Mediterranean Sea Circulation R - Prof. Allan R. Robinson

Mediterranean Sea Circulation

Allan R. Robinson, Wayne G. Leslie, Division ofEngineering and Applied Sciences, Department ofEarth and Planetary Sciences, Harvard University,29 Oxford Street, Cambridge, MA 02138, USAAlexander Theocharis, National Centre for MarineResearch (NCMR), Aghios Kosmas, Hellinikon 16604,Athens, GreeceAlex Lascaratos, Department of Applied Physics,Oceanography Group, University of Athens,University Campus, Building PHYS-V, Athens 15784,Greece

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0376

Introduction

The Mediterranean Sea is a mid-latitude semi-en-closed sea, or almost isolated oceanic system. Manyprocesses which are fundamental to the general cir-culation of the world ocean also occur within theMediterranean, either identically or analogously.The Mediterranean Sea exchanges water, salt, heat,and other properties with the North Atlantic Ocean.The North Atlantic is known to play an importantrole in the global thermohaline circulation, as themajor site of deep- and bottom-water formation forthe global thermohaline cell (conveyor belt) whichencompasses the Atlantic, Southern, Indian, and Pa-ciRc Oceans. The salty water of Mediterranean ori-gin may affect water formation processes andvariabilities and even the stability of the global ther-mohaline equilibrium state.

The geography of the entire Mediterranean isshown in Figure 1A and the distribution of deep-seatopography and the complex arrangement of coastsand islands in Figure 1B. The Mediterranean Sea iscomposed of two nearly equal size basins, connectedby the Strait of Sicily. The Adriatic extends north-ward between Italy and the Balkans, communicatingwith the eastern Mediterranean basin through theStrait of Otranto. The Aegean lies between Greeceand Turkey, connected to the eastern basin throughthe several straits of the Grecian Island arc. TheMediterranean circulation is forced by water ex-change through the various straits, by wind stress,and by buoyancy Sux at the surface due to fresh-water and heat Suxes.

Research on Mediterranean Sea general circula-tion and thermohaline circulations and theirvariabilities, and the identiRcation and quantiRca-tion of critical processes relevant to ocean and cli-

mate dynamics involves several issues. Conceptual,methodological, technical, and scientiRc issues in-clude, for example, the formulation of multiscale(e.g., basin, sub-basin, mesoscale) interactive nonlin-ear dynamical models; the parameterization ofair}sea interactions and Suxes; the determination ofspeciRc regional processes of water formation andtransformations; the representation of convectionand boundary conditions in general circulationmodels. A three-component nonlinear ocean systemis involved whose components are: (1) air}sea inter-actions, (2) water mass formations and transforma-tions, and (3) circulation elements and structures.The focus here is on the circulation elements andtheir variabilities. However, in order to describe thecirculation, water masses must be identiRed anddescribed.

Multiscale Circulation andVariabilities

The new picture of the general circulation in theMediterranean Sea which is emerging is complex,and composed of three predominant and interactingspatial scales: basin scale (including the thermoha-line (vertical) circulation), sub-basin scale, andmesoscale. Complexity and scales arise from themultiple driving forces, from strong topographicand coastal inSuences, and from internal dynamicalprocesses. There exist: free and boundary currentsand jets which bifurcate, meander and grow andshed ring vortices; permanent and recurrent sub-basin scale cyclonic and anticyclonic gyres; andsmall but energetic mesoscale eddies. As the scalesare interacting, aspects of all are necessarily dis-cussed when discussing any individual scale. Thepath for spreading of Levantine Intermediate Water(LIW) from the region of formation to adjacent seastogether with the thermohaline circulations areshown in Figure 2; where the entire Mediterraneanis schematically shown as two connected basins(western and eastern). The internal thermohalinecells existing in the western and eastern Mediterra-nean have interesting analogies and differences toeach other and to the global thermohaline circula-tion. In the western basin (Figure 3A) the basin-scale thermohaline cell is driven by deep waterformed in the Gulf of Lions and spreading fromthere. Important sub-basin scale gyres in the mainthermocline in the Alboran and Balearic Seas havebeen identiRed. Intense mesoscale activity exists andis shown by instabilities along the coastal current,mid-sea eddies and along the outer rim swirl Sow of

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a sub-basin scale gyre. The basin scale thermohalinecell of the eastern basin is depicted generically inFigure 3B and discussed in more detail in the nextsection. The basin scale general circulation of themain thermocline is composed of dominantly ener-getic sub-basin scale gyres linked by sub-basin scalejets. The active mesoscale is shown by a Reld ofinternal eddies, meanders along the border swirlSow of a sub-basin scale gyre, and as meandering jetsegments. The Atlantic Water jet with its instabili-ties, bifurcations, and multiple pathways, whichtravels from Gibraltar to the Levantine is a basinscale feature not depicted in Figure 3 this also per-tains to the intermediate water return Sow.

Large-scale Circulation

Processes of global relevance for ocean climate dy-namics include thermohaline circulation, water massformation and transformation, dispersion, andmixing. These processes are schematically shown inFigure 4A and B for the western and the easternbasins. The Mediterranean basins are evaporationbasins (lagoons), with freshwater Sux from the At-lantic through the Gibraltar Straits and into theeastern Mediterranean through the Sicily Straits.Relatively fresh waters of Atlantic origin circulatingin the Mediterranean increase in density becauseevaporation (E) exceeds precipitation (advective sa-linity preconditioning), and then form new watermasses via convection events driven by intense localcooling (Q) from winter storms. Bottom water isproduced: for the western basin (WMDW) in theGulf of Lions (Figure 4A) and for the eastern basin(Figure 4B) in the southern Adriatic (EMDW, whichplunges down through the Otranto Straits). Recentobservations also indicate deep water (LDW) forma-tion in the north-eastern Levantine basin duringexceptionally cold winters, where intermediatewater (LIW) is regularly formed seasonally. Evid-ence now shows that LIW formation occurs overmuch of the Levantine basin, but preferentially inthe north, probably due to meteorological factors.The LIW is an important water mass which circu-lates through both the eastern and western basinsand contributes predominantly to the efSux fromGibraltar to the Atlantic, mixed with some EMDWand together with WMDW. Additionally, intermedi-ate and deep (but not bottom) waters formed in theAegean (AGDW) are provided to the eastern basinthrough its straits. As will be seen below, that waterformerly known as AGDW, is now identiRed asCretan Intermediate Water (CIW) and Cretan DeepWater (CDW). Important research questions relateto the preconditioning, formation, spreading, disper-

sion, and mixing of these water masses. These in-clude: sources of forced and internal variabilities;the spectrum and relative amounts of water typesformed, recirculating within the Mediterraneanbasins, and Suxing through the straits, and the ac-tual locations of upwelling.

A basin-wide qualitative description of the ther-mohaline circulation in the western basin of theMediterranean Sea has recently been provided byMillot (see Further Reading). Results based oncruises in December 1988 and August 1989 in-dicated that the deep layer in the western Mediterra-nean was 0.123C warmer and about 0.33 PSU moresaline than in 1959. Analysis of these data togetherwith those from earlier cruises has shown a trendof continuously increasing temperatures in recentdecades. Based on the consideration of the heat andwater budget in the Mediterranean, the deep-watertemperature trend was originally speculated to bethe result of greenhouse gas-included local warming.A more recent argument considers the anthropo-morphic reduction of river water Sux into the east-ern basin to be the main cause of this warmingtrend.

Since the beginning of the twentieth century,when the Rrst investigations in the MediterraneanSea took place (1908), up to the mid-1980s, boththe intermediate and deep conveyor belts of theeastern basin presented rather constant character-istics. The Adriatic has been historically consideredas the main contributor to the deep and bottomwaters of the Ionian and Levantine basins, thusindicating an almost perfectly repeating cycle inboth water mass characteristics and formation ratesduring this long period. Roether and Schlitzer foundin 1991 that the thermohaline circulation in theeastern basin consists of a single coherent convectivecell which connects the Levantine and Ionian basinsand has a turnover time of 125 years below 1200m.Their results indicated that the water formed in theAdriatic is a mixture of surface water (AW) andintermediate Levantine water (LIW) from the Medi-terranean. The Aegean has also been reported asa possible secondary source, providing dense watersto the lower intermediate and/or deep layers, name-ly Cretan Intermediate Water (CIW), that affectedmainly the adjacent to the Cretan Arc region of theeastern Mediterranean. Since 1946 increased densit-ies were observed in the southern Aegean Sea in1959}1965 and 1970}73. These events occurred un-der extreme meteorological conditions. However,the quantities of the dense water produced werenever enough to affect the whole eastern Mediterra-nean. The traditional historical picture of waterproperties is illustrated in Figure 5A by a west}east

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vertical section of salinity through the eastern Medi-terranean.

After 1987, the most important changes in thethermohaline circulation and water propertiesbasin-wide ever detected in the Mediterranean oc-curred. The Aegean, which had only been a minorcontributor to the deep waters, became more effec-tive than the Adriatic as a new source of deep andbottom waters of the eastern Mediterranean. Thissource gradually provided a warmer, more saline,and denser deep-water mass than the previouslyexisting Eastern Mediterranean Deep (and bottom)Water (EMDW) of Adriatic origin. Its overall pro-duction was estimated for the period 1989}95 atmore than 7 Sv, which is three times higher thanthat of the Adriatic. After 1990, CIW appeared tobe formed in the southern Aegean with modiRedcharacteristics. This warmer and more saline CIW(less dense than the older one) exits the Aegeanmainly through the western Cretan Arc Straits andspreads in the intermediate layers, the so-called LIWhorizons, in the major part of the Ionian Sea, block-ing the westward route of the LIW.

These changes have altered the deep/internal andupper/open conveyor belts of the eastern Mediterra-nean. This abrupt shift in the Mediterranean ‘oceanclimate’ has been named the Eastern MediterraneanTransient (EMT). Several hypotheses have been pro-posed concerning possible causes of this uniquethermohaline event, including: (1) internal redis-tribution of salt, (2) changes in the local atmo-spheric forcing combined with long term salinitychange, (3) changes in circulation patterns leadingto blocking situations concerning the ModiRed At-lantic Water (MAW) and the LIW, and (4) vari-ations in the fresher water of Black Sea origin inputthrough the Strait of Dardanelles.

The production of denser than usual local deepwater started in winter 1987, in the Kiklades pla-teau of the southern Aegean. The combination ofcontinuous salinity increase in the southern Aegeanduring the period 1987}92, followed by signiRcanttemperature drop in 1992 and 1993 caused massivedense water formation. The overall salinity increasein the Cretan Sea was about 0.1 psu, due to a per-sistent period of reduced precipitation over theAegean and the eastern Mediterranean. This me-teorological event might be attributed to larger scaleatmospheric variability as the North Atlantic Oscil-lation. Moreover, the net upper layer (0}200 m) salttransport into the Aegean from the Levantine wasincreased one to four times within the period1987}94 due not only to the dry period but also tosigniRcant changes of the characteristic water masspathways. This was a secondary source of salt for

the south Aegean that has further preconditioneddense water formation. The second period is charac-terized by cooling of the deep waters by about0.353C, related to the exceptionally cold winters of1992 and 1993. The strongest winter heat loss since1985 in the Adriatic and since 1979 in the Aegeanwas observed in 1992. During this winter an almostcomplete overturning of the water column occurredin the Cretan Sea. The density of the newly formedwater, namely Cretan Deep Water (CDW), reachedits maximum value in 1994}95 in the Cretan Sea ofthe southern Aegean. The massive dense water pro-duction caused a strong deep outSow through theCretan Arc Straits towards the Ionian and Levantinebasins. Interestingly, the peak of the productionrate, about 3 Sv, occurred in 1991}92 when the 29.2pT

isopycnal was raised up to the surface layer.While its deep-water production in the Aegean isbecoming more effective with time, that in theAdriatic stopped after 1992. Conditions in 1995 areillustrated in Figure 5B by a west}east vertical sec-tion of salinity through the eastern Mediterranean.

The period 1995}98 is characterized by continu-ous decrease of CDW production, from 1 to 0.3 Sv.The level of the CDW in the straits (i.e.,) is foundapproximately at the sill depths (800}1000m). Thedeep outSow has also been weakened, especiallyfrom the western Cretan Straits. Moreover, the den-sity of the outSowing water is no longer sufRcient tosink to the bottom and therefore the water comingfrom the recent Aegean outSow has settled abovethe old Aegean bottom-water mass, in layers be-tween 1500 and 2500 m. On the other hand, theAegean continues to contribute the CIW to the in-termediate layers of the eastern Mediterranean. Sa-linity in the eastern Mediterranean in 1999 is shownin Figure 5C.

The intrusion of the dense Aegean waters hasinitiated a series of modiRcations not only in thehydrology and the dynamics of the entire basin, butalso in the chemical structure and some biologicalparameters of the ecosystem. The dense, highly oxy-genated CDW has Rlled the deep and bottom partsof the eastern Mediterranean, replacing the oldEMDW of Adriatic origin, which has been upliftedseveral hundred meters. This process brought theoxygen-poor, nutrient-rich waters closer to the sur-face, so that in some regions winter mixing mightbring extra nutrients to the euphotic zone, enhanc-ing the biological production. After 1991, the deepCretan outSow began to be compensated by theintrusion into the southern Aegean of the upliftedmid-depths old eastern Mediterranean waters thatare found shallow enough in the vicinity of theCretan Straits. These waters, namely Transitional

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Mediterranean Water (TMW), gradually formeda distinct intermediate layer (150}500m) in thesouth Aegean, characterized by temperature, salinityand oxygen minima, and nutrient maxima. This hasenhanced the previously weak stratiRcation and en-riched with nutrients one of the most oligotrophicseas in the world. This new structure preventswinter convection deeper than 250 m. Finally, in1998}99, the presence of the TMW was muchreduced, mainly as a result of mixing.

The simultaneous changes in both the upper anddeep conveyor belts of the eastern Mediterraneanmay affect the processes and the water character-istics of the neighboring seas. The contributionof the Aegean to the intermediate and deep layers isstill active. The variability in the intermediatewaters can alter the preconditioning of dense waterformation in the Adriatic as well as in the westernMediterranean. On the other hand, the changes inthe deep waters can affect the LIW formation char-acteristics. Whether the present thermohaline regimewill eventually return to its previous state or arriveat a new equilibrium is still an open question.

Sub-basin Scale Circulation

Figure 6 shows the patterns of circulation in thewestern Mediterranean for the various water types.The AW in the Alboran Sea Sows anticyclonically inthe western portion of the western basin, whilea more variable pattern occurs in the eastern por-tion. The vein Sowing from Spain to Algeria isnamed the Almeria-Oran Jet. Further east, theMAW is transported by the Algerian Current, whichis relatively narrow (30}50 km) and deep(200}400m) in the west, but it becomes wider andthinner while progressing eastwards along the Alger-ian slope till the Channel of Sardinia. Meanders offew tens of kilometers, often ‘coastal eddies’ aregenerated due to the unstable character of the cur-rent. The cyclonic eddies are relatively superRcialand short-lived, while the anticyclones last forweeks or months. The current and its associatedmesoscale phenomena can be disturbed by the ‘opensea eddies’. The buffer zone that is formed by theMAW reservoir in the Algerian Basin disconnectsthe inSow from the outSow at relatively short time-scales typically.

Large mesoscale variability characterizes theChannel of Sicily. In the Tyrrhenian Sea both theSow along Sicily and the Italian peninsula andthe mesoscale activity in the open sea are the domi-nant features. The Sows of MAW west and east ofCorsica join and form the so-called Liguro-Provenco-Catalan Current, which is the ‘Northern

Current’ of the Basin along the south-west Europeancoasts. Mesoscale activity is more intense in winter,when this current becomes thicker and narrowerthan in summer. There is also strong seasonal varia-bility in the mesoscale in the Balearic Sea. Intensebarotropic mesoscale eddy activity propagates sea-ward from the coastline around the sea from winterto spring, and induces a seasonal variability in theopen sea.

There is evidence that the Winter IntermediateWater (WIW) formed in the Ligurian Sea and theGulf of Lions can be larger than that of WesternMediterranean Deep Water (WMDW). This WIWcan Sow out at Gibraltar with LIW more easily thanthe WMDW. Furthermore, apart from the LIWthere are also other intermediate waters of easternMediterranean origin that circulate and participatein the processes of the western Basin. Once theWMDW has circulated and accumulated at depthsgreater than &2000 m in the Algero-Provencalbasin, the easier route for it is towards the deepTyrrhenian Sea (&3900m). The amount of un-mixed WMDW in the western Mediterranean andespecially in the south Tyrrhenian Sea is automati-cally controlled by the density of the cascading Sowfrom the Channel of Sicily and thus from the densewater formation processes in the eastern Mediterra-nean. The south Tyrrhenian is a key place for themixing and transformation of the water masses; theprocesses within the eastern Mediterranean play adominant role in the entire Mediterranean Basin.

In the eastern basin energetic sub-basin scale fea-tures (jets and gyres) are linked to construct thebasin-wide circulation. Important variabilities existand include: (1) shape, position, and strength ofpermanent sub-basin gyres and their unstable lobes,multi-centers, mesoscale meanders, and swirls; (2)meander pattern, bifurcation structure, and strengthof permanent jets; and (3) occurrence of transienteddies and aperiodic eddies, jets, and Rlaments.Figure 7 shows a conceptual model in which a jet ofAtlantic Water enters the eastern basin through theStraits of Sicily, meanders through the interior ofthe Ionian Sea, which is believed to feed the Mid-Mediterranean Jet, and continues to Sow throughthe central Levantine all the way to the shores ofIsrael. In the Levantine basin, this Mid-Mediterra-nean Jet bifurcates, one branch Sows towardsCyprus and then northward to feed the Asia MinorCurrent, and a second branch separates, Sows east-ward, and then turns southward. Important sub-basin features include: the Rhodes cyclonic gyre, theMersa Matruh anticyclonic gyre, and the south-eastern Levantine system of anticyclonic eddies,among which is the recurrent Shikmona eddy south

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of Cyprus. The diameter of the gyres is generallybetween 200 and 350 km. Flow in the upper ther-mocline is in the order of 10}20cm s~1. A tabu-lation of circulation features in the easternMediterranean and their characteristics is presentedin Table 1.

During the period 1991}95, a large three-lobeanticyclonic feature developed in the south-westernLevantine (from the eastern end of the Cretan Pas-sage, 263E up to 313E), blocking the free westwardLIW Sow, from the Levantine to the Ionian, andcausing a recirculation of the LIW within the westLevantine Basin. Although, multiple, coherent anti-cyclonic eddies were also quite common in the areabefore 1991 (as the Ierapetra and Mersa-Matruh),the 1991}95 pattern differs signiRcantly, with threeanticyclones of relatively larger size covering theentire area. This feature seems to comprise theMersa-Matruh and the Ierapetra Anticyclone. More-over, the 1998}99 infrared SST images indicatedthat the area was still occupied by large anticyclonicstructures. The data sets collected in late 1998 andearly 1999 indicated that this circulation patternhad been reversed to cyclonic, conRrming the transi-ent nature of these eddies. Consequently, the Atlan-tic Ionian Stream (AIS) was not Sowing from Sicilytowards the northern Ionian, but directly eastwardscrossing the central Ionian towards the Cretan Pas-sage (Table 2).

The seasonal variability of the circulation of thelate 1980s in the south Aegean Sea has been re-placed by a rather constant pattern in the period ofthe EMT (1991}98). Therefore, the Cretan Seaeddies were in a seasonal evolution in the 1980s(always present), while in the 1990s there was aconstant succession of three main eddies (onecyclone in the west, one anticyclone in the centralregion and again one cyclone in the east) that pre-sented spatial variability.

Mesoscale Circulation

The horizontal scale of mesoscale eddies is generallyrelated to, but somewhat larger than, the Rossbyradius of deformation. In the Mediterranean theinternal radius is O(10}14) km or four times smallerthan the typical values for much of the world ocean.The study of mesoscale instabilities, meandering,and eddying thus requires a very Rne resolutionsampling. For this reason, only recently differentmesoscale features were found in both the westernand eastern basins including the mesoscale variabili-ties associated with the coastal currents in the west-ern basin and open sea mesoscale energetic eddies inthe Levantine basin.

In the western basin, intense mesoscale phe-nomena have been detected using satellite informa-tion and current measurements. Mesoscale activityoccurs as instabilities along the coastal currents (i.e.,the Algerian Current) leading to the formation ofmesoscale eddies which can eventually move acrossthe basin or interact with the current itself. Alongthe Algerian Current cyclonic and anticyclonic ed-dies develop and evolve over several months as theyslowly drift eastward (a few kilometers per day).The anticyclonic eddies generally increase in sizeand detach from the coast. Some may drift near thecontinental slope of Sardinia, where a well-deRnedSow of LIW exists. Here they are able to pullfragments of LIW seaward. Old offshore eddies ex-tend deep in the water column and last from severalmonths to as much as a year. They sometimes enterthe coastal regions and interact with the AlgerianCurrent. In the coastal zones the mesoscale currentsappear to be strongly sheared in the vertical.

This clearly indicates that eddies can modify thecirculation over a relatively wide area and for rela-tively long periods of time. The coastal eddies alongthe Algerian coast can be especially vigorous, induc-ing currents of 20}30 cm s~1 strength for periodsof a few weeks. More complicated variations of thecurrents have also been measured at 300 m andsometimes at 1000 m.

Mesoscale activity has been observed in thenorthern basin (i.e., along the western and northernCorsican Currents). Coastal Corsican eddies are typ-ically anticyclonic and located either offshore oralong the coast of Corsica. A number of experi-ments were conducted to investigate the mesoscalephenomena in the Ligurian Sea. The results of a 1-year current meter array are shown for the southerncoastal zone in the Corsican Channel (Figure 8D).Mesoscale currents are characterized by permanentoccurrence and by a baroclinic structure with rela-tively large amplitude at the surface, moderate atthe intermediate level and still noticeable at depth,thus indicating large vertical shear of the horizontalcurrents.

Dedicated high-resolution sampling in the Levan-tine basin led to the discovery of open ocean meso-scale energetic eddies, as well as jets and Rlaments.This was conRrmed by a mesoscale experiment inAugust}September 1987 in the eastern basin. Meso-scale eddies dynamically interacting with the generalcirculation occur with diameters in the order of40}80 km. From this analysis a notable energeticsub-basin/mesoscale interaction in the Levantinebasin and a remarkable thermostad in the Ionianhave been revealed.

Figure 8A shows a temperature cross-section from

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XBT proRles collected in summer 1987 across theMid-Mediterranean Jet, West Cyprus Gyre, MMJ,and the northern border of the Shikmona eddy (sec-tion ABCD shown in Figure 8C). Figure 8B showsthe identical XBT section after a pyramid Rlter, withhorizontal inSuential distance of 50 km has beenapplied. The Rlter has removed very small scalefeatures while maintaining the mesoscale structure.

Modeling

Vigorous research in the 1980s and the developingpicture of the multiscale Mediterranean circulationwere accompanied by a new era of numericalmodeling on all scales. Modeling efforts included:water mass models, general circulation models, anddata assimilative models. Dynamics in the modelsinclude: primitive equations, non-hydrostatic formu-lations, and quasi-geostrophy. The assimilation ofthe cooperative eastern Mediterranean surveys ofthe 1980s and 1990s into dynamical models playeda signiRcant role in the identiRcation of sub-basinscale features. The numerical model results shownin Figure 9 depict the existence of numerous sub-basin scale features, as schematized in Figure 7.

In recent years numerical modeling of the generalcirculation of the Mediterranean Sea has advancedgreatly. Increased computer power has allowed thedesign of eddy resolving models with grid spacing ofone-eighth and one-sixteenth of a degree for thewhole basin and higher for parts of it. An exampleoutput from such a model is shown in Figure 10.Many of these models incorporate sophisticatedatmospheric forcing parameterizations (e.g., inter-active schemes) which successfully mimic existingfeedback mechanisms between the atmosphere andthe ocean. Studies have been carried out using per-petual year atmospheric forcing, mostly aimed atstudying the seasonal cycle, as well as interannualatmospheric forcing. Studies have mainly focused onreproducing and understanding the seasonal cycle,the deep- and intermediate-water formation pro-cesses and the interannual variability of the Medi-terranean. They have shown the existence of astrong response of the Mediterranean Sea to sea-sonal and interannual atmospheric forcing. Bothseasonal and interannual variability of the Mediter-ranean seems to occur on the sub-basin gyre scale.The Ionian and eastern Levantine areas are found tobe more prone to interannual changes than the restof the Mediterranean. Sensitivity experiments to at-mospheric forcing show that large anomalies in win-ter wind events can shift the time of occurrence ofthe seasonal cycle. This introduces the concept ofa ‘memory’ of the system which ‘preconditions’ the

sea at timescales of the order of one season to1 year.

In the deep- and intermediate-water formationstudies, the use of high frequency (6 h) atmosphericforcing (in contrast to previously used monthly forc-ing) in correctly reproducing the observed convec-tion depths and formation rates was found to becrucial. This shows the intermittent and often viol-ent nature of the phenomenon, which is linked toa series of speciRc storm events that occur duringeach winter rather than to a gradual and continuouscooling over winter. The use of high-resolution nu-merical models in both the western and the easternMediterranean allowed the study of the role ofbaroclinic eddies, which are formed at the peripheryof the chimney by instabilities of the meanderingrim current, in open ocean convection. These eddieswere shown to advect buoyancy horizontally to-wards the center of the chimney, thus reducing theeffectiveness of the atmospheric cooling in produ-cing a deep convected mixed layer. These results arein agreement with previous theoretical and laborat-ory work.

The LIW layer which extends over the wholeMediterranean was found to play an important roleboth in the western (Gulf of Lions) and the eastern(Adriatic) deep-water formation sites and more spe-ciRcally in ‘preconditioning’ the formation process.It was shown that the existence of this layer greatlyinSuences the depth of the winter convection pen-etration in these areas. This is related to the factthat the LIW layer with its high salt content de-creases the density contrast at intermediate layers,thus allowing convection to penetrate deeper. Thisresult shows the existence of teleconnections andinter-dependencies between sub-basins of the Medi-terranean.

A number of numerical models have been de-veloped to simulate and understand the origins andthe evolution of the Eastern Mediterranean Transi-ent. These models indicate that the observed cha-nges can be at least partially explained as a responseto variability in atmospheric forcing. The observedshift, with deep waters of very high density pro-duced within the Aegean Sea, has been very wellreproduced by the models. Sensitivity experimentswithout the precipitation anomaly, conRrm that thisfactor was a signiRcant contributor to the occur-rence and evolution of the Eastern MediterraneanTransient, since it acted as a ‘preconditionner’ to thelatter by importantly increasing the salinity in thearea.

The enhanced deep water production in theAegean has implied a deposition of salt in the deepand bottom layers with a simultaneous decrease

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higher up. As the turnover rate for waters below1200 m has been estimated to exceed 100 years, thisextra salt will take many decades to return into theupper waters. Its return, however, might well inducechanges in the thermohaline circulation, consideringthe dependence of the two potential sources ofdeep water on the salinity preconditioning. It willtherefore take many decades before the easternMediterranean returns to a new quasi-steady state.An interesting question in this connection is whetherthe system will recover its previous mode of opera-tion with a single source of deep-water productionin the Adriatic or evolve into an entirely different,perhaps even an unanticipated direction.

Conclusion

The Mediterranean Sea is now known to havea complex thermohaline, wind, and water Sux-driven multi-scale circulation with interactivevariabilities. Recent vigorous research, both experi-mental and modeling, has led to this interesting andcomplex picture. However, the complete story hasnot yet been told. We must wait to see the storyunfold and see how many states of the circulationexist, what changes occur and whether or not condi-tions repeat.

See also

Air-Sea Interaction: Heat and Momentum Fluxes.Convection: Deep Convection (Labrador Sea andChimneys); Open Ocean Convection. Elemental Distri-bution: Overview. Models: Coastal Circulation Models;Data Assimilation (Physical/Interdisciplinary); NumericalModels (The Forward Problem); Regional Models (In-cluding Shelf Sea Models). Ocean Circulation: GeneralProcesses; Surface/Wind Driven Circulation; Thermoha-line Circulation; Water Types and Water Masses. OceanCurrents: Mediterranean Sea (Overview of Basin andCurrent Systems). Turbulence and Diffusion:Meddies and Sub-surface Eddies; Mesoscale Eddies.Upper Ocean Structure: Time and Space Variability;Ocean Fronts and Eddies. Ocean Process Tracers:Tracers of Ocean-Atmosphere Exchange.

Further ReadingAngel MV and Smith R (eds) (1999) Insights into the

hydrodynamics and biogeochemistry of the SouthAegean Sea, Eastern Mediterranean: The PELAGOS(EU) PROJECT. Progress in Oceanography 44 (specialissue): 1}699.

Briand F (ed.) (2000) CIESM Workshop Series no.10, TheEastern Mediterranean Climatic Transient: its Origin,Evolution and Impact on the Ecosystem. CIESM.

Chu PC and Gascard JC (eds) (1991) Elsevier Oceanogra-

phy Series, Deep Convection and Deep Water Forma-tion in the Oceans. Elsevier.

Lascaratos A, Roether W, Nittis K and Klein B (1999)Recent changes in deep water formation and spreadingin the Eastern Mediterranean Sea. Progress inOceanography 44: 5}36.

Malanotte-Rizzoli P (ed.) (1996) Elsevier OceanographySeries, Modern Approaches to Data Assimilation inOcean Modeling.

Malanotte-Rizzoli P and Eremeev VN (eds) (1999) TheEastern Mediterranean as a Laboratory Basin for theAssessment of Contrasting Ecosystems, NATO ScienceSeries } Environmental Security vol. 51, Dordrecht:Kluwer Academic.

Robinson AR and Golnaraghi M (1994) The physical anddynamical oceanography of the Mediterranean Sea. In:Malanotee-Rizzoli P and Robinson AR (eds). Proceed-ings of a NATO-ASI, Ocean Processes in ClimateDynamics: Global and Mediterranean Examples, pp.255}306. Dordrecht: Kluwer Academic.

Malanotte-Rizzoli P, Manca BB, Ribera d’ Acala M et al.(1999) The Eastern Mediterranean in the 80s and inthe 90s: The big transition in the intermediate anddeep circulations. Dyn. Atmos. Oceans 29: 365}395.

Millot C (1999) Circulation in the Western Mediterra-nean Sea. Journal of Marine Systems 20: 423}442.

Pinardi N and Roether W (eds) Mediterranean Eddy Re-solving Modelling and InterDisciplinary Studies (MER-MAIDS). Journal of Marine Systems 18: 1}3.

POEM group (1992) General circulation of the easternmediterranean. Earth Sciences Review 32: 285}308.

Robinson AR and Brink KH (eds) (2) The Sea: TheGlobal Coastal Ocean, Regional Studies and Syntheses,vol. 11. John Wiley and Sons.

Robinson AR and Malanotte-Rizzoli P (eds) PhysicalOceanography of the Eastern Mediterranean Sea, DeepSea Research, vol. 40, Oxford: Pergamon Press.

Roether W, Manca B, Klein B. et al. (1996) Recent cha-nges in the Eastern Mediterranean deep waters. Science271: 333}335.

Theocharis A and Kontoyiannis H (1999) Interannualvariability of the circulation and hydrography in theeastern Mediterranean (1986}1995). In: Malanotte-Rizzoli P and Eremeev VN (eds) NATO Science Series} Environmental Security vol. 51, The Eastern Medi-terranean as a Laboratory Basin for the Assessment ofContrasting Ecosystems, pp. 453}464. Dordrecht:Kluwer Academic.

0032



Table 1 Upper thermocline circulation features

Feature Type ON85 MA86 MA87 AS87 SO91 JA95 S97 ON98

AIS P * * Y Y Y Y Y NMMC P Y Y * Y Y Y * YAMC P Y Y * Y Y Y * YCC R Y N Y N * * * *

Se Lev. Jets T Y Y * Y * * * *

Rhodes C P Y Y * Y Y Y * YWest Cyprus C P Y Y * Y Y Y * *

MMA P Y Y * Y Y Y * YCretan C P Y ? * Y Y * Y YShikmona AC R Y Y * Y Y * * *

Latakia C R Y N N Y * * * *

Antalya AC R ? Y * N * * * *

Pelops AC P * Y Y Y Y * Y YIonian eddies AC T * * * Y Y * Y NCretan Sea eddies T Y Y * Y * Y * YIerapetra R Y N Y Y Y Y * Y

Table 2 Mediterranean water masses

Water mass name Acronym

Aegean Deep Water AGDWAdriatic Water AWCretan Deep Water CDWCretan Intermediate Water CIWEastern Mediterranean Deep Water EMDWEastern Mediterranean Transient EMTLevantine Deep Water LDWLevantine Intermediate Water LIWModified Atlantic Water MAWTransitional Mediterranean Water TMWWinter Intermediate Water WIWWestern Mediterranean Deep Water WMDW

a0376tbl0001

a0376tbl0002



45

40

35

305 0 5 10 15 20 25 30 35

Usbon

Gibraltarstrait

Tangier

Alboran AlgiersTunis

Straitof

sicily

Sicily

Barcelona

Marseille

Lionsgulf

Ligurian

Corsica

Balearic Saroinia

Tyrrhenian

Genoa

Venice Trieste

Adriatic

Strait of

Otranto

IonianAegean

Rhooes

Crete

Cretan Passage

Levantine

Alexandria

Majorca

Cyprus

Haifa

Instanbul

Western Mediterranian Eastern Mediterranian

40 N°

30 N°

10 W° 0 E° 10 E°

10 E°0 E°

20 E°

20 E°

30 E°

30 E°

40 E°

Naples

10

(A)

(B)

WMDW EMDW

?

2000

1000

200

200

200 1000GDW

2000

1000

200

1000

2000

Figure 1 (A) The Mediterranean Sea geography and nomenclature of the major sub-basins and straits. (B) The bottomtopography of the Mediterranean Sea (contour interval is 1000 m) and the locations of the different water mass formations.

Gibraltar

Balearic

Sicily

Adriatic Aegean

MawLiw

LevantineIonian

Figure 2 Schematic of thermohaline cells and path of Levantine Intermediate Water (LIW) in the entire Mediterranean.

a0376fig0001

a0376fig0002



Sub-basinscale

Mesoscale

(A)

Sub-basin scale

Mesoscale

Basinscale

(B)

Figure 3 Schematic of the scales of circulation variabilities and interactions in (A) western Mediterranean, (B) eastern Mediterra-nean.

a0376fig0003

10 OCEAN CURRENTS/ Mediterranean Sea Circulation


s

Mw

E Q

Liw

MawLiw

EMDW?Sicily

Gibraltar

(A)

QQ

AdriaticEAegean

τ

LiwLiw

??

Ldw

LevantineIonian

Emdw?

Emdw

Sicily MawAgdw

(B)

?

?

?

AW

Figure 4 Processes of air}sea interaction, water mass formation, dispersion, and transformation. (A) Western Mediterranean, (B)eastern Mediterranean, (C) eastern Mediterranean (post-Eastern Mediterranean Transient).

a0376fig0004



Maw

EmdwAdriaticorigin

Liw

Old EmdwAdriaticorigin

EmdwAegeanorigin

(C)

Figure 4 Continued



0

0

200

200

400

400

600

600

800

800

1000

1000

1200

1200

1400

1400

1600

1600

1800

1800 2000

Di t (k )

_ 4000

_ 4000

_ 4500

_ 3500

_ 3500

_ 3000

_ 3000

_ 2500

_ 2500

_ 2000

_ 2000

_ 1500

_ 1500

_ 1000

_ 1000

_ 500

_ 500

780779

778777

773761

760758

756 751723

72672772

8729

734739

740741

742

39.05

39.00

38.9038.95

38.8538.8038.7538.70

38.6738.6638.5038.0037.50

39.0539.00

38.9038.95

38.8538.8038.7538.7038.6838.6538.5038.0037.50

0 12 13 30 34 50 54 58 57 67 62 60 74 72 76 776110

Pre

ssur

e (d

bar)

Pre

ssur

e (d

bar)

(B)

(A)0

Figure 5 West}east vertical sections of salinity through the eastern Mediterranean: (A) 1987, (B) 1995, (C) 1999.a0376fig0005



0 200 400 600 800 1000 1200 1400 1600 1800 2000

Distance (km)

_ 4000

_ 3500

_ 3000

_ 2500

_ 2000

_ 1500

_ 1000

_ 500

Pre

ssur

e (d

bar)

39.0539.00

38.9038.95

38.8538.8238.8038.7538.7038.6538.5038.0037.50

(C) 78 7 6 5 4 34 3 1 2

Figure 5 Continued



MAW-WIW

: more or less steady paths



: mesoscale currents throughout the year



: wintertime mesoscale currents



: wind-induced mesoscale eddies

: the north balearic front

: 0 m isobath

: 0 m and 200 m (thick) isobaths

: 0 m and 1000 m (thick) isobaths

TDW-WMDW

LIW-TDW

_5 0 5 1035

40

_5

_5

0

0

5

5

10

10

35

35

40

40

15

15

(A)

(B)

(C)

15

Figure 6 Schematics of the circulation of water masses in the western Mediterranean. (A) MAW}WIW; (B) LIW}TDW; (C)TDW}WMDW (Reproduced with permission from Millot, 1999).

a0376fig0006



AIS

MAW

MAW

MIJ

ISW LSW

LSW

PA

CC Lérapetra

Rhodes

Mersa-Matruh

W

cyprus

AMC CC

Shikmona

MMJ

12°E

40°

38°

36°

34°

32°

16°E 20°E 24°E 28°E 32°E 36°E

AIS = Atlantic-Ionian Stream IA = Ionian Anticyclones PA = Pelops AnticycloneMIJ = Mid Ionian Jet MMJ = Mid-Mediterranean Jet CC = Cretan CycloneMAW = Modified Atlantic Water ASW = Adriatic Surface Water AMC = Asia Minor CurrentISW = Ionian Surface Water LSW = Levantine Surface Water

ASW

IA

MMJ

Figure 7 Sub-basin scale and mesoscale circulation features in the eastern Mediterranean (Reproduced with permission fromMalanotte-Rizzoli et al., 1997).

a0376fig0007



A B C D

75

150

225

300

375

450

75

150

225

300

375

450

15.0

15.5

0.91

15.5

15.0

14.5

14.5

14.0

14.0

15.5 14.5

14.5

14.5_

15.0

14.0

14.5

15.0

16.015.5

14.516.0

16.5

250200150

150

100

100

50

50

50 50

0

0

0 0

_ 50

_ 50

_ 50 _50

_ 100

_ 100

_ 150

12Jul81

1Aug81

1Sep81

1Oct81

1Nov81

1Dec81

1Jan82

1Feb82

1Mar82

1Apr82

1May82

1Jun82

1Jul82

(A) (B)

(D)(C)250200150

150

100

100

50

50

0

0

_ 50

_ 50

_100

_100

_150B

0.00

D

0.06

A

C

Figure 8 (A) Mesoscale temperature cross-section from XBTs in AS87 POEM cruise along section ABCD. (B) Filtered temper-ature cross-section using a pyramid filter with 50 km influential radius. (C) Location of the cross-section superimposed on thedynamic height anomaly from AS87 survey (excluding XBTs). (D) Velocities from a current meter array in the Corsican Channel.

a0376fig0008



50.0

Figure 9 Velocity field for the eastern Mediterranean at 10 m depth from an eddy-resolving primitive equation dynamical model.

_ 5 0 5 10 15 20 25 30 35

Temperature at depth = 10 m, Year = 2, Month = 6, Day = 15, Min = 15.9703, Max = 23.2415

30

32

34

38

40

42

44

46

Temperature distribution and currents at 10 m depth during summer in the mediterranean sea

16 17 18 19 20 21 22 23

Figure 10 Temperature and superimposed velocity vectors at 10 m depth from a numerical simulation of the entire MediterraneanSea.

a0376fig0009

a0376fig0010



_ 5 0 5 10 15 20 25 30 35

Temperature at depth = 10 m, Year = 2, Month = 6, Day = 15, Min = 15.9703, Max = 23.2415

30

32

34

38

40

42

44

46

Temperature distribution and currents at 10 m depth during summer in the mediterranean sea

16 17 18 19 20 21 22 23

Colour Figure for Figure 10



Documents

Mediterranean Sea Circulation R - Prof. Allan R. Robinson