VOL. 84, NO. C7 JOURNAL OF GEOPHYSICAL RESEARCH JULY 20, 1979

Laboratory Simulationof' the Gyre in the Alboran

JOHN A. WHITEHEAD, JR., AND A. R. MILLER

WoodsHole OceanographicInstitution, WoodsHole, Massachusetts02543

A laboratoryexperiment is describedwhich appearsto exhibitflows which are similarto the flow- counterflowin the Strait of and, for certain valuesof the parametersinvolved, to the gyre and front in the westernA!boran Sea.The experimentis transientin natureand is madewith two connecting basinson a rotatingturntable. A slidingdoor is fitted into the channelconnecting the two basins.Each basin is filled with water, the door is closed, and salt is added to one side so that the two waters have differentdensities. After the watershave spun up to restin the rotatingframe, the dooris opened.A flow, drivenby thedensity imbalance, is observed shortly thereafter, the lighterfluid risingup overthe heavier fluid andpushing into the basincontaining the heavyfluid. Likewise the heavyfluid pushes into the basin containinglighter fluid. For very rapid rotation theseflows are violentlyunstable. For lessrapid counterclockwiserotation both currentsstay confined to a narrowjet whichclings to the right-handwall of the basinswhich they are entering.At somelower rate of rotationthe jet cannothold to a sufficiently curvedwall, and thejet separatesfrom the wall--a gyreis observedbetween the jet and the wall. The gyre and thejet initiallyare botha Rossbyradius in size,but graduallythe gyregrows larger. Growth of the gyreseems to resultfrom an accumulationof fluid from thejet asit returnsto thewall. Scalingarguments and estimatesof buoyancy,Coriolis, and wind forcesare advancedin supportof the conceptthat this laboratory-producedgyre and the gyre in the A!boranSea share the samedynamics.

INTRODUCTION ually to the right with a radiusof the Alboran gyre, which is It is well known that the water in the is 2.5 times larger than the most liberally estimated Rossby radius. saltier than the water in the Atlantic and that the The purpose of this paper is to describesome laboratory resultingdensity imbalance at the createsa experimentswhich offer an explanationfor the formation of distinctdensity-driven flow. Mediterraneanwater flows out of the Mediterranean near the bottom of the connectingpassage, the gyre,the reasonwhy it is largerthan a Rossbyradius, and while Atlantic water flows near the surface into the west- some observationsof interaction of this gyre with the sharp, ernmost basin of the Mediterranean Sea--the Alboran Sea. cold jet which borders it off the Spanishcoast betweenthe The water of Atlantic origin is easily identified as having strait and Malaga. salinitylower than 37.5%0.A curiousfeature of the water in the There weremany reasons for conductingthis study. For one Alboran Sea is that the Atlantic water covers much of the thing, the presenceor absenceof the gyreis stronglyinfluential A!boran Sea (at least the western portion)' to a depth of upon the water masscharacteristics of the water in the A!bo- ran Sea.If the gyre werenot thereand thejet of Atlantic water approximately200 m. Figure 1, from Lanoix [1974], shows were to veer southwardalong the coastof , for instance, salinity in a north-south section at 4ø00'W. The water of the salinitystructure of the westernAlboran Seawould be very Atlantic origin lies in an anticyclonicgyre which hasa clearly differentfrom the presentstructure to a depth of 200 m. Since identifiablesalinity and temperaturefront on its northernside. there is presentlylittle overlapbetween our understandingof Figure 2, also from Lanoix, is perhapsthe bestknown illustra- ocean dynamicsand our present knowledgeof water mass tion of the gyre itself. It is over 100 km wide and roughly circular. characteristics,it is hoped that our small study can slightly bridgethis gap, at leastfor this very specificproblem. The gyre tends to move about, and there has been some debateabout whetherit occasionallyvanishes or reverses.It is Another reasonfor studyingthis problem lies in the impor- known, for instance,that the currentsin the Strait [Lacombe, tant influencewhich the gyre has upon nutrient upwelling, lateral advection, and frontal structure within the A!boran 1961; Lacombeet al., 1964] and the Alboran gyre and frontal Sea. Not only are these features important for a variety of systemare somewhattime dependent[Donguy, 1962; Grousson biological and environmentalproblems, but they might also and Faroux, 1963; Cano Lucaya and DeCastillego,1972; La- stronglyinfluence the dynamicsof deepercurrents, which may noix, 1974; Ovchinnikovet al., 1976;Cheney, 1978a, b]. How- have some fundamental influenceupon the deepwatercharac- ever, it appearsthat the gyre is more or less a permanent feature of the western Alboran Sea. teristics of the western Mediterranean, as suggested,for in- One might not predict the existenceof such a gyre from stance,by Stomrnelet al. [1973]. dynamicalprinciples because mechanics of rotating fluids The front is intenseenough to be at the far limit of the quasi- would lead one to believe that the Atlantic water would veer to geostrophicapproximation, where nonlinear terms are impor- tant. Hence numerical or analytical approachesare particu- the rightas it enteredthe Alboran Sea and wouldhug the coast larly challenging.A quasi-steadyexperiment was found to be of Africa as a jet with a width of approximatelya Rossby perfectlytractable and appearedto be a usefulstarting point radius, which is about 20-40 km. The jet would be able to for studieswhich may be planned in the future. The experi- curve around the African shoreline with approximately the ment is describedin the next section.Further experimentswith same length scale. Indeed, a jet of Atlantic water doesexist, with a size of approximately35 km, accordingto Cheney geometricallyrealistic basin shapes suggest that the interaction of the jet with the African coastlinemay produce a simple [1978b], but it lies off the coast of and bordersthe Alboran gyreon the northernand easternsides, curving grad- mechanismfor stoppingthe growthof the gyreand ultimately may determinethe final sizeof the gyre. It will be shownthat Copyright¸ 1979 by the AmericanGeophysical Union. two dimensionlessnumbers govern the flow and that when Paper number 9C0464. 3733 0148-0227/79/009C-0464501.00 3734 WHITEHEADAND MILLER: ALBORAN SEA GYRE

SOUTH NORTH o (a) •

200']J :5001 4001 5001 ." ß •- 1 (b) 30;m. 700• 10001 1:;'001 1500-1

Fig. 1. North-southsection of salinityat 4ø00'W(data taken from Lanoix [ 1974],Figure 11). (c) thesenumbers are closeto the onesappropriate for the Albo- ran Sea, the laboratoryjet is approximatelythe same size (suitablyscaled) as the real cold jet in theAlboran Sea and the laboratorygyre is the samescaled size as the gyre in the Alboran Sea.Finally, somecomparison is madebetween the Fig. 3. Sketchof the threebasin geometries which were used in the presentideas and various features of thecurrents in theAlbo- experiment. ran Sea. the edge of the sliding gate at one end and connectedto a THE EXPERIMENT straightartificial wall at the other,as shownin Figure3a. The The2-m diameter turntable at WoodsHole Oceanographic second had a straightchannel 30 cmlong which then joined Institutionwas fitted with a watertightdividing wall along its quarter-circlearcs which joined a straightwall, as shown in centralchord so that the basinwas separated into two equal Figure3b. In both of the abovegeometries the openingwas semicircularbasins. The bottomof the tank wasflat and level 10.2 cm wide. The third had styrofoamwalls which were to I mm.At thecenter of thisdividing wall a verticallysliding carvedto give a roughapproximation of the AlboranSea- watertightdoor was fitted so that when the door was opened, Straits-Bay of Cadizshoreline, reduced so that 1 cmin the watercould pass from one basin to theother. When the door laboratoryequaled about 6 kmin the Alboran Sea, as sketched wasclosed, the two basins were essentially isolated. Artificial in Figure3c. The openingwas 3 cmwide, which is slightly wallswere also placed in bothbasins. Three artificial wall widerthan the (scaled) width of thenarrowest opening of the geometrieswere used, as sketched in Figure3. Thefirst had Straitof Gibraltar. wallswhich were quarter-circle arcs of radiusr connectedto All experimentswith the geometries shown in Figures3a

SPAIN ,

• 37o

!., • 36 ø

•c=ApE"/O

0,6ø 05 ø 04 ø 03 ø 02 ø 07 ø lOIVGITUDE WEST Fig.2. Dynamicheight in centimeters inrelation to200 dB for the Alboran Sea (data taken from Lanoix [1974], Figure

25). \ Wmv•.H•.^O^NO MJ•.•.•.R: A•.BO•^N S•.^ Gv•tE 3735

Fig.4. Surfacecurrents, asmarked bystreak photographs offloating pellets, which demonstrate thefour principal ways theflow was observed toevolve. Width of the channel is10.2 cm. Salty water, representing theMediterranean water and dyedblack, lies at the top of the picture. The first column shows very rapid rotation where there is violent instability. R - 2.9cm. The second column shows rapid rotation where the flow went unstable and generated small gyres which were propelleddownstream. R - 5.8 cm. The third column shows intermediate rotation, where no gyre was observed. R - 7.3 cm,and the radius of the wall is 7.6 cm. The fourth column shows slow rotation, where one distinct gyre was observed to build up. R - 14.5 cm, and the radius of wall is 15.2 cm.

lONG (b ) SHORT (a ] and 3b (hereafterreferred to as geometries3a and 3b) were conductedas follows:the slidingdoor was opened,and the G G J G G G entiretank was filled with water at restto a depthof 5 cm.The artificial walls were not sealedwatertight on the bottom and sides, so the area behind the artificial walls was also filled. The watertightsliding door wasclosed, and 150g of salt (sodium G G J G G J chloride)and 50 ccof saturatedpotassium permanganate solu- tion wereadded to oneside. This wasthoroughly mixed into the water. Specialcare was taken to insurethat water behind the artificialwalls wascompletely mixed with the waterin the d d i i d main basin. Hydrometerand refractometerreadings were I d i i i taken routinelyto checkthe resultingproperties of the water. In experimentswith geometry 3c the water was also 5 cmdeep, and 47 g of salt and 50 cc of saturatedpotassium per- I I I I I I manangante were added in the 26-1 . The resultingsolution is 0.195%solute by weightand has a density difference of 0.0014 + 0.0001 from the fresh water in the 1. I I i I 1 6 15.2 50.4 7.6 15.2 50.4 adjoining basin. r, cm. r, cn•. Fig.5. Summaryof resultsfor runs with geometries 3aand 3b. G Next, the turntable wasbrought to the desiredrate of rota- indicatesthat a gyrewas observed, J indicates that a jet was observed, i tion and wasleft for at least25 min.This is manytimes longer indicatesthat weak instability was observed, so the jet retainedits than the spin-uptime of 167 s, which correspondsto the integrity,and I indicatesthat strong instability was observed. slowest-usedrotation rate. In orderto aid thespin-up process, 3736 WHITEHEAD AND MILLER: ALBORAN SEA GYRE

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...... ;:•?:::.•.::.:•.,::....-,•-**:;-•---:•.::----.-:--.-;:;..5::•-' ' .•-.*..-•-....:.,:;.':'.;::'"*'*•'":'*::•;:•;'::...•-•***';4-•:]...:..:?: ...... 2 I 4, , I I i ! 10] •0 i 40 ß.:.:: :.::,...y:--•-,?.-;::.;..•?:•...... '..' /•o, 6'/77. -•.:ß .. ;:•-•:•:**..•,•: '-.. ,,.•. .:.: ,....-:.--*--%::;:}:..: Fig. 6. Size of the jet Rj in centimetersas a function of Rossby :' ,•::..? .•:,-.....:•.,. :::...... radiusR0 = (gAoh/o]•)m. Error is +30% owingto causesmentioned ß:•:•:.½•.;;.***;'• ,....-/::•:-. "-'.'ß.....ß 7'-:-;;;; ...... in the text. ..,::;*:.':"•:.... ';'"•• ?'"'•:.• ...... '.• :-•* :..: ... . "•; ..,•;ß...... r-:...... "..' ,.:-.:"C'- .,.',..-:½-*:;:" ß"' ::.:

...... the water was stirred for a few moments before the turntable ,f•:•-.;.•-:•...... :f::::.':::::'•;.,..:...... • :..•-.,.•"•::;•,: -. .½'•*.".-..•::;,,,,:: •.....:... ,,,., :.:- . ..,...::.,.-•.•'%;::::,::,•.:,•:,.:½,..;,;.:::'•;.•>.,.... was started.In this way the interior fluid possessessome initial •;::'.'• ';*-. % ...... ,.••4 .;*.... ., ...... 4:,':,,'-'-:-•* ::•:•: ,;.:.•...:...... 8:..•?,• :...•:...::. ::,,::.::. vorticity so that the rotating spin-upmechanism, as discussed, .i::,•:•;•,:..:'*' .....-...... : ß., ,,,•, ,'•'...... ,. }.. •...... •..-.½.,:,:.:•_.•, •-.;...... ,.,•, -..... •..,..,.:•.,...... -- ,- ...... --.--,.... for instance,by Greenspan[1968], will immediatelyoperate, :?::.-:.,•"*':..•:.; ...... ,,.,.•*•,;• .,::-:•,•.•;.:.:.:.:::,..,.,....•..:.: , ;,.-: and the usual problems of spin-up from rest, where the fluid .:'::"'-"*'•: ..... ' '*"'"-'"-**--:• '*'...•'c-'- ":'*":a:/'""/•(; :".'-*'L'"":-'*:•;::"': •,-•'. initially has no vorticity, will not be encountered.Before re- :•:..:•:.;:;•--::,..... ß,,-...,--.-':-;••**.'?.'"'"':: ,,:..:., , .'.,,,..... -':"'•-:::'•.:-..;•".•;;-..' ...,. ;,::.::;•:.•... ,.:.:;•,...,:, ...... f-'.::::,f'*:•.:..::....:....:...... • •'::•.½,':'*d½ moving the barrier a small amount of dye was injectedinto :-:}4'-'.;..-;;:....:::.•*,; ,•*•:,:,-.;-...:...:-;. •...... : '•. . .•...... :-.,•.?./,:<.½.• •:.:** .•.. ?. •., ,.•: ::,.'•-'.;.?'.':.L'.... "': :"::.:--::';:,;...... ?::':::::'•';':'?:'?"..;.':.....'::'")r::::...... *'-'.:..._....:.:....,.;-..2::::.' :.-::*:,:• •" *'..,,,.:. each side in order to double check whether there was any ..." •:,'::-"½' ,..•.;:..,, ', ...... ,-,•:, ....C.;:.(>,.:.,-.--:.•.::•:.:?;...... ,;.6.?..-....--• .,.½e, remnant motion. None was found for any experiment.Next, . ß ... -...... • .... ß;..•:.•;: , -../.. :...,. --& ...... ,.:::..,:.:::-*-:::-..:&•..._.,•- .. small paper pellets were scattered over the surfacesof both Fig. 8. Streak photographsshowing the developmentof a new basins.The experimentbegan when the door was drawn up stagnationpoint and subsequentfebranching of the jet. Rotation and completelyremoved from the tank. Immediately there- period is 140 s; times aRer initiation are 215 and 322 s. after a 6-s time exposurephotograph was taken by a camera that was rotating synchronouslywith the table by meansof a of 14, 28, 35, 70, and 140 s; artificial barrier radii r of 7.6, 15.2, servo system. Subsequentphotographs were taken at fixed and 30.4 cm. Experimentsfor geometry 3c were conductedat intervals. Experimentsfor geometries3a and 3b were con- rotation periodsof 10, 14, 17, 20, 28, 35, 56, 110,and 220 s and ductedfor each of the followingparameters: rotation periods also at zero rotation.

RESULTS

x xß•l Xo - Scaling x II ooo ß o o + First, the experimental observationson geometries 3a and 5O x ß + - ß o + x ß 3b will be reported. In order to more easily visualize the o o+ + ß - various dynamicsinvolved, all resultswill be scaledas follows: q- the natural length scale associatedwith the density-driven current is the Rossbyradius, R = (gAph/pf2)•/2, whereg is force of gravity, Ap is the density differencebetween the salty and freshwater, h is a suitablychosen depth of the water,and f is 2 times the angular rotation rate of the fluid. This not only colnesout of scalingarguments but arises from density-driven flow-counterflow through the Strait [Whitehead et al., 1974, _ equation6.6]. In the experiments,Ap, g, andp werefixed and f was varied. The value of h was not so easilypicked, sincedepth

_ actually varied with radius owing to centrifugal force. The

I i i i i i i i I i i i i valuesof R shownhere are presentedusing a valuefor h of 5 20 loo 500 cm. Fluid depth actually gets as small as 4.0 cm at the center TIME, SECONDS and 6.0 cm near the outer wall for the fastest rotation rate, and Fig. 7. Length (parallel to the artificial wall) and breadth of gyres one might prefer to usea value of R basedon the true depth at as a function of time for three experiments.For a period of 143 s (R = the center(or someother depth). If one usesa value of h at the 29 cm), crossesdenote length and plussesdenote breadth; for a period of 70 s (R = 14.4 cm), open circlesdenote length, and open squares center, values of R should be reducedby 11, 5, 3.5, and 2.5% denotebreadth; for a period of 73 s(R = 14.5cm), solid circlesdenote for rotation periodsof 10, 14, 17, and 20s, respectively.Longer length and solid squaresdenote breadth. periodshave correctionsless than 2.5%. These correctionsare WHITEHEADAND MILLER: ALBORANSEA GYRE 3737 all small enoughfor us to feel they are unwarrantedin the reproducible,and mostwere run more than once,but all one presentwork, whereR spansa rangeof 10. There are three can say at presentis that the transitionfrom G to J occurs lengthsin the experimentalgeometry: width of the opening, approximatelywhen the wall radiusis lessthan R. radius of the artificial wall, and outer radius of the tank. To determinewhether the width of thejet wasscaled to the Experimentalparameters were selectedso that the first two Rossbyradius, measurements were made of the width. The geometriclengths were sometimes greater and sometimesless data are shownin Figure6. To getthese data, the width of the than the Rossbyradius. The outer radiusof the tank was jet as determinedby the streakphotographs was measured alwaysgreater than the Rossbyradius (except for the onerun with an opticalmicrometer in two or threeplaces that were at zerorotation). consideredtypical. The measurementsare not extremely precise owing to a ResultsforGeometries3aand3b numberoffactors and should only be believed to +30;, as Theobserved flows were easily divided into three categories. given by thesample error bars in Figure6. Theprincipal factor If rotationwas sufficiently rapid so that R wasless than the causingimprecision is the decisionas to exactlywhich streak width of the connectingchannel, a sharpshear zone would line constitutesthe edge of thejet. Also,in caseswhere the jet form withinthe channel,which would go violentlyunstable separatedfrom the wall it wasdifficult to determinewhich was and developsmall turbulenteddies (of both rotationalsign jet and which was gyre. Furthermore,the experimentwas andof Rossbyradius size), as shown in Figure4, firstcolumn. transient,and the front or noseof the jet was consistently Therewould be little transportfrom one basin to another,and observedto be greaterthan the fully developedjet. In spiteof theradius of the artificialwall wouldnot affectthese results. If theseproblems the data in Figure6 exhibita consistentten- rotationwas less rapid so that the Rossbyradius was greater dencyfor thejet to becomeRossby radius in width.The data than the width of the opening,light freshwater wouldflow for the largestRossby radius could only be taken after a intothe salty basin as a narrowjet propagatingalong the right- substantialgyre had formed,and it may not be correctto hand wall, as shownin Figure4, third column.The heavy comparethem too closelywith the other data, whichwere dyed,salty water would behave like thistoo andwould wedge takenbefore a substantialgyre had formed. alongthe right-handbottom of the freshwatertank. The Measurementswere taken of thelength of fiveclear particle wedgeswould be veryclose to a Rossbyradius in width.The trajectoriesin all experimentswith a clearjet. Therewas no dividingline betweena jet flow and a turbulentbreakdown systematic change of thevelocity in thejet withrotation rate of wasrather indistinct in theseexperiments. The secondcolumn the turntable,and the velocity averaged 1.9 cm/s with a stan- in Figure4 showsa weakinstability in whichone eddy formed darddeviation of 0.8 cm/s.Inertial-rotational theory such as but was transporteddownstream. If the Rossbyradius was that of Whiteheadet al. [1974] predictsthat velocityis of equalto or lessthan curvature of the wall,the jet wouldhave magnitude(gAt•hl•)2/2, which for thisproblem has a magnitude no troublestaying next to the wall. However,in othercases, of 2.6 and is consistentwith the measurements.Since the jet usuallywhen the Rossbyradius was greater than wall curva- is observedto scaleas Rossbyradius, the Rossbynumber of ture,the jet wouldseparate from the curvingartificial wall, as the jet basedupon jet width is closeto 1, a resultconsistent shownin Figure 4, fourth column.In this casea steadily with Rossbyadjustment ideas. growinggyre was observed in the separationregion, the gyre It is clearfrom the sequenceof photographsin the fourth initiallybeing a Rossbyradius in sizeand largerthereafter. columnof Figure4 andother flows where a gyrewas generated The growthof the gyreappeared to be producedby gradual that the gyrewas initiated by separationof thejet from the accumulationof water,caused by splittingof theseparated jet wall but thatonce the gyre was initiated it continuedto grow. as it impingedon the wall. The right-handbranch (looking Figure7 showsmeasured length (right to left) and breadth downstream)would flow backtoward the opening,where it (topto bottomin photos)of thegyre as a functionof timefor wouldbe deflected to itsright and curl around again and again threecases in whicha gyrewas clearly identifiable. Measure- to forma gyrewhich lies between the wall and the jet. The left- mentswere made from contactprints of the streakphoto- handbranch would continue to flowalong the artificialwall graphswith an opticalmicrometer. The gyreslooked in all andgo left andflow alongthe outerwall aroundthe whole caseslike theone in Figure4, fourthcolumn. The sizeof the tank until it finallyreturned to the opening,where it would gyrestarts out greaterthan a Rossbyradius in extentand flowalongside the jet of clearwater coming in fromthe open- increasesas the squareroot of time,which implies that the ing.At thispoint the experiment was terminated, although the volumeof thegyre is growingat a constantrate. subsequentevolution of the currentscould have readily been Examination of the float streak lines in the photographs observed. revealedthat the jet split into a right- and left-handbranch as Figure 5 shows a summary of the results of the runs. In it impingedon the artificial wall. It is possiblethat the stagna- points labeledG a steadilygrowing gyre was clearly observed tion point dynamicshave quite a lot to do with the evolution to form as in Figure4, fourth column.J denotesthat a jet, as in of the gyre. A clue to this effect comes from the sequence the third column of Figure 4, was observed.I denotes an shownin Figure 8, in which the gyre got so big that it beganto instabilityso violent that the originaljet washardly discerned, graze the outer radius of the tank. As soonas this happened, as in the first column of Figure 4. The letter i denotesan the jet developeda new stagnationpoint on the outer wall, instabilityless violent, so thejet retainedits integrity,as in the which dramatically rebranchedthe flows. secondcolumn of Figure 4. There was at least one eddy that These experimentstherefore have produceda plausiblede- wasswept downstream. The exactvalues of R and radiusof scriptionof theinitiation and growth of a gyrenext to a strait thewall whichdenote the transitionfrom G to J are beyond whichconnects water masses of differingdensity. They suggest the scopeof theseexploratory experiments and may be func- thatif Atlanticwater were to veersouthward as it passedPoint tionsof the width of the opening,Ekman number, detailed Almina,the jet wouldseparate from the coastof Africaand roughnessof thewalls, and perhaps even details of theway the wouldsubsequently impinge upon the coastof Moroccoand gate is opened.'Inthese experiments the resultswere clearly branchinto an eastwardand westwardflowing current. The 3738 WIm'EI4E^I) ^NI) M•LLEn: AL•On^N SE^ GvnE

: ..

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,.••. '"":;; -"•'•::'.?•,:::.,•. '•7.•.'!::•.:. - • ß.::?,--•:•.•:.....• ...... -•;...;.'.,"' .':•'• ß .• %.. .,,•:...,•...---. ,•.:.:,...: !•."- •. :. .',•,. ;,,.3*'.:•:.' "*.," "."'.'::. ."::".:"::'•'•'" ':.-.•,, f•',, " • ':•%:-4'•.•'';•:

...... -.:? , , ..3... • , • ..•. * "•.,..•'"•• :•. •,. ".:-•..,• &• ...... ½.•'';.•':-•-, •" ': ' "t.':• ½ ,.•:::•..-;.•..:....•.•:;•.,...... -::' ' :.:• ?::."' .•,...... '•'"'.':•.',"...... ½•s•mc.:•,.ß ..': •.'::7•:: .,. *•' , %,.-. ,,•' ,-•• • •,.• ;..• • ,• ...... ß':'..½•;-:'::-'U;•;.:•?•:•:½•:..•.. • e,---,.::•.....;--,,..•e::•..;;-':;'-::-:.•, ' ,e,. •, '•' '"( .:...,;•.. , '::"•"'"•;'::, '(•'•½•;•::•;•;::•',*•E½;;'.:•;:;.•½:::•:•:..:•.."..• ;:'.•;'•-•.•%' ,•. ' '":'•?••-.•.•.•'•"'..•½'•"..%•....;: '"'.:• •;5• '• • • • ,"• . •.c .--:,.....•; :':5:?A.;• •. •..; -.;...... ::•:½:•. .•:..:•... •.;...... • • ½*• • ....•:• ß.... e, • ½ .,.. ":':...... '•":"."•:.":.:•:•": .•..:"'•..•;'.(•-(I ½•'"/•;•' ,½ • •: • ;• •..•...... %'½ • .... :•..... •:,:;'•:.• <. ;..•:•.. • ...... ,•, ...... ::•...... ½'•:½'W•.?•:" • •.•.•,'"•: '•'•:;•.•.: ....•...... :,,,::..½:... ., ..;;5:'• ....:..•-•....•,•,•:;;;;; •:.'.•: .•:.;..::...:,.:'.':..• ...'.•.:.:•;::•;•,...:• (:'•.• "'';•:•:. ":½•Z•:•. ., ::.. Fig.9. Sequenceofstreak photographs ofthe formation ofa gyrein an experiment witha shorelinemodeled after the BayofCadiz-Strait of Gibraltar-AlboranSea . Scaled Rossby radius is 35 km. Timesafter start are, from left to right and top to bottom, 28, 196, 532, and 952 s.

eastward flowing branch would continue into the Mediterra- impingeseast of a topographicfeature that correspondsto nean,but the westwardflowing branch would beginto fill the CapeTres Forcasin ,at whichpoint the capeappears region between the jet and the coast of Africa with water of to deflectthe entirejet eastwardso that the gyre-fillingprocess Atlantic origin and initiate a graduallygrowing gyre. is terminated. One might envisionthat such a shoreline-in- ducedflow is even stabilizing,since if the gyre were to get R esultsforGeometry 3c biggerand expandfurther to the east,the capewould scoop In the previousexperiments the gyrecontinually grows until out some of the fluid in the gyre and send it eastward, thus it fills a substantialportion of the basin. Becauseof this, one reducingthe size of the gyre. might expectthat a largeportion of the westernMediterranean As a caution, one should note that has a would be substantiallycovered by Atlantic water. This is not long shallow shelf due north of Cape Tres Forcas. This shelf --the case,however; the gyreseems to remain confinedto that doesnot cometo the surfacebut only protrudespartially into portion of the Alboran Sea west of Alboran Island. The fol- the bottom of the Atlantic gyre.The experimentshere do not lowingset of experimentsindicates that thegrowth of thegyre incorporatethat shelf and cannot provide any information stopswhen the easternportion of the Alboran Basinis filled. It concerningthe role of that shelf exceptto say that the gyre suggestsone mechanismby whichthe continuedgrowth of the apparentlycan form without its presence. gyrecan be curtailed.In theseexperiments the coastalgeome- The parametersused in the run shownin Figure 9 were as try of theAtlantic and eastern Mediterranean was modeled by follows:density difference, 0.0014; rotation period,28 s;water styrofoamland areas.It was necessaryto smooththe Cape depth, 5 cm. This yieldsa Rossbyradius (gAph/pff) •/• of 5.84 Tres Forcaswhich projects up from the southto avoidsepara- cm. Whiteheadet al. [1974] found that this parametricgroup tion of the noseof thejet of Atlanticwater in early stagesof specifiesthe width of a two-layershear zone, and the data in the experiment.The resultingcoastline more nearly resembles Figure 6 show that jet width follows this. At the scale of the 100-mbathymetric contour. The growthand evolutionof a 1 cm -- 6 km, 5.84 cm representsa 35-km shear zone. Since gyre in one experimentis shownin the sequenceof photo- Cheney[ 1978b,p. 4597]gives a typicalwidth of the coldjet as graphsin Figure9. It is evidentin thephotographs that thejet 35 km, it is felt that this experimenthas the parameterswhich beginsto follow a sequenceof gradualgyre growth similar to optimize the modeling of the Alboran Sea-Strait of Gibraltar that shownin Figure4, fourthcolumn. That is,the jet initially dynamics. veerssouthward but separatesfrom the African coasteast of Figure 10 showsthe flowsin experimentswith other ratesof the strait.As it reimpingeson theAfrican coast, the jet bifur- rotation after havingrun for 10 min exceptfor the run with a catesso that water beginsto accumulatein an anticyclonic scaledR of 12 km, which had small, tightly wound eddies gyrethat forcesthe jet northward.This continuesuntil thejet whichare difficultto seein the picture.For R - 17km thegyre WHITEHEADAND MILLER: ALBORAN SEA GYRE 3739

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Fig.10. Streakphotograph offlow in experiments with a modeledshoreline and with different rotation rates. Owing to camerajiggle in somepictures, the photographs were selected for quality (lack ofjiggle) rather than selected at a consistent time,so times vary from 5 to 12min after the start of the experimental run.Scaled Rossby radii are, from top to bottom on the left, 17, 22, 25, and 45 km' on the rightthey are 70, 138,and 276km. 3740 WHITEHEAD AND MILLER: ALBORAN SEA GYRE

- i • I ' ' ' I I i I i ' ' $'1 showsthat initially the gyresgrew quite rapidly, but whenthe gyres attained approximately 25 km and rotation period was .* lessthan 111 s, they essentiallystopped growing; at least the • n & ß, • &•O•OO rate was far lessthan a t•/•' power law. A line with slope•} is .+• shownfor Comparison. Only experiments with periods of 111s or longerexhibited gyres which continuedto grow. The most striking result is that the Alboran gyre grew until it attained approximately the same size for the rather wide range of rotation rates correspondingto Rossby radii of 21-70 km, too •ooo while those with larger Rossby radii continued to grow. The Fig. il. Siz• of th• gyresas a functionot tim• for runs in th• reason for the gyre not exceedingthis size is undoubtedly •xp•rim•ntal g•om•try whichduplicates th• coastline.On• cm in th• connectedwith the geometryof the basinin someway, but it is laboratoryscal•s to 6 km in the ocean.Scaled R in kilometersis: dots, not entirelyclear which featureof the geometryis mostimpor- 21: opensquares, 2S; triangles, 3S; pluss•s, 4S; opencircles, 70• solid tant. If our conjectureis correctthat the positionof the stagna- squares,138: solid circles,276. tion point with respectto coastline featuresis central to the growth mechanismof the gyre, then possiblythe geometryof doesnot form; instead,the jet goesunstable to smallturbulent the African coaststrongly influences the final sizeof the gyre; eddieswhich are approximatelya Rossbyradius in size. For it is interestingthat the gyre alwaysgrew until the southward R = 21 and 25 km there was a gyre and, in addition, lots of flowing jet on the eastern side of the gyre impinged on a smallereddies in the Mediterranean.The photographfor R -- feature which correspondsto Cape Tres Forcas in Morocco. 35 km was already shown in Figure 9 and has a gyre in the DISCUSSION A!boran Sea and another further to the east. For R = 45 km therewas a gyrein the Alboran Sea and anothervery weakone Observationaldata which hint that there is indeeda stagna- further to the east. For R = 70 km there was one gyre in the tion point and a bifurcationof the flow in the regionnear Cape A!boran Sea with no gyresto the east. For R = 138 and 276 Tres Forcas do exist. A persistent westward current exists km there wasa gyre whichextended substantially further into along the coastof Africa east of Cape Tres Forcas, according the Mediterranean.One experimentwas done at zero rotation. to the study of nearshorecurrents as deducedfrom photo- It differed little from the flow with a scaled shear zone of 276 graphic analysesby Stanley et al. [1975]. km. The presenceof one gyre in this casesuggests that the The trajectory of drifting buoy M arisond B03 of the Labo- obliquity of the axis of the strait playssome role in the weak ratoired'Oc6anographie Physique, Paris, supplied by P. Tch- rotation gyre. ernia (personalcommunication, 1978), is shownin Figure 12. Figure 11 showsthe sizes,as definedbelow, of the gyresas a This is also shown,in modifiedform, by Petit et al. [1978]. It function of time for all the experimentalruns, with the ex- shows a well-developedgyre in which the buoy made one ceptionof the two runs in which Rossbyradius was smaller completecycle and then hoverednear the proposedpoint of than the width of the strait. These were omitted because small impingementnorth of Cape Tres Forcas for approximately turbulencewith gyresgoing in both directionswas observed, 4 days. almost exactly as shown in first column in Figure 4, and no Another observation that is consistentwith a stagnation gyre existedwhich was steadyon a time scalelong in com- point lying north of the capeis the satelliteinfrared imageryin parison to the circulationtime of the water going into the Figure 13, taken June 11, 1978. It showsan anomalous tem- basin. Measurementswere taken with an optical micrometer perature distribution from the usual well-definedgyre - and were made of the distancebetween the place correspond- served from satellites.Here the jet of Atlantic water skews ing to Point Aimina and the outer edgeof the gyre. Figure 11 eastwardand then bendsto impingeand deflectleft and right

37 ø

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, •• . • __ h . • .... '• 9(P• o•...... :'... 35•06 • 0 • • 04I • 03I • ß 'l02 • ' ' O1• Fig. 12. Trajectoryof the drifting buoy MarisondeB03, whichindicates both the rapid speedof the gyre and the possibilityof a stagnationpoint north of Cape Tres Forcas.Data courtesyof P. Tchernia. WHITEHEAD AND MILLER: ALBORAN SEA GYRE 3741

the Coriolis parameter to be 10-4 s-•, and total force is easily calculated to be 2.1 X 10•ø N. What wind velocity would be neededto exert this amount of force?Using a drag coefficient of 2 X 10-3, which appearsto be a typical value for moderate wind conditions [Bunker, 1976, p. 1124], a surface area of the eastern Alboran Sea waters of 10'ø m2, and approximating densityof air as I kg/m 3, we find that a wind speedof 45 m?sis necessary,which is absurd. The observedvalue of 8 m/s for wind speedis not only in the wrong direction but is more than I order of magnitudetoo weak to hold the jet along the coast of Spain against Coriolis force. Nof[1978] has recently suggesteda mechanismfor the de- flection of the Atlantic water to the left as it leaves the Strait of ..•.•::.•- ...... '...... •,.• Gibraltar. This mechanismhinges upon a theoretical predic- tion of adjustmentof a steadyfree jet in a rotating coordinate system.I t is predictedthat deflectionto the left will occurif the Fig. 13. Infrared photograph showing the splitting of the jet at entering current possessesa suitable shear acrossthe stream. approximately3øW. Photo courtesyof J. Gallagher. We cannot at this time determinehow stronglythe N of theory as it hits the African coast. A detailed though interpretive bearsupon our experiments,since they were transientand our constructionof surface circulation from Huang and Stanley control over the detailed velocity profile was nonexistent.We [ 1971] also showsa well-developedgyre which splitsat 3ø west can say, however, that the internal Froude number in our and indicates small transient eddies to the east of . experiments(and probably in the Strait of Gibraltar) attains a Modification of the horizontal velocityprofile of thejet as it value of 1, whereasthe analysisof Nor concernssmall Froude rubs againstthe coastof Spain is another possiblemechanism numbers.Both studiesemphasize the need for a clearerunder- which could influencethe ultimate sizeof the gyre. The effects standingof ageostrophicjets. of sidefriction upon a baroclinicjet, as in theseexperiments, is CONCLUDING REMARKS poorly understood.It has been known that the Spanishcoast experiencesstrongly variable currents[Stanley et al., 1975]and The experimentsclearly indicate that the initiation of a gyre that the front lies tens of kilometers south of the coast [Che- results from separation of the southern edge of the jet of ney, 1978b], hinting that turbulence and/or local interaction Atlantic water as it leaves the Strait of Gibraltar. The sub- with bottom topography may be important. The counterparts sequentgrowth of the gyre to a size many timesgreater than a of local eddiesnear Spain, however, were not observedin this Rossby radius of approximately35 km appearsto be linked laboratory model. with a dynamical filling processwhich is not clearly under- The above experiments assumethat the gyre is essentially stood, but it may be linked with the relative position of the produced by a buoyancy-driven process, while others have stagnation point of the jet with respect to coastal features suggestedits origin is in the wind field. where it impingeson the coastof Africa. The obliquity of the The thesisthat wind conditions promote the formation of strait axiswith respectto the axisof the Alboran Sea may play the Alboran gyre by forcing the jet northward to the coastof a role in this process.. Spain is at odds with prevailing wind conditions as measured The notion that the propertiesof the stagnationpoint region over the past 30 years.Mean windsby Miller [ 1977],as shown of a jet control the filling behaviorof that jet appearsto be new in Figure 14, illustrate that prevailing winds come from the in rotating fluid mechanics.We have beenunable to locate the northwest,consistent with the overall pattern of surfacewinds existenceof any studiesof the properties of stagnationpoint about the Iberian coast.This pattern changeslittle when bro- flows in baroclinic, rotationally influencedjets. The dynamics ken down into monthly means.Mean wind force varies from 5 of flows in the vicinity of stagnationpoints is not new in to 8 m/s, averaging about 6 m/s. Moreover, there is no in- nonrotating fluid mechanics,and interaction of stagnation dication of wind curl with inversemagnitudes of a few days, pointswith the basingeometry lies at the heart of the designof which would be necessaryto drive a gyre directly by curl if it fluidic amplifiers.The notion that theseprocesses control the were to spin the buoys, as in Figure 12. filling propertiesof large jets which are streaminginto large If anything, the mean winds probably promote a strong seasor lakes and determine whether one or another water type shear in the northern sector,along with some upwelling along the Spanish coast. In this area, actual wind conditions vary considerablyboth with location and season.The downpouring air flowing from the land to the south is probably influencing the Bay of Cadiz as well, since the mean pattern shows a divergence to the southwest west of Gibraltar and to the southeast east of Gi•a•ar. It would appear that the winds have insu•cient force to hold thejet away from the African coasteast of Point Almina. To see this, let us compare the total Coriolis force exerted upon a jet 35 km wide, 2• m deep, 100 km long, and flowing at 0.3 m/s, which is perhapsa slight overestimatebased upon thedata of Lanoix[1974]. This co•?sponds to newAtlantic water flowing into the Mediterranean and does not include Fig. 14. Averageannual surfacewind distributionof watersabout water recyclingin the gyre. For purposesof simplicitywe take • southwesternEurope, from Miller [1977]. 3742 WHITEHEAD AND MILLER: ALBORANSEA GYRE will be found on the surfaceof that body may be worthy of Huang, T. C., and D. J. Stanley, Western Alboran Sea: Sediment more field observationsand theoretical, laboratory, and nu- disposal,ponding and reversalof currents,in The Mediterranean Sea.' A Sediment Laboratory, pp. 521-559, Dowden, Hutchinson, merical modeling.Such a study is beyondthe scopeof this and Ross, Stroudsburg,Pa., 1971. laboratorystudy. Possibly some day, if theseideas are borne Lacombe, H., Contribution a l'&ude du regime du D&roit de Gibral- out by suchinvestigations, surface water typesin lakesand tar, I, Etude dynamique, Cah. Oceanogr.,13, 73-107, 1961. even seascan be controlled in a beneficialway by giant stagna- Lacombe, H., P. Tchernia, C. Richez, and L. Gamberoni, Second tion point deflectors. contribution to the study of the Strait of Gibraltar region (in French), Cah. Oceanogr.,16, 283-314, 1964. Lanoix, F., Project AIboran: Hydrologic and dynamic study of the Acknowledgments.Support was received under grant 04-8-M01- 117pursuant to the authorityof publiclaw 95-224,the FederalGrant AIboran Sea (in French), Tech. Rep. 66, N. Atl. Treaty Org., Brus- andCooperation Agreement Actof 1977,interagency agreement sels, 1974. B41706between theNational Oceanic and Atmospheric Administra- Miller, A. R., The sea surface wind and sea surface temperature field tionand the Department ofState, and the Treaty of Friendship and aboutIberia (abstract), Rapp Comm. Int. Mer. Medit., 24, 2, 1977. Cooperationbetween Spain and the United States,dated September Nor, D., On geostrophicadjustment in sea straitsand estuaries,The- 21, 1976.Photographs were taken and laboratoryassistance was given ory and laboratory experiments, II, Two-layer system,J. Phys. by RobertFrazel, whom we thank.Contribution 4290 of the Woods Oceanogr., 8, 861-872, 1978. Hole OceanographicInstitution. Ovchinnikov, I. M., V. G. Krivosheya, and L. V. Maskalenko, Anom- alousfeatures of the water circulationof the Alboran Seaduring the REFERENCES summer of 1962, Oceanology,15, 31-35, 1976. Petit, M., V. Klaus, R. Gelci, F. Fusey, J.-J. Thery, and P. Bouly, Bunker,A. F., Computationsof surfaceenergy flux and annualair-sea l•tude d'un tourbi!lonoc6anique d'echelle moyenne en mer interactioncycles of the North AtlanticOcean, Mon. WeatherRev., d'Alboran par emploi conjoint techniques spatiales et ocean- 104, 1122-1140, 1976. ographiques,C. R. Acad. Sci., 287, 215-218, 1978. CanoLucaya, N. C., andF. F. De Castillejo,Contribution to under- Stanley, D. J., G. Killing, J. A. Vera, and H. Shinz, Sands in the standingof the Alboran Sea:Variations in the anticyclonicgyre (in Alboran Sea: A model of input in a deep marine basin, Smithson. Spanish),Bol. Inst. Espan.Oceanogr., 157, 3-7, 1972. Contrib. Sci., 15, 51, 1975. Cheney,R. E., Wind-inducedmigration of the Alboran Sea anti- Stommel, H. M., H. Bryden, and P. Manglesdoff, Does someof the cyclonicgyre (abstract), EOS Trans.AGU, 59, 301, 1978a. Mediterranean outflow come from great depth? Pure Appl. Cheney,R. E., Recentobservations of theAlboran Sea frontal system, Geophys.,105, 874-889, 1973. J. Geophys.Res., 83, 4593-4597, 1978b. Whitehead, J. A., A. Leetmaa, and R. A. Knox, Rotating hydraulics Donguy,J. R., Hydrologyof the Alboran Sea (in French),Cah. of strait and sill flows, Geophys.Fluid Dyn., 6, 101-125, 1974. Oceanogr.,8, 573-578, 1962. Greenspan,H. P., The Theoryof RotatingFluids, 327 pp., Cambridge University Press,New York, 1968. (ReceivedJanuary 28, 1979; G rousson,R., and J. Faroux, Surface current measurementsin the revised March 19, 1979; Alboran Sea (in French), Cah. Oceanogr.,15, 716-721, 1963. acceptedM arch 20, 1979.)