review paper circulation in the western...

Ž .Journal of Marine Systems 20 1999 423–442

Review Paper

Circulation in the Western Mediterranean Sea

Claude Millot )

Laboratoire d’Oceanographie et de Biogeochimie, Centre d’Oceanologie de Marseille, Antenne LOBrCOMrCNRS, BP 330,´ ´ ´F-83507 La Seyne, France

Received 20 October 1996; accepted 1 July 1997

Abstract

During the last decade, a considerable amount of work has been made and definite results obtained about the circulationwin the Western Mediterranean Sea. The diagrams presented 10 years ago Millot, C., 1987a. Circulation in the Western

xMediterranean. Oceanol. Acta, 10, 2, 143–149. have been confirmed and complemented, mainly in the south where allwater masses appear to flow anticlockwise along the continental slope, as they do everywhere else in the sea. Definitiveresults have also been obtained about the mesoscale phenomena in the Algerian Basin which induce a dramatic variability ofthe circulation of all water masses, as far as around the Balearic Islands and through the Channel of Sardinia. Extremelyinteresting observations have been collected between Sicily, Tunisia and Sardinia about the hydrological and dynamicalcharacteristics of the waters entering the Tyrrhenian Sea. Finally, the dense water formation processes have been specified,mainly since some uncommon situations have been encountered. As a whole, the relatively large importance of the seasonalŽ . Ž .resp. mesoscale variability in the north resp. south has been documented. Together, the observations in nature, thelaboratory experiments and the numerical models have thus provided a more thorough understanding of the sea dynamics.Nevertheless, uncertainties remain about the amount of the waters formed in the sea mainly at intermediate depths, for thesewaters are often not easily distinguished from the surrounding ones. This is of major importance for, a priori, intermediatewaters can flow out of the sea more easily than deep waters. In any case, the southern Tyrrhenian Sea appears to be a keyplace for the functionning of the whole sea. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: water mass; circulation; sea dynamics

1. Introduction

The paper continuously referred to in the follow-Ž .ing Millot, 1987a was written to express ideas

about the analysis of all the in situ and remotelysensed data that are based on simple theoreticalarguments and lead to circulation features markedlydifferent from those suggested by the diagrams of

Ž . Ž .Wust 1961 and Ovchinnikov 1966 . One argument¨

) Corresponding author. Tel.: q33-4-94-30-48-84; Fax: q33-4-94-87-93-47; E-mail: [email protected]

is that a water mass spreading from a source area isforced by the Coriolis effect to follow the isobaths,thus flowing mainly anticlockwise along the conti-nental slope in such an enclosed sea. Another argu-ment is that the major forces driving the circulationgenerally display smooth spatial variations, so thatno permanent branching can occur in the middle of asea. A basic belief was that the major currents aregenerally structured like unstable veins, rather thanlike wide and smooth flows, generally following thecoast andror the continental slope, rather than cross-ing the sea. Another belief was that the mesoscale

0924-7963r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0924-7963 98 00078-5

( )C. MillotrJournal of Marine Systems 20 1999 423–442424

phenomena mainly resulted from the instability ofthese coastal currents and could have a dramaticimportance for the circulation of all water masses.

Ten years ago, a relatively large amount of in situmeasurements was available in the northern part ofthe sea, collected either to study the dense waterformation processes or in the framework of regularprogrammes conducted by coastal laboratories.There, the phenomena were thus better understood,and the 1987 circulation diagrams were rather de-tailed, especially as the circulation itself is ratherstable. In the southern part of the sea, hydrologicaldata were very scarce and current time series notavailable, so that it was mainly on the basis of theremotely sensed data, thought consistent with all theavailable in situ data, and on the basis of the afore-mentioned theoretical arguments that the diagramswere drawn. This explains why they were so incom-plete and essentially hypothetical there.

Nevertheless, these circulation diagrams were pre-sented as ‘new references for future works, with theobvious understanding that they will be certainlycomplemented quite shortly and, if necessary, cor-rected’. As a matter of fact, important experimentswere conducted all over the sea, mainly by teams of

Žthe EUROMODELrMAST-1,2 group namelyCOM-La Seyne, ICM-Barcelona, SO-La Spezia and

.UIB-Palma in association with most of the othergroups interested in the Western Mediterranean Sea,while sophisticated laboratory experiments and nu-merical models were performed, mainly by other

Žteams of this group see EUROMODEL Group, 1995,.for a general view .

Among the experimental studies, some are essen-tial as they have allowed a marked improvement ofthe circulation diagrams. During the MEDIPROD-5experiment, conducted in the Algerian Basin in1986–1987 within the framework of the Western

ŽMediterranean Circulation Experiment La Violette,.1990 , we performed more than 100 CTD casts

during a 1-month cruise, launched five drifting buoysand deployed 28 current meters on eight mooringsduring ;9 months. Then, we participated in 1990–1991 in the PRIMO-0 experiment during which morethan 60 current meters were set on ;15 mooringsfor several months between the channels of Corsicaand Ibiza, while CTD data were regularly collectedŽseveral papers were published in a special issue; see

.Millot, 1995, for a general view . Our group alsoparticipated in 1993–1994 in the PRIMO-1 experi-ment and made interesting observations between Sar-

Ždinia, Tunisia and Sicily Bouzinac et al., 1999;.Sammari et al., 1998; Sparnocchia et al., 1999 .

Simultaneously, we took part to the THETIS-2rMAST-2 experiment and launched twice a monthbetween France and Algeria especially calibratedXBT’s that are analysed together with other data setsŽ .Send et al., 1997; Fuda et al., 1998 . Finally, duringthe ALGIERS and ELISArMAST-3 cruises con-ducted in 1996–1997 close to Algeria, we inten-

Žsively sampled various mesoscale phenomena Font.et al., 1998, and not yet published data .

These specific hydrological data sets closely fit inŽ .with historical Guibout, 1987 and most recent

Ž .Picco, 1990; Brasseur et al., 1994 ones, and the lastcurrent measurements are consistent with thoseavailable 10 years ago. A marked development of the1987 diagrams is thus allowed, and the major objec-tive of this paper is to present and discuss the newschematized circulation features. The paper also pro-poses new arguments about the formation, circula-tion and mixing of the intermediate and deep watersmainly, which might modify ideas generally ac-cepted about the working of the whole Mediter-ranean Sea. It first describes the circulation featuresof the various water masses along their course, fromsurface to bottom, and then comments possible im-plications of the new results. Repeating referencesmore than 10-year old has been avoided and only themost significant recent ones have been selected. Wedid not follow the suggestion of the referees to showsome of the data, as they are too numerous to makean objective selection and as the most significantones are presented elsewhere, either in already pub-lished papers or in this special issue, or will bepublished soon.

( )2. Water of Atlantic origin Fig. 1

2.1. The Alboran Sea

It is first necessary to mention that, mainly thanksto the Gibraltar Experiment, a lot of papers were

Žpublished about the dynamics of the strait Bryden.and Kinder, 1991 . Among the definitive results

( )C. MillotrJournal of Marine Systems 20 1999 423–442 425

Ž . Ž .Fig. 1. Circulation of the Modified Atlantic Water MAW and the Winter Intermediate Water WIW . Symbols meanings are as follow. p :more or less steady paths; : mesoscale current throughout the year; : wintertime mesoscale currents; : wind induced mesoscalecurrents qq: the North-Balearic front; r: O m isobath.

already obtained, it can be retained that: the flowsmight be significantly smaller than previously esti-

Ž .mated Bryden et al., 1989 , the exchange tends tobe maximal early in the year and submaximal later

Ž .on Garrett et al., 1990 , the Mediterranean watercan be uplifted from a few hundreds of metersŽ .Kinder and Bryden, 1990 and most of the barotropicsignal is induced by the atmospheric pressure forcingŽ .Candela, 1991 .

The Atlantic water in the Alboran Sea describes,as previously schematized, a quasi permanent anticy-clonic gyre in the west and a more variable circuit inthe east. Several experiments were conducted when

Žthis circuit was cyclonic e.g., Folkard et al., 1994;.Viudez and Tintore, 1995 , while some infrared im-´

ages clearly indicate that filaments can extend east-ward from Cape of Gata for days, seemingly in

Žrelation with strong westerly winds Le Vourch et.al., 1992 . Nevertheless, the circuit in the east might

Žbe anticyclonic most of the time e.g., Tintore et al.,´.1988; Davies et al., 1993; Viudez et al., 1996 . In

such a situation, the vein flowing from Spain to

Algeria is now commonly named ‘the Almeria-Oranjet’, and its eastern side was often chosen for frontal

Ž .dynamics studies Prieur and Sournia, 1994 . SmallŽcyclonic eddies were also depicted there Davies et

.al., 1993 , as well as all around the western gyreŽ .Tintore et al., 1991, 1994 , while variations in the´structure of the gyres have been documented, espe-cially with infrared images by Heburn and La Vio-

Ž . Ž .lette 1990 and by Vazquez et al. 1996 who alsoconsidered altimetric data. Recent numerical modelsŽ .Speich et al., 1996 and laboratory experimentsŽ .Gleizon et al., 1996 underline the coupling betweenthe regime of the Strait of Gibraltar, the generalpattern of the Atlantic flow in the Alboran Sea, andthe circulation of the underlying Mediterranean wa-ter.

Ž .The name ‘Modified Atlantic Water’ MAW isnow systematically used to refer to the surface waterall over the Mediterranean Sea. Except in someplaces, MAW forms a 100–200 m layer character-ized by salinities that increase, due to evaporationand mixing, from ;36.5 at Gibraltar to 38.0–38.3


in the north of the Western Mediterranean Sea, andby a mean temperature, below the mixed layer, ofusually 14–158C. A crucial point is the thickness ofthe recent MAW layer in the Alboran Sea that is

Žthere generally greater than ;200 m e.g., Tintore.et al., 1988 , even if markedly variable in time and

space. This concerns the part of old MAW thatcompletes in this region its anticlockwise circuit in

Ž .the sea as indicated in the 1987 diagrams , and theshallowest intermediate waters that proceed south-

Žward along the Spanish continental slope as detailed.later on . At depths shallower than ;200 m, these

waters encounter the inflow of recent MAW and arethus deflected eastward.

2.2. The Algerian Basin

Further east, the MAW flow forms what is nowŽcommonly named ‘the Algerian Current’ see Millot,

.1985 . As estimated from a unique MEDIPROD-5data set collected with a few-kilometre samplinginterval along sections that reached the Algeriancoast, and now supported by the ALGIERS data set,

Žthe flux of this current was ;1.7 Sv Benzohra and.Millot, 1995a , in agreement with several other infer-

Ž .ences e.g., Viudez et al., 1996 . This current isŽ . Žrelatively narrow 30–50 km and deep 200–400 m

.at the coast near 08E, but it becomes wider andŽthinner while progressing eastward Benzohra and

.Millot, 1995a .Thereafter, its unstable character sometimes leads

to the generation of meanders a few tens of km inwavelength, but the current continues flowing along

Žthe Algerian slope till the Channel of Sardinia see.Morel and Andre, 1991 . Most often, this character´

leads to the generation of a series of ‘coastal eddies’Ž .50–100 km in diameter , clearly evidenced withinfrared images, drifting buoys and ship drifts, aswell as with in situ current and temperature recordsŽ .Millot, 1991; Millot et al., 1990, 1997 . Roughly as

Ž .hypothesized from satellite images Millot, 1985 ,eddies of both signs are most of the time generatedeastward from 1–28E, the cyclones being relativelysuperficial and short-lived, and they propagate at aroughly constant speed of 3–5 kmrday. Therefore,only the anticyclones can get a noticeable amplitudeand last for weeksrmonths, being associated with anupwelling that is definitively not wind-induced. These

features are consistent with some specific conditionsŽstudied with both numerical models Beckers and

.Nihoul, 1992; Mortier, 1992 and laboratory experi-Ž .ments Chabert d’Hieres et al., 1991 . As a whole,`

this quasi permanency of eddies increases the mixingbetween the resident and newly-entered surface wa-ters.

Nevertheless, not all of them have a sufficientlylarge vertical extent to markedly modify the circula-tion of all water masses. Indeed, two anticycloniceddies, sampled with the guidance of the infraredimagery, were clearly associated with more than 50cmrs surface currents and a clear hydrological sig-

Žnature in the surface layer at least Millot et al.,.1990 , without inducing any significant current at

100 m. Moreover, the hypothesis put forward in1985 that the coastal eddies could extract more andmore energy from the mean current, so that theywould be advected eastward more slowly and growdeeper, has to be rejected since their diameter, depthand kinetic energy fluctuate without any special ten-

Ž .dency Millot et al., 1997 . In fact, among the tens ofmesoscale anticyclonic eddies evidenced with satel-lite data during the 9-month MEDIPROD-5 experi-ment, only one induced significant currents at depths

Ž .of 100 m and more 1000 m at least . To get acoherent interpretation of the whole data set, it was

Ž .proposed Millot, 1994 to consider such an ‘event’basically as an isolated meander of the AlgerianCurrent that generates in the surface layer, betweenits inner edge and the coast, an anticyclonic eddy

Žpreviously named ‘coastal eddy’ diameter of 50–100. Žkm and, in the deeper layer, a larger diameter

.;150 km anticyclonic eddy, the axes of both ed-dies being initially not centered. The 2-layer struc-ture of this event has similarities with mid-latitudeatmospheric systems, and is in agreement with some

Ž .other laboratory experiments Obaton et al., 1998 ,without having been reproduced up to now by anynumerical model. As specified hereafter, such eventsare of dramatic importance for the circulation of allwater masses.

The alongslope progression of both the AlgerianCurrent and its associated mesoscale phenomena canbe disturbed by what we called ‘open sea eddies’,i.e., eddies wandering in the middle basin. On thesole basis of infrared images, Taupier-Letage and

Ž .Millot 1988 described a situation when two such


eddies of ;200 km in diameter were clearly ob-Žserved from June to October 1984 at least these

months correspond respectively to the increase andthe decrease of the overall sea surface temperature

.gradient between newly-entered and resident waters ,thus accounting for relatively long lifetimes. Accord-ing to these images, the Algerian Current interactedwith the westernmost eddy during the whole periodand spread directly from the Algerian slope towardthe Balearic Islands. Note that the inflow of recentAtlantic water through the Balearic Channels, underthe influence of the mesoscale Algerian eddies, wasschematized in the original circulation diagram.These inflows have been further documented, essen-tially with another set of infrared images and hydro-logical data collected near the Balearic IslandsŽ .Lopez-Garcia et al., 1994 . Moreover, such a situa-tion was encountered during most of theMEDIPROD-5 experiment: the whole current inter-acted for months with an open sea eddy, spreading

Žseaward before it reached ;68E Benzohra and.Millot, 1995a; Millot et al., 1997 . These interactions

Žstopped when the event as described in the previous.paragraph , progressing eastward, came close to the

open sea eddy. The latter was pushed seaward andŽthus no longer interacted with the current Millot et

.al., 1997 . A detailed analysis of this open sea eddyŽ .Benzohra and Millot, 1995b , and the feeling thatthe energy is available more from the current thanfrom the eddy, lead us to think that, in such situa-tions, the current is not passive, and thus not di-verted: more probably, the eddy disturbs the currentthat becomes unstable and provides energy to theeddy through interactions.

As already mentioned, it is clear that our defini-tion of coastal eddies has evolved, since we firstthought they continuously increase in diameter, depthand kinetic energy, while we now believe some ofthem are the upper part of an event, which rapidlygets a mature stage. Indeed, the MEDIPROD-5 eventwas followed for a while, through the whole current

Ž .meter array 0 to 58E from July to December 1986Ž .and then more eastward till ;88E in January 87

with infrared images. The diameter of the surfaceeddy varied between ;50 to ;100 km, sometimeswithin a few weeks, but did not show any overallincrease. Similar events were sampled in the eastern

Ž .part of the Algerian slope near 88E with 760-dbar

XBT probes, from February to October 1994 duringŽ .the THETIS-2 experiment Fuda et al., 1998 . The

25-km sampling interval was relatively large, but theevents in the East are seemingly more barotropicthan in the West, as if the axes of both the surfaceand the deep eddies were more centered. Off theeastern Algerian slope, the events might thus bemore similar to open sea eddies. Their hydrological

Ž .structure 100–200 m deepening of the isotherms ,clearly indicative of an anticyclonic motion, is asso-

Žciated with intense surface currents several tens of.cmrs from the ship drifts and large elevations of

Žthe sea surface 10–20 cm from the TOPEX-.POSEIDON and ERS-1 altimeters .

In our 10-year old papers, open sea eddies weredepicted as late stages of coastal eddies. Fuda et al.Ž . Ž .1998 and Vignudelli 1997 clearly demonstratethat the events follow the circuit first depicted by

Ž .Millot 1985 , that is eastward along the Algerianslope, then northward toward Sardinia and finallywestward in the middle Algerian Basin. It is stillassumed that the topography of the Channel of Sar-dinia, that should not allow deep structures toprogress eastward, plays a major role for such acircuit; the role of the b-effect and of non-linearities,

Ž .hypothesized by Taupier-Letage and Millot 1988should probably not be neglected when consideringthe westward motion of the eddies, especially whentheir diameter reach 100–200 km. Let us specify that

Ž .the long lifetime several months and the largeŽdiameter several tenths of the Algerian Basin di-.mensions of these mesoscale eventsreddies imply a

low rate of generation, of the order of one perseveral months, that is the rate expected from theMEDIPROD-5 experiment. Therefore, and as sup-ported by all the analyses of the altimetric data setsŽe.g., Larnicol et al., 1995; Ayoub et al., 1998;

.Vignudelli, 1997 , the eastern part of the AlgerianŽ .Basin is one of the most energetic at mesoscale

place in the whole Mediterranean Sea.As initially hypothesized, the Algerian Basin ap-

pears to work as a reservoir of MAW, thus forming abuffer zone that disconnects, at relatively shorttimescales, the inflow from the outflow. The inflow

Ž .comes from Gibraltar as the Algerian Current andfrom the Northern Current; both components can berather accurately estimated, either due to the relation-ship with the surface slope or to the absence of


continental shelf that makes geostrophic computa-tions easier. This is also the case for the part of theoutflow following the western slope of Corsica thatis relatively stable, even if sometimes disturbed by

Ž .mesoscale phenomena Millot, 1991 . But this is notthe case in the Channel of Sardinia, due to thecomplex topography and to the large intensity of themesoscale Algerian eventsreddies. This must beconsidered as a major and definitive handicap toachieve flux estimations. Indeed, let us neglect evap-oration, precipitation and dense water formation inthe Western Mediterranean Sea, that are generallyexpected to be only a few tenths of the flux atGibraltar. On a yearly average for instance, the

Ž .fluxes F are such that F Channel of Sardinia sŽ . Ž . ŽF Gibraltar qF Northern Current yF West Cor-

.sica Current , the second term being larger than theŽ . Žthird one, andror equals F Sicily qF East Cor-

.sica Current . The published flux values spread overrelatively wide ranges that can generally be consid-ered as 1–2 Sv for the Gibraltar, Northern Currentan Sicily fluxes, and 0.5–1 Sv for the West and EastCorsica Currents ones. Therefore, the flux throughthe Channel of Sardinia exceeds, by ;0.5 Sv atleast, those through any other strait or channel in thesea.

2.3. The Tyrrhenian Sea

The large mesoscale variability in the Channel ofŽSicily schematized in the 1987 diagram and already

.supported by Manzella et al., 1990 has been docu-mented with hydrological measurements collected bythe EUROMODEL and INSTOPrTunis groupsŽ .SALTOrMAST-2 programme in the whole

ŽTunisia–Sardinia–Sicily area Astraldi et al., 1996;.Sammari et al., 1998 . Obviously, the MAW flux

Žtoward the Eastern Mediterranean Sea is and will.always be! very difficult to estimate directly, with a

lot of current meters for instance, furthermore as theflow sometimes spreads over the whole width of thechannel and sometimes is constrained along theTunisian side. When considering this MAW flux,both indirect estimations from some hydrological

Žstations and current time series at depth see Astraldi.et al., 1996, for an overview and computations from

numerical models are very scattered, ranging fromless than 1 Sv to more than 3 Sv, without providing a

coherent picture of the seasonal variability. SomeŽ .models e.g., Harzallah et al., 1993 provide mean

values that compare well with an average of thoseŽestimated from the data, and others e.g., Pinardi et

.al., 1997 account for a large seasonal and annualvariability which might explain the dispersion of thevarious estimations. In any case, these estimationswill not be easily specified, due to the occurrence of

Ž .low-frequency oscillations Astraldi et al., 1996 andthe presence of wide continental shelves.

In the Tyrrhenian Sea, the mesoscale turbulencein the middle sea and the flow along Sicily and theItalian peninsula were schematized on the basis ofthe aforementioned theoretical arguments and in-

Žfrared data sets. They are supported only during the.winter for the alongslope flow by more recent in-

Ž .frared data Marullo et al., 1994 but not by in situŽ .measurements Astraldi and Gasparini, 1994 . In the

central and northern Tyrrhenian Sea east of the Straitof Bonifacio, a cyclonic eddy expected to occuroccasionally has been intensively studied. It is clearlyassociated with an upwelling phenomenon inducedby the westerly winds, and it displays a seasonalvariability related to the general circulation in the

Ž .whole sea Artale et al., 1994; Marullo et al., 1994 .In the Channel of Corsica, the current time seriescollected since 1985 by the SO-La Spezia teamaccount for a significant variability of the MAW flux

Žon both seasonal northward flux maximum in win-. Žter and annual more or less continuous decrease of

.the transport over the decade scales. These variabili-ties are mainly attributed to the atmospheric–climatic

Žconditions over the basin Astraldi and Gasparini,.1992; Astraldi et al., 1994; Astraldi et al., 1995 and

are in agreement with historical data.

2.4. The Liguro–ProÕencal Basin

The flows of MAW west and east of Corsica joinŽ .and form what the Italian and our original paper

named ‘the Ligurian Current’, that is actually anentity flowing along the continental slope at least asfar as the Channel of Ibiza. Since the French namedit ‘the Liguro–Provencal Current’ and the Spanishthe ‘Catalan Current’, we first proposed the name‘Liguro–Provenco–Catalan Current’. However, thiscurrent in fact originates before the Ligurian Sea andcontinues south of the Channel of Ibiza, which wouldlead to an extremely long name. Considering that


such a current is a characteristic of the northern partof all the semi-enclosed seas, the name ‘NorthernCurrent’ was proposed, which implies to add an

Žadjective when dealing with different seas Millot,.1992 .

According to several studies conducted off theŽFrench coasts Alberola et al., 1995a; Conan and´

.Millot, 1995; Sammari et al., 1995 , its flux isŽmaximum 1.5–2 Sv down to ;700 dbar, the limit

between MAW and the intermediate waters being.not easily defined during a relatively long winter

Ž .season roughly from December to May , and itsstructure markedly changes seasonally. In summer,the Northern Current is relatively wide and shallow,and it displays a reduced mesoscale variability.Northwesterlies frequently blow in the Gulf of Lionsso that the surface water there is relatively cool.Conversely, the Balearic Sea is far less windy, due toa sheltering effect of the Pyrenees, so that themixed-layer water comes to be the warmest encoun-tered in the Western Mediterranean Sea. A veryintense thermal front, which is generally not welldifferentiated from the North Balearic Front, is thuscreated roughly in the axis of the Pyrenees. As aconsequence, the relatively cool Northern Currentflows in a direction perpendicular to this front, below

Žthe relatively warm mixed layer Lopez-Garcia et al.,.1994 . In winter, the Northern Current becomes

thicker and narrower and it tends to flow closer tothe slope. At this time, it develops relatively intensemesoscale meanders with both amplitudes and wave-lengths of a few tens up to one hundred of km. Thesemeanders have phase speeds of 10–20 kmrday andinduce an extremely large variability.

The seasonal variations of the mesoscale variabil-ity associated with this current has also been studiedin the Balearic Sea with 5-year current time seriesŽ .Font et al., 1995 . They appear to be consistent withthat first described by Taupier-Letage and MillotŽ .1986 and supported by the aforementioned papers.This variability increases dramatically in late autumnand decreases suddenly in late winter. A soft de-crease trend is then observed from spring to autumn.

ŽIt has also been confirmed Alberola et al., 1995a;´.Sparnocchia et al., 1995 that an associated intense

and barotropic mesoscale turbulence slowly propa-gates seaward, from winter to spring, and induces atremendous seasonal variability in the open sea.

The major characteristics of the Northern Currentitself, as well as those of its mesoscale meanders,can now be considered as close to well-described.During the 1990–1991 winter, the dense water for-mation was studied with ADCP’s moored in the

Ž .Ligurian Sea Astraldi et al., 1995 , while the flux ofthe Northern Current was estimated with CTD tran-sects and classical current meters moored off NiceŽ .Alberola et al., 1995a . Episodes of water sinking´were rather scarce and the transport did not varyclearly, probably because this winter was relativelymild. Therefore, we are not able yet to stand for anyaccurate seasonal variation of the Northern Currenttransport that might be indicative of its relationshipswith the dense water formation, a process exten-sively studied with analytical and numerical modelsŽ .Crepon et al., 1989; Madec et al., 1991a,b . The old´hypothesis that the current might be forced by thewind stress has been checked with numerous numeri-cal models without providing clear information,whereas the freshwater discharge has been men-tioned as a candidate for the Northern Current forc-

Ž .ing by Bethoux et al. 1988 .´

2.5. The Balearic Sea

The branching of the Northern Current in thesouth of the Balearic Sea indicated in the 1987diagram has been supported by various studies con-

Žducted during the last decade Font et al., 1988;.Pinot et al., 1995; Salat, 1995 . In the middle sea,

mesoscale structures have been generally linked toinstabilities of the alongslope circulation due tobathymetric features, but they might also be due tointeractions with recent MAW entering through the

ŽBalearic channels La Violette et al., 1990; Maso andTintore, 1991; Garcia-Ladona et al., 1994; Lopez-´

.Garcia et al., 1994; Pinot et al., 1994 . In winter, thesurface waters in the Gulf of Lions are swept awaysouthward by the northwesterlies, thus leading theNorth Balearic Front to be along the Balearic IslandsŽ .Lopez-Garcia et al., 1994 . This is probably themost southern position of the front, and thus themore reduced northward spreading of the recentMAW, as the wind stress effect is disturbed by theorography of the islands. Part of the Northern Cur-rent continues southward from the Channel of Ibiza,but with less and less energy and an increasing


Žmesoscale variability Lopez-Garcia et al., 1994;.Lopez-Jurado et al., 1995 . Sooner or later, it tends

to enter the Alboran Sea and it encounters the ener-getic flow of recent MAW, thus being deflectedtoward the Algerian Basin.

2.6. The Winter Intermediate Water

This water, which was not explicitly described inthe original paper, might have an importance under-estimated up to now, so that it is described in aspecific sub-section.

In winter, in the whole northern part of the West-ern Mediterranean Sea mainly and during moderatelystrong and cold gusts of northwesterly winds, MAWcan be cooled without any intense mixing with thewaters below. This leads to the formation of a spe-cific water mass, named ‘Riviera Winter Water’ by

Ž .Lacombe and Tchernia 1960 , with temperatures-12.48C and salinities of ;38.3. A similar waterŽ .12.5–13.08C, 38.1–38.3 , named ‘Winter Interme-

Ž . Ž .diate Water’ WIW by Salat and Font 1987 andexpected to be formed on the continental shelves, inboth the Gulf of Lions and the Balearic Sea, is

Žcommonly found in this latter sea below MAW alsomentioned by Lopez-Jurado et al., 1995, and Pinot et

.al., 1995 .In fact, such a temperature minimum can be

encountered in several other places. A simple mecha-nism for its formation has been proposed on thebasis of observations routinely collected in both the

ŽGulf of Lions Conan and Millot, 1995; Fuda et al.,. Ž .1998 and the Ligurian Sea Alberola et al., 1995a .´

It is clear that MAW locally cooled during cold windŽevents in the Ligurian Sea it should be the one

.already named ‘Riviera Winter Water’ becomesdenser, and will sooner or later be overlaid by warmerand less dense MAW coming from the surroundings.The cooled MAW would thus be prevented from anyinteractions with the atmosphere and recognized later

Ž .on as WIW. Fuda et al. 1998 show that WIW couldbe formed as late as early April under specificmeteorological conditions, and then be recognizedtill the following winter, even though more and moremixed and reduced.

Ž .Note that Lacombe and Tchernia 1960 describedthe cooling of MAW only in the Ligurian Sea, andnot in the Gulf of Lions. This might be due to the

fact that the meteorological conditions are generallymore severe in the latter place, so that dense waterformation processes there often involve deeper watermasses. The coldest WIW is obviously found in thenorth. Then, it is expected to follow mainly the pathof MAW, along the Spanish continental slope and

Žacross the Algerian Basin Perkins and Pistek, 1990;.Benzohra and Millot, 1995a . The former authors

depict this circulation as relatively diffuse whereasthe latter consider it as being more constrained bythe topography and more dependent on the thicknessand circulation of the recent MAW in the AlboranSea. Another route, directly southward from the Gulfof Lions to the Algerian Basin, is expected to beinduced by frontal eddies associated with the North

Ž .Balearic Front Fuda et al., 1998 . Note that WIWŽwas recognized in the Strait of Gibraltar Gascard

.and Richez, 1985 . Finally, let us mention the occur-rence in the northeast of the Minorca island of ananticyclonic eddy, ;100 km in diameter, which

Žmainly involves WIW Aleynik and Mikhin, pers..com. so that we name it ‘Weddy’. It is clearly seen

on infrared images during months and from year toŽ .year see Le Vourch et al., 1992, for instance and is

expected to be induced by the shear of the north-westerly wind stress field.

In any event, it appears from our data sets that arelatively large amount of MAW can thus be trans-

Ž .formed into an intermediate water WIW accordingto processes that seem roughly similar to those oc-curring in the Eastern Mediterranean Sea, betweenRhodes and Cyprus, where a saltier MAW is tran-

Ž .formed into Levantine Intermediate Water LIW .The transformation of MAW into WIW is an impor-tant stage of the transformation of Atlantic water intoMediterranean waters, as it produces a water denserthan MAW which will eventually be able to pene-trate into the Eastern Mediterranean Sea and will bemore easily mixed with LIW in the WesternMediterranean Sea. Since LIW is characterized in thelatter sea, between ;200 and 1000 m, by maximumtemperatures of 14.0–13.28C and salinities of 38.7–

Ž38.5 generally found at 300–400 m the temperaturemaximum being obviously shallower than the salin-

.ity one , it is rather easily differentiated from WIW.Nevertheless, the various transformation and mixingprocesses concerning these intermediate waters occuron a wide range of space and time scales, so that


adequate numerical models are needed to help quan-tifying the phenomena.

3. Water originated in the Eastern Mediterranean( )Sea Fig. 2

Twenty-year old measurements collected with hy-drological bottles evidenced the heterogeneity of thewater flowing into the sea from the Channel ofSicily. They were not enough considered and arereferred to in this section since they are supportedand complemented with recent data.


The hydrological section across the deeper part ofthe Channel of Sicily performed by Garzoli and

Ž . Ž .Maillard 1976 , and reproduced by Guibout 1987 ,Žclearly displays not only LIW per se maxima )

.14.58C and ;38.7 at ;200 m along the SicilianŽ . Žslope, but also a cooler -14.08C and denser by

. Ž .;0.1 water with roughly the same salinity at thebottom and along the Tunisian slope. This distribu-

Žtion is supported by more recent data Schlitzer etal., 1991; Astraldi et al., 1996; Sammari et al.,

.1998 . As suggested by Schlitzer et al., the latterwater, even if it is strongly mixed, is partly com-posed of the waters formed in the southern Adriaticandror Aegean Seas. The distribution of these twowater masses in the Channel of Sicily is similar tothe one described, in the original paper, for theintermediate and deeper water masses in the Strait ofGibraltar, up to now thought to be LIW and WMDWŽ .Western Mediterranean Deep Water , respectively.In this latter place, such a distribution is now sup-

Žported by several authors e.g., Kinder and Bryden,.1990 , but important comments can be made about

Žthe origin and nature of the denser water see Section.5 . These distributions clearly indicate that the vari-

ous water masses formed, either in the EasternMediterranean Sea or in the Mediterranean Sea as awhole, do not mix completely within the sea andflow out separately. As these waters are formed and

Ž . Ž .Fig. 2. Circulation of the Levantine Intermediate Water LIW and the Tyrrhenian Dense Water TDW . Symbols meanings as in Fig. 1Ž .except for rr: 0 and 200 m thick isobaths.


evacuated from the sea at different rates, they proba-bly flow out of the sea with different velocities.Therefore, they might be differently deflected to theright by the Coriolis force, the most rapid, thatshould be LIW, being incidentally the shallowest. Inany event, a large part of the deep flow in theChannel of Sicily has a sigma–theta )29.10, so thatit is as dense or even denser than the dense watersformed in the Western Mediterranean Sea. In the

ŽTyrrhenian Sea, values at similar depths 200–400.m range between 29.00 and 29.05, which implies a

noticeable cascading of this deep flow.Ž .Sparnocchia et al. 1999 have performed CTD

sections across the route they expected for LIW, i.e.,along the northern slope of Sicily. Note that, eventhough convenient in situ data were not available atthat time, the alongslope route was indicated in the

Ž1987 diagram on the basis of the aforementionedtheoretical arguments and of in situ data collected in

.other parts of the sea . These data show that, whilethe part of this inflow corresponding to LIW per seis recognized from ;200 to 600–1000 m by rela-tively large temperatures and salinities, a consider-able volume sinks to more than 1800 m, where itfollows the isobaths. The mixing processes modify-ing this intermediate flow and its progressive deep-ening along the slope appear to be relatively stableyear round. Sparnocchia et al. also measured intenseeastward currents in the same area over a wide depthrange as, for instance, the several-month mean is;20 cmrs and maximum values reach ;40 cmrsat ;1200 m. Nevertheless, contrary to what Gas-

Ž .parini et al. 1994 said, the flow cascading from theChannel of Sicily has a structure markedly differentfrom the one cascading from the Strait of Gibraltar,for the latter continues to be relatively thin far away

Ž .from the sill O’Neil Baringer and Price, 1997 . ThisŽ .is clearly due to i the density range of both flows

that is larger for the Sicily flow than for the GibraltarŽone maximum values are ;29.12 for the former

and ;29.08 for the latter, minimum values being. Ž .roughly of the same order and ;29.05 and ii the

difference in density between the flows and theresident waters they encounter at similar depths that

Žare smaller for the Sicily flow 29.0–29.05 in the.Tyrrhenian Sea, i.e., a difference of ;0.1 than for

Žthe Gibraltar one 26.5–27.5 in the Gulf of Cadix,.i.e., a difference of ;2–3 . These data also empha-

size a feature, already evidenced in the atlas ofŽ . ŽGuibout 1987 : the deeper part below 1800–2000

.m of the whole Tyrrhenian Sea is filled with aŽ . Ž .relatively cool -12.88C and fresh -38.45 water

that is nothing else than WMDW discussed in thenext section.

Obviously, such a cascading from depths shal-lower than 400 m down to ;2000 m induces atremendous mixing not only of the inflow itself, butalso of the inflow with the Tyrrhenian Sea waters.The mixing processes are very complex, furthermoreas an important inflow of intermediate and even deepwaters, from the Algerian Basin through the Channel

Žof Sardinia, has to be taken into account see Sam-.mari et al., 1998 . Nevertheless, the less dense part

Ž .of the inflow mainly LIW per se spreads overalmost similar depths and is thus expected to en-counter a mixing less intense than the denser part of

Žthe inflow. The latter waters partly originated mainly.from the Aegean and Adriatic Seas encounters a

more marked cascading and thus an intense mixingwith the waters originated from the Algerian Basinand with those resident in the Tyrrhenian Sea. What-ever the mixing processes are, we will consider thatthe result leads to a water called ‘Tyrrhenian Dense

Ž .Water’ TDW . Note that the TDW acronym hasŽ .already been used by Hopkins 1988 for ‘Tyr-

rhenian Deep Water’, but the invoked mixing pro-cesses were not similar and the presence of WMDWin the deepest part of the Tyrrhenian Sea impliesdifferentiating dense from deep. The 200–400 minflow from the Channel of Sicily is thus trans-formed into a 200–2000 m flow basically composedof LIW per se in its upper part, and of TDW in itslower part. The limit between the two waters isobviously not well defined, but they might roughlyfollow the same path in the Western MediterraneanSea, so that the acronym LIW used hereafter must bethought as LIW per se plus TDW.

The circuit of LIW in the Tyrrhenian Sea schema-Žtized in the 1987 diagram anticlockwise along the

.slope is supported by hydrological data collectedŽ .near Rome Cristofalo and Serravall, pers. com. and

Ž .is said by Brasseur et al. 1996 to be in agreementwith the historical data set. Such a circuit can also becompared with the two-cell circuit proposed by Hop-

Ž .kins 1988 . The difficulties to define a mean circuitpartly come from the turbulence associated with the


cascading from the Channel of Sicily. They alsocome from the instabilities that can disturb analongslope flow and from the fact that the Tyrrhe-nian Sea can be considered as a relatively closed hallfor LIW, which might tend to accumulate there. Inany case, the diagram is consistent with hydrological

Ždata collected between Sicily and Sardinia e.g.,.Garzoli and Maillard, 1976; Sparnocchia et al., 1999 ,

Ž X .and with north–south near 8850 E hydrologicalsections across the Channel of Sardinia and currentmeasurements down to 2000 m south of SardiniaŽ .Bouzinac et al., 1999 . Therefore, LIW and TDWclearly flow out from the Tyrrhenian Sea along theslope of Sardinia.

3.2. The Algero–ProÕencal Basin

Not yet published THETIS-2 and ELISA data setsŽclearly support this diagram basically a vein flowing

.anticlockwise along the continental slope and mightalso emphasize some dramatic changes of the veinnear the Sardinia southwestern corner. South of Sar-dinia, and as estimated on two occasions with the

Ž13.48C isotherm, the vein was relatively narrow ;. Ž . Ž50 km , deep ;800 m and unmixed maxima of

;13.98C at 250–300 m and values of 13.88C over a.200 m by 50 km section . West of Sardinia, the vein

Ž . Ž .was wider ;120 km and shallower ;750 m andmaxima of 13.88C are found over a 50–100 m by10–20 km section only. Therefore, the vein becamewider, thinner and cooler within ;100 km along theslope. Comparatively, all along its course of nearly2000 km along the Italian, French and Spanish slopes,it will undergo much less modifications, with roughlythe same shape and maxima that evolve slowlyŽ;13.48C in the Gulf of Lions and 13.28C in the

.Strait of Gibraltar .Several phenomena could account for these

changes around the Sardinia southwestern corner. Asthese changes seem to be frequent if not permanent,they could be linked to the overall distribution of thedensity and circulation features in the TyrrhenianSea and the Algerian Basin; but this first phe-nomenon, which considers an intermediate veinflowing into a basin of specific characteristics, hasnot been studied yet. A second one, first foreseen by

Ž .Millot 1987b , is now supported mainly by Fuda etŽ . Žal. 1998 and by the ELISA data sets not yet

.published : it is obvious that some Algerian eddiesthat leave the Algerian slope interact with the LIWvein there, and entrap andror dispatch pieces of it inthe middle Algerian Basin where maximum valuesof 13.6–14.08C and 38.55–38.70 are encountered.Another hypothetical phenomenon considers that thevein is unstable and generates anticyclones: this could

Žbe due to the slope angle to the right Pichevin and.Nof, 1996 , to the fact that LIW flows out from the

Tyrrhenian Sea more or less as an intermediate jetŽ .Afanasyev and Fillipov, 1996 , andror to an in-crease in the vertical density gradient between theTyrrhenian Sea and the Algerian Basin that wouldreduce the thickness and create negative vorticity.Note that eddies can also be generated through insta-bility processes that disturb a vein of intermediatewater flowing along a vertical and straight wallŽ .Baey et al., 1995 . To make a parallel with theMeddies generated by the Mediterranean outflowfrom Gibraltar, and although the mechanisms couldbe different, I proposed to name ‘Leddies’ thesehypothetical anticyclones, since they should be ed-dies mainly composed of LIW. The data sets that

Žhave been collected during the ELISA Eddies and.Leddies Interdisciplinary Study in the Algerian basin

campaign, will certainly provide a better understand-ing of the various phenomena.

From the Ligurian Sea and the Gulf of LionsŽwhere typical values range from ;13.5 to 13.48C

.and 38.55 to 38.50 to the Balearic and Alboran SeasŽwhere typical values are ;13.28C and less than

.38.5 , nothing has to be changed on the diagram.Moreover, it has been supported in the Gulf of LionsŽ .Durrieu de Madron et al., 1990 , the Balearic SeaŽ . ŽFont, 1987 , and the Alboran Sea Parrilla et al.,

.1986 . Note that the seaward spreading of the vein inwinter has been verified during all the campaignsconducted off the French coasts.

Very determinant information concerning thesouthern part of the Algerian Basin has been pro-vided by several experiments. First, an eastward flowalong the Algerian slope is clearly indicated at 300–400 m by the MEDIPROD-5 current time seriesŽ . ŽJune 1986 March 1987 between 0 and 58E Millot

.et al., 1997 and by the THETIS-2 and PRIMO-1Ž .ones November 1993–October 1994 between 5 andŽ .88E not yet published data . In a complementary

way, the MEDIPROD-5 hydrological data show that


the maxima associated with LIW first are, near 08,the lowest found in the whole Western Mediter-

Žranean Sea typical values are 13.1–13.28C and.38.47–38.50 , and second increase both eastward

Ž .and seaward. Benzohra and Millot 1995a havedemonstrated that this is consistent with a deflectionof a part of the LIW vein, from the Spanish slope tothe Algerian one, probably due to the Almeria-Oran

Ž .jet see Tintore et al., 1988 or to the western´anticyclonic gyre, when the jet and its associatedeastern anticyclonic gyre are not present in the Albo-ran Sea. Once off Algeria, and although LIW is nomore structured as a vein, it proceeds again along theslope, i.e., eastward, and mixes with the more recentLIW found offshore and deviated from the slope ofSardinia by the mesoscale phenomena already de-scribed. LIW will thus become warmer and saltier,and this can be misleading if the historical hydrolog-ical data are averaged without any caution, since thiscan suggest an overall westward flow across thewhole basin. Therefore, the diagram in Fig. 2 depictssome extremely modified LIW entering the Tyrrhe-nian Sea where it is mixed with the outflow from the

Ž .Channel of Sicily see Sammari et al., 1998 .It must be specified that the mean path of LIW is

still debated in several key places. On the basis ofnumerical models and data analyses, several col-

Ž .leagues e.g., Roussenov et al., 1995 think that alarge part of the outflow from the Channel of Sicilydirectly proceeds to the Channel of Sardinia, i.e.,without flowing round the Tyrrhenian Sea. In theAlgerian Basin, all recent analyses disagree with the

Ž .diagrams published in the 1960s e.g., Wust, 1961 ,¨that depict a LIW branch flowing northward alongSardinia and another one flowing westward off Alge-

Ž .ria. Perkins and Pistek 1990 support the occurrenceof a LIW vein along the slope of Sardinia, and olderpapers provide complementary information about thenon-occurrence of such a westward branch off Alge-

Ž .ria Katz, 1972 and about the importance of theŽ .mesoscale eddies there Burkov et al., 1979 . Never-

Žtheless, several numerical models e.g., Wu and.Haines, 1996, and Herbaut et al., 1996 reproduce

the two branches without providing any clear under-standing of the processes involved. In our opinion,considering the basic forces that are the internalpressure gradient and the Coriolis effect lead torefute any branching toward the middle part of a

basin, provided there is no marked bathymetric fea-Žture. Fortunately, at least one model Alvarez et al.,

.1994 takes into account mainly the Neptune effect,and depicts an anticlockwise alongslope circulationin the whole Western Mediterranean Sea very similarto the diagram in Fig. 2.

As for the MAW fluxes through the Strait ofGibraltar and especially through the Channel ofSicily, the LIW fluxes have not been accuratelyestimated yet. Current meters can be deployed moreefficiently in the LIW flow than in the MAW one,

Ž .but the data collected by Astraldi et al. 1996 haveshown that estimating the fluxes is still a difficulttask, due to a complex bathymetry and the occur-rence of large low-frequency variations. Neverthe-less, these data are consistent with geostrophic com-putations which indicate an annual flux of ;1 Sv.Attempts to establish relationships between thesefluxes and the water masses content in the Western

Ž .Mediterranean Sea Manzella and La Violette, 1990Ž .do not seem promising Millot et al., 1992 .

4. Western Mediterranean dense and deep waters( )Fig. 3

4.1. The Liguro–ProÕencal Basin

In the 1987 paper, the major concern of thissection was about WMDW per se, i.e., the waterformed during the winter, mainly in the Gulf ofLions, and generally characterized, in the nineties,

Žby values of 12.75–12.808C and 38.44–38.46 Schott.et al., 1994 . It also indicated that waters having

markedly different characteristics could also beformed and identified for a long while close to the

Ž Ž .bottom Lacombe et al. 1985 named them ‘Bottom.Water’, BW . As will be resumed later on, the

processes of dense water formation have been speci-Žfied see Leaman, 1994, and Schott et al., 1994, for

.recent general views , and it is clear that the meancharacteristics at depths of ;2000 m and even lessare continuously changing at a decadal scale. Never-theless, focus will be on the circulation featuresevidenced in the southern part of the Algerian Basinand the Tyrrhenian Sea.

The Gulf of Lions has been sounded during sev-Žeral winter experiments with new techniques tomog-


Ž . Ž .Fig. 3. Circulation of the Tyrrhenian Dense Water TDW and the Western Mediterranean Deep Water WMDW . Symbols meanings as inŽ .Fig. 1 except for rr: 0 and 1000 m thick isobaths.

. Ž .raphy and instruments ADCP’s , and thus newinsights about dense water formation processes are

Žprovided Leaman and Schott, 1991; Schott and Lea-.man, 1991; Schott et al., 1996 . Vertical mixing

appears to be essentially due to plumes of a fewhundreds of meters in diameter and associated withvertical speeds of ;10 cmrs, that develop in aconvection region characterized by a mean down-ward motion of the order of 1 mmrs. Meanwhile,baroclinic instability leads to the development offew-km eddies that tend to re-stratify the region. It isconfirmed that the amount of newly formed WMDWis markedly smaller than the amount of water enter-ing the basin through the Channel of Sicily. Evi-dences were given, seemingly for the first time, thatduring mild winter newly formed dense water doesnot reach the bottom and thus forms intermediatewaters that lie at depths shallower than ;1500 mŽalso reported for the Ligurian Sea by Sparnocchia et

.al., 1995 . From a numerical modeling point of view,it was shown that the convection could be consideredas non–penetrative and that relationships might exist

Žwith the general alongslope circulation Madec et al.,.1991a,b .

The hydrological characteristics of the deep wa-ters in the Gulf of Lions, for instance, are muchmore variable than previously thought. For what

Žconcerns the mesoscale variability of BW Lacombe.et al., 1985 , it must be considered that dense waters

are formed here and there, i.e., under different envi-ronmental conditions, even during the same windevent. Note that the most dense waters will reach thebottom, and will thus be more prevented from mix-ing than the less dense waters, which will be moreeasily mixed with waters of similar density. La-combe et al. also evidenced a marked variability atan annual scale, now commonly admitted since me-teorological conditions are clearly different from yearto year. Even though not especially emphasized byLacombe et al., the data they reported clearly ac-

Žcount for an increase of both the temperature ;. Ž .0.038Cr10 years and the salinity ;0.02r10 years

near 2000 m that has continued during the lastdecade. An increase of the LIW temperature and


Ž .salinity ;0.18C and 0.02r10 years is also reportedŽ .by Sparnocchia et al. 1994 . Different hypotheses

Ž .have been put forward by Bethoux et al. 1990 who´invoke climatic changes, whereas Leaman and SchottŽ . Ž .1991 and Rohling and Bryden 1992 rather believein an effect of the Nile and Black Sea rivers daming.

4.2. The Algerian Basin

The anticlockwise circulation along the northerncontinental slope has been confirmed by 1-year di-rect measurements we made between 5 and 68E in

Ž .the Gulf of Lions not yet published data and issupported by temperature time series collected in the

Ž .Balearic Sea Send et al., 1996 . In the western partŽ .of the Algerian Basin 38W to 58E , several-week

Žthe SEGAMO experiment, Vangriesheim and Made-. Žlain, 1977 and several-month the MEDIPROD-5.experiment current time series clearly account for a

Ž .significant circulation along the Spanish westwardŽ . Žand Algerian eastward slopes see Millot, 1994, for

.a detailed analysis . Other several-month current timeseries collected between 5 and 88E off Algeria,

ŽSardinia and Corsica not yet published THETIS-2.and PRIMO-1 data also clearly account for a anti-

clockwise circulation along the slope, round thewhole Algero-Provencal Basin. It must be empha-sized that alongslope mean speeds, close to thebottom at depths larger than 2500 m, are generally ofthe order of 3–4 cmrs. Another result, which comesfrom all our current measurements in the whole sea,is that the mean currents are generally lower at;1000 m than deeper. Although they are collected

Ž .with classical i.e., not very sophisticated instru-ments, these values are accurate, as demonstrated bythe ability of these instruments to measure tidalcurrents with amplitudes of only a few mmrsŽ .Alberola et al., 1995b . We also demonstrated that´the mesoscale eddies in the Algerian Basin induceintense currents over the whole deeper layer and

Ž .even close to the bottom Millot et al., 1997 .


Ž .As the deep and shallowest too! waters circulateanticlockwise along the continental slope in the wholeAlgero-Provencal Basin, the Channel of Sardinia is a

key place for the circulation of WMDW. Indeed,depths as large as ;2000 m allow waters flowingoff Algeria at less than ;2000 m to continue east-ward towards the Tyrrhenian Sea, while deeper wa-ters progress northward off western Sardinia. Consis-tently, potential temperatures of 12.8–13.08C andsalinities of 38.44–38.48 were found in the deeper

Žpart of the channel and along the Tunisian slope up. Xto ;1000 m , during surveys conducted near 8850 E

Žmainly during the 1993 and 1994 autumns see.Bouzinac et al., 1999; Sammari et al., 1998 . These

values are roughly those encountered in the Gulf ofŽ .Lions at similar depths 1000–2000 m . Neverthe-

less, 1-year current measurements collected in 1993–1994, at 1800 m near 8850X E in the middle part ofthe channel, do not show any significant circulationŽ .Bouzinac et al., 1999 . Such a discrepancy mightindicate that, at the time these observations weremade, the eastward circulation of WMDW within a1000–2000-m depth interval was non significant,thus accounting for the intermittency of this circula-tion. It could also be due to the specific location ofthe current meter in the middle of the channel:waters entering the Tyrrhenian Sea at such a largedepth must go out of the sea, eventually modifiedŽ .see Section 3 , roughly at the same depth, so thatthe circulation might be intense only along the slopesand non significant in between.

Similar measurements were performed in autumnŽ .1984 Hopkins, 1988 . Temperatures and salinities of

Ž;12.738C and 38.43, associated with a low ;1.cmrs but significant eastward flow, were found in

the deeper part of the channel. These hydrologicalvalues are much more similar than those alreadymentioned to the values characteristic of the lowerpart of WMDW in the Gulf of Lions, both nowŽ . ŽSchott et al., 1994 and several years ago Lacombe

.et al., 1985 , and in the deep Tyrrhenian Sea tooŽ .Guibout, 1987 . First, the Hopkins’ observationsprove that relatively dense WMDW can flow throughthe deeper part of the Channel of Sardinia, from theplace where it is formed toward a place where it willbe trapped. Second, all these observations supportthe large variability, probably at a yearly scale, ofthe characteristics of the WMDW flow through theChannel of Sardinia. This is consistent with thevariability, from year to year, of the dense waters

Ž .formed in the Gulf of Lions e.g., Schott et al., 1996


which makes, if one also considers the variabilityexpected for the waters coming in through the Chan-

Ž .nel of Sicily Roether et al., 1996 , any budgetestimations over the different sub-basins rather diffi-cult.

5. Discussion

From a more general point of view, the data setscollected during the last decade indicate that the roleof the formation and mixing of intermediate watersin the Western Mediterranean Sea is essential andmarkedly underestimated. Indeed, during mild win-ters and probably during other winters too, theamount of WIW formed in the Liguro–ProvencalBasin can obviously be larger than that of WMDW.Similarly, LIW per se is clearly not the sole watermass entering through the Channel of Sicily andmixing in the Tyrrhenian Sea, so that a large amountof water circulating in the Western MediterraneanSea, while not being characterized by especially

Ž .large temperature values i.e., TDW , partly origi-nates from the Eastern Mediterranean Sea too. Theseintermediate waters have not been considered yet inthe general circulation models.

There are simple reasons explaining such a defi-ciency of our present knowledge. Basically, watersof intermediate density necessarily mix easily withthe surrounding waters so that, even though theircore can generally be identified, their limits cannot.Therefore, without any accurate estimation of theamount of these various intermediate waters, andeven though their circulation can be specified, adefinite understanding of the working of the wholesea is not yet achieved. To synthetize the majorquestions into one, and deal with a point alreadyaddressed several times, it should be interesting todiscuss the processes leading a deep water such asWMDW, that lies at depths of ;2000 m and more,to be lifted up to ;300 m at the Gibraltar sill.

Several processes can be invoked. One is diffu-sion with the intermediate waters, the major processfor the world ocean conveyor belt, but the ratiobetween the surface of the whole sea and that of theconvection region does not seem large enough, andthere are no reasons to use larger diffusion coeffi-cients. Mixing in the convection region could also be

invoked to reduce the WMDW density, but it occursduring a few daysrweeks, probably not every year,and it involves relatively small amounts of water.

Another process proposed by Stommel et al.Ž .1973 is the Bernouilli effect, but Kinder and Par-

Ž .rilla 1987 demonstrated it could be effective forsucking water over no more than ;1000 m, that isŽ .intuitively! already a rather great depth. Even ifthey report extreme values of 12.758C and 38.42Ž .typical of the lower part of WMDW in the westernAlboran Sea, they consider 12.98C and 38.43 to berepresentative values of WMDW at the sill. Asroughly similar values are encountered near 1000 min most of the sea, it appears that only waters

Ž .‘assimilated to the upper part of WMDW’ sic canbe lifted up by such a process. Therefore, it isnecessary to question whether or not the water foundnear 1000 m in the sea andror together with LIW inthe Strait of Gibraltar is WMDW per se, i.e., thewater directly originated from the deeper part of theGulf of Lions.

It is obvious that during mild winters, only WIWand more generally intermediate waters shallowerthan ;1500 m are formed in the Gulf of Lions andthe Ligurian Sea so that, even during cold winters

Žwhen deep waters that reach the bottom i.e., WMDW.per se and BW are clearly observed, intermediate

Ž .waters too are formed even if less clearly identified .These intermediate waters, that ‘can be assimilatedto the upper part of WMDW’, will flow out atGibraltar more easily than the WMDW per se.Meanwhile, other intermediate waters can be encoun-tered in the Gulf of Lions and the Ligurian Sea at

Žless than 1500 m that have a different origin theycan come from the Tyrrhenian Sea and be identified

.as TDW andror that have undergone various mix-ing processes all around the sea. Contrary to theformer, these intermediate waters ‘cannot be assimi-lated to WMDW’. Nevertheless, both intermediatewaters can flow into the Alboran Sea and be upliftedat Gibraltar.

Therefore, care must be taken when dealing withthe origin of the waters encountered below LIW inthe Strait of Gibraltar. In any case, our presentunderstanding of the circulation accounts for a rela-tively large variability of the hydrological parameters

Žalong the mean path of a specific water mass see.Garcia-Lafuente et al., 1995 and emphasizes the


necessity to collect as many as possible long timeseries.

What about WMDW per se and how is it evacu-ated from the sea? In other words, once WMDW hasfilled the whole deeper part of the Algero-Provencal

Ž .Basin maximum depths of ;2900 m , is it easierfor it to flow up till 300 m through the Strait ofGibraltar or down from 2000 m through the Channelof Sardinia? Although I did not perform any quanti-tative estimations of the water masses mixing, didnot run any dedicated numerical model and did notconducted any laboratory experiment, I imagine thatthe easier route for WMDW, once it has circulatedand accumulated at depths larger than ;2000 m inthe Algero–Provencal Basin, is toward the deep

Ž .Tyrrhenian Sea down to ;3900 m . Obviously, theupper part of WMDW will mix with the intermediatewaters but, according to Zodiatis and GaspariniŽ .1996 , the vertical fluxes related to the well-knownstep structure in the Tyrrhenian Sea play a minorrole in the exchanges between WMDW and thewater above. Therefore, how can WMDW be up-welled?

Ž .Considering the great depths ;2000 m reachedŽ .by the flow cascading all year long! from the

Channel of Sicily, most of the upwelling of WMDWmight occur through turbulent mixing in the farsouthern Tyrrhenian Sea. Without considering anychanges in the characteristics of the cascading flow,it is clear that the larger the amount of WMDW inthe sea, the shallower its upper part and the moreintense the mixing. If this amount is low, the cascad-ing flow reaches rather smoothly an equilibriumlevel and ‘floats over’ the resident water, as depicted

Ž .by Sparnocchia et al. 1999 . But if this amount islarge, the turbulent mixing between WMDW and thecascading flow will be intense. Since we know that

Žthe amount of WMDW both formed according to.the studies in the Gulf of Lions and resident in the

Žsouth according to the studies in the Algerian Basin.and the Channel of Sardinia displays a large vari-

ability at seasonal and longer scales, it is clear thatthe upwelling of WMDW displays a similar variabil-ity. Note that the amount of unmixed WMDW in the

ŽWestern Mediterranean Sea more especially in the.southern Tyrrhenian Sea is automatically controlled

by the density of the cascading flow, and thus by thedense water formation processes in the Eastern

Mediterranean Sea. In any case, TDW might be amajor stage in the transformation of WMDW.

An important parameter to be considered whendealing with these mixing phenomena is the amount

Žof rather old intermediate waters i.e., WIW, LIW.and even TDW that, after they have proceeded

round the Algero-Provencal Basin and finally east-ward along the Algerian slope, enter the TyrrhenianSea and mix with the cascading flow. According tothe amount of these old intermediate waters, thecascading flow will be more or less lightened, andwill thus reach more or less easily the WMDW layer.One can also consider that part of the flow of the oldintermediate waters is forced into the Tyrrhenian Seaby these mixing phenomena. In any case, it is clearthat most of the intermediate and deep waters formedin the Western Mediterranean Sea can be upwelledby the flow cascading from the Channel of Sicily upto a level from where they can be uplifted moreeasily through the Strait of Gibraltar. Therefore, thesouthern Tyrrhenian Sea is actually a key place forthe working of the whole Western MediterraneanSea.

Only considering the phenomena occurring in thiskey place, it is clear that the Eastern MediterraneanSea has a dominant role in the whole MediterraneanSea. This is furthermore evidenced by the variousflux estimations which consider that the values in theStrait of Gibraltar and in the Channel of Sardinia areroughly similar, and thus account for a rather smallamount of dense water formed in the WesternMediterranean Sea. For what concerns the circulationof the water masses, and although I still consider thatthe Western and the Eastern Mediterranean Seasroughly function in the same way and display similar

Ž .basic circulation features Millot, 1992 , it is clearthat the specific phenomena occurring in the south-ern Tyrrhenian Sea induce major differences. Essen-tially, the permanent and large mixing processesoccurring in this region lead the deep waters in the

Ž .West except in the very deep Tyrrhenian Sea tohave a renewal rate faster and a circulation moreintense than in the East.

The different ages of the deep waters in theWesternrEastern Mediterranean Seas certainly hassignificant consequences on their bio-geochemicalcharacteristics and maybe, through mixing processes,on those of the intermediate and even surface waters


too. But more dedicated data and models must beperformed to check whether or not the differentproductivity of the surface waters in theWesternrEastern Mediterranean Seas could resultfrom the mixing phenomena occurring in the south-ern Tyrrhenian Sea.

Acknowledgements

Most of the results presented in this paper havebeen obtained within the framework of the EURO-

ŽMODELrMAST-1,2 grants MAST-CT 920041 and.930066 , and I warmly thank all my EUROMODEL

colleagues, especially Corinne Alberola and Isabelle´Taupier-Letage of the COMrLa Seyne team, andJoaquin Tintore from UIBrBarcelona for their very´fruitful and friendly collaboration. I do not forgetthat numerous crew members kindly helped us incollecting what must now be considered as a ‘signifi-cant’ data set.

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