Jacques Nihoul

The influence of the “tidal stress” on the residual circulation

The residual current field in the sea is defined as the mean velocity field over a time sufficiently long to cancel transitory wind currents and tidal oscillations. The hydrodynamic equations governing the residual circulation are established and it is shown that, in the regions of intensive tides, the tidal motion has a cogent influence on the residual flow pattern. This effect which arises from the non linear terms is equivalent to the application of a “tidal stress” which combines with the wind stress to drive the water motion. The tidal stress is calculated in the Southern Bight of the North Sea from the results of a numerical tidal model and the residual circulation is computed. The comparison with the circulation obtained when neglecting the tidal stress shows determinant differences enforcing the theory.

Modelling the general circulation of shelf seas by 3Dk-ε models

Abstract One examines the modifications which must be made-and the limitations which must be set-to classical k-e models to extend their application to the simulation of marine mesoscale, synopticscale and macroscale processes which compose the weather-like and general circulations of the sea. The case of the general circulation—for which sub-grid scale fluctuations include such semi-organized motions as tides and storm surges-is discussed in more detail. A 3D k-e model appropriate to the study of the general circulation in a shallow stratified sea is presented and illustrated with the results of a simulation of the general summer circulation in the Northern Bering Sea, made in the scope of the NSF ISHTAR (“Inner Shelf Transfer and Recycling”) Program.

Perspective in three-dimensional modelling of the marine system

Publisher Summary This chapter discusses the three-dimensional modeling of the marine system. The marine system is extremely complex, and it is an overwhelming task to predict the environmental effects of mans activities and, a fortiori , to set limits to such activities. The first step in modeling a marine system is the demarcation of the system. This includes the definition of its support—that is, its extension in physical space—and of its scope—that is, its deployment in state space. The first characteristic of a model is its object—that is, the geographical area, the dates and the specific events or processes—to be investigated. The second characteristic of a model is its span—that is, its dimension in physical space and instate space. The state space can be divided in several sectors corresponding to hydrodynamical, chemical, biological processes, and separate hydrodynamical, chemical, and biological models can be conceived with the necessary input–output links between them. The chapter also discusses marine weather equations.

Reconnaissance of the main Black Sea's ecohydrodynamics by means of a 3D interdisciplinary model

Abstract A 3D interdisciplinary model has been used to test the sensitivity of the Black Seas ecosystem to physical processes. The hydrodynamical model of the general circulation has been built up, using the GHER primitive equation model. A model with 15 km horizontal resolution and 25 vertical levels is used to compute the typical seasonal cycle. The model is forced by climatological monthly mean fields of temperature, salinity and wind stress at the air–sea interface; the river discharges of the Danube, Dnestr and Dnepr are taken into account. An ecosystem model at basin scale is then defined by a nitrogen cycle considering several phytoplankton and zooplankton sizes and including the microbial loop. The ecosystem model is embedded on-line into the 3D hydrodynamical model with a superimposed cycle for the light intensity. This model must be regarded rather as a first tool for testing the coupling of hydrodynamic and ecosystem submodels, while acquiring some preparatory assessment of the effect of physical processes on the ecodynamics. The results display a highly three-dimensional aspect with important horizontal and vertical variations, obviously imparted to the system by the physical processes (horizontal and vertical advection, vertical mixing and diffusion, upwelling…) associated with light limitation at depth and sinking of dead organisms. In this paper, the results are described emphasizing the effects of the hydrodynamic constraints on the space–time distribution of the primary and secondary production.

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.

Circulation of the western Mediterranean: From global to regional scales

A free-surface, three-dimensional, primitive equation model has been implemented with a horizontal resolution of 4.6 km to study the ocean circulation in the Gulf of Lions at time scales ranging from weeks to seasons. Numerical experiments have been conducted, in which the regional model is nested into a basin-scale model of the whole western Mediterranean. The global model is operated with a relatively coarse resolution (16 km) and provides boundary conditions at the open-sea boundaries of the regional domain. There is, however, no feedback loop from the regional to the global model. The simulations are consistently driven with atmospheric fluxes computed from the output of the French PERIDOT meteorological forecasting system, between August 1988 and 1989. In addition to the initial conditions, in situ measurements of temperature and salinity are assimilated in the simulation of the general circulation, adopting a simple nudging technique to prevent an excessive drift of the model against climatology. The response of the regional model below and above the thermocline is discussed in the context of the prevailing meteorological situations. Some experiments give indications that a double-gyre system may develop from wind regimes that exhibit a cyclonic/anticyclonic wind stress curl. Advection-diffusion of passive tracers are also examined on the basis of the local hydrodynamic features, because this work has been conceived with the aim of determining the physical conditions in which ecological and biochemical processes develop at the interface between river mouths and the open ocean.

Hierarchy and scales in marine ecohydrodynamics

Abstract Recent investigations reveal, in marine ecosystems, an ecohydrodynamic hierarchical organization resulting from the different rates of ecological processes confronted to a multi-scale physical environment. Major marine hydrodynamic processes are briefly analyzed here in an ecohydrodynamic perspective, emphasizing the effects they have on ecosystems at different levels of hierarchy and identifying appropriate “spectral windows” for modelling. A case study application to the Northern Bering Seas Summer Ecohydrodynamics is given in illustration.

A Non-Linear Mathematical Model for the Transport and Spreading of Oil Slicks

Abstract A non-linear mathematical model is presented for forecasting the transport and spreading of oil slicks on the surface of the sea. The model differs from previous ones by its capability of taking simultaneously gravity, surface tension, friction and weathering processes of the oil into account and by the introduction of a new parameterization of surface tension and friction, better adapted to real field conditions. The new parameterization induces slightly different similarity laws but asymptotic similarity solutions for the surface tension - and gravity - dominated regimes, appear to be consistent with observations and, in the surface tension regime at least, in better agreement with experimental data than previous theories. The model is summarized by an evolution equation - describing oil transport and spreading - for the oil layers thickness. This equation contains all essential features of the phenomenon and can be solved numerically for any oil spill situation whether the released volume is large or small.

Influence of the residual circulation on the physico-chemical characteristics of water masses and the dynamics of ecosystems in the Belgian coastal zone

Abstract Classical models of the residual circulation in the North Sea predict a north-bound residual flow in the Southern Bight. A more refined model, taking into account the mesoscale Reynolds stress exerted in the mean on the residual flow by the non-linear interactions of mesoscale processes (tides, storm surges, etc.), shows on the contrary off the Belgian coast a south-bound coastal current in relation with a residual coastal gyre. Observations of the physico-chemical characteristics of coastal waters confirm the existence of the gyre. The dynamics of coastal ecosystems here are found to be determined by the gyre; successive stages of the pelagic food chain displaying a typical spatial distribution along the deflected plume of the Scheldt estuary around the gyre ‘outerlagoon’.

Influence of Climate Change on the Changing Arctic and Sub-Arctic Conditions

The current warming trends in the Arctic may shove the Arctic system into a seasonally ice-free state not seen for more than one million years. The melting is accelerating, and researchers were unable to identify natural processes that might slow the deicing of the Arctic. Such substantial additional melting of Arctic and Antarctic glaciers and ice sheets would raise the sea level worldwide, flooding the coastal areas where many of the world s population lives. Studies, led by scientists at the National Center for Atmospheric Research (NCAR) and the University of Arizona, show that greenhouse gas increases over the next century could warm the Arctic by 3-5C in summertime. Thus, Arctic summers by 2100 may be as warm as they were nearly 130,000 years ago, when sea levels eventually rose up to 6 m higher than today.