Circulation of the western Mediterranean: from global to regional scales

Deep-Sea ResearchII,Vol 44,No.34,pp.531-549, 1997 C 1997 Elsev~er Saence Ltd All rights reserved. Prmted in Great Britain 0967-0645/97517.00+0.00

Pergamon

PII: SO9674645(96)0009&2

Circulation of the western Mediterranean: from global to regional scales J.-M.

BECKERS,*

P. BRASSEURT

and J. C. J. NIHOUL$

(Received 5 January 1995; in revisedform 27 October 1995; accepted 21 March 1996)

Abstract-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 opensea 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 ofdetermining the physical conditions in which ecological and biochemical processes develop at the interface between river mouths and the open ocean. 0 1997 Elsevier Science Ltd.

INTRODUCTION Environmental conditions in the Mediterranean Sea are of growing concern, especially in the coastal zone which is the most vulnerable area affected by human activities. One of the various objectives of the EROS-2000 programme was to study the impact of hydrodynamic constraints on ecological and biochemical processes taking place in coupled river-ocean systems. Due to its particular position, the Gulf of Lions exemplifies a system where local, regional and global processes interact. The plume of the RhGne River and its associated local features develop in the regional environment of the Gulf of Lions, which is itself embedded into the global Mediterranean. As a consequence, modelling the ocean circulation in the Gulf of Lions cannot be performed independently from its global Mediterranean context.

* Research Associate, National Fund for Scientific Research, Liege, Belgium. t LEGI, UMR 5519 du CNRS, BP 53, F-38041 Grenoble, Cedex 03, France. $ GeoHydrodynamics and Environment Research (GHER), University of Liege, Sart-Tilman Belgium. 531

B5, 4000 Liege,

532

J.-M. Beckers et al

From the physical point of view, the Gulf of Lions can be conceived as a dynamical system that is driven by three distinct forcing mechanisms: the atmosphere at the seasurface, the river inflow, and the global ocean at the open-sea boundaries of the regional domain (Fig. 1). The mechanical energy injected into the system is dissipated by lateral diffusion and bottom friction. From a numerical point of view, this is reflected by the need to provide the model with appropriate boundary conditions specifying the role of the “external world”. At the air-sea interface, the output of meteorological models can be used to compute heat and momentum fluxes from the atmosphere to the ocean. For the regional model, lateral open boundaries have a quite significant extension, and in situ measurements are usually too sparse to provide the relevant information. Concerning the semi-enclosed western Mediterranean basin, this problem is less acute since the open boundaries in the Gibraltar and Sicily Straits can be monitored reasonably well by field observations. The present problem is an ideal case to apply the concept of nested models: firstly, the general circulation is simulated over a domain covering the whole western Mediterranean (with appendages in the Atlantic Ocean and the eastern Mediterranean) with a rather coarse horizontal resolution; then, the results of the global model are used as boundary conditions for the regional model implemented over the Gulf of Lions. The horizontal resolution of the regional model can then be refined in a way that presently available computers can not afford over the global domain. The nesting approach is implemented without any feedback loop by which the regional model might affect the large-scale ocean dynamics (and also the meteorological model dynamics). The underlying assumption is that small scale processes do not significantly affect large scale dynamics, although this is not necessarily true. In this paper, the general circulation model is first described, including its implementation in the western Mediterranean and a synthesis of the main results obtained from it. Special

‘7”w

2%.

w

16” E (GHER

Fig. 1.

Numerical

grid of the general circulation model with the position isodepths are drawn every 200 tn.

of the regional

1994)

model;

533

Circulation of the western Mediterranean: from global to regional scales

attention is paid to the need to complement the model simulations with historical data sets and climatological analyses. Then, the regional model covering the Gulf of Lions is presented, focusing on the boundary conditions derived from the basin-scale model. Finally, the results of the regional simulations are discussed, and conclusions are given.

SIMULATIONS

Implementation

OF THE

GENERAL CIRCULATION MEDITERRANEAN

of the general circulation

IN THE

WESTERN

model

Model formulation. The three-dimensional ecohydrodynamic primitive equation model developed at the GeoHydrodynamics and Environment Research Laboratory (GHER) of the University of Liege has been described in various earlier publications (e.g. Nihoul et al., 1989; Beckers, 1991; Nihoul and Beckers, 1992; Beckers et al., 1994). The GHER model, in its general formulation, consists of two sectorial submodels: (1) the hydrodynamic model, the state variables of which are the three components of the velocity vector, the pressure, temperature, salinity and the turbulent kinetic energy; the hydrodynamic model is calibrated to the study of marine weather-like processes from currents mesoscale to synoptic structures (fronts, eddies. . .) and macroscale slowly-varying forming the general circulation; (2) the plankton ecosystem model, whose variables are the concentrations of nutrients, the biomasses of phytoplankton, zooplankton, bacterioplankton and the concentration of organic matter (interactions with the benthic ecosystem are treated in the bottom boundary conditions) (Nihoul et al., 1993). The evolution equations are all of the same general form, expressing that the rate of change of any state variable is the result of horizontal and vertical advection, local production or destruction, and diffusion by horizontal sub-grid scale motions and vertical turbulence. They differ from one another by the specific expressions of the rates of production-destruction (related to different processes: mechanical forcing, radiation. . . and translocations,-i.e. material fluxes between state space compartments) (Nihoul, 1975; Nihoul et al., 1989; Nihoul and Djenidi, 1991). In the present study, the generic ecosystem model has been implemented in a most simple configuration to analyse the behaviour of three different passive tracers; the actual ecosystem model, with its full biological equations, has not been used at this stage. Except for the air-sea interaction and the open sea boundaries, which need a special treatment described later, classical boundary conditions are applied at the bottom, where heat and salt fluxes are assumed to be zero, but the flux of momentum (bottom stress) is based on the logarithmic velocity profile approximation for the computation of the quadratic bottom friction. At the coasts, heat and salt fluxes are set to zero except at the mouths of main rivers like the Rhone. The velocity perpendicular to the coast is zero and a quadratic friction law is imposed for the tangential velocity. The numerical discretization of the model equations is performed by means of a vertical coordinate change and a finite volume method on the Arakawa C-grid; here, the implementation of the vertical coordinate change is such that the Mediterranean Sea is cut into two regions superposed vertically. One region covers the deeper parts of the sea and a

534

J.-M.

Beckers et al.

second region covers the shelf and the region above the deep region. In each of these regions, a classical sigma coordinate change is introduced. Additional details on the numerical scheme can be found in Beckers (199 1) and Beckers (submitted). The discrete equations are resolved on a 16 km x 16 km horizontal grid with 20 vertical levels, resulting in 125 x 62 x 20 computation points for the domain covering the whole western Mediterranean (including the appendages in the Atlantic and the Eastern Basin, Fig. 1). A mode-splitting technique allows computation of the baroclinic component of the flow with a larger time step (1800 s) than the barotropic one (36 s). In the momentum and scalar equations, horizontal diffusion terms are made effective in the transformed space with rather small diffusivity coefficients (400 m2/s for velocity fields and 140 m2/s for scalars). Atmospheric conditions. High-quality atmospheric data are needed to reproduce the main features of the Mediterranean circulation (i.e. deep water formations, thermocline depths as well as thermohaline and wind driven circulations). For the ocean model, fluxes at the airsea interface are prescribed a priori. In this work, data from the french PERIDOT meteorological model were made available on a daily mean basis, over a 15-month period starting 21 July 1988. The data contain atmospheric pressure, evaporation, winds, sensible heat and downward radiation fluxes. Infra-red upward flux was computed from lo-day averages of sea-surface temperature fields. The resolution of the atmosphere data is approximately 0.4” x 0.3”. An analysis of the total heat flux averaged over one year shows a heating excess of 5 W/m2, but based on budgets in the straits the climatological heat loss over the basin is 7 W/m2 (Garrett et al., 1993). There is thus an excess heating of 12 W/m* in the 198881989 data set compared to climatology. In short-term simulations of the mesoscale activity, it is crucial to take into account the mesoscale signal of heat and momentum fluxes stemming from atmospheric models, while erroneous warming or cooling trends that might exist in long-term averages of the fluxes are not very dangerous. Conversely, systematic errors in the heat balance are known to be disastrous in long-term simulations of the general circulation. In using the 1988-1989 PERIDOT data for a perpetual year simulation, the daily fluxes have been corrected by increasing the wind speed slightly, because the PERIDOT wind speeds are usually found to be smaller than climatological estimates of Garrett et al. (1993). By doing so, the latent and sensible heat fluxes are increased to the values quoted by Garrett et al. (1993) and the overall budget becomes coherent with the climatology. Precipitation values given in the PERIDOT data set are not precise enough to determine surface “salinity fluxes” unambiguously, and corrections are introduced as restoring fluxes towards monthly-mean surface salinity fields interpolated in time. The combination of restoring salinity fluxes and imposed heat fluxes in some cases could lead to instability problems (e.g. Huang, 1993) which didn’t occur in our simulations. Data analysis and assimilation. Significant numbers of hydrographic data have been collected in the Mediterranean during this century, providing the basic ingredients needed to construct a climatological picture of the general circulation. This “background” picture is an important element for developing operational models of the circulation at various time scales. Several historical data sets gathered in different institutions have been identified and merged together after elimination of duplicated profiles and careful check of the data quality (Brasseur et al., 1996).

Circulation

of the western Mediterranean:

from global to regional

scales

535

Climatological analyses of temperature and salinity data have been performed at the seasonal and monthly scales, using a variational method and a finite element numerical technique. In special cases, the variational formulation is demonstrated to be equivalent to objective analysis, and a hybridation of the statistical and variational methods allows computation of 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 data sets (Brankart and Brasseur, 1996). The variational formulation of the data analysis problem, however, is more general and flexible than the traditional objective method, since additional constraints can be implemented to take physical processes into consideration. In Brasseur (1991), an advective constraint was implemented in the scheme to introduce anisotropic correlations, consistent with the flow field in which the data were collected. Initial conditions for the present model are derived from the climatological analyses as described below. In addition, a simple nudging assimilation method has been used to make the simulations coherent with the observational picture of the general circulation. In the nudging scheme, the evolution equations for temperature and salinity are modified so as to include a Newtonian relaxation term which acts as a local source/sink term depending on the difference between the predicted and observed values. The monthly-mean climatological analyses are linearly interpolated in time before the actual assimilation by nudging, in order to avoid phase-lag and Gibbs phenomena in the model response. The relaxation time scale has been fixed at 35 days in the interior of the basin, and it gradually increases to infinity when approaching the coast and the bottom. Initialisation. The simulations are initialised with the monthly mean temperature and salinity fields of August. Geostrophic currents are computed to initialise a barotropic salinity and baroclinic velocities). This adjustment run (with frozen temperature, adjustment is performed to eliminate spurious external gravity waves excited during the initialisation phase. After adjustment, the turbulent kinetic energy is initialised following a local equilibrium hypothesis. The system then adjusts rapidly to the imposed atmospheric and relaxation fields, reaching a quasi-periodic state after 10 years of simulation. In the following, we will analyse the eleventh year of this perpetual year simulation. Straits. In the buffer zones set in the Atlantic and eastern Mediterranean, the relaxation towards monthly-mean climatological values of temperature and salinity is intensified near the open boundary. Nevertheless, open boundary conditions are still needed: the normal gradient of scalars, tangential velocity and the baroclinic part of the normal velocity are imposed to vanish, while the mean normal velocity is computed following a radiation boundary condition of Orlanski’s type. As a result, the Gibraltar and Sicily Straits are part of the computational domain, but care should be taken when analysing the model results there. Results of the simulations

of the general circulation

The general circulation model reproduces the following elements with a fairly good degree of realism (e.g. Nihoul and Beckers, 1992; Beckers et al., 1992, 1994; Brasseur et al., 1996):

536

J.-M. Beckers et al.

(1) The Atlantic surface water flows eastwards along the North African coast, forming the Algerian Current (Arnone et al., 1990; Heburn and La Violette, 1990; Millot, 1987,199l; Millot et al., 1990). The Alboran Sea, the westernmost basin in the Mediterranean Sea, is directly concerned with the exchanges with the Atlantic Ocean. The Atlantic water flows through the narrow and shallow Strait of Gibraltar (20 km width, 300 m depth) into the Alboran Sea forming a jet of more-or-less 30 km width, with surface speeds near the Strait reaching 1 m/s. The penetration of the Atlantic water into the Alboran Sea is marked by meandering of the inflowing jet giving rise to one or two anticyclonic gyres with characteristic remote sensing signatures (Gascard and Richez, 1985; La Violette, 1994). (2) Large cyclonic circulations develop in the Thyrrhenian and in the Liguro-Provencal basins. The Gulf of Lions, extending from Toulon to the border between France and Spain, is mostly occupied by Modified Atlantic Water. The northern branch of the cyclonic circulation of the so-called Liguro-Provencal-Catalan Current (e.g. Euromodel and Primo, 1995) flows along the shelf-break and constitutes the large-scale feature of the dynamics of the continental shelf s outer boundary. Although this current follows the continental slope southwestwards most of the time, intrusions onto the shelf may occur, especially during northwesterly wind events. (3) The summer heating of the surface layer leads to the formation of a seasonal thermocline. During the winter, under the effect of dry and cold continental winds and of the difference of temperature between air and sea, the evaporation and heat transfer from the sea to the air becomes very important; hence the density of the surface layer increases. The consequent mixing and convection gives rise to deep water formation (Jones and Marshall, 1993). One observes this phenomenon in the northwestern Mediterranean (like in the model results), but also in the Adriatic Sea and in the Levantine Basin. (4) The water formed during winter in the Levantine Basin crosses the Sicily Strait (at the open boundary) and spreads in the Western basin, describing mainly cyclonic circulations (the so-called Levantine Intermediate Water) and flows out to the Atlantic through the Strait of Gibraltar (Hopkins, 1985). The Levantine Intermediate Water is mostly confined to the northern part of the basin along the Spanish coast, but its distribution is irregular: it flows westwards towards the Gibraltar Strait and forms more-or-less one third of the total outflow. (5) Below, the major part of the basin is occupied by the Mediterranean Deep Water which flows very slowly at a speed of approximately 2.5 cm/s, roughly in a cyclonic way. Part of it contributes to the outflow through the Gibraltar Strait, following the bottom topography in the form of a narrow boundary current, as a result of hydraulic uplifting (Venturi aspiration). Some of these broad features are easily recognisable in Fig. 2a, which shows the mean annual circulation at 10 m depth computed by the GHER Model in a typical climatic situation. One notices, in particular, the inflowing Atlantic Waterjet, the Alboran Sea gyres, the Algerian Current, the large cyclonic circulation in the Liguro-Provencal basin, the gyre in the central Tyrrhenian Sea produced by the year-round eastward wind jet from the Strait of Bonifacio, the Liguro-Provencal frontal current and its separation into two branches in the Balearic Sea and around the Balearic Islands, which act as a buffer zone limiting the interactions between the Alboran and Liguro-Provencal basins. Illustrative of the variability of the system is, for instance, the comparison between Fig. 2a (circulation averaged over one year of simulation) and Fig. 2b, which shows the weekly-

Circulation

of the western Mediterranean:

from global to regional

537

scales

Mean Circulation

Longitude

(b)

-6

-2

2

6

10

14

Longitude Fig. 2. (a) Mean-annual circulation at 10 m depth computed by the GHER Model (typical year). (b) Weekly average current field at 10 m depth computed by the GHER Model at the end of October

(typical year). averaged current field in October. Particularly revealing is the deformation of the main gyres, the presence of a temporary wind-induced northward water transport in the Algerian Basin and the evidence of the northward intrusion of recent Modified Atlantic Water through the sills of the Balearic Islands. Figures 3a and 3b display the autumn and winter salinity distributions at 100 m depth,

538 and shows Water in Proven9al modifying

J.-M. Beckers et al.

how, from one season to the next, the conflicting confluence of incoming Atlantic the Alboran Sea and returning Modified Atlantic Water from the Liguroo Basin and the Balearic Sea is rearranged according to seasonal conditions, the relative subduction and by-passing of the two water masses.

THE Implementation

REGIONAL

MODEL

OF THE

GULF

OF LIONS

of the regional model

From the results of the general circulation model, it is clear that a higher resolution model is needed on the shelf if the river impact has to be studied. A regional model has thus been implemented in the region covering the Gulf of Lions by using the same mathematical and numerical model as described above, but with the following specificities: Model formulation and data. The choice of the domain covered by the nested model is guided by the topography of the region and by the characteristics of the Liguro-Provencal current, which flows south-westward along the shelf break with incursions on the shelf. The Liguro-Provencal current is a permanent feature of the basin’s general circulation, with, however, significant modulation ofintensity (almost doubling during winter as a result ofdeep water formation, Fig. 4) and has a major impact on the shelf circulation. Figure 4 indicates that the flow in the eastern section is almost barotropicand increases during the winter. On the southern boundary, a baroclinic transport appears as a consequence of deep water formation in the Gulf of Lions and induces an anticyclonic tendency in the deeper region. The relation between the current and the buoyancy distribution in the northern basin (e.g. Hopkins, 1985) requires that the regional model be provided with density fields compatible with the current. Surface velocities are of the order of 0.1 mjs: the exploitation of the hydrodynamic model for the simulation of biogeochemical processes lasting several weeks sets the lower limit of the grids outer-scale at a few hundred kilometres. Indeed, the surface covered by the model is 25,000 km2 with a mean depth of 740 m which gives a mean residence time (estimated with Ligurian currents intensity calculated with the basins model) of the order of 50 days. The mesh size is 4.6 km and there are 14 vertical levels in the transformed-coordinates space. The mode-splitting technique has been used with the baroclinic time-step of 120 and a barotropic one of 8 s. Horizontal diffusion is 300 m2/s for velocity and 60 m2/s for scalars. The model is initialised by interpolating results of the general circulation model. Gravity waves are damped initially as described in the general circulation model setup. The actual simulation starts in August, forced by the PERIDOT atmospheric fluxes. Tracer model. In addition to the hydrodynamic model, a passive tracer model was introduced to compute typical residence times and patterns of tracers. In practice, three different tracers are computed: one released by the Rhone River, a second one carried by the Liguro-Provencal current, and the last one is present initially in the deep ocean. The tracer model is analysed in more detail in Beckers (submitted), but it is mentioned here because, as we will see later, some figures show tracer distributions which exhibit similar patterns to satellite pictures of the region. Concerning the release by the RhGne River, the water flux is imposed directly at the river

Circulation

of the western Mediterranean:

from global to regional

scales

Autumn Sahity

-29

1.1

5.1

9.1

Longitude

Winter Salinity

(b)

35.1 -6.9

-i.Q

Longitude Fig. 3.

(a) Autumn salinity field at 100 m depth in the western Mediterranean Basin. (b) Winter salinity field at 100 m depth in the western Mediterranean Basin.

539

540

J.-M. Beckers et al.

End of August, River Nitrate

End of September, River Nitrate

3.2

3.4

3.6

3.1

4

4.2

414

46

4.8

5

5.2

5.4

5.6

5

SCALE

Fig. 5. Current field at 10 m depth in the Gulf of Lions superimposed on the distribution pattern of a passive tracer released by the RhBne River (arbitrary units for the tracer). (a) End of August; (b) end of September; (c)end of October; (d) end of March: (e) end of June.

Circulation

of the western Mediterranean:

from global to regional

541

scales

End of October, River Nitrate

42.5

42.3 j.2

is

i8

i

i.2

i.4

j.4

4.6

Longitude

SCALE

7

0

t .75

(4

--

3.50

5.2s

End of March, River Nitrate

Lmghdc

SCALE

b

3.;0 Fig. 5.

(Continued.)

5125

j.8

542

J.-M. Beckers et al.

End of June, River Nitrate

3.2

34

3.6

3.8

4

4.2

4.4

4.6

4.8

5

52

Longitude

3.50

Fig. 5.

(Continued.)

5.25

,.*

3.0

3.11

Circulation

of the western

Mediterranean:

from global to regional

543

scales

End of September, Ligurian Nitrate

(4

43.1 B E % c)

42.9

42.3 s

3.2

3.4

3.11 4

3.6

4.2

4.4

4.6

4.8

5

5.2

3~

5.6

sa

Longitude

SCALE

8

I

0

1.75

3.50

5.25

7

End of November, Ligurian Nitrate

(b)

42.3 3

3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

4.8

5

5.2

5.4

5.6

58

Longitude

SCALE

I :7s

3150

Fig. 6. Current field at 50 m depth superimposed on the distribution pattern of a passive tracer carried along by the Liguro-ProvenGal Current. (a) End of September; (b) end of November; (c) end of March.

J.-M. Beckers et al.

544

Longitude

Fig. 6.

(Continued.)

End of October, Deep Nitrate

Longitude

Fig. 7.

Current

field at 100 m depth at the end of October superimposed on the distribution of a passive scalar coming from below 200 m.

pattern

545

Circulation of the western Mediterranean: from global to regional scales

Eastern Gulf of Lions Flux (Sv)

4’ 7/W88

1111188

I/389 I/1/89

I/9/88

l/7/89 I/Y99

Day

Southern Gulf of Lions Flux (Sv)

7/8/88

l/Ii/w

1/9/8B

l/3189 I/I/89

I/7/89 I/w39

Day

Fluxes through the eastern and southern entrance of the Gulf of Lions Fig. 4. Water transport in the Gulf of Lions computed by the GHER Model for the period 1August 1988-31 July 1989. In the eastern section, the total tranport (shown in dark grey, negative Flux-axis) is the sum ofthe eastern (counted positively and shown in black) and western (counted negatively and shown in light grey) flows. In the southern section, the total transport (shown in dark grey, negative Flux-axis) is the sum of the northern (counted positively and shown in black) and southern (counted negatively and shown in light grey) flows.

mouth. As short and strong discharge discharge data used were not climatic R/z&e) from a typical year (1988). Open sea boundary

conditions.

in detail in Beckers (submitted),

events carry a major part of the tracers, the river averages but daily data (Compagnie Nationale du

The conditions imposed at open-sea can be summarised as follows.

boundaries,

discussed

J.-M. Beckers ef al.

546

As we use a procedure in which the information is transferred only one way from the general circulation model towards the local model, the nesting is simply a problem of applying open sea boundary conditions to the local model. In particular, the problem of different scales and processes resolved by the models now must be taken into account at the open boundary. This is only possible if the open sea boundaries are somehow permeable to the outflowing, small scale information from the local model, while applying coarse resolution background fields for the inflow conditions. This is achieved in the following way. For temperature and salinity, the GCM results (GCM subscript) interpolated to the local grid are used at the boundaries as a background field to which the boundary values are relaxed. The effect of an outflow is modelled by using an upwind advection scheme and a boundary condition such that at the open boundary the temperature and salinity fields are modified according to ar _+,,.vT__(TTGCM)

at

5

For an inflow, the advection is simply discarded. This scheme is similar to Orlanski’s boundary conditions, except that the phase velocity is the real horizontal velocity. For the barotropic part of the velocity field, a 2-D radiation condition is used, but the resulting normal transport is corrected in such a way that the total inflow is equal to the one computed by the general circulation model (as shown on Fig. 4). Finally, for the baroclinic part, a zero normal gradient is imposed. This has the advantage of avoiding spurious vertical velocities at the open boundaries. Results

qf the

regional model simulations

As mentioned already, the Gulf model was used to appraise the inputs, and subsequent deployment in the Gulf, of nutrients carried along the Liguro-ProvenGal Current, brought by the Rh6ne or recirculated from the bottom sediments (Beckers, submitted). In the following, attention will be restricted to the general circulation in the Gulf of Lions. However to make this circulation more visible, current velocities will be plotted on passive tracer distribution patterns (exemplative of nitrate distributions in periods of negligible consumption) corresponding to one of the three sources (Liguro-ProvenGal current, river, deep ocean) solely operating (the other sources switched ofT). Surface hydrodynamics. The surface circulation can be completely disconnected from the main Ligurian flow. Especially on the shelf, a strong baroclinic current is present which can change direction frequently. A structure which reappears regularly is the one observed in Fig. 5a (end of August). The main Liguro-ProvenGal current is south-westward, but on the shelf a recirculation brings some off-shore waters onto the shelf near the Spanish border. Furthermore, the Rhbne discharge is entrained between the recirculation and the main current. This feature is however modified by wind conditions and the intensity of the LiguroProvenGal current (itself depending on the deep water formation and the heat losses). In September (Fig. 5b), for example, a clear entrainment of the surface waters by the wind is observed, thus creating upwellings along the coast. When the Liguro-ProvenGal current intensifies, the whole circulation on the shelf can be reversed and follows a general south-westward path (October situation, Fig. 5c) before showing again a recirculation on the shelf in March and June (Figs 5d and e).

Circulation of the western Mediterranean: from global to regional scales

547

The atypical river plume spreading directly seaward observed in the simulations needs some explanation (the classical river plume picture would be a righthand turn with horizontal scales of the order of the inertial radius). The characteristic pattern of Fig. 5a also has been detected from satellite pictures (Morel et al., 1990). According to Cruzado and Velasquez (1990) and Cruzado (1995), this entrainment is due to the joint action of an anticyclonic gyre to the west and a cyclonic gyre to the southeast which he derived from field observations and geostrophic calculations. His description of the currents is thus very similar to ours. The reason for the anticyclonic tendency can be searched for in the wind fields. Indeed the average curl of the wind stress in this region is clearly negative on the western shelf region and positive on the southeast (Pinardi and Navarra, 1993; Roussenov et al., 1995) so that this would favour the observed circulation pattern. We tried to verify this conjecture by cutting off the direct wind forcing in the local model, and indeed, the anticyclonic circulation on the shelf was generally weakened, but it did not always disappear. This, of course, can be due to open boundary conditions that still carry the information of the general circulation, which itself was always operated with the full atmospheric data. The attenuated anticyclonic tendency in the regional model comforts the idea that the wind fields in the northern basin control the recirculation on the shelf and the entrainment of the river plume. Below thermocline hydrodynamics ( - 50 m to - 100 m). Here the currents are relatively stable along the shelf break (Ligurian current), but on the shelf, recirculation (e.g. September, Fig. 6a and March, Fig. 6c) or direct intrusions (e.g. November, Fig. 6b) of these currents are predicted by the model, depending on the strength of the LiguroProvengal inflow. Furthermore, from the tracer distribution, a clear signature of the shelf break can be observed on Fig. 6b. This is probably due to upwelling across the shelf break, bringing up the higher tracer concentrations from below. Similarly, the cyclonic circulation in the southeast favours the upwelling there. Figure 7 shows also the possible intrusion of deep waters onto the shelf in the southwest of the domain. Intermediate waters (-400 m). The currents are much weaker than in the upper layers. This is confirmed by observations, and the velocity of a few cm/s is comparable to those quoted by Millot (1991). Generally, the current is very stable and directed southwestward.

CONCLUSIONS The combination of hydrographic data sets, the general circulation model and nested modelling can be used to realise a regional model of a marine sector strongly influenced by the open ocean, atmospheric forcings and river discharges. The global model is capable of reproducing the main physical processes found in the Mediterranean Sea, and provides a numerical picture of the general circulation that synthesises field observations and process studies conducted for years in the Western basin. In addition, this numerical spin-up achieved at the global scale delivers open-boundary conditions to the regional model of the Gulf of Lions. When refining the model resolution, the traditional picture of a general cyclonic circulation developed in the northwestern basin is slightly revisited: the simulation of an annual cycle in typical conditions indicates a rather strong variability over the region extending from the shelf to the deep ocean, especially in the upper layer which is mostly

548

J.-M. Beckers et al.

affected by atmospheric forcings. Typical flow patterns simulated in the sub-surface layer confirm the episodic existence of a double (cyclonic and anticyclonic) gyre system, in good agreement with both remote and in situ observations. This variability is of the utmost importance in assessing the deployment of the Rhone River plume and the associated dispersion of nutrients and pollutant substances. Though the method suggested in this paper works reasonably well in the present case, the possibility of really coupled (rather then the one-way nesting) models needs further investigation, especially when using full biological models for which very accurate open boundary conditions are needed. Acknowledgements-The authors wish to express their gratitude to the Commission of the European Union for its financial support in the scope of the Environment and Marine Science and Technology Programs, They are indebted to the Belgian Ministry for Science Policy (SeaSEx Program), the National Fund for Scientific Research (Belgium) and IBM for their assistance in providing means for parallel and vector computing. The German Alexander von Humboldt Foundation is also acknowledged.

REFERENCES Arnone, R. A., Wiesenberg, D. A. and Saunders, K. D. (1990) The origin and characteristics of the Algerian Current. Journal of Geophysical Research, 95, 158771598. Beckers, J. M. (1991) Application of a 3D model to the Western Mediterranean. Journal of Marine Systems, 1, 315-332.

Beckers, J. M. (submitted). 3D Numerical Simulation of the Circulation and Tracer Dispersion in the Gulf of Lions. Continental Shelf Research. Beckers, J. M., Djenidi, S. and Nihoul, J. C. J. (1992) The western Mediterranean. From hydrology to hydrodynamics. Wafer Pollution Research Reports, 28, 19-37. Beckers, J. M., Brasseur, P., Djenidi, S. and Nihoul, J. C. J. (1994) Investigation of the western Mediterranean’s hydrodynamics with the GHER three-dimensional primitive equation model. In Seasonal and Interannual VariaMity of the Western Medirerranean Sea, ed. Paul E. La Violette, Coastal and Estuarine Studies, Vol. 46, pp. 2877324. AGU Publ. Brasseur, P. (1991) A variational inverse method for the reconstruction of general circulation fields in the Northern Bering Sea. Journal of Geophysical Research, 96(C3), 48914907. Brankart, J. M. and Brasseur, P. (1996) Optimal analysis of in situ data in the Western Mediterranean using statistics and cross-validation. Journal of Atmospheric and Oceanic Technology, 13, 477 -491. Brasseur, P., Beckers, J. M., Brankart, J. M. and Schoenauen, R. (1996) Seasonal temperature and salinity fields in the Mediterranean Sea: climatological analyses of an historical data set. Deep-Sea Research I, 43, 159% 192.

Cruzado, A. (1995) Nutrient distribubons in the Gulf of Lions (northwestern Mediterranean) during the cruise of R.R.S. Discovery (July 1993). Water Pollution Research Reports, 32, 69-77. Cruzado, A. and Velasquez, 2. R. (1990) Nutrients and phytoplankton in the Gulf of Lions, northwestern Mediterranean. Continental Shelf Research, 10, 93 l-942. Euromodel and Primo groups (in press) Progress in understanding the general circulation of the Western Mediterranean. Oceanologia Acta. Garrett, C., Outerbridge, R. and Thompson, K. (1993) Interannual variability in Mediterranean hand buoyancy fluxes. Journal of Climate, 6, 900-910. Gascard, J. C. and Richez, C. (1985) Water masses and circulation in the western Albordn Sea and in the Strait of Gibraltar. Progress in Oceanography, 15, 157-Z 16. Heburn, G. W. and La Violette, P. E. (1990) Related variations in the structure of the anticyclonic gyres in the Alboran Sea. Journal of GeophysicalResearch, 95(C2), 159991613. Hopkins, T. S. (1985) The physics of the sea. In Western Mediterranean, ed. R. Margalef, pp. 100-125. Pergamon, New York. Huang, R. X. (1993) Real freshwater flux as a natural boundary condition for the salinity balance and

Circulation

of the western

Mediterranean:

from global to regional

scales

549

thermohaline circulation forced by evaporation and precipitation. Journal of Physical Oceanography, 23, 2428-2446. Jones, H. and Marshall, J. (1993) Convection with rotation in a neutral ocean: a study of open-ocean deep convection. Journal of Physical Oceanography, 23, 1009-1039. La Violette P. E. (ed.) (1994) Seasonal and interannual variability of the Western Mediterranean Sea. Coastal and Estuarine Studies, 46, 373 pp. AGU Publ. Millet, C. (1987) Circulation in the Western Mediterranean Sea. Oceunologia Acta, 10, 143-149. Millet, C. (1991) Mesoscale and seasonal variabilities of the circulation in the Western Mediterranean. Dynamics of Atmospheres and Ocean, 15, 179-214. Millet, C., Taupier-Letage, I. and Benzohra, M. (1990) The Algerian eddies. Earth Science Review, 27, 203-219. Morel, A., Bricaud, A., Andre, J. M. and Pelaez-Udlet, J. (1990) Spatial/temporal evolution of the Rhone plume as seen by CZCS imagery--consequence upon the primary production in the Gulf of Lions. Waler Poltution Research Reports, 20, 45-62. Nihoul, J. C. J. (1975) Modelling of marine systems, Elsevier Publ., Amsterdam. Nihoul, J. C. J. and Djenidi. S. (1991) Hierarchy and scales in marine ecohydrodynamics. Earth Science Revieua, 31, 255-277. Nihoul, J. C. J. and Beckers, J. M. (1992) Diagnostic/metagnostic modelling of the Western Mediterranean’s general circulation with a 3D primitive equations k-c model. In Avancees en Oc&nologie MPditerrunPenne. Vol. 11, pp. 99-l 12. Bulletin de I’Institut Oceanographique, Monaco. Nihoul, J. C. J., Deleersnijder, E. and Djenidi, S. (1989) Modelling the general circulation of shelf seas by 3D kmodels. Earth Science Review, 26, 163-189. Nihoul, J., Adam, P., Brasseur, P., Deleersnijder, E., Djenidi, S. and Haus, J. (1993) Three-dimensional general circulation model of the northern Bering Sea’s summer ecohydrodynamics. Continental Shelf Research, 13, 509-542. Pinardi, N. and Navarra, A. (1993) Baroclinic wind adjustment processes in the Mediterranean Sea. Deep-Sea Research I, 40, 1299-1326. Roussenov, V., Stanev, E., Artale, V. and Pinardi, N. (1995) A seasonal model of the Mediterranean Sea general circulation. Journal of Geophysical Research, 7.