Signatures of the Mediterranean Outflow from a North Atlantic Climatology 1. Salinity and Density Fields

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. Cll, PAGES 25,985-26,009, NOVEMBER 15, 1999

Signatures of the Mediterranean outflow from a North Atlantic climatology 1. Salinity and density fields Michaela Ciobotaru Iorga Department of Mechanical Engineeringand Material Science,Duke University, Durham, North Carolina

M. Susan Lozier Earth and Ocean Sciences,Duke University,Durham, North Carolina

Abstract. Using historicaldata from the National Oceanic Data Center, the climatology of the easternNorth Atlantic basin has been investigatedfor the purposeof detailing the Mediterranean outflowwater in terms of its salinity,density, and flow patterns.Part 1 of this work is a descriptiveanalysis of the fate of the MediterraneanWater once it flows out of the Strait of Gibraltar. Tracing the salinity and densitysignatures, high-resolution maps of the climatologicaloutflow are presented,with an emphasison the continuityof the water from its source.From the climatologicalfields a continuoussignal of Mediterranean Water is tracked northwardto ---50ø20'N,yet its westwardadvection is limited to the Tagus Basin. Recirculationsof Mediterranean Water in the Gulf of Cadiz and in the Bay of Biscay,deduced from property signals,are also detailed.Iorga and Lozier [this issue] presentabsolute velocity fields from a diagnosticmodel constrainedby geostrophic dynamics,conservation of mass,and no-flux boundary conditions.

1. Introduction McCartneyand Mauritzen, submittedmanuscript, 1999). With this latter scenario,Mediterranean Water is relegated to an Warm and saltywaters flowing from the Mediterranean Sea indirectrole on the sourcewaters in the Norwegian/Greenland into the North Atlantic Ocean form one of the most pro- Sea since it is assumedthat the waters carried by the Gulf nouncedtongue-like property distributions in the globalocean. Stream/NorthAtlantic Current Systemgain saltby mixingwith At ---1000-1200 m the temperatureand salinitysignatures of the adjoining Mediterranean Water at subtropicallatitudes MediterraneanWater nearlyfill the entiretyof the subtropical [Lozier et al., 1995]. Ambiguity also surroundsthe issue of North Atlantic basin(Figure 1). The distincttemperature and whether the MediterraneanWater extendsinto the subtropical salinitycharacteristics of Mediterranean Water can be traced basin(where it is likely to mix with the Gulf Streamor North westward to the Bermuda Rise [Armi and Bray, 1982] and Atlantic Central Waters) by advectiveor diffusivemeans. Reid northwardto the Rockall Channel [Reid, 1979;Harvey, 1982]. [1994] has suggesteda westwardadvective pathway for Medi- During the past 2 decadesan interestin MediterraneanWater terranean Water in the range 35ø-45øN, and this has been has focusedon its possibleinfluence on deep water formation supportedby the model resultsof Hogg[1987] andBogden et al. processesin the northernNorth Atlantic. Althoughit hasbeen [1993].However, from a studyof zonaltransports in the eastern suggested[Reid, 1979, 1994; Harvey, 1982] that the source North Atlantic,Maze et al. [1997]argued that therewas no direct waters to the formation sitesare influencedby the warm and advection of Mediterranean Water into the ocean interior. Model saline Mediterranean Water at middepths, the particular resultsfrom Paillet and Mercier [1996] supportthis argument. venue of this influence remains unknown. The influence is Before the question of whether and how Mediterranean assumedto be either directly through advectionor indirectly Water influencesdeep water processesin the North Atlantic throughmixing. The directroute hypothesis,first promulgated can be answereda detailed analysisof the fate of the Medi- by Reid [1979], is that a branch of the Mediterranean outflow terraneanoutflow is needed.Since this influenceis presumably penetrates northward along the eastern boundaries to the on a climatologicalscale, we have chosento investigatehistor- Greenland-Scotland Sill where its salt content influences the ical hydrographicdata in order to detail the climatological productionof deep watersin the Nordic Seas.Alternatively, it signatureof the Mediterranean Water in the eastern North is proposedthat the warm and saltysource waters in the Nor- Atlantic. Specifically,we use a recent high-resolutionclimato- wegian/GreenlandSea derivemainly from the upper watersof logicaldatabase [Lozier et al., 1995] of the North Atlantic to the North Atlantic Current, which carry saltywaters into the examinethe salinityand densityfields of the easternbasin. Our Nordic Seas (M. S. McCartney and C. Mauritzen, On the primarygoal is to establishthe continuityof the Mediterranean origin of the warm water inflow to the Nordic Seas,submitted signalinto the open Atlantic on a climatologicalscale. High- to Deep-SeaResearch, Part I, 1999, hereinafter referred to as resolutionisopycnal maps and meridionaland zonal crosssec- tionsof salinity,density, potential temperature, and oxygenare Copyright1999 by the American GeophysicalUnion. presentedin this descriptiveanalysis of the climatologicalflow Paper number 1999JC900115. pattern of the MediterraneanWater. Of particularinterest are 0148-0227/99/1999JC900115509.00 the possible northward and westward penetrations of this

25,985 25,986 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

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Figure 1. Propertydistributions at middepthin the North Atlanticbasin. These property fields were created usingthe hydrographicdatabase described by Lozieret al. [1995].The spatialresolution of thesefields is 0.5ø. water mass.An attempt has been made to compareand con- 2. Background trast the climatologicalsignals to thosederived from synoptic surveysor, as is the casewith Reid [1979, 1994], from quasi- Paststudies concerning the fate of the MediterraneanWater synopticstudies. Iorga and Lozier [this issue](hereinafter re- have generallyfocused on synopticdescriptions of the flow in ferred to aspart 2) presentresults from a diagnosticmodel that the local vicinity of the Strait of Gibraltar and/or the Gulf of usesgeostrophy and mass conservation, in conjunctionwith the Cadiz. A brief surveyof the collectiveknowledge from these Lozier et al. [1995] database,to estimatethe flow field in the studiesis given here as backgroundfor the larger context of easternNorth Atlantic. Backgroundfor our study is given in our study.The reader is referredto Daniaultet al. [1994]for a section2, followedby a discussionof our databaseand meth- more completesummary of thesestudies and to Figure 2a for ods in section3. Section4 containsthe resultsof our analysis, the location of topographicfeatures discussed in the text. A and a summaryis found in section5. detailedmap of the bathymetryin the Gulf of Cadiz is givenin IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,987

I I I I I I I ..! I .. I !•. ß ' 55øN

• .. Promontorjr I:ß St.Vincent Canyon CapeSt. Vincent ' .•/! Sto...VincentSpur ...... Cape St. Maria

Bank ....• ...... • •. .•••.• Horseshoe A.P. -.. :. '/:•.•;•';?C"•nyon

Bank:'•:•;•2).. A.P.Seine StraitofGibraltar I 35øN '•'..... ;.• •...•4•?:: :•:...... ::?/•' ':'•:•' I 30'N I

I I

Figure 2a. Topographicfeatures in the eastern North Atlantic basin.

Figure 2b. Mediterranean Water exitsthe Strait of Gibraltar as 1997], while a secondcore, generally transportinga larger a single, dense plume with typical properties of (0, S) = volume,is guidedby variouschannels down to ---1200m [Zenk (13øC,38.4 practicalsalinity units (psu) [Baringer,1993; Bar- and Armi, 1990; Baringer,1993; Boweret al., 1997]. The two ingerand Price, 1997]. The plume movesdown the northern branches,or cores,converge near Cape St. Vincent. A more continentalslope of the Gulf of Cadiz,flowing around complex detailed synopticdescription of the outflow in the Gulf of topographyas a bottom-drivenboundary current. As the out- Cadiz is presentedby Zenk [1975].Briefly, Zenk identifiesfour flow spreadsnorthwestward along the southernSpanish coast, branchesof MediterraneanWater that are determinedby the it slowly loses its high salinity as it mixes with fresh North local bathymetry:a shelfbranch in the depth rangeof 400-600 Atlantic Central Water [Madelain, 1967, 1970; Zenk, 1975; m; a main offshorebranch, at a depthvarying between 800 and Ambar, 1984, 1985; Zenk and Armi, 1990;Baringer and Price, 1000 m; an intermediate branch described as the southern core 1997; Bower et al., 1997]. By the time the flow reachesthe of the shelf branch;and a jet-like branch,guided by a narrow vicinity of Cape St. Vincent it is neutrally buoyant [Baringer, channelnear the Straitof Gibraltar(36øN, 7ø08'W). Ochoa and 1993; Ochoaand Bray, 1991;Zenk and Arrni, 1990]. Bray [1991] alsoreport severalbranches that flow down small Downstream from the Strait of Gibraltar, two main cores of local canyonsand eventuallyrejoin the main branchhalfway to the outflowhave been observed.The upper core of the outflow Cape St. Vincent. follows the northern slope of the Gulf of Cadiz at a depth After the coresconverge in the vicinity of Cape St. Vincent rangingfrom 600 [Zenkand Arrni, 1990]to 750 m [Boweret al., the Mediterraneanwater flowsthrough the gap betweenGet- 25,988 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

38øN creatinga broad, irregular slope in the easternGulf of Cadiz. Near Cape St. Maria (---8øW)the isobathsconverge sharply, creatinga steepslope that extendsto Cape St. Vincent. In the vicinityof Cape St. Vincent the continentalslope protrudes in a feature called the Cape St. Vincent Spur that creates the 37øN southernwall of the wide St. Vincent's Canyon. The slope is irregular and relativelybroad north of this canyon.At ---38øN the Lisbon and Setubal Canyonsextend far into the slope. Farther north, the slope turns northwestwardand steepens 36øN along the southern flank of the Estremadura Promontory, knownalso as Tejo or TagusPlateau, and then sharplyreverses direction, following its northern flank. Nazar• Canyon is located to the north of this promontory.Galicia Bank, west of 35øN the Iberian Peninsulaat --•43øN,is anothermajor topographic feature in this region that risesto the Mediterranean Water's depth. The Seine and HorseshoeAbyssal Plains, which are north-

34øN easternextensions of the larger Cape Verde-Madeira Abyssal Plain, are locatedsouth of 36øN.They are separatedfrom the Tagus AbyssalPlain in the north by a quasi-zonalridge at 36ø-37øN,the summit of which is the Gorringe Bank, which rises to within 25 m of the surface. Located between the Gor- 33øN 10øW 9'W 8'W 7'W 6'W 5'W ringe Bank and Cape St. Vincent is the wide St. Vincent's Canyon,with a depth of --•3800m. Finally, the TagusAbyssal Figure 2b. Bathymetry in the Gulf of Cadiz. The contour interval is 100 dbar. Plain communicatesto the north with the Iberian AbyssalPlain through a sill of depth 4600 m, just west of the Estremadura Promontory. tysburgBank and Cape St. Vincent [Madelain, 1967, 1970; 3.2. Hydrographic Database Zenk, 1975;Ambar, 1984, 1985; Zenk and Armi, 1990; Bower et Our data source is a recently assembleddatabase of the al., 1997], termed St. Vincent's "gateway"by Zenk and Armi North Atlantic [Lozieret al., 1995], which consistsof the cli- [1990].From thisgateway, part of the outflowis trappedalong matologicalmean property fields averagedon isopycnalsur- the continental slope, flowing northward [Reid, 1979, 1994; faces.Data from ---144,000hydrographic stations in the North Daniault et al., 1994],while anotherpart hasbeen reportedto Atlantic, spanningthe period 1904-1990, were retrievedfrom flow westwardinto the oceaninterior [Reid,1994; Daniault et the National Oceanic Data Center and quality controlled to al., 1994;Arhan et al., 1994].Daniault et al. [1994] describea eliminate erroneous data. The reader is referred to Lozier et al. reservoir of Mediterranean Water situated east of 13øW, be- [1995] for a detailed discussionof the quality control and tween 37ø and 40øN, with a westward exit and several north- processingof the station data. Whereas Lozier et al. [1995] ward exits. The northward penetration of dilute Mediterra- focusedon the North Atlantic basinin its entirety,our focuson nean Water as an eastern poleward boundary current as far the Mediterraneanoutflow dictatesthe use of different isopy- north as PorcupineBank (---50øN)is generallyaccepted [Reid, cnal surfaces,different projections,and generallyhigher reso- 1979, 1994; Bogden et al., 1993; Hill and Mitchelson-Jacob, lution grids. Specifically,we use the quality-controlled,un- 1993;McCartney and Mauritzen, submittedmanuscript, 1999]. evenlydistributed set of stationdata to investigatethe details However,its further polewardpenetration into the Norwegian/ of the salinityand densityfields in the easternNorth Atlantic, Greenland Sea is doubtedby thosewho contestthe possibility which for our purposeis defined as 25ø-60øNand 40ø-0øW. of a rise of the isopycnalsfrom ---1200 m northwest of the The spatialdistribution of the 26,533stations in our domainof Porcupine Bank to <500 m over the sill of the Wyville- interest are shown in Figure 3a. The irregularly distributed Thomson Ridge [McCartneyand Mauritzen, 1999; Hill and data are binned and ensemble-averagedto produceproperty Mitchelson-Jacob,1993]. maps on isopycnalsurfaces, as well as meridional and zonal cross sections of the eastern North Atlantic basin. Details 3. Database and Methods concerningmapping techniquesand resolution are given in section 3.3. 3.1. Bathymetry The ensembleaveraging used to producethe maps in this To facilitatethe descriptionof the propertyfields in section paper does not take into accounttemporal changesin data 4, the complexbathymetry in our spatial domain is briefly density.To guardagainst the possibilitythat featuresobserved discussedin this section.The reader is againreferred to Figure in the climatologicaldatabase are the resultof samplingover a 2b for the detail of the bathymetryin the Gulf of Cadiz. The shorttime period only, the database'stemporal representative- Gulf of Cadiz has an irregular bottom topographyalong the nesswas examined. Figure 3b showsthe spatialcoverage of the continentalslope of Portugaland immediatelyoffshore, where data for each of the 8 decadesrepresented in the database. the majority of the Mediterranean Water has been observed. Even though the databaseis temporallyweighted toward the The slopevaries in steepnessand is deeplyindented by several last 4 decades,there are no spatial domainsthat were exclu- submarinecanyons. Downstream from the Strait of Gibraltar, sivelymeasured during one time period only. In an effort to be the isobathsdiverge as the coast rounds to the north, thus more explicitabout the temporalcoverage of our databasewe IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,989

60øN have attached an appendix that showsthe data coveragefor each crosssection presented in section4. Our concernabout temporal aliasingis also allayedby a statisticalanalysis of the data that showslittle changein the climatologicalmean when propertieswere averaged such that all years received equal weighting[Lavine and Lozier, 1999].

3.3. Mapping

50•N Isopycnalproperty maps and zonal and meridional cross sectionsof the water propertiesare producedusing a variable grid size to accountfor differencesin local data density.Gen- erally, a smoothingradius of 1/10ø is used near the Strait of Gibraltar. This radius increases toward the interior of the oceanto a maximumof 30'. The verticalincrement of the grid varies between 25 m in the Gulf of Cadiz and increases to a 40øN maximum of 100 m in the interior of the ocean. Because our analysisof the Mediterranean Water dictates much smaller grid increments(to resolveboundary currents, for example) than a basin-wideclimatology [Lozier et al., 1995],data density is compromisedin some locales.When analyzingthe fields, data gapswere masked,as seenin Figures4c and 4d for two 30øN representativecross sections. These masked sections were then comparedto sectionsproduced from a smoothingof the local data (Figures4a and4b). A subjectivedetermination was made 40•W 30øW 20øW 10øW 0 ø as to whether the smoothedrepresentation of the data was sufficientlysupported by the maskedplot. For the two exam- Figure 3a. Spatial distributionof the stationsin the eastern ples presented,one (8ø15'W) was selectedas representative, North Atlantic for the period 1904-1990. while the other (37øN)was selectedas unrepresentative.We

1910-1920 1920-1930 1930-1940 1940-1950

60'N 60'N

50'N 50'N

40'N

30'N 30'N

40 'W 30'W 20'W 10'W 0' 40'W 30'W 20'W 10'W 0' 40'W 30'W 20'W 10øW 0' 40'W 30'W 20'W 10'W 0'

1950-1960 1960-1970 1980-1990

60'N 60'N

50'N 50'N

40'N 40øN

30'N 30'N

40'W 30'W 20'W 10'W O' 40'W 30'W 20'W 10'W O' 40'W 30'W 20'W 10'W O' 40'W 30'W 20'W 10'W O'

Figure 3b. Spatial distributionof stationsin the easternNorth Atlantic for selecteddecades, as marked. 25,990 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

Figure4. (a)and (b) Salinity fields plotted without masking for two representative cross sections. (c) and (d) Samefields as in Figures4a and4b but with datagaps shown. have chosento showthe smoothedfields in this paper to in a largercontext, the readermay find the schematicof some facilitatethe representationof the flow and its continuity. aid. However,the fieldswe presenthave been selected only in the casethat theyare representativeof the maskedproperty fields. 4.1. Plan View of Salinity Signal Becauseof thischoice, the sectionswe showin thispaper are Salinityfields for eightisopycnals, which span the depthof not alwaysevenly spaced, rather they are selectedto maximize the Mediterraneanoutflow (from •o.5 = 29.40at -500 dbarto data densityand representativeness.The locationsof each of •o.5 -- 30.10at -1400 dbar), havebeen usedto composea the 46 crosssections discussed in this paper are shownin high-resolutionplan view of MediterraneanWater spreading Figure 5. into the Gulf of Cadiz(Plate 1). Salinities>36.25 psu, which approximatelydefine the minimum limits of Mediterranean 4. Results Water in the vicinityof its outflow[Baringer and Price, 1997], aremarked on themap. Specifically, for eachisopycnal surface In the followingsubsections the Mediterraneanoutflow sig- all 0.1ø grid elements that have a salinity>36.25 psu have been nal is analyzedon the basisof the climatologicalsalinity sig- assigneda color that correspondsto the depthof that mea- nature of the Mediterranean Water in the eastern North At- surement.The isopycnalsurfaces are thenstacked one on top lantic.An overallview of the salinitysignal in the Gulf of Cadiz of the other,with the deepestisopycnal at the bottomand the is givenin section4.1, followedby a collectionof meridional shalloweston the top.This arrangement creates a planview of andzonal cross sections of densityand salinity fields and isopy- the spreadingof the saltsignal. As seenin Plate1, the high- cnalmaps of pressure,salinity, and oxygen. These salinity and salinity Mediterranean outflow leaves the Strait of Gibraltar densityfields are investigatedin sections4.2-4.5 to establish and flowsover the Camarinaland SpartelSills as a trapped the continuityof the Mediterraneansignal into the openAt- vein. The outflow, sinking rapidly, turns northward at lantic. Our discussionbegins at the Strait of Gibraltar and -35ø40'N and 6ø40'W and then follows the 400 m isobath to proceedsdownstream. A schematic(Plate 2), presentedas part -6ø50'W and 36ø20'N. The northward turn of the outflow has of section5, providesa large-scaledistillation of our results.To beenattributed to topographicsteering [Ochoa and Bray, 1991] placethe followingdiscussions (sections 4.1-4.5) of localareas andto a Coriolisdeflection JAmbar and Howe, 1979; Baringer IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,991

35 øW

Figure 5. Plan view of the locationand spanof all crosssections presented and discussedin section4.

and Price, 1997; Ochoa and Bray, 1991]. Further downstream, 4.2. Gulf of Cadiz Observations the northern edge of the outflowdescends slightly and follows 4.2,1, The Mediterranean outflow subdivision into two the isobathsbetween 500 and 700 m, while the southernedge cores, Tracking the signal of Mediterranean Water through descendsbelow 1000 m. The onshore,shallow portion of the the easternGulf of Cadiz is facilitatedby a sequenceof three outflowis commonlydesignated as the "upper core,"while the meridional crosssections of salinitystarting near the Spartel offshore,deep portionis designatedas the "lower core" [Zenk Sill at 6ø30'W and progressingwestward at an interval of 20' andArmi, 1990;Daniault et al., 1994;Bower et al., 1997].As the (Figure 6). Five isopycnals,chosen to representthe upper flow followsthe continentalslope, the plume widens,reaching its maximum width in the Gulf of Cadiz of over 50 km at (O'o.s= 29.50), middle (O'o.s = 29.70 and O'o.s = 29.80), and 7ø30'W.This width is in roughagreement with the Baringerand lower (O'o.s = 29.90 and O'o.s = 30.00) portion of the Mediter- Price [1997] estimate of 65 km at this site, which they also ranean outflow, are superposedon the salinityfields for this describe as a local maximum. The agreement between the and all subsequentcross sections. synopticand climatologicaljet's width presumablyattests to The crosssection at 6ø30'W (Figure 6a) showsthe outflow the strongboundary trapping of this outflow. From this loca- just downstreamof the Spartel Sill. The outflow is bottom- tion of maximum width both cores narrow in the horizontal trapped,lying between ---400 m and the bottom (at ---600m) plane as the flow approachesCape St. Vincent. Even though and has a maximumsalinity of 38.00 psu. The outflow main- the data are sparsenear Cape St. Vincent, the vein of Medi- tains a fairly constantthickness of ---250m, from section6030 ' terranean Water turning northward along the Iberian Penin- to ---6ø50'W(Figure 6b), but by 7ø10'Wit hasthickened to over sula, at a depth below 900 m, is evident. 500 m. The maximumsalinity decreases to 37.60 psuby 6ø50'W 25,992 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS and to 37.00 psu by 7ø10'W. In addition to a fresheningand thickeningof the plume as it movesdownstream, the sequence of crosssections shows the plume gainingbuoyancy and, by 7ø10'W, existingas two connectedbut distinctcores. In sum, over a horizontal distance of--•100 km the climatological plume is diluted, thickens,and is verticallydifferentiated. There are three main hypothesesfor the subdivisionof the Mediterranean outflow into two vertical cores. First, Madelain [1970] suggestedthat variablebottom topographyand subma- rine canyonsare responsiblefor subdividingthe outflowalong different paths. Second,Siedler [1968] and later Zenk [1975] proposedthat tidal mixing in the Strait of Gibraltar produces waterswith two distinctpeaks in temperatureand salinityre- lationshipsthat then move out onto the continental slope. Finally, the Gulf of Cadiz Experiment data indicatedthat two distinct cores of outflow evolve through differential mixing within the Gulf of Cadiz [Baringer,1993; Baringerand Price, 1997]. Because the climatologicalflow exits the strait as a single,dense plume, the secondhypothesis is apparentlynot supportedby our work. However,it mustbe kept in mind that spatialand/or temporal averagingof the flow field couldcreate a smoothingof two cores suchthat they could not be distin- guishable.This possibility,however, is weakenedby the fact that the useof the samesmoothing scales downstream does not prevent the appearancethere of separate,discernable cores. However, the difference rests on the relative vertical distance between the cores at each site and on the strength of the temperatureand salinitydifferences at eachsite. Further support for the argumentthat two coresare not exportedfrom the Strait of Gibraltar is providedby an analysisof the historical O-S relationship,which doesnot showany bimodality. While our analysiscannot discernthe effect of bottom to- pography on the evolution of the cores, our analysisdoes suggest,as will be argued, that the Mediterranean outflow could be subdivided into two cores as a result of differential mixingwith North Atlantic waters.The use of the climatological salinityfield allowsfor a studyof the Mediterranean outflow in the contextof the surroundingwaters. As seenin Figure 6, fresh North Atlantic waters overlie the Mediterranean outflow along the southernIberian coast.Additionally, relatively freshwaters (--•35.6 psu), centered at --•600m, are foundat the outflow'ssouthern boundary, as seen at the southernedge of each crosssection in Figure 6. The sourceof this low-saline water at the southern edge of the Mediterranean outflow is evidentfrom the maps of salinityand oxygenon the isopycnal surface,0-0. 5 = 29.50,which lies at a nominal depth of 500 m in the Gulf of Cadiz (Figure 7). Low-oxygenwaters extend westward into the open Atlantic from the northwesternAfrican coast, centered along 14øN [Reid, 1994; Lozier et al., 1995]. Along the African coast,north of 25øN, the oxygenisopleths are directednorthward along the coastline,suggesting the existenceof a poleward current advectingwaters low in salinity (--•35.60psu) and oxygen (--•3.5 mL L -h) alongthe continental slope(Figure 7 inset). The continuationof this boundarycurrent into the Gulf of Cadiz is supportedby the southwardrise Figure 6. Meridional cross sections in the eastern Gulf of of the isopycnalsurface 0-0. 5 = 29.50 near the African conti- Cadiz showingthe Mediterranean outflow subdivisioninto two nental slope, seen on the meridional section at 7ø10'W. A cores.The backgroundwaters are contouredwith dashedlines poleward flowing undercurrentoff northwestAfrica was first with an interval of 0.1 psu. Salty waters are contouredwith noted by Woosterand Reid [1963], and nearly two decades shading,every 0.05 psu, startingwith a minimum salinityof later, Tomczakand Hughes[1980] reported the existenceof a 36.0 psu.Dashed shaded lines designate areas where there are data gaps.The isopycnals,0-0. 5 = 29.50, 0-0.5 = 29.70, 0-0.5 = core of freshwater of southernorigin advectedpoleward along 29.80, 0-0.5 = 29.90, and 0-0.5 = 30.00, are designatedby the the continentalslope at severallatitudes. They found tracesof solid lines in this and all subsequentcross sections. southernwater north of 20øNat depthsdown to at least 600 m. IORGA AND LOZIER: MEDITERRANEAN OUTFLOWmSALINITY AND DENSITY FIELDS 25 993

11 øW 10øW 9øW 8øW 7øW 6øW

Estremadura

39ON Promontory 39øN IBERIAN PENINSULA 38ONI •, 38ON .• CapeSt. Vincent

37øN ! / CapeSt.MariaI37øN 400 rn • • •00ß •'0o,•

Gorringe 36 øN [' Bank ' 36 øN

Gulf of Cadiz

35øN 35øN

11 øW 10øW 9øW 8øW 7øW 6øW

1500 m 900 rn 100 rn

Plate 1. Plan view of the Gulf of Cadiz, showingthe spreadingof Mediterranean Water with a salinity greaterthan 36.25 practicalsalinity units (psu). The depth of salinityis indicatedby the color code.

The existence of this undercurrent as far north as 32øN at -700 flow, an intersectionthat allowsfor horizontal,cross-isopycnal m has been reported by Barton [1989]. The depth of the cli- mixing.The upper waters are simultaneouslymixing with the matologicalboundary current, at 600 m, is within the range overlyingwaters derived from the open Atlantic. This differ- reported from synopticstudies of this current [Barton, 1989]. ential mixingis one potential mechanism,as explainedby Bar- Thus we hypothesizethat the sourceof the freshwaters at this ingerand Price [1997], for the creation of two coresfrom the depth in the Gulf of Cadiz is the boundarycurrent along the Mediterranean plume, observedat 7ø10'W. Additionally, at African coast. this site the upper core now has a maximumsalinity of-36.3 Downstreamchanges in salinityindicate mixing between the psu, and the lower core has a maximumsalinity of-37.0 psu. outflowwaters, the overlyingsurrounding waters, and thoseto 4.2.2. The Mediterranean outflow in the western Gulf of the south.The 36.00 psu salinityisopleth (the outer shaded Cadiz. Westwardpenetration of the Mediterranean outflow contourdefining the plume of Mediterraneanwaters) lies on alongthe continentalslope in the Gulf of Cadiz is represented the isopycnalsurface fro. s = 29.50 at 6ø30'W (Figure 6a), but by a sequenceof six meridionalcross sections in Figure 8. For changesits positionto fro.s = 29.70 by 6ø50'W as the outflow a better understandingof the vertical distributionof the salin- penetratesinto the Gulf of Cadiz(Figure 6b). At -6ø50'W and ity signal the same five isopycnalsurfaces as in Plate 1 are 36øN(Figure 6b) the Mediterraneanoutflow reaches the same superposedon the salinityfields. The outflowreaches its max- depth as the polewardflowing fleshwater. The meridionalsec- imum width of over 50 km at -7ø30'W, before it encounters tion at 6ø50'W indicates the intersection of the fresh Atlantic the steep slope of Cape St. Maria (at 8øW). Three salinity water with the southwesternedge of the Mediterranean out- maxima, at -400 and 900 m and a new one at -1200 m, are 25,994 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

lOW

80øW 60'W ..•O'W 40'W 30'W 20'W 10'W O' 70 'W ttO'W 50'W 40'W 30'W 10'W

10*W 9'W 8*W 38'N 38'N

3TN

36W

10'W 9'W 8*W 7*W

10'N

70'W 60'W $O'W 40'W 30'W 20'W 10'W O*

Slit

Figure 7. Pressure,salinity, and oxygen in the NorthAtlantic on the isopycnalsurface cro. s = 29.50.Insets showdetails of the oxygenand salinityfields. IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,995

-> No• -> North

Figure 8. Meridionalcross sections in thewestern Gulf of Cadiz.Contouring is as used in Figure6. The solid lines designatethe isopycnals,as in Figure 6. found from 7025' to 7ø40'W.It is possiblethat the coexistence 1993].Zenk [1975]noted the existenceof a slopebranch at this of the three local salinitymaxima is createdby the temporal sitewith two verticalsalinity maxima at -400 and 700 m [Zenk, variabilityof the flow; however,an examinationof the data 1975, Figure 3] and a main branchfound between-7ø30 '- doesnot supportthis possibility. Instead, these cores appear to 7ø40'Wand 36ø10'-36ø20'Nat -1000 m (moorings20 and 22) be climatologicalmanifestations of coresobserved from syn- [from Zenk, 1975, Figures4 and 7]. These depthsapproxi- opticsurveys [Zenk, 1975; Ambar, 1983; Hinrichsen and Rhein, matelymatch those found from the climatologicalrepresenta- 25,996 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS tion. Of note in Figures 8a and 8b is the southwardpenetration saddlethat connectsthe Tagus and HorseshoeAbyssal Plains. of the lower core'ssalinity signal across the Gulf of Cadiz. This They describeda southwardrecirculation of MediterraneanWa- extendeddeep salinitymaxima could result from the presence ter, with a diameterof -150 km, in thisarea. The slightdifference of a jet-like branch of dilute Mediterranean Water that is in size and locationbetween the climatologicalfeature and Da- created by a sharp canyonupstream of 7ø25'W at 36øN and niaultet al.'s[1994] observation might be the resultof discrepan- 7ø08'W [Zenk, 1975;Baringer, 1993; Baringer and Price, 1997]. ciesbetween synoptic and climatologicaldata. Alternatively,the southwardpenetration could result from lateral mixing induced by strong velocities associatedwith the 4.3. Mediterranean Water in the interior of the North Atlantic southernedge of the main branch[Zenk, 1975].It is notedthat as the outflow turns sharplywestward following the continental The sequenceof crosssections used in section4.2 is further slope around 7øW, meridional sectionsand the stream'saxes analyzedto detail the path of the Mediterranean Water into are not orthogonal; therefore events from upstream/down- the interior of the North Atlantic. Additional information is stream of one meridional sectioncould be averagedonto the providedby a sequenceof six meridional sectionsextending same section. Such averaging could explain why a jet-like towardthe interior of the North Atlantic (Figure 11) and six branch of Mediterranean Water observed at -7ø08'W and zonalsections spanning the TagusBasin (Figure 12). The same 36øN [Zenk, 1975] is not detectedon the 7ø10'Wclimatological five isopycnalsurfaces used in section4.1.1 and Figure 6 are representationof the outflow (Figure 6c), yet its dilute saline superposedover the salinityfields for all these sections. signalappears downstream from the formationsite (Figure 8) 4.3.1. The Mediterranean Water in the Tagus Basin. The as a deep salinitymaximum centered on the isopycnalsurface meridional sectionsin Figure 9 show the northwardpenetra- •ro.s = 30.00. tion of Mediterranean Water along the westernIberian shelf, As reportedby Daniault et al. [1994],two salinitymaxima are concentratedat 1100-1200 m. Also apparent from these sec- sometimesencountered on a singlevertical profile downstream tions is the dilution of the salinity signal (-0.15 psu) from of Portimio Canyon.The climatologicalfields (Figures 8d and Cape St. Vincent to Cape Finisterre,a distanceof -400 km. 8e) also displaytwo salinitymaxima in this locale.At approx- The salinitymaximum at -37øN and 1200 m, shownin Figure imately this longitude the lower core of the outflow detaches 9a, can be traced westward,appearing in the meridional sec- from the continental slope, becoming neutrally buoyant at tions 10ø30', 11ø30', and 12ø30'W.The plume is diluted west- -1100-1200 m. From Figure 8f it is apparent that the upper ward by -0.15 psu (from 36.3 to 36.15 psu) over a horizontal core is flowing along the continentalshelf with its maximum distanceof -200 km (Figures9b, 9c, and 9d). This westward salinity located slightlybelow •ro.s = 29.80 (-850 dbar), in penetrationof Mediterranean Water along the northern flank agreementwith synopticmeasurements [Daniault et al., 1994]. of the GorringeBank is consistentwith a westwardcurrent (6 The lower core, carryinga substantiallylarger volume of Med- cm s-•) northof GorringeBank, reported by Daniaultet al. iterranean Water, is centered on the isopycnalsurface •ro. s = [1994] and by Maz• et al. [1997]. 30.00 (---1250dbar), the deepestisopycnal shown in Figure 8. Another feature of interestin the sequenceof crosssections This deep isopycnalis approximatelyequivalent at this site to in Figures 9 and 11 is the appearanceof another Mediterra- the isopycnal•r• = 32.25, which was used as the targeted nean core between 39 ø and 39ø30'N from 10ø30 ' to 18øW. The isopycnalfor the deploymentof floats used in A Mediterra- distinction between this core and the branch of Mediterranean nean UndercurrentSeeding Experiment (AMUSE) [Boweret Water that flowswestward along the Gorringe Bank is partic- al., 1997]. In sum, three salinity maxima, at 400, 900, and ularly evident at 12ø30'W. Here the core at -39øN can be 1200 m, are detected from 7025' to 7ø40'W; yet by Cape St. clearlyidentified and distinguishedfrom the generallydiffuse Maria, only two salinitymaxima, centered at ---850and 1250m, are found. salinity signal from 35ø to 41øN. That the salinity maximum 4.2.3. The eyeIonic circulation of Mediterranean Water defining this secondcore is located on the same isopycnal south of 36øN. A sequenceof four meridional sectionsbe- surface(rr0. s = 29.90) as the Mediterraneancore penetrating tween9030 ' and 12ø30'W(Figure 9) and three zonalsections at westwardalong the northern flank of the Gorringe Bank and 33050', 34030', and 35øN (Figure 10) are usedto describethe that this core is lesssaline than the westwardcore suggestan Mediterranean signalsouth of 36øN.These crosssections also anticyclonicturning of the westwardflowing Mediterranean have five isopycnalsurfaces superposed on the salinity fields Water. This anticyclonicpathway creates a "reservoir"of Med- (•ro.s = 29.50, •ro.s = 29.70, •ro.s = 29.80, •ro.s = 29.90, and iterraneanWater in the TagusBasin, similar to Daniault et al.'s •ro.s = 30.00). The deepest isopycnalsurface, •ro.s = 30.00 [1994] description.A more detailed analysisof the Mediterra- (equivalentto % = 32.25), hasbeen definedas the locationof nean Water pathwaywest of Iberian Peninsulawill be provided the lower (main) core of the Mediterraneanoutflow in the in section 4.3.3, where the possibilityof a mean westward western Gulf of Cadiz [Boweret al., 1997;Arhan et al., 1994]. Mediterranean flow acrossthe Atlantic is investigated. In the meridionalsection at 9ø30'W(Figure 9a), whereMed- 4.3.2. The Mediterranean Water exit from the Tagus Basin iterranean Water has two local salinitymaxima, 36.15 psu at into the Horseshoe Basin. On the basis of observations from -35ø30'N and 36.05 psu at -34ø30'N, the isopycnalslope the Bord-Est 3 program,Daniault et al. [1994] suggestedthat indicatesa local cyclonicflow, assuminga shallowlevel of no Mediterranean Water flowsnorthwestward into the Tagus Ba- motion at •ro.s = 29.50. Similar elongatedsalinity signals, as- sin, circulatescyclonically, and then exitsthe basin southward, sociatedwith the bowlingof the isopycnal,can be identified in west of the Gorringe Bank. Float trajectoriesfrom AMUSE the zonal sectionsat 35øN and, substantiallydiluted, at 34030' [Boweret al., 1997] illustratethat this southwarddeflection out and 33ø50'N between -10030 ' and 8øW (Figures 10a-10c). of the TagusBasin is a preferred Meddy pathway.Our analysis This apparentcyclonic gyre is centeredat -35øN and 9øWand also finds evidence for a southward exit of Mediterranean has a diameterof ---250km (Figure 10c).Daniault et al. [1994] Water from the Tagus Basin, although this exit is believed to alsofound southwardgeostrophic flow along36øN, south of the be associatedwith an anticyclonicturning of the waters in the IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,997

d ½

4i000

Figure 9. Meridionalcross sections in the easternNorth Atlantic basin from 9030 ' to 12ø30'W.Contouring is asused in Figure6, exceptthat the minimumsalinity contour plotted with shading is 35.90psu. The solid lines designatethe isopycnals,as in Figure 6.

TagusBasin, rather than a cyclonicturning. Such a circulation exhibitstwo salinitymaxima, south and north of the seamount is supportedby the diagnosticmodel presented in part 2. Ev- that definesthe northernwall of the HorseshoeAbyssal Plain. idencefor the southwardMediterranean Water/Meddy path- We can associate the southernmost maximum with a westward waycomes from the climatologicalsignal of highsalinity cen- flowalong the southernflank of the GorringeBank, while the teredat ---15øW(Figure 10c) and at ---14øW(Figure 10b) along northernmostmaximum is assignedas its counterpartalong 34ø30'N.Such an exitcan explain these salinity maxima as well the northernflank of the GorringeBank. The convergenceof as the local salinitymaximum centered at ---11ø30'Won the these branchesinto a single branch centered on --•36øNis zonal sectionat 36ø15'N(Figure 12b). The southwardpene- evidenton the meridionalsections at 15ø and 18øW (Figures tration of this water is apparentlyinfluenced by the Ampere lib and 11c).The fate of this outflowbranch is of particular Bank(located along 35øN latitude), which has a seamountthat interestas it representsa potentialadvective pathway for Med- reachesto the Mediterraneanoutflow at 13øW(Figure 10c).A iterranean Water into the interior of the Atlantic, as suggested series of zonal cross sections south of this locale does not by Reid [1994].Our climatologicalanalysis, however, suggests exhibita continuoussignal of southwardpenetrating Mediter- that this branchdoes not penetratemuch past 22øW,as evi- ranean water. Thus we are led to believe that this southward dencedby the extremelyweak signal at 26øW(Figure lie) and exitis principallyreserved for Meddiesand doesnot constitute 30øW(Figure 11f). As mentionedearlier, it is believedthat a mean advectivepathway. thesewaters turn anticyclonicallywithin the Tagus Basin to 4.3.3. Westward penetration of Mediterranean Water into rejoin the branchof northwardflowing Mediterranean Water the North Atlantic. A branch of Mediterranean Water also near the EstremaduraPromontory. Evidence for this supposi- existsalong the southernflank of the GorringeBank, as de- tion is found in the crosssections of Figures11 and 12. The scribedin this section.The elongated36.0 psucontour on the elongatedtongue on 36ø15'N(Figure 12b) exhibitsa local 35ø50'N(Figure 12a) and 36ø15'N (Figure 12b) sections illus- maximum at 15ø30'W that can be associated with the local trate thisbranch. The meridionalsection at 14øW(Figure 11a) salinitymaxima also at 15ø30'Won the 36055' and 37ø15'N 25,998 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

zonal sections(Figures 12c and 12d). This signalsuggests that the branch of Mediterranean Water present along the southern flank of the Gorringe Bank turns cyclonically after -15ø30'W. The distributionof salinitymaxima on the meridional sectionsat 14ø, 15ø, and 18øW supportsthis view. The local salinitymaximum centered at -39øN on all thesesections is believed to indicate the return pathway of the westward branch defined by the local salinity maximum centered at •37øN. The westernlimit of this anticyclonicdeflection of the MediterraneanWater is seen at •22øW (Figure lid) as a single local salinity maximum of 35.7 psu between 37ø and 39øN.The crosssections west of this site (Figureslie and 11f) showa generallydiffuse salinity signal (35.5 psu) rather than a distinct core of Mediterranean Water. Our interpretation of this spatial changein the salt core is that the salinity signal carried westwardbeyond 20øW is mainly spread by diffusive processesrather than by a direct advectivepathway. Addition- ally, salt could be spreadwestward by Meddies asArhan and King's [1995] conceptual model suggests.Arhan and King [1995] found that turbulentmixing by mesoscaleeddies is the main causeof the observedlarge-scale westward penetration of Mediterranean salt. Our conclusionof no advectivepathway past •20øW for waters emanatingfrom the Strait of Gibraltar is in agreementwith the circulation estimatedby Maillard [1986]and by Pailletand Mercier[1996], and it is supportedby resultsof the diagnosticmodel in part 2.

4.4. The Fate of Mediterranean Water North of the Estremadura Promontory A sequenceof 10 zonal sectionsand six meridionalsections that span the easternNorth Atlantic basin north of 40øN are collectedin Figures13, 14, and 15. The climatologicalsalinity fieldsnorth of 39ø25'N(Figures 13a-13f) showa divergenceof the flow that could be associatedwith a divergenceof the isobathssouth of Galicia Bank. A comparisonof the salinity field at 39ø25'N(Figure 12f) to that at 40ø50'N(Figure 13a) showsa westwardspreading of the salinitysignal as this branch movesnorthward. Apparently, such spreading contributes to a bifurcation of the flow south of the Galicia Bank, as is evident in the sequenceof sectionsin Figure 13. The bifurcation,with branches on either side of Galicia Bank, is exhibited most clearlyin the 42ø50'N section.The offshore,western branch is visible between 15ø and 11ø30'W, with salinities of 35.8-35.85 psu,while the easternbranch remainstrapped along the continental shelf,preserving its higher salinityof more than 36.00 psu.The westernbranch of MediterraneanWater, more dilute than the along-shelfflow, is the climatologicalmanifestation of a branchobserved to the westof Galicia Bank byDaniault et al. [1994]. The weaker salinity signal carried by this western branch,relative to the shelfbranch, is possiblydue to its longer advectivepathway from the Strait of Gibraltar and/or due to strongerlateral mixing processes.North of the Galicia Bank (43006' and 43ø30'Nsections, Figures 13e and 13f, respective- ly), the salinityfield showsa convergenceof the westernand eastern branches into a wide vein. Figure 10. Zonal crosssections showing the cyclonicrecir- The zonal sectionat 44ø30'N(Figure 13f), whichfalls along culation of the Mediterranean Water, south of 36øN. The con- the northern shelf of the Iberian Peninsula, and the six merid- tour intervalsfor the backgroundwaters and for the Mediter- ional sectionswithin the Bay of Biscay(Figures 14a-14f) show ranean outfloware the sameas Figure 6 with 35.90 psu as the the eastwardturn of the Mediterranean Water into the Bay of minimum salinityplotted with shading.The solid lines desig- Biscay.West of Cape Ortegal (section9øW, Figure 14a) the nate the isopycnals,as in Figure 6. maximumsalinity signal is offshore.As the Cape Ortegal shelf protrudesinto the ocean (section 8ø30'W, Figure 14b), the Mediterranean Water vein is trapped again near the upper IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,999

'"' 11'x

30 3• 32 33 34 35 3• •C/ 3t• • 40 41} 4• 4S 44 45

Figure 11. Meridionalcross sections in the easternNorth Atlantic basin from 14ø to 30øW.Contouring is as usedin Figure9. The solidlines designate the isopycnals,as in Figure6. slope,flowing eastward along the northernIberian coast (sec- In the Bay of Biscay,north of 46øN(section 46ø06'N, Figure tions7055 ' to 5ø10'W,Figures 14c-14f). The dilution of the 15a, and section47øN, Figure 15b), salinitymaxima are found localmaximum salinity from ---35.9psu at 7ø55'W(Figure 14c), near the shelf at ---6øW and offshore at ---8øW and at 11- near the Iberian shelf,to ---35.8psu at 5ø10'W(Figure 14f) 12ø30'W.The isopycnalslopes in thesesections, from ---6ø to indicatesthe eastwardpenetration of the flow alongthe north- 8øW,are consistentwith a local cyclonicrecirculation of Med- ern Iberian shelf. iterraneanWater in the Bay of Biscay,if a shallowlevel of no 26,000 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

East e

40 -9 .,•

East --> East f

o ;35.8

-> East *-> East

Figure 12. Zonal crosssections showing Mediterranean Water pathwayswest of the IberianPeninsula. Contouringis asused in Figure9. The solidlines designate the isopycnals,as in Figure6.

(slow)motion is assumedat cro.5 = 29.50.It isbelieved that the thesesalinity maxima are isolatedand found only duringthe two salinitymaxima at 12030' and 11øWin the 46ø06'Nzonal early 1950s.Meddies in this vicinity are not as common as section(Figure 15a) resultfrom the presenceof Meddiesat those found to the south and west of Gibraltar, but some have theselocales. This suppositionis deducedfrom the fact that been noted. In a study of historicaldata, Richardsonet al. IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,001

• a 60'N

upper core of MW Rockall Plateau lower core of MW

reservoir of MW .. . 55ON preferredMeddy pathways

conjecturedpathways Porcupine Bank possible flow of non-Mediterranean water

50'N

45'N

IBERIAN PENINSULAI 40'N

I 35'N

AFRICA

• • " ! ß-. I -- • ,,,, ,,,, ,,,, • • • a25'N 35'W 30'W 25'W 20'W 15øW 10øW 5øW 0 ø Plate 2. Compositesummary of the Mediterranean Water pathwaysin the easternNorth Atlantic basin.

[1991] found 3 Meddles in the region boundedby 44ø-46øN psu at 47øN to 35.675 psu at 47ø30'N and to 35.65 psu at and 15ø-18øW.In regionsof low data density a mean may be 48ø30'N (Figures 15b, 15c, and 15d). Past 48ø30'N there is no skewedby the presenceof a Meddy. longer a strong distinct core of Mediterranean Water in the As the continentalshelf protrudes into the oceanbasin north climatologicalmean; yet a weak salinitysignal can be traced to of 47øN, Mediterranean Water penetratespoleward, confined ---50ø20'N,as shownin the 14øW crosssection (Figure 16b). along the continentalslope. The northern edge of the cyclonic This alongshoreclimatological salinity signal, found near Por- recirculation of Mediterranean Water in the Bay of Biscayis cupine Bank, is confirmationof the northward penetration of also identified offshore in this zonal cross section. Farther MediterraneanWater reported by Arhan et al. [1994]. north, the salinity signal can be traced along the continental The crosssections in Figure 16 also reveal a salinity maxi- slope up to 48ø30'N (Figures 15c and 15d). However, the mum to the southof the alongshoremaximum. The steepslope salinitymaximum is substantiallydiluted from more than 35.7 of the isopycnalsin the 11ø40'W meridional section (Figure 26,002 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

C f

4•

Figure 13. Zonal crosssections in the easternNorth Atlantic basin,north of EstremaduraPromontory. Contouringis asused in Figure6. The minimumsalinity contour plotted with shadingis 35.85psu. The solid lines designatethe isopycnals,as in Figure 6.

16a) associatedwith the two distinctsalinity maxima indicates the westwardbranch converges with the deep North Atlantic a possiblebifurcation of the MediterraneanWater as it flows Current, thus eventuallyentering the Rockall Channel. out of the Bay of Biscay.One branchis apparentlydeflected westward,while the other branch penetratesnorthward as a 4.5. The Mediterranean Outflow: A Cross-Isopycnal Flow boundarycurrent [Harvey,1982]. As will be demonstratedin Near the Strait of Gibraltar the isopycnalsurface fro.s = part 2, a diagnosticmodel of the easternbasin indicates that 29.50 is approximatelyat the depth of the interfacebetween IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,003

a d

44 45 4• 47 48

40OO

Figure 14. Meridionalcross sections in the Bayof Biscay.The contourinterval is 0.1 psufor thebackground watersand 0.05 psu for theMediterranean Water. The minimumsalinity contour plotted with shadingis 35.70 psu.Two additional shaded contours (35.65 and 35.675 psu) are used to showthe Mediterranean Water along the northerncoast of the Bay of Biscay.The solidlines designate the isopycnals,as in Figure6. the inflow of North Atlantic waters and the outflow of Medi- salinitymaximum is shiftedupward and centeredon fro.s = terranean Water. In the interior of the Gulf of Cadiz the main 29.90, indicatinga gain of buoyancyfor the Mediterranean core of the Mediterraneanoutflow (corresponding to the low- Water. Past this promontory,as the flow turns eastward,north er-salinitymaximum) is centeredon the isopycnalfro. 5 = 30.00. of 45øN,the salinitytongue is foundcentered on the isopycnal Downstream,north of the EstremaduraPromontory, the local surfacefro. s - 29.80.A changein isopycnalsfrom rrI = 32.23 26,004 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

46 ø 06'

->East

Figure 15. Zonal crosssections showing the fate of MediterraneanWater in the Bayof Biscay.The contour intervalfor the backgroundwaters is 0.1 psu.For the Mediterraneanoutflow the contourinterval is 0.025psu, and the minimumsalinity plotted with shadingis 35.7 (46ø06'N),35.65 (47ø and 47ø30'N)and 36.6 psu (48ø30'N).The solidlines designate the isopycnals,as in Figure6.

(1200dbar, equivalent to -0-0.5 = 30.03)to oh = 32.12(1000 bottom at -600 m. Downstreamof the SpartalSill, the clima- dbar, equivalentto -0-0.5 = 29.92) for the maximumsalinity tologicalmaximum salinity is 38.00 psu.The outflowof Med- anomalyassociated with the lower core of the Mediterranean iterraneanWater maintainsa fairly constantthickness of -250 Water is alsodescribed by Arhan et al. [1994].They suggesta m, from 6030' to -6ø50'W, but by 7ø10'W it has thickenedto relationshipbetween this changeand the presenceof the un- over 500 m. The maximum salinity decreasesfrom 38.00 to derlyingLabrador Sea Water north of 44øN. 37.60psu by 6ø50'Wand to 37.00psu by 7ø10'W.In additionto a fresheningand thickeningof the plume as it movesdown- 5. Summary stream,the plume gainsbuoyancy and, by 7ø10'W,splits into two connectedbut distinctcores. Our analysisof the climato- A summaryof this climatologicalanalysis, given below, is logicaldata supportsthe hypothesisthat two distinctcores of aidedby a schematicof the pathwayof the salinitysignal from outflowevolve through differential mixing within the Gulf of its source(Plate 2). The schematicrepresents the overallpath- Cadiz, one of the mechanismssuggested by Baringerand Price way of the climatologicalMediterranean salinity signal based on an analysisof historicalhydrographic data. It is essentially [1997].The sourceof freshwaters involved in the horizontal, a compositeof the 46 sectionsshown and discussedin this cross-isopycnalmixing process appears to be tropicalwaters, paper.As sketched,the MediterraneanWater entersthe Gulf advectednorthward along the African continentalslope into of Cadiz throughthe Strait of Gibraltar,below 150 m. It rap- the Gulf of Cadiz at 550-650 m depth. Additionally, it is idly sinksas it turnsnorthward, flowing along the continental suggestedthat the upperlayer of MediterraneanWater verti- slopeas a bottomtrapped vein, lying between -400 m andthe callymixes with the inflowingNorth Atlantic Central Water, IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,005

cyclonicrecirculation that is centered at ---34ø30'N, 9ø30'W and is ---250km in diameter.This recirculationacts to spread the salinitysignal south of 34øN. From Cape St. Vincent a part of the Mediterranean outflow turnsnorthward and entersthe TagusBasin through Zenk and Armi's [1990]gateway. Within thisbasin the climatologicalflow diverges,with part of the water flowing northward along the Iberian continental slope and the other part deflected westward. The westwardflow is dividedby the Gorringe Bank into a southernand northern component.The collectivewestward branch turns anticyclonicallywithin the Tagus Basin, creat- ing a reservoir of Mediterranean Water. After the anticy- clonic turn this branch rejoins the northward, shelf branch of Mediterranean Water west-northwest of the Estremadura Promontory. From the EstremaduraPromontory the Mediterranean Wa- ter penetratesnorthward along the continentalslope and di- b vergesdownstream into two branchesat Galicia Bank. North of Galicia Bank, the two branchesconverge and mainly turn eastwardinto the Bay of Biscay,following the northern Iberian slope. The Mediterranean Water branch reaches Porcupine Bank after it has penetratedpoleward along the continental shelf and after it has been partially recirculatedin the Bay of Biscay. Its saline signal is tracked as far north as 50ø20'N. Finally, we speculatethat south of the Porcupine Bank, a branch of Mediterranean Water is deflected westward until it convergeswith the North Atlantic Current, while the rest of it continuesits northwardpenetration as a boundarycurrent into the Rockall Channel. Additional support for this hypothesis will be providedin part 2, which providesresults from a diag- nostic model of the ocean circulation in the eastern North Atlantic basin. The main contributionof this work is the placementof the Mediterranean outflow signalin a broad temporal and spatial Figure 16. Meridional cross sections showing the fate of context. The strength of the climatological signal lies in its Mediterranean Water south of PorcupineBank. The contour spatialcontinuity. Though this continuityis compromisedto a interval for the backgroundwaters is 0.1 psu. For the Medi- degree by nonuniform data density,the overall signal is not terranean outflow the contour interval is 0.025 psu and the seriouslyaffected. The fields shownhere are resolvedwith a minimumsalinity plotted is 35.575psu for 11ø40'Wand 35.525 grid spacingof no more than 0.5ø , far exceedingthe expected psu for 14øW.The solid lines designatethe isopycnals,as in lengthscale of climatologicalflows. Specific main contributions Figure 6. from this work are the verification of the northward penetration of the Mediterranean Water into the Rockall Channel and the suggestionof their possiblepenetration poleward over the sill of the Wyville-ThomsonRidge. Additionally, this analysis, while the deeperlayer mixeswith the watersto the south.It is in conjunctionwith the diagnosticmodeling work presentedby supposedthat this differential mixingof the upper and lower Iorga and Lozier (submittedmanuscript, 1999) suggeststhat layersof Mediterranean Water could produce the two clima- the westward penetration of Mediterranean Water past tologicalcores. ---20øWis principallydiffusive on climatologicalscales. As the Mediterranean Water flows along the continental shelf from 7ø25' to 7ø40'W, three salinity maxima, at ---400, 900, and 1200 m, are found. It has been suggestedthat these Appendix cores result either from strong mixing processesassociated with a jet-like branch of MediterraneanWater [Baringerand The climatologicaldata were grouped into four time peri- Price, 1997;Zenk, 1975], from mixinginduced by a swift main ods, each containing2 decades.Data coverageat each grid in branch [Zenk, 1975], or from topographicsteering [Baringer a given crosssection is indicated by a gray shade that corre- and Price, 1997]. By Cape St. Maria the Mediterraneanwaters spondsto its measurementdate. Becausethe time periodsare have convergedagain to two verticalsalinity maxima centered superposedon each other (beginningwith the most recent at ---800and 1200 m. These corescorrespond to the two cores period), some data are partially covered. A data coverage of Mediterranean outflow extensivelymentioned in the litera- sectionis provided for each crosssection shown in section4 ture [Baringerand Price, 1997;Bower et al., 1997; Ochoaand either in Figures A1, A2, or A3. Color imagesfor all figures Bray, 1991; Daniault et al., 1994]. The climatologicalsalinity can be accessedat www.eos.duke.edu/Faculty/Lozier/lozier. field in the western Gulf of Cadiz containsa signatureof a html. 7o25 ' W

B •

FigureA1. (a) The temporalcoverage of the crosssections presented in Figure6. The two-decadetime periodsare color coded as follows: 1970-1990, solid; 1950-1970, dark shading; 1930-1950, light shading; and 1910-1930,medium shading. The dots have been sized to giveadditional information on the temporal span of the data. The size of the dot indicateswhen the measurementwas made relative to the first measurement (smallestdot) and the latest measurement (largest dot) in each section. So that the data from one time period donot cover data from another time period the largest dots have been plotted first and the smallest last. (b) Sameas for FigureAla butfor Figure 8. (c) Sameas Figure Ala butfor Figure9. (d) Sameas Figure Ala but for Figure 11. IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,007

-9600

4OOO

-,S -,7 -16 -,S -,4 4* .,2 -, -m -9 a -* -•

-> East

4 I; -14 43 -12 11 -10 -9 ß -7 4i A -1,47 -I, -->Ea6t

i - i - i - i - i ß i ß

=> Nod1 ->

Figure A2. Sameas FigureA1 but for Figures(a) 10, (b) 12, (c) 13, and (d) 14. 26,008 IORGA AND LOZIER: MEDITERRANEAN OUTFLOWmSALINITY AND DENSITY FIELDS

ary of the subtropicalNorth Atlantic, J. Phys.Oceanogr., 12, 384- 387, 1994. Ambar, I., A shallow core of Mediterranean Water off western Por- tugal, Deep Sea Res.,Part A, 30, 677-680, 1983. Ambar, I., Seismeses de medicoesde correntes,temperatufas e salin- idadesna vertente continentalas largo da costaAlentejana, Tech. Rep. 1/84, 46 pp., Groupo de Oceanogr.,Univ. de Lisboa, Lisbon, Portugal, 1984. Ambar, I., Seismeses de medicoesde correntes,temperatufas e salin- idadesna vertente continentalPortuguesa a 40øN, Tech.Rep. 1/85, 39 pp., Groupo de Oceanogr.,Univ. de Lisboa, Lisbon, Portugal, 1985. Arebar, I., and M. R. Howe, Observations of the Mediterranean outflow, I, Mixing in the Mediterranean outflow,Deep SeaRes., Part A, 26, 535-554, 1979. Armi, L., and N. A. Bray, A standardanalytical curve of potential temperatureversus salinity for the westernNorth Atlantic, J. Phys. Oceanogr.,24, 1295-1316, 1982. Baringer,M. O., Mixing and dynamicsof the Mediterraneanoutflow, Ph.D. thesis,224 pp., Mass.Inst. of Technol./WoodsHole Oceanogr. Inst. Joint Program,Cambridge, 1993. Baringer, M. O., and J. F. Price, Mixing and spreadingof the Medi- terranean outflow,J. Phys.Oceanogr., 27, 1654-1677, 1997. Barton,E. D., The polewardundercurrent on the easternboundary of the subtropicalNorth Atlantic, in PolewardFlows Along Eastern OceanBoundaries, edited by S. J. Neshybaet al., pp. 82-92, Spring- er-Verlag, New York, 1989. Bogden,P., R. E. Davis, and R. Salmon,The North Atlantic circulation: Combiningsimplified dynamics with hydrographicdata, J. Mar. Res., 51, 1-52, 1993. Bower,A., L. Armi, and I. Ambar, Lagrangianobservations of meddy formationduring A MediterraneanUndercurrent Seeding Experi- ment, J. Phys.Oceanogr., 27, 2545-2575, 1997. Daniault, N., J.P. Maze, and M. Arhan, Circulation and mixing of MediterraneanWater west of the Iberian Peninsula,Deep Sea Res., Part I, 41, 1685-1714, 1994. Harvey, J., 0-S relationshipsand water massesin the easternNorth Atlantic, Deep Sea Res.,Part A, 29, 1021-1033, 1982. Hill, A. E., and E.G. Mitchelson-Jacob,Observations of a poleward- ... ':•:?.:;;:.:.:} flowingsaline core off the continentalslope west of Scotland,Deep Sea Res., Part I, 40, 1521-1527, 1993. Hinrichsen,H.-H., and M. Rhein, On the origin and spreadingof the shallowMediterranean Water core in the Iberian Basin,Deep Sea Res., 40, 2167-2177, 1993. 11040 ' W Hogg, N. G., A least-squaresfit of the advective-diffusiveequations to Levitus Atlas data, J. Mar. Res., 45, 347-375, 1987. Iorga, M. C., and M. S. Lozier, Signaturesof the Mediterranean outflow from a North Atlantic climatology,2, Diagnosticvelocity . fields,J. Geophys.Res., this issue. Lavine,M., and M. S. Lozier, A Markov randomfield spatio-temporal analysisof oceantemperature, Environ. Ecol. Stat., in press,1999. .: , Lozier, M. S., W. B. Owens, and R. Curry, The climatologyof the ß::: North Atlantic, Prog. Oceanogr.,36, 1-44, 1995. Madelain,F., l•tudehydrologique aularge de la P•ninsuleIb•rique, Cah. Oc•anogr.,19, 125-136, 1967. Madelain, F., Influencede la topographiedu fond sur l'6coulement m6diterran6enentre le D6troit de Gibraltar et le Cape St.-Vincent, 14 ø W Cah. Oc•anogr.,22, 43-61, 1970. Maillard, C., Atlas hydrologiquede l'AtlanticNord-Est, report, 32 pp., B _>• Inst. Fr. de Rech. pour l'Exploit de la Mer, Brest, 1986. Maz6, J.P., M. Arhan, and H. Mercier, Volume budgetof the eastern Figure A3. Sameas Figure A1 but for Figures(a) 15 and (b) boundary layer off the Iberian Peninsula,Deep Sea Res., 44, 1543- 16. 1574, 1997. Ochoa,J., and N. A. Bray, Water massexchange in the Gulf of Cadiz, Deep Sea Res.,Part A, 38, suppl. 1, S465-S503, 1991. Paillet, J., and H. Mercier, An inverse model of the eastern North Acknowledgments. Supportfrom the National Oceanicand Atmo- Atlantic general circulationand thermoclineventilation, Deep Sea spheric Administration (NA 46GPO184) and the National Science Res., Part I, 44, 1293-1328, 1996. Reid, J. L., On the contribution of the Mediterranean Sea outflow to Foundation (OCE-96489) is gratefullyacknowledged. the Norwegian-GreenlandSea, Deep Sea Res., Part A, 26, 1199- 1223, 1979. References Reid, J. L., On the total geostrophiccirculation of the North Atlantic Ocean: Flow patterns,tracers, and transports,Prog. Oceanogr.,33, Arhan, M., and B. King, Lateral mixingof the MediterraneanWater in 1-92, 1994. the eastern North Atlantic, J. Mar. Res., 53, 865-895, 1995. Richardson,P. L., M. S. McCartney, and C. Maillard, A searchfor Arhan, M., A. Colin de Verdi•re, and L. M•mery, The easternbound- meddiesin historicaldata, Dyn. Atmos. Oceans,15, 241-265, 1991. IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,009

Siedler, G., Die Haufigkeitsverteilungvon Wasserartenin Ausstrom- nean Water off the Portuguesecontinental slope, Deep SeaRes., 37, bereichvon Meeresstrassen,Kiel. Meeresforsch.,24, 59-65, 1968. 1805-1823, 1990. Tomczak, M., and P. Hughes,Three dimensionalvariability of water massesand currentsin the CanaryCurrent upwellingregion, Meteor M. C. Iorga, Department of MechanicalEngineering and Material Forschungsergeb.,21, 1-24, 1980. Science,Duke University, Durham, NC 27708. Wooster,W. S., and J. L. Reid, Eastern boundarycurrents, in The Sea: M. S. Lozier, Earth and Ocean Sciences,Duke University,Durham, Ideas and Observationson Progressin the Study of the Seas,vol. 2, NC 27708. ([email protected]) edited by M. N. Hill, pp. 253-280, Wiley-Interscience,N.Y., 1963. Zenk, W. On the Mediterranean Outflow west of Gibraltar. Meteor Forschungs-erge,Reihe A, 16, 23-34, 1975. (ReceivedJanuary 8, 1998;revised February 15, 1999; Zenk, W., and L. Armi, The complexspreading pattern of Mediterra- acceptedFebruary 24, 1999.)