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Laboratory Simulation of the Gyre in the Alboran

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Laboratory Simulation of the Gyre in the Alboran VOL. 84, NO. C7 JOURNAL OF GEOPHYSICAL RESEARCH JULY 20, 1979 Laboratory Simulationof' the Gyre in the Alboran Sea 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 Gibraltar 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 the densityimbalance, 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 Mediterranean Sea is 2.5 times larger than the most liberally estimated Rossby radius. saltier than the water in the Atlantic Ocean and that the The purpose of this paper is to describesome laboratory resultingdensity imbalance at the Strait of Gibraltar 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 Africa, 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 Spain 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
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