both the western and eastern Mediterranean basins. Seasonal graphical visualisations of the variational inverse model results are presented for nitrate concentrations. Besides providing a new insight into the distribution of nitrates in the Mediterranean Sea, our results confirm the need for more up-to-date and good quality data before any definite conclusion concerning long-term seasonal modelling and the fate of nutrients can be drawn.

Application of a 3-D variational inverse model to the analysis of ecohydrodynamic data in the Northern Bering and Southern Chukchi Seas

Abstract In the scope of the Inner SHelf Transfer And Recycling Program, extensive oceanographic surveys were conducted in the Northern Bering and Southern Chukchi Seas. A vast amount of hydrographic, chemical and biological data were collected in order to increase the information available on the system and to test if biological observations are indeed compatible with the dominant hydrographic regimes. An original model, based on the variational inverse method, was developed with the aim of reconstructing realistic three-dimensional fields of the variables of interest and giving an interpretation of the observations consistent with the physics of the general circulation. The basic formulation of the model is quite general and provides a very helpful tool in the context of interdisciplinary studies. In a first stage, the vertical and horizontal structures of physical variables are reproduced and discussed. Secondly, a sketch is proposed to explain the driving mechanisms of the primary productivity in the Northern Bering Sea. In particular, one shows that the hydrodynamic regimes strongly condition the behaviour of the ecosystem and are mainly responsible for the very fertile environment that characterizes the Southern Chukchi Sea.

Free data offered to researchers studying the Mediterranean

Several data products, analysis software packages, and visualization tools have been released by the The Mediterranean Oceanic Data Base (MODB), a group that prepares hydrographic data sets and climatological fields for distribution to scientists working in the Mediterranean Sea. The climatological distributions of temperature and salinity in the Mediterranean that are available account for the regional properties of the water masses(Figure 1). These measurements should help refine the Mediterranean sector of the Climatological Atlas of the World Ocean published by S. Levitus in 1982. The Mediterranean gridded fields are represented in ASCII format on a 1/4-degree grid and 19 levels on the vertical. The data sets also include the hydrographic profiles from which the analyzed fields were derived.

Mathematical Visualisation of the Northern Bering Sea’s Summer Ecohydrodynamics

An extensive survey of the Northern Bering Sea was carried on for five years in the scope of the NSF ISHTAR Program. The observations were concentrated in the summer months with the objective of determining the main physical, chemical and biological characteristics of the system in typical summer situations.