THE AND SURROUNDING WATERS' AT THE CROSSROADS

T. Rossby Graduate School of Oceanography University of Rhode Island, Kingston

Abstract. The North Atlantic Current is a well-defined the Mid-Atlantic Ridge. The currentsalong the north- westernboundary current that flowsnorth alongthe east ward flowingfront are quite swift,with typicalmaximum side of the Grand Banks from 40ø to 5 iøN, where it turns averagespeeds in the upper300 m near1 m s-• (= 2 sharplyto the east and beginsits journey acrossthe knots). The current meanders almost as wildly as a ocean.The currentis uniquein transportingwarm trop- "snaking"river, but unlike steep meandersin the Gulf ical waters to much higher latitudes than any other Stream these meanders appear to be stable, and with westernboundary current and thusplays a crucialrole in one exceptionhave not been observedto break off to amelioratingthe climateof the Europeansubcontinent. form pools of warm and/or cold waters as frequently The North Atlantic Current originates in the Gulf occursin the .The meandersappear to be Stream when the latter curves north around the South- inducedby major topographicfeatures along the path of east NewfoundlandRise, a major submarineridge that the current,namely, the SoutheastNewfoundland Rise, stretches SE from the Grand Banks. A well-defined the NewfoundlandSeamounts, and FlemishCap. Strong front delineatesthe path of the currentas long asit flows recirculationsdevelop on the concaveside of the mean- north as a western . After the current ders.One of these,the "Mann eddy" at the first meander turnseast in the north, it broadensinto a wideningband crest of the North Atlantic Current, shouldbe regarded of eastwarddrift without a sharpor permanentfront in as a permanentfeature of the North Atlantic circulation. the senseof the eastwardflowing Gulf Stream after it Other meanders also contain recirculations that can separatesfrom Cape Hatteras.The North Atlantic Cur- persistfor months.Under certain conditionsthese can renttransports more than 40 Sv(1 Sv = 106m 3 s-1) in merge togetherto form an extendedSW flow (recircu- the southand about 20 Sv by the time it flowseast across lation) just east of the North Atlantic Current.

INTRODUCTION subpolarlatitudes. Thanks to this rapid advection,the temperature of the surface waters of these currents Southeast of Canada and the Grand Banks, some- almost always exceedsthat of the surroundingwaters thing happens to the Gulf Stream that has puzzled and overlyingatmosphere. Should this rapid poleward oceanographersfor years.Rather abruptlyafter it passes transportdecrease or turn zonal at midlatitudesas in the the 50øW meridian, it undergoesa transformationor other oceanbasins, the mean temperaturesof the North breakdown that has posed a formidable challengeto Atlantic and European landmasseswould drop precipi- describeand understand.Speculation has ranged from tously. the Gulf Stream'ssplitting into two or more branches,to The characterof these currentsvaries stronglywith the current switchingor alternating between different region. The Current, constrainedby southern directionsof flow, to its dissolutioninto a turbulent field Florida on one side and the Bahamas on the other, of eddy motion. While there hasbeen much controversy showslittle path variability. The Gulf Stream, on the in the past,we now knowthat a significantfraction of the other hand, after it separatesfrom the coast at Cape Gulf Stream turns north as the North Atlantic Current Hatteras, flowseast as a free meanderingjet. It entrains (NAC). This current is unique in transportingwarm waters from both sides until it reaches a maximum watersto much higher latitudesthan in any other ocean. transport of about 150 Sv [Hogg, 1992] near 60øW The uniquenessof the North Atlantic Current gainsin (1 SV: 106 m3 s-•; seethe glossary following the main significanceowing to its role in moderatingthe climate text). Southeastof the Grand Banks where the Gulf over the northernNorth Atlantic and Europeansubcon- Streamflows along the SoutheastNewfoundland Rise, it tinent [Krauss,1986]. undergoesa poorly understoodtransformation or split- TheNorth Atlantic Current, in broad perspective, isa ting process.In my view, the baroclinicflow in the Gulf crucial link in a seriesof connectedboundary currents Stream breaksup into two parts, a broad, perhapsdif- that swiftly(<< 1 year) transportwarm tropicalwaters to fuse return flow to the south and west feeding a recir-

Copyright1996 by the American GeophysicalUnion. Reviewsof Geophysics,34, 4 / November 1996 pages 463-481 8755-1209/96/96 RG-02214515.00 Paper number 96RG02214 .463 464 ß Rossby:THE NORTH ATLANTIC CURRENT 34 4 / REVIEWSOF GEOPHYSICS

5O 40 30 20

60 60

50 50

40 .... 40

3O

50 I0 40 30 20

Figure 1. The major currentsof the northernNorth Atlantic adaptedfrom Krauss[1986]. The left slanted shadingindicates the region where the warm waters of theSubpolar Front can be found.The shadingin the southrepresents waters of the subtropicalgyre. The numbersindicate the volume transportin sverdrups (l Sv - 106 m3 s-I). Abbreviationsare NAC, NorthAtlantic Current; SPF, Subpolar Front; IC, ; LC, ; EGC and WGC, the East and West Greenland Currents; and AzC, .Isobaths are drawnat 200, 1000,2000, 3000, and 4000 m. The topographicridge extendingSE from the Grand Banks is the Southeast Newfoundland Rise.

culation back toward the Gulf Stream at and down- venient below to refer to the eastward flow as the Sub- stream of Cape Hatteras [Hogg, 1992], and a polar Front. northeastward branch, the North Atlantic Current. Ad- The eastward flowing Subpolar Front marks the ditional waters break off, presumablyin the form of northernmostextent of the subtropicalcirculation sys- streamers,pools of water, or eddies,which drift away as tem. So far as is known,it is not a sharplydefined front part of a large-scaleeastward drift. In the easternAt- in the senseof the Gulf Stream after it separatesfrom lantic some of this drift concentrates into a shallow but the coast,but a limited swathor regionwhere the main well-defined baroclinic front known as the Azores Cur- thermoclineshoals to the surface,along which a stronger rent [Klein and Siedler,1989]. baroclinictransport is sustainedthan to either the south The North Atlantic Current flowsnortheast along the or north. From numerous drifter studies the eastward continental rise past Flemish Cap and continuesNW drift waters appear to be generallydifluent, with waters toward what has become known as the Northwest Cor- peelingoff to the southeastand northeast,some gradu- ner [Worthington,1976], where the current turns east ally or poorly defined, and some in perhapsstronger, toward the Mid-Atlantic Ridge (MAR). West of the transientflOWS. If there is a singlepermanent branch, it NAC along the continental shelf edge, the Labrador wouldbe the warmwaters continuing NE towardthe gap Current flows south toward the Tail of the Grand Banks, between Scotlandand Iceland. Figure 1, publishedby where some waters round the corner and continue west Krauss [1986], provides a schematicoverview of the while the greaterpart turn northeastand joins the NAC North Atlantic circulation.It is basedon a large number alongits inshoreedge. As a result,warm, salinewaters of of surfacedrifter observations obtained throughout the tropical-subtropicalorigin and very cold, fresh waters early 1980s.The meridionallyshaded area indicatesthe from the Labrador Sea come into direct contactalong a region of predominant!ynorth and eastwardflowing more than 1000 km front between the start of the North waters. In some ways, Figure 1 is a simplificationof Atlantic Current and the Northwest Corner, where the earlier descriptions[cf. Sverdrupet al., 1942;Dietrich et flow turns east at 50ø-52øN.Although many authors al., 1975]in whichthe circulationtended to be depicted refer to continuingflow to the east acrossthe Atlantic as in termsof discretebranches of flow. The descriptionsof the North Atlantic Current as well, we will find it con- the branchingvary significantly,suggesting that these 34, 4 / REVIEWSOF GEOPHYSICS Rossby:THE NORTH ATLANTIC CURRENT ß 465 most likely should be interpreted not as permanent THE NORTH ATLANTIC CURRENT currents but as indicators of what may really be an inherentlyvariable or restlessregion of the ocean.Most The debate about the origin of North Atlantic Cur- recently,Krauss [1995] has suggestedthat much of the rent haswaxed and waned over the years.The first paper watersentering the Nordic Seasmay do so by continuing to put the location and size of the current into clear north in the Irminger Current to Iceland and branching focuswas publishedby Iselin [1936]. Although he cau- to the SE toward the Faroe Islands(62øN, 7øW) rather tioned that the region was so complexthat many addi- than east along 60øN as is indicated in Figure 1. Al- tional data would be needed,his plot of the depth of the thoughthe North is often said to be the 10øCisotherm [Iselin, 1936, Figure 47] (shownhere as oceanwe know best, our knowledgeand understanding Figure 2) revealsalmost prescient similarity with more recent studiesof the region [Lozier et al., 1996]. The of its circulationare still very limited. turning north of the contoursat the Tail of the Grand The purposeof this articleis to reviewrecent progress Banks (the SE tip of the shallow Grand Banks) and in our understandingof the North Atlantic Current and rapid shoalingof the isotherm near 50øN indicate the its relationshipto the surroundingwaters. First, we sur- path of the current.The strengthof the currentincreases vey the major advancesthat have taken place in our in rough proportion to the slope of the temperature knowledgeof the region south and east of the Grand surface.The 10øCsurface represents reasonably well the Banks. This is followed by a first review of a major Pt = 27.2 densitysurface in the main thermocline.Only Lagrangian study of the North Atlantic Current and where that surface shoals to less than a few hundred neighboringwaters. A number of observationshave meters and waters come into contact with cold, fresh emergedthat we hope will lead to an improvedunder- waters from the north do the two surfacesdeviate sig- standingof this major yet elusivecurrent. We beginwith nificantly:the densitysurface does not shoal as rapidly. a few remarkson observationalmethods. Worthington[1962] publisheda controversialpaper in Most of our ideas and understandingabout large- which he argued that the North Atlantic Current does scaleocean circulation have been interpreted from data not originate in the Gulf Stream but forms a link in a collectedby the manyhydrographic sections and surveys separateanticyclonic gyre north of the main subtropical oceanographershave undertakenover the years. From circulation of the North Atlantic. Worthington con- these observations,which consistof vertical profiles of structed his "two-gyre" hypothesisto account for the temperature, salinity, and other properties, one can higheroxygen levels in the NAC than in the Gulf Stream computefor instancethe densityand hencethe pressure that presumablyfed it. Specifically,the higherdissolved field, from which the speed of the current can be esti- oxygen (02) levels suggestedrecent contact with the mated. The appeal of this technique, known as the atmosphere.He proposeda separategyre in which the dynamicmethod, lies in its simplicityof use: from two northwardflows shoalclose to the surfaceand presum- hydrographicstations separated a few tensof kilometers ablypick up oxygen,then turn eastand southand finally or more, one can determine the horizontal pressure back west toward the NAC. In supportof his two-gyre thesishe pointed to observationsof a trough extending differenceas a functionof depthand thusthe shape(not SE from the Grand Bankswith flows in oppositedirec- the absolutevalue) of the velocityprofile betweenthe tions to either side.At the time Worthingtonwrote his two stations. However, pressurevariations also arise paper, oceanographersgenerally portrayed the ocean from slightbut hitherto unmeasurablevariations in sea circulationin laminar, steadyterms. In a sense,they had level (<1 m). This means that our knowledgeof the little choice because the few observations available did strengthand variability of oceancurrents consists only of not permit characterizationof oceanvariability. More to that part observablein the hydrographicfield. In deep the point, however, oceanographershad constructed waters or in the north, where the waters have lost their over the yearswhat appearedto be quite robustdescrip- heat and the densityfield becomeshorizontally uniform, tionsof the mean circulationfrom widely spacedsurveys the dynamicmethod alone is unable to determine the (see Figure 2), thereby reinforcing confidencein the magnitudeand directionof flow. In suchsituations one hydrographic survey technique to meet their needs. often resortsto tracingwater properties(such as salinity, Ideas about eddies and their role in lateral mixing had oxygen,nitrates, tritium, etc.) to determinehow waters yet to becomepart of the working vocabulary. spreador might flow. Unfortunately,tracer techniques Well awareof Worthington'sstudy, Mann [1967]pub- have their own limitations because the observed distri- lished the results of two hydrographicsurveys of the butionsresult not only from steadyflow patternsbut also watersSE of the Grand Banksin an attempt to examine from mixing processes,which we cannot quantifywith- the questionof where and how the North Atlantic Cur- out independentobservations of how they operate.The rent takesshape. His surveysshowed a bifurcationof the paucity of direct measurementsof currents and trans- Gulf Stream with a southern branch forming, as he ports has greatlyincreased the challengeof putting our stated it, the NE boundary of the SargassoSea and a knowledgeof the oceancirculation and its variabilityon northern branch that fed into the North Atlantic Cur- a quantitativefoundation. rent, which also receivedwaters directly from what he 466 ß Rossby:THE NORTH ATLANTICCURRENT 34, 4 / REVIEWSOF GEOPHYSICS

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GH•LENGER ß ß *,• • ' / ' ' ' : I

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i

Figure2. The depthof the 10øCisotherm in the NW Atlanticaccording to Iselin[1936, Figure 47]. The Gulf Streamcan be identifiedby the closespacing of the isopleths(indicating rapid flow) from the FloridaStraits to SE of Newfoundland.Where the isoplethsturn north marksthe beginningof the North Atlantic Current. The sharpturn to the eastat 50øNis knownas the NorthwestCorner.

termed the Slope Water Current, a separateeastward [1936]paper. In it he reiteratedhis earlierbelief that the flowingcurrent south of the continentalshelf and north high oxygenlevels relative to the T-S propertiespre- of the Gulf Stream.The NAC appearedto be fed by two clude the possibilityof the North Atlantic Current's branchesof the Gulf Stream, therebysuggesting a more originatingin the Gulf Stream.He acknowledgeddiffi- complexsystem of bifurcationor splittingthan was as- cultyin constructinga transportfield or circulationthat sumedpreviously. Mann cited the presenceof an anti- maintainedconsistency with the observeddensity field. cycloniceddy in the NewfoundlandBasin as a sourceof Specifically,he couldnot find a returnflow to matchhis oxygen-richwaters (due, he implied,to wintertimecon- computedtransports in the NAC. However, the high vection)that couldmix with the North Atlantic Current oxygenlevels in the NAC impressedhim, sufficientlyso and therebyincrease the 02 levelswithout changingthe that he preferredto "violate"the dynamicmethod and temperature-salinity(T-S) characteristicsof the waters. recirculate waters from the NE rather than admit waters However, Mann's studydid not settle the matter. from the Gulf Stream in the SW, the diametricallyop- Worthington[1976] describedhis monographOn the positedirection. Perhaps he thoughtor hopedthat there North Atlantic Circulationas an update of the Iselin might existin his northerngyre a barotropiccomponent 34, 4 / REVIEWSOF GEOPHYSICS Rossby:THE NORTH ATLANTIC CURRENT ß 467 that could not be resolved or detected in the hydro- parable to that of Worthington's17 years earlier except graphicobservations. that they allowed for a northward continuationof the Clarkeet al. [1980] discussedin detail the resultsfrom Gulf Stream,specifically, the 12 Sv neededto supplythe a major three-shipsurvey of the region SE of the Grand waters that eventuallysink at high latitudes and return Banks that took place in spring 1972. They showed south along the deep continentalmargin as part of the conclusivelythat givena reasonableamount of isopycnal thermohaline overturningof the world ocean. Schmitz mixing, one could reconcilethe observedproperty dis- and McCartney[1993] did not offer any suggestionas to tributionswith a geostrophicflow patternof Gulf Stream how the northern gyre might be sustained.We can rule waters feeding into the North Atlantic Current. They out winds as a drivingmechanism. The southwardmass made an important clarifying point by distinguishing transport T in a wind-driven gyre can be obtained from between the mean fields of the SargassoSea and New- the Sverdrupbalance, which we approximatehere as foundlandBasin, which do differ in oxygenand T-S on a mean torque or wind stresscurl times the width of the same isopycnal,and the propertiesof fluid parcels that enter the Newfoundland Basin with the North At- the gyre divided by the planetaryvorticity gradient, or T = (V x ,)L/• where -r and X7x -r representthe wind lantic Current where fluid parcelsof Gulf Stream origin stressand its curl. Assuminga wind-driventransport of are found and noted that the NAC graduallyloses its subtropicalproperties (relatively low oxygenand high -25 X 109 kg s-•, an east-westscale L of 106 m, and salinity) downstreamthrough gradual mixing with the [3= 2 x 10-• m-• s-•, the requiredtorque would be fresherwaters inshore. Worthington's difficulty, they ar- -0.5 x 10-6 N m-3. Isemerand Hasse [1987, Chart 156] gued, consistedin adheringtoo strictlyto the conserva- show a negativewind stresscurl over the conjectured tion of water massproperties and not appreciatingor gyre, but the magnitudeis much lessthan that required allowingfor the role of lateral mixingprocesses that are to sustainthe supposedcirculation. We will return to importantin water massmodification. For manyphysical this questionof recirculationsin the discussion. oceanographersthe Clarke et al. [1980] study persua- Much instructiveinsight into the North Atlantic Cur- sivelyput to rest the hypothesisof two completelydis- rent and its origin comes from two synoptic surveys tinct Gulf Stream and NAC gyres. [Krausset al., 1987, 1990] of the Gulf Streambranching Whereas the aforementioned studies relied almost region and north to Flemish Cap. Both studiesinclude, exclusivelyupon the hydrographic surveys of water in additionto hydrography,direct measurementsof cur- massesand dynamic computationsof currents,Krauss rentsusing surface drifters drogued at 100 m. The drift- [1986] assumeda very differentapproach and used sat- ers add significantlyto the surveysby mappingthe spa- ellite-trackedsurface drifters to map out the circulation tial structure of the currents. Krauss et al. [1987] of the upper oceanof the North Atlantic. These drifters analyzeda set of 13 sectionsspanning the North Atlantic had their droguesattached at 100-m depth to minimize Current both southand north/eastof Flemish Cap. The drift due to the action of wind, whether directly or dynamictopography at 100 relative to 1500 (100/1500) indirectly through the wind-driven Ekman boundary dbar revealedan unusuallyextended high-pressure ridge layer. One objectiveof this major drifter programwas to (with superimposedpeaks near 42øN, 45øW, and 46øN, assess or confirm two distinct schemes for the North 38øW) extendingNE alongthe current.The decreasein Atlantic circulation, namely, those of Dietrich et al. dynamicheight east of the ridge implied a substantial [1975] and Worthington[1976]. In his analysisof 90 southwestwardtransport of water. Four transectsacross drifter tracks, Krauss drew two important conclusions the NAC SW of Flemish Cap showedabout 35-40 Sv with respectto the North Atlantic Current. First, Krauss could not confirm the elaborate scheme of Dietrich et al. movingnorth and 15-30 Sv returningSW [Krausset al., [1975] of permanent branch flows peeling off from the 1987,Figure 8]. The driftersadded considerable value to SubpolarFront both north and south.This conclusionis the studyby tracingthe structureof the eddyfield during significantbecause it underscoresthe transientnature of and after the conclusionof the survey.The drifter tra- flows at high latitudes and sends a cautionary signal jectoriesindicated that (1) the branchingat FlemishCap against"overinterpreting" enhanced gradients in hydro- must have a transientcharacter, (2) significantvolumes graphicsections as permanentcurrents. Second, Krauss of water recirculateto the SW, and (3) the eddyfield SE found no evidence for waters recirculatingback west of the NAC has a lifetime of several months. In the toward the NAC as required by the Worthington two- secondstudy, Krauss et al. [1990] took three long hydro- gyre hypothesis.His conclusionsreinforce those of graphicsections to enclosea triangularregion between Clarke et al. [1980] and gain in significancebecause of the Tail of the Grand Banks, Bermuda, and the Azores. the study's systematicuse of drifters to make direct They estimateda Gulf Streaminflow of 46 Sv relative to measurementsof fluid motion in the upper ocean. 2000 m, of which 31 Sv continued NE as the North More recently, Schmitzand McCartney [1993] par- Atlantic Current, with the remainderfeeding the Azores tially revivedthe two-gyrehypothesis in their review of Current and Gulf Stream recirculation.The trajectories the North Atlantic Ocean circulation. They gave the of numerousdrifters drogued at 100 m confirmed the hypothesizednorthern gyre a shapeand transportcom- transition into these two branches. 468 ß Rossby:THE NORTH ATLANTIC CURRENT 34, 4 / REVIEWSOF GEOPHYSICS

THE SUBPOLAR FRONT AND FLOWS ACROSS 60 THE MID-ATLANTIC RIDGE

In the mid-1980s, European oceanographers mounteda major coordinatedstudy of the North Atlan- 55 tic circulationin the general area of the Mid-Atlantic Ridge. The program,known as TOPOGULF, included arraysof current meters spanningthe ridge [Colin de Verdibreet al., 1989], long meridional hydrographicsec- 50 tions along each side of the MAR [Harveyand Arhan, 1988;Arhan, 1990], and surfacedrifter studies[Krauss, 1986]. The westernsection, from 53ø to 24øN,traverses the eastern side of the Newfoundland Basin at about 45 33øW.The high densityof hydrographicstations permits a detailed analysis of the property fields. A major strengthof the Harveyand Arhan [1988]study was their

systematicuse of isopycnalanalyses of the hydrographic 40 sections.Plotting one property such as density against pressuregives information on the dynamicalstructure, but other propertiessuch salt, 02, and potentialvorticity

often become more informative when plotted versus 35 density,since mixing and water massmodification in the -55 -50 -45 -40 -35 -30 -25 -20 interior of the ocean takes place along isopycnals,not lines of constantdepth. Harvey and Arhan identified Figure 3. Trajectoriesof five drifters droguedat 50-m depth three frontal regions which they labeled the Azores and deployedin 1989 by the InternationalIce Patrol (liP) to studythe motion of icebergsdrifting southfrom the Labrador Current (36øN),and two branchesof the SubpolarFront Sea. Each trajectorylasts 310 days.Small circlesare plotted at 47ø and 52øN,respectively. However, at none of these every10 days.The 200-, 2000-, and 4000-misobaths are shown can one find a correspondingwater property front or as thin lines.FC is FlemishCap. (CourtesyD. Murphy, liP.) changein stratificationexcept possibly very closeto the surface,which suggeststhat the fronts have a transient or evanescentcharacter (which may enhance mixing). As the NewfoundlandBasin. The IFM group concentrated Harveyand Arhan [1988] point out, a difficultywith any on gettinga wide area descriptionof surfacecurrents by single hydrographicsection consistsin distinguishing releasingclusters of drifters in different regionsof the between permanent and transient features. Thus, the North Atlantic. As was mentionedearlier, they drogued evidence for a permanent jet at 47øN remains very their drifters at 100-m depth in order to minimize the sketchy. [Krausset al. [1987] did not find it.] Many impactof the Ekman layer, i.e., the wind-drivensurface studieshave documentedthe SubpolarFront [cf.Krauss, waters.(As an aside,it shouldbe noted that the surface 1986] and the Azores Current east of the MAR [Ki•se water drift cannot be avoided altogether. During the and Sledlet,1982; GouM, 1985].Krauss [1986] viewed the winter months,when the mixed layer depth exceedsthe SubpolarFront as the only permanent(zonal) feature depth of the drogue,the drifter motion will reflect the with a concentratedtransport. sum of the directlydriven surfacewaters and the under- lying geostrophicmotion. The impact is not seriousin strongcurrents, but eastof the NAC, where the geostro- VELOCITY FIELD phic flow is weak, the southwesterlywinds might set the driftersfarther to the ENE owingto the direct actionof Direct observationsof the velocity field east of the the wind and Ekman drift than otherwise would be the Grand Banks have been collected by several groups. case.)The clustersreleased near or in the NAC on the These include the satellite-tracked near-surface drifters warm water side revealed rapid downstreamadvection by the Institut ffir Meereskunde(IFM, Kiel, Germany) by the current. Significantly,virtually none of the IFM [Krauss,1986; B•gge, 1995], the Woods Hole Oceano- drifters escapedwest or north from the NAC into the graphic Institution (WHOI, Woods Hole, Massachu- subpolargyre. Other clusters,launched within the New- setts)[Richardson, 1983], and the InternationalIce Pa- foundland Basin, show a clear tendencyto drift to the trol (IIP, Washington,D.C.) [Murphyand Hanson, east toward the MAR and beyond. Less well known, 1989]. Moored current meter studies include arrays each Spring since 1979 the IIP has deployeddrifters acrossthe MAR [Colin de Verdibreet al., 1989] and the (drogued at 50 rn to simulate the center of drag of Gulf Stream bifurcation area southeast of the Grand icebergs)in the LabradorCurrent to monitorand model Banks [Fofonoffand Hendry, 1985]. Schmitz[1985] and the motion of icebergs.As an example,Figure 3 shows Owens[1991] discusstheir work with SOFAR floats in 310-daytrajectories of five IIP drifterslaunched in 1989 34, 4 /REVIEWS OF GEOPHYSICS Rossby:THE NORTH ATLANTIC CURRENT ß 469 in the region 49ø-52øN, 49ø-51øW. All five drift east, 55 either directlyor after drifting southalong the continen- tal escarpmentbefore turning north on the inshoreside of the North Atlantic Current. Significantly,they remain inshoreof the NAC all the way to 50øN before turning eastaround the NorthwestCorner. In a studyunder way, M. E. Carr (personalcommunication, 1996) has com- bined all IIP and IFM drifters to examinepathways of 50 ...... motion and cross-NAC exchange.It appears that less than 10% of the IIP drifters cross over to the warm side of the NAC into the NewfoundlandBasin. Instead, they '?-'L>:'z:.'""",,-'"-*.,-,',••.. •., :•!i½' ::-"-.-:•D•!½!::•ii[½•;.--;.-_.':•i•!g::[:::•i?:gi[ii½[i•!i::i::i--'ii[::[i!?:[:i::;L :::: .,!::::::::!ii•:•!:_-_-'i- follow the NAC past Flemish Cap, north of which some drifters turn east while the majority continue to the Northwest Corner. There is very little exchangeof wa- 45 .--.::..-.::•-: .... '.!i• ":*'•i :•;;.'-'.'.4!• ters across the NAC near the surface. We have frequentlyreferred to the NorthwestCorner as if it were a self-evident feature of the North Atlantic Current. It can be seen already in Iselin's [1936] map (Figure 2). It showsup clearly in the North Atlantic mapsof temperatureand salinityat 200 rn publishedby ..::,... Fuglister[1954] and in the Lozieret al. [1996]climatology 4O of the North Atlantic. It doesnot appear in the Levitus -50 -45 -40 -35 -30 [1982] climatologyowing to the heavy smoothingem- ployed. The sharplycurved flow around the Northwest Figure 4. Eddy kinetic energy (EKE) at 50- to 100-m depth, shadedin incrementsof 250 cm2 s-2, constructedfrom 217 Corner emerges clearly in surface drifter trajectories drifters deployedbetween 1979 and 1992 by the International [Krauss,1986]. A recentpaper by Lazier [1994] on direct Ice Patrol and the Institut fdr Meereskunde(IFM) in Kiel, measurements of currents in the Northwest Comer Germany. The data have been groupedinto 1ø x 1ø bins. The clearly definesits NW limit. Using a SE-NW array of data were very kindly made availableto us by W. Kraussat the four equally spaced(57 km) current meter moorings IFM and D. Murphy at the IIP. The 200-, 1000-, 2000-, and (between51.8øN, 45.7øW, and 50.7øN, 44øW) and drifters 4000-m isobathsare shown.This figurewas prepared by M. E. (droguedat 100m), Lazier showedthat strongbaroclinic Carr. currentspass through the two SE mooringson at least three occasionsfor monthlongperiods whereas the two NW mooringsremain in cold quieter waters during the In general, Figure 4 mimicsrather well the surfaceEKE entire8•5-month observation period. These observations distributions computed from the Geosat mission[Le- limit the NW penetration of the Northwest Corner to Traon et al., 1990], but the drifter estimateshave not the SE of the third mooring at 51ø30'N, 45øW. Lazier been smoothed,and thus Figure 4 reveals more local estimatedthe volume transportacross the entire section structure along the path of the NAC. The EKE levels to be 50 _+ 23 Sv. The large variancemay result from exceedthose reported by Kraussand Kiise [1984]. We lateral shifting(and thushow muchof it passesthrough attribute this to the significantlylarger data set. Specif- the array) and variationsin the strengthof the current, ically,the IIP driftersthat drift southand turn north just whereasthe large mean may indicatesignificant recircu- inshoreof the NAC may accelerateto very high speeds lation of waters within the Northwest Corner as indi- comparedwith those launchedon the warm water side. catedby the near-circulartrajectories of severaldrifters. As part of the French-GermanTOPOGULF project The eddykinetic energy(EKE) in the regionwest of in 1983-1984, four clustersof current meters spanned the MAR exhibitsstrong spatial gradients.At the sur- the MAR along 48øN for 1 year to study cross-ridge face,EKE averages600 cm2 s-2 in the NAC andde- mean flows and eddy variability.The westernmostclus- creases to about <100 cm2 s-2 to the south and east ter at 35øW exhibited very high EKE levels: at 350-m accordingto Kraussand Kiise [1984],while at 700 rn the depththe averageEKE, 349cm 2 s-2, is almosta factor EKE rarelyexceeds 50 cm 2 s-2 fromthe SOFAR data in of 2 larger than the EKE at 50- to 100-m depth in Figure the NAC [Owens,1991]. Figure 4, showsthe EKE from 4. Colin de Verdi•reet al. [1989] found evidenceof an 217 IFM and IIP drifters deployedbetween 1979 and upgradient(i.e., toward warmer water) eddy heat flux 1993. A band of very high EKE followsthe path of the west of the MAR and converselyeast of the ridge. This NAC from the Southeast Newfoundland Rise in the flux, reminiscientof the upgradientfluxes south of Cape southto the NorthwestCorner near 51øN.All alongthis Hatteras noted by Webster[1961], could be accounted envelopethe EKE drops very rapidly and uniformly to for by a downstreamdecrease in amplitude of the me- the east. Even the baroclinicSubpolar Front, the con- andering of a baroclinic jet [Rossby,1987]. In the tinuationof the NAC, showsonly slightlyelevated EKE. TOPOGULF case, increasingtopographic control by 470 ß Rossby'THE NORTH ATLANTIC CURRENT 34, 4 / REVIEWSOF GEOPHYSICS

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6(Y'"W 55øW 5()"W 45øW 4ff'W

Plate 1. A compositeview (spanning2 weeks)of seasurface temperature from early May 1994using advanced very high resolutionradiometer (AVHRR) imagesfrom NOAA polar-orbitingsatellites. Note the wavy thermal front extendingNE from the troughat 42øN,50øW (compare Figure 2). This figurewas preparedby S. Schollaert.

the MAR might cause a decreasein meanderingof the the Gulf Stream south and the NAC east of the Grand eastwardflow as it crossesthe ridge. Banks.A tongue of cold water extendsSE from the Tail IR imageryof sea surfacetemperature (SST) east of of the Grand Banks along the SoutheastNewfoundland the Grand Bankshas providedless information than we Rise. This trough may correspondto the cold tongue have become used to elsewhere because of >80% cloud extending to the SE [Worthington,1962; Clarke et al., coverage most of the year [Peixotoand Oort, 1992]. 1980;La Violette,1982], and aroundwhich the northern Spotwise,the imagescan provide useful SST, but only branch of the Gulf Stream curves to the NW and the rarely do they offer spatial information. Thus they have beginning of the NAC. Warm waters can be seen ex- contributedvery little to our knowledgeabout the me- tendingto the north with sharplydefined meander crests soscalefield and its temporal evolution in the New- near 440-45ø and 46øN along the NAC. foundland Basin and southern Labrador Sea. In the Gulf Stream this limitation is circumventedby superimposing severalimages from a limited period of time, including RECENT OBSERVATIONS OF THE NAC only the warmest observedtemperature for each loca- tion. Using the same approach and compositing IR Over the last decade,our group here at the University images over a couple of weeks or even a month [e.g., of Rhode Island (URI, Kingston) has gained consider- Krausset al., 1990],one can find sequencesthat permit a able experiencein the use of Lagrangiantechniques to reasonablyclear view of the North Atlantic Current off studyocean currents.These subsurfacedrifters or floats the Grand Banks. Plate 1 showsa 16-day compositeIR can be adapted to a variety of applications.We have image (from early May 1994) of SST of the region used them in the Gulf Stream, a rapid, highlybaroclinic aroundthe Grand Banks.Note the strongmeandering of or surface-intensifiedflow, as well as in the deep, slow 34, 4 / REVIEWSOF GEOPHYSICS Rossby:THE NORTH ATLANTIC CURRENT ß 471 waters of the South Pacific. In baroclinic flows such as accuratelyan isopycnalsurface (1) by matchingits com- the Gulf Stream it had not been clear to what extent pressibilityto that of seawaterto within -0.5 +_2%, and water parcelsadvecting downstream move up and down (2) by usinga borosilicate(Pyrex) glass housing that has alongthe steeplyinclined isopycnals and to what extent a very smallcoefficient of thermal expansion(<10% of suchinclined motionsmight be stochastic,perhaps due water). The first requirementensures that no buoyancy to small-scalelateral stirringprocesses (a turbulentjet), or ballast resultsfrom a changein pressurethat would or systematic,reflecting the dynamicsof the meandering cause the float to rise or sink relative to the water, and current.To addressthese issues, we developedthe isopy- the secondrequirement ensuresthat the float remains cnal RAFOS float, which can mimic or follow the verti- on the samedensity surface regardless of any tempera- cal motion of fluid parcelsof a particulardensity [Rossby ture or salinity changethat might occur as a result of et al., 1985; Goodman and Levine, 1990]. From two small-scalemixing. This follows from the fact that nei- studies, one conducted in 1984-1985 and the other in ther salinity nor temperature variations can alter the 1987-1989, it became evident the lateral and vertical volume of the float. In practical terms, a float that motionscould be quite large and that thesedepend very downwellsacross the NAC 500 dbar with a compress- closely upon the curvature of the current such that ibility mismatchof 1% should follow the isopycnalto betweentroughs and crestswith path curvatureturning within 15 rn for a typical densitystratification of 0.1 (r, from positiveto negative,floats will alwaysupwell, and units/75rn or 0.2øCassuming 10øC/(r, unit. (One (r, unit viceversa between crests and troughs[Bower and Rossby, is equalto 1 kg m-3, wherethe densityof seawateris 1989; Songet al., 1995]. --103 kg m-3.) Cross-frontaland cross-gyre variations in We thoughtthe float technologymight offer an effec- temperaturegreatly exceedthis uncertainty,as we shall tive means to study the NAC and its interactionswith see. The floats "track" themselvesby listeningfor and the surroundingwaters. It is clear from the preceding storing in their microprocessormemories the arrival review that many questionsremain about the NAC. times of acousticsignals transmitted from four moored Could it be regardedas a simplyconnected front as we sound sourcestwice per day. The sourcesconsist of a had cometo view the Gulf Stream,i.e., would a parcelor resonantpipe projectordeveloped by Spartonof Canada float in the center of the current remain in it as it drifted and an electronics-powerpack driver developed by downstreamthrough several meander wavelengths,or Webb ResearchCorporation [Rossbyet al., 1993]. The shouldthe current and surroundingwaters be viewed as mooring deploymentsand first release of floats took a turbulent transition region between the subtropical place in July-August1993. This acousticnavigation sys- waters in the NewfoundlandBasin and subpolargyre tem covered most of the North Atlantic Ocean between farther north? In other words, to what extent does the 30ø and 60øN.The field programended in summer1995, NAC act to separatethe two water masses(the barrier when all four soundsource moorings were retrieved. concept), and to what extent might an energeticeddy Perhaps nowhere in the world ocean do waters of field promote exchangeof waters instead (the blender suchcontrasting properties as the Gulf Stream and La- concept)?Do waters leave the NAC preferentially in brador Sea water come into direct contact with each zonaljets ashas been conjectured (the SubpolarFront at other. To illustrate this, Figure 5 showsa sectionacross 52øN; a jet at 47øN east of Flemish Cap; the Azores the NAC at 43øNin four panels:density versus pressure Current), or is the lossmore gradual or stochastic?To and temperature,salt, and oxygen,the latter three plot- addressthese and related issues,since July 1993we have ted against density.The choice of at as the ordinate been trackinga total of 100 isopycnalRAFOS floatsthat highlightsthe property contrastsacross the current in havebeen deployedin the currentin severallocations as light of the fact that mixing, where it occurs,does so well as in both gyresto either side. We have had the preferentiallyalong isopycnalsurfaces. The deepening good fortune to work with R. A. Clarke of the Bedford isopycnalsin Figure 5a reflectthe strongbaroclinicity of Institute of Oceanography,who hasa programof repeat the current.The strongestslope of the (rt surfacesat 275 hydrographyof the NAC betweenthe Tail of the Grand km indicatesthe approximatelocation of the axisof the Banksand FlemishCap. Thanksto hisprogram, we have current. The temperaturefield in Figure 5b bringsout been ableto deployfloats in the regionon three separate two importantfeatures. First, the principalthermal con- occasionswhile alsohaving a detailedhydrographic de- trast, i.e., the property front, is quite sharp and in this scriptionof the stateof the NAC andsurrounding waters sectionis about 150 km inshoreof the baroclinic(or at the time. In addition, Clarke and R. Watts of the dynamic)front for (rt < 27.2 and about 100 km farther University of Rhode Island have recentlyrecovered an west on the deeper densitysurfaces. Second, below at = array of mooredcurrent meters that wasdeployed across 27.2 there is more variabilityalong most of the section, the NAC for a 2-year period. We provide here early suggestinggreater exchangeon deeper than shallower salient resultsfrom the float program that may aid in isopycnals.In principle,the salinitydistribution in Fig- understandingthe characterof the NAC. ure 5c addsno informationbeyond that of temperature, We summarizebriefly here the characteristicsof the sincesalt and temperaturetogether determine density. RAFOS float (seeRossby et al. [1986] for a more com- Nonetheless,because temperature plays the major role plete systemdescription). The float tracks reasonably in settinga fluid parcel'sdensity (hence the horizontal 472 ß Rossby:THE NORTH ATLANTIC CURRENT 34, 4 / REVIEWSOF GEOPHYSICS

Density

26.6

500 26.8

' 2 - 27 27.2 27.4

27.6

2000 •000'O ..... 0 0 ( 0 200 400 600 200 40• (•0 a Distance(km) b Dist•e(k•)

Salinity Oxygen

26.6 26.6

26.8 26.8

27 27 •27.2 27.2

27.4 27.4 27.6 27.6 • '

0 200 400 600 0 200 400 600 c Distance(•) d Distance(kin)

Figure5. Hydrographicsection across the NAC at 43øN:(a) density(or,) versus pressure, (b) temperature (degreesCelsius) versus crt, (c) salinity(practical salinity units, PSU) versus c&, and (d) oxygen(milliliters per liter) versuscr t. The thin circlesindicate the stationlocations.

layeringof the temperaturefields), salt tends to act more 265 continuessouth to 39øN(15% decreasein planetary like a tracer. As a result, the salinityfields bring out vorticity)before turning north again.The singleloop more clearly the exchangeof fresh and salty waters duringthe long southwardtransit during which float 265 between the Labrador Current side and the Newfound- shoals200 dbar (day 130) revealsa cyclonicstructure land Basin. Note also that the salinity field reveals a that also appearsin the trajectoryof float 256 (not weak propertyfront coincidentwith the dynamicalfront shown)at the samesite about a month earlier. (As an on shallowisopycnals. The 02 distributionin Figure 5d aside,the tiny saw-toothedwiggles in the trajectoryjust highlightsthe strongcontrast between the low-oxygen north of the loop indicateinertial motion.The apparent waters originating from the south and the cold, more period of ---2 days with anticlockwiserotation results recently aerated waters from the Labrador Sea. from aliasingthe ---17-hourinertial period (= 12 hours/ sin (latitude)) by the 12-hour positionsampling rate. Becauseof the well-knownspectral gap betweenthese THE LAGRANGIAN VIEW energetichigh-frequency motions and the geostrophi- cally balanced motions on longer timescales[see Figure 6 showsthe 10-monthtracks of floats 254 and Webster,1968], the inertialoscillations stand out clearly.) 265, alongwith the correspondingtemperature and pres- One of the most impressivetrajectories must be that suretime seriesof float 265. The floatswere deployedas of float 260 (Figure 7). During its 10-monthdrift on the a pair to examinehow longthey might continue together a t = 27.2+ surface,it ceaselesslyloops anticyclonically before separating.This pair is the most extreme of all, within a 300-km box centered at 42øN, 44øW. At various stayingclose together for almost3 months.(Other pairs timesduring the 10-monthperiod the float shoalsduring typicallypart ways after 3 weeks.) Immediately after its western excursions into the NAC. This float confirms launchin the NAC (near 250 km in Figure5) the floats the persistenceif not permanenceof the anticyclonic upwelltoward the coldwater side,but after 15 daysthey circulationin this region [cf. Mann, 1967;Clarke et al., start accelerating to the east and cross the NAC as 1980].Very high resolutionanalysis of all hydrographic indicatedby the 500-dbar downwellingto >950 dbar. data in this regionshows unequivocally a deepeningof Sixtydays after the start at 48øN,they reversedirection the thermoclinecentered at 42øN, 44øW [Cart et al., and head south.About 15 dayslater theypart ways,with 1995;Keams, 1996]. 254 making N-S excursionsbetween 46 ø and 49øNwhile Immediately to the north a cycloniccirculation dom- 34, 4 / REVIEWSOF GEOPHYSICS Rossby:THE NORTH ATLANTIC CURRENT ß 473

)1oo 45

265

.)50

40

-•:•, a -50 -45 -40 -35 -30

600 '" / : ..-, ,.z \ / 8ool- / :. / "-•' ••'"• \ / 1000•-""'" " ' "".'"-,•'•'•""-,-.,,.•"q / I I I i I 0 50 100 150 200 250 300 Elapsed time(days)

Figure6. (a) Trajectoriesof floats254 and 265 on the % = 27.5 surface,and (b) temperatureand (c) pressure as a function of time for float 265. The thin lines in Figure 6a indicatethe 200-, 2000-, and 4000-m isobaths; FC is Flemish Cap. inatesthe flow field. Float 272 on the o- t : 27.5+ surface alwaysin conjunctionwith upwellingmotion toward the typifiesthis quite well (Figure 8). Launchedat the same cold, fresh waters from the Labrador Sea between the site as 260, float 272 at first drifts south with 260 but NAC and the continental shelf. After day 150 the tem- continuesquite a bit farther south before getting en- perature showslittle further change. The near-perma- trained back to the north over the SE Newfoundland nence of this cyclonicregion or trough in the NAC has Rise. The transition toward the cold side occurs around the effect that no float in the NAC or on the warm side day 70 when the float risesabout 300 m. After it loops can get north of 44øNwithout gettingcaught in it unless throughthe trough at 41.5øN,it winds up in a cyclonic it movesoffshore to at least42øW (as in Figure 6). Floats region startingaround day 110. Only 20 daysbefore the on the cold side can drift north inshoreof the trough. end of its 300-daymission does the float break off to the Between the trough and Flemish Cap to the north, north.Another float, 285 (not shown),indicates that this floats often upwell through a tight anticycloniccrest cyclonicregion persistsfor at least another 5 months. before continuingeast past Flemish Cap, as is evidenced The temperature record from float 272 (not shown) by the sharpclockwise turn of floats 254 and 265 SE of showssignificant drops around days 70, 90, and 140, Flemish Cap in Figure 6. The downstreamincrease in 474 ß Rossby-THE NORTH ATLANTICCURRENT 34, 4 / REVIEWSOF GEOPHYSICS

55 depth east of Flemish Cap appearsto induce strong cyclonic curvature to the current. The current turns north andwest toward Flemish Cap at 47ø-48øN,where it againturns offshore.Float 307 (Figure 9) illustrates this well. Launchedinshore of the NAC on the (rt = 27.5- surface,this float traces out the cyclonicand

5O anticyclonicpatterns discussed above (including a "fig- ure 8"). It then loopsoffshore of FlemishCap, turns $o west, and weaves its way northward in an onshore- offshorepattern. This float hasthe unusualdistinction of maintaininga highspeed (>50 km d-•) muchof the time after day 30, when it entersthe cyclonicloop at 44øN,and day ---130,when it leavesthe NorthwestCor- 45 ner at 52øN and heads east. North of---47øN,the thermoclinestarts shoaling more rapidly towardsthe north, (Figure 2). The Subpolar Front coincides with the northernmost extent and out-

9 croppingof the subtropicalthermocline. Geostrophi- cally speaking,this requires an eastwardmean flow in the upper layers.However, there is significanteddy 40 i -50 -45 -40 -35 motionthroughout this region such that the float trajec- tories (compareFigs. 3, 6, and 9) will exhibit more Figure 7. Trajectory of float 260 on the (r, = 27.2 surface.The complexmotion than just a uniform eastwarddrift. orbital periodwhen the float is near the centeris ---14days. At the risk of some exaggeration,it is instructiveto The thin lines indicate the 200-, 2000-, and 4000-m isobaths; summarize what we have observed from the above and FC is Flemish Cap. many other float trajectoriesin the form of a cartoonof

5o

0

45

4o

35 -50 4O i i -50 -45 -40 -35 -30 Figure 8. Trajectory of float 272 on the (r, = 27.5 surface.The cyclonicmotions at 44ø-45øNcontinue for over 6 months.The Figure 9. Trajectory of float 307 on the (r, = 27.5 surface.The thin lines indicatethe 200-, 2000-, and 4000-m isobaths;FC is thin lines indicatethe 200-, 2000-, and 4000-m isobaths;FC is Flemish Cap. FlemishCap. 34, 4 / REVIEWSOF GEOPHYSICS Rossby:THE NORTH ATLANTIC CURRENT ß 475 the mean path of the NAC. Likely pathwaysof fluid loss from the NAC, as well as locations where fluid tends to recirculate,are also indicated.The heavyline in Figure 500 10 indicatesthe mean path of the current and its off- shore-onshoremeandering. Perhaps the single most striking aspectof the NAC to emerge from this study consistsin the stabilityof the meander path: the mean- 1500 ders do not propagate. They grow and recede, and disappearentirely, but it appearsthat the locationof the 2000 crestsand troughsdepends upon the bathymetryaround 0 5 10 15 20 the Grand Banks and Flemish Cap. Unlike the Gulf Ternp(øC) Stream, which flows east as an unboundedfree jet, the NAC flows north as a western boundary current, with Figure 11. Two temperature profiles from the Mann eddy the detailsof its path most likely governedby the inter- [Mann, 1967]centered at 41øN,43øW. One (station58, pluses) is from the center, and the other (station60, circles)is from action between the current and the local bathymetry outsidethe eddy, 50 km to the east. [Warren,1969; Shi and Chao, 1994].Strong recirculating flows develop on the concave side of the meanders, presumablyas a consequenceof the stationarymean- ders.Further, from a numberof float tracks(Figures 7 mum, the flow on the convex or outer side of the current and 8) it has become apparent that the recirculating becomesincreasingly difluent, meaning that the velocity flowsin the meandersof the NAC last much longerthan vectorsdiverge. Waters closeto the center of the current anythingever observedin the Gulf Stream [Bowerand may continue through the meander, whereas waters Rossby,1989; Song et al., 1995].Where the floatsindicate closeto the outer edge may recirculateback upstream, sustainedor coherent recirculations,Figure 10 showsa and waters in between may drift away from the current closedloop at the correspondingconcave location of the toward the interior (indicatedby the arrows pointing meander.Where the NAC approachesa meanderextre- awayfrom the NAC). We now seehow exchangecan take placebetween the current and surroundingwaters. Regardlessof the dy- 55 namicalreasons, if a stationarymeander decaysor pulls back, a fluid parcel entering the point of maximum curvaturewill have a velocity componentthat takes it away from the current toward the slackwaters outside. Trajectoriesand streamlinescan coincideonly in steady flows,as illustratedby the simpleyet elegantkinematical ideasoriginating with Flierl [1981]and adaptedby Bower 50 [1991] for a meanderingjet. The Bower model provides a powerful analyticaltool for interpretingparticle mo-

A/C tions in a meanderingjet. Becausethese kinematical studieswork with simpleyet rather realisticmodels of a meanderingcurrent, one can, by tuning the model from observations,reproduce many observedpatterns quite well [e.g.,Dutkiewicz and Paldot, 1994]. A/C 45 The extraordinarypersistence of the trapped anticy- clonicmotion as evidencedby float 260 (Figure 7) be- trays a remarkable,possibly permanent circulationfea- ture. Mann [1967] first observed this extraordinary

9 intenseanticyclonic eddy (but see also Barahoy [1984] for a summaryof other observations),and for thisreason we call it the Mann eddy. Figure 11 showstwo temper- 40 ature profiles,one from station58 in the eddy and one -50 -45 -40 -35 from station 59, 50 km to the east. In the center of the eddy the upper layer of uniform temperature(14.7øC) Figure 10. Path of the NAC and adjacentrecirculation cells. reachesdown to 1000-m depth, below which the tem- The arrowsindicate probablepathways and directionsof loss from the NAC to the surroundingwaters. The dashedline in perature drops off quite rapidly, but even at greater the north indicates an additional meander (or partially at- depthsthe isothermsare deeper than elsewhere.Quite tachededdy) that appearsin someof the deeperfloat trajec- possibly,nowhere in the world ocean has such light tories. The thin lines indicate the 200-, 2000-, and 4000-m water (S = 36.15 PSU (practicalsalinity units); •rt = isobaths;FC is Flemish Cap. 26.9) been observedat suchgreat depths.Mann [1967] 476 ß Rossby:THE NORTH ATLANTIC CURRENT 34, 4 / REVIEWSOF GEOPHYSICS

assumedthat wintertime coolingcaused the deepening [1994] took a very different approachby deployinga of the mixed layer, but this doesnot explain the 50-km linear array of four current meter mooringsacross the scaleof the eddy (roughlythe radius of deformation), Northwest Corner to examine the stability and NW which is smallerby an order of magnitudethan that of extent of the flow. We can add to Lazier's observations the typical cold and dry continentalair massesrespon- by notingthat severalfloats (not shown)became trapped siblefor the heat losses.Further, as a mixed layer deep- in that region for varyingperiods of time. ens,its temperaturewill decreasesuch that it constantly matches that of the waters immediately underneath without significantlydeepening them. Insofar asthe con- DISCUSSION vective cooling goes, the deep densitysurfaces should remain level, contraryto the remarkabledeepening of The cartoonof the NAC (Figure 10) with its largebut the deep temperaturesurfaces observed at station58. In nonpropagatingmeanders is distilled from the major short,the warm, very deep mixed layer and thermocline featuresof float trajectoriesin the area. It accountsfor reflect a substantialgain rather than lossof heat: some- all recirculations inside meander extrema that have been thing other than atmosphericprocesses is driving the observed,although not all meandershave had trapped Mann eddy. recirculatingfloats. As was stated earlier, the crest at Without doubt, coolingand mixing did take place in 42øN shouldbe considereda permanent feature of the the eddy. At a salinity of 36.15 PSU, the temperature NAC. Similarly,the extendedtrough at 44øNappears in shouldbe about IøC higherto matchthat of Gulf Stream many of the float tracks. The meander was quite ex- waters.Nonetheless, the warm, salinewaters of the eddy tendedin 1993-1994,whereas by 1995it had diminished point to the Gulf Stream as its source.The extraordi- considerablyin amplitude.The eastwardflow between narily deep thermocline at station 58, indicatessome the Mann eddyand this troughis highlydifluent, and the form of pumping mechanism or spin-up process. radiusof curvatureof the trough at times so sharpthat Whether continuouslyor in bursts,it would appearthat perhapsthis portion of the NAC is very "1ossy,"i.e., the the eddy picks up warm water with negative relative probability of escape is high. The meanderswobble vorticityfrom the NAC. This would happenwhen pack- aboutand vary considerablyin amplitude.The "figure8" ets of water in the NAC have so much negative shear trajectoryof float 307 in Figure 9 (betweendays 60 and and curvaturevorticity that when they enter the cyclonic 70) suggeststhat the trough became semidetachedto trough immediatelyto the north with its strongpositive open a pathway back south. Indeed, the float tracks curvaturevorticity, the parcelscannot adjust (whether by inside the trough reveal a tendencyfor severalcyclonic increasingthe negative shear and kinetic energy even recirculationsto developinside it, suggestinga compe- further, or through increasedstretching and potential tition betweena larger-scaleprocess driving the trough energy)and thusleave the currentto join or loop around and a local trend or preference toward axisymmetric the eddy. Depending on the relative vorticity of the recirculation,i.e., coherenteddies. This wobblingof the envelopingwaters in the NAC, thesewill strengthenor meandersnotwithstanding, the meanderingNAC differs weaken the eddy over time through lateral exchange. substantiallyfrom the Gulf Stream,where we sawlittle if Evans [1989]has described numerical experiments of the any tendency toward sustainedrecirculations on the formationof long-livedanticyclonic vortices at meander concaveside of meanders[Bower and Rossby,1989; Song troughs.Whatever the processthat sustainsit, the Mann et al., 1995].The reason,we believe,is that Gulf Stream eddy clearlystands out as a most remarkableand prob- meanders propagate whereas in the stationaryNAC ably permanentfeature of the North Atlantic. meandersthe waters can be "spun up" in place. If one acceptsthe cartoonof a NAC with its station- The wavelengthof the meandersis mostlikely deter- ary meandersin Figure 10 then it followsthat the Mann mined by the shapeof the topographyalong the Grand eddy has a number of "cousins"farther downstream. Banks.Specifically, the meandersreflect the bathymetry Thesemay not havethe samedegree of permanence,but along the way: the Southeast Newfoundland Rise the same causative factors for their existence should (trough), the Newfoundland Seamounts near 44øN apply: a sharply curved flow continuallyrecirculates a (trough), and finallythe trougheast of FlemishCap. La semitrappedvolume of water. Another, very striking Violette [1982] has emphasizedthe importanceof the anticyclonicfeature dominatesthe NAC in the far north NewfoundlandSeamounts to the path of the NAC. at the NorthwestCorner. Already Iselin [1936]noted the Exchangeof watersbetween the NAC and surround- warm watersin this region (see Figure 2). Lozier et al. ing waterscan still take place,even though the meanders [1996] showthat even after averagingall hydrographic are nonpropagating,either from their wobblingmotions data spanninga 50-year period, the Northwest Corner or throughgrowth and decayof the meanders.Song et al. remainsin sharpfocus. That a highlybaroctinic current [1995] noted how this can happenin the Gulf Stream.In flowingnorth and then of[ to the east(actually southeast the NAC a growingmeander would pick up parcelsin at first) shouldstand out after observationsfrom many previously quiescentwaters and sweep them down- decadesof time are averagedtogether implies substan- stream. Conversely,in a decaying meander, parcels tial spatial stability.As was mentioned earlier, Lazier would be displacedtoward the edge and left behind. 34, 4 / REVIEWSOF GEOPHYSICS Rossby'THE NORTH ATLANTICCURRENT ß 477

55

50

45

40 -50 -45 -40 -35 -30 -50 -45 -40 -35 -30

55 55

50 50 %'%

45 45

40 40 -50 -45 -40 -35 -30 -50 -45 -40 -35 -30

Figure12. Dot distributionof floatspeeds in excessof (left)40 and(right) 60 cms -• for floatson the (top) 27.2and (bottom)27.5 (r t surfaces.The thin linesindicate the 200, 1000,2000, and 4000 m isobaths.The percentagesin eachpanel indicate the fractionof all observationsexceeding the threshold.

From the few eventsobserved thus far, the majority of terns in the eddy field. With higherthresholds the me- transitsacross the entire NAC havetaken placefrom the andering pattern emerges with preferentially high cold to warm side between the crest at 42øN and the speedsaround the meanderextrema, such as the troughs troughat 44øN.With onepossible exception, not a single near 44ø and 47øN.The percentagesin eachpanel indi- float has evincedcross-frontal transfer of water through cate the fraction of all observations exceeding the ring formation,whereas several such events were ob- threshold. servedin the courseof our Gulf Streamwork [Bowerand The loss of fluid along the NAC upstream and at Rossby,1989; Song et al., 1995]. meanderextrema can help explainwhy the eddykinetic The fact that floats do not cross the entire NAC can energylevels drop so rapidly to the eastof the NAC. The be seenas compellingevidence for a simplyconnected flow is so difluent that fluid parcelsrapidly decelerate front between the Southeast Newfoundland Rise, Flem- uponleaving the current.Further, this difluenceor dis- ish Cap, and the NorthwestCorner. A numberof floats persalmay help explainthe lack of a clearindication of maintainsustained speeds over substantialdistances be- semipermanenteastward flows or jets, suchas a jet at tweenthese points (the trough at 44øNnotwithstanding). 47øN as has been hypothesized or evena well-definedjet High speedsmean stronglateral gradients,i.e., a ba- at the SubpolarFront. The EKE pattern in Figure 4 roclinicfront alongwhich the parcelstravel. Further, all showsonly little enhancedEKE along the Subpolar along the NAC it consistentlyserves as a water mass Front, suggestinga broadeningand weakeningof the boundarybetween the coldLabrador Sea waters and the zonal flow. High EKE in zonal flowsappear to be asso- warm subtropicalwaters from the south.Floats consis- ciated with the meanderingof well-definedfronts [cf. tentlyshow high temperatures in the NAC and offshore, Rossby,1987; Johns et al., 1995]. and consistentlycold temperatureson the inshoreside The proposedpathway for the NAC and resulting for the sameisopycnal. The highspeeds in the NAC can standingeddies in Figure 10 makesit easierto under- be illustratedby indicatingall those points where a stand the Krausset al. [1987] report of an extended velocityexceeds some threshold value. In Figure 12 we recirculationbetween the Mann eddy at 42øNand Flem- showall observationswhere the speedsexceed 40 and 60 ish Cap. At thosetimes when the troughat 44øNis not cms -• for floatson the (rt = 27.2 (toppanels) and (rt = extended and the course of the NAC is more linear, it 27.5 (bottompanels) density layers, respectively. The appearsthat the anticycloniccells at 42øN (the Mann concentrationof high speedsto the west indicatesthe eddy)and 45øNcan join to form an extendedrecircula- high speedsalong the mean path of the NAC. A nice tion cell. In addition, detailed hydrographicanalysis feature of thesefigures is how they can emphasizepat- [Keams,1996] revealsthat there is also,on average,an 478 ß Rossby:THE NORTH ATLANTIC CURRENT 34, 4 / REVIEWSOF GEOPHYSICS

anticycloniccell east of Flemish Cap. These three lati- Bathymetriccontrol? As the Gulf Stream flows tudescoincide exactly with thosereported by Krausset east and makes bottom contact with the Southeast New- al. [1987].These cells, individually and whenjoined into foundland Rise, conservation of potential vorticity an extended one, must be driven by the expulsionof forcesat leastpart (the inshorepart) of the currentto negativerelative vorticity (and/or large layer thickness) turn north alongthe bathymetry[Warren, 1969; Shi and from the NAC. In its extendedstate this resultingrecir- Chao, 1994].Our observationthat the NAC meandersso culation will assume the scale of a small westward- tightly along the topographysuggests that bathymetry intensifiedgyre with a N-S extent of---600 km. It was plays a major role in governingthe Gulf Stream-NAC shown earlier that the available wind stress curl was transition.Indeed, the flow of the NAC NE pastFlemish insufficientto drive sucha gyre,but expulsionof anticy- Cap with its retroflectionto the NW (and with some clonicvorticity by the meanderingcurrent may provide waters that return SW) seemsto mimic what we have an alternativedriving source.The propertiesof plane- observed of the Gulf Stream at the Southeast New- tary wave radiationmight explainwhy the recirculation, foundlandRise. The point is that bathymetryseems to when it occurs, is limited to a narrow band east of the exert a very stronginfluence on the courseof the cur- NAC itself rather than filling the entire Newfoundland rent. What, then, might result from shifting the Gulf Basin as was suggestedschematically by Worthington Stream a bit farther south so that it no longer makes contact with the Southeast Newfoundland Rise? Would [1976].The float data suggestthat the individualcells are the Gulf Stream continue east as does the Kuroshio connectedinto an extendedcell only infrequently.The Extension in the Pacific? Reconstructions of sea surface extendedcell doesnot appearin high-resolutionhydro- graphicclimatologies [Lozier et al., 1996;Keams, 1996]. temperaturesfrom the lastglacial maximum suggest that the Gulf Stream did just this [Mcintyreet al., 1976]. A major question about the Gulf Stream-North Atlantic Current transition that should be addressed is Winds? We have previouslyargued that windscan not be responsiblefor the hypotheticalnorthern gyre. why the Gulf Stream turns north and the NAC flowsso But this doesnot precludethe possibilitythat the NAC far north as a western boundary current. The high- as part of the larger subtropicalgyre cannotextend the resolutionclimatology of Keams [1996] showsthe NAC Gulf Stream as the westernboundary current farther to transporting40 Sv (relativeto 2000 dbar) into the New- the north. Theoretical arguments[Sverdrup, 1947; Gill, foundland Basin. No other western boundary current 1982] indicatethat zero wind stresscurl lines separate transportssuch volumes of warm watersso far poleward. wind-drivengyres from eachother suchthat in subtrop- This issuemay be connectedto the coupledwind-driv- ical systemsthe wind stresscurl inducesan equatorward en-thermohaline system,for it is here off the Grand transport, and vice versa in subpolar gyres. Broadly Banks that the two meet and cross each other, both in speaking,the NAC-Subpolar Front systemdoes coin- the horizontal and in the vertical; hence the subtitle "at cide with the zero wind stress curl line. However, the the crossroads."In the horizontal the NE flowing NAC mean wind stresscurl is relativelyweak comparedwith and the cold waters from the Labrador Sea attached to its annualand interannualvariability [Isemer and Hasse, it on the inshoreside define the boundarybetween the 1987], so that it seemsunlikely that the NAC's sharp wind-drivensubtropical and subpolargyres. In the ver- turn to the north and penetrationto 52øNresults directly tical the NAC transportsthe warm, salty waters of the from suchvariable forcing.Perhaps the oceanleads the upperlimb to the north,while the lower limb flowssouth atmospherein this region? along the continental rise immediately underneath. What is it that setsthis systemor region apart from the Kuroshio in the western Pacific, which certainly shows SUMMARY no suchtendency to flow north? A number of possible causative factors can be mentioned. AlthoughIselin [1936]perhaps did not realizeit at the Demands of the ? The time, his depiction of the path of the North Atlantic convective overturning of warm saline waters in the Current and the waters in the Newfoundland Basin was Labrador, Greenland, and Norwegian Seas demands a remarkablyaccurate. The reasonfor this is simple:The northward transport of about 12 Sv [Schmitzand Mc- path of the NAC is quite stable, extendingthe Gulf Cartney,1993] (Broecker[1991], on the other hand,pre- Stream northeastwardalong the continentalslope east fers more like 20 Sv). These waterssubsequently flow of the Grand Banks.Even sparse,infrequent sampling south along the deep western boundary and sink to will resolvea pattern that doesnot evolve.The Interna- replenishthe deepwaters of the world ocean.The trans- tional Ice Patrol has maintaineda stronginterest in the port of warm, salty waters by the NAC to the high position and currentsof the NAC in order to provide latitudeswhere deep convectioncan take place is gen- accurateinformation on icebergmovements. Thanks to erally acceptedas the upper limb of the thermohaline their activitiesthere is a very substantialhydrographic overturning[Broecker, 1991], but this does not explain and drifter databaseof the region east of the Grand why the watersflow north as a westernboundary current Banks that otherwisewould not exist. Oceanographers, as far as 51øN before turning east. on the other hand, have tended to focus on the fate of 34, 4 / REVIEWSOF GEOPHYSICS Rossby:THE NORTH ATLANTIC CURRENT ß 479 the Gulf Stream along the SoutheastNewfoundland the trajectorydata. This accordswith the Krauss[1986] Rise, stimulatedperhaps in part by the two-gyrecontro- study. versy.But the structureof the current in this region is rich in detail, posing a formidable challenge to the hydrographicsampling techniquesavailable to chart GLOSSARY ocean currents.This may, in the end, be the reason for the controversy:we have not had the measurementor Anticyclonicmotion: Flow patternsin which the samplingtechniques appropriate for the task. Besides, directionof flow turnsto the right (i.e., clockwise)in the shipsare slow. northern hemisphereand to the left (anticlockwise)in Droguedsurface drifters have proved to be very help- the southernhemisphere. ful in charting the spatial and temporal structuresof Baro½linic flow' Horizontal motion with vertical currents,especially when geostrophicand boundarylay- shearthat is dynamicallyrelated to transversehorizontal ers are carefullydistinguished. By systematicallydeploy- gradientsof the densityfield. ing drifters in different regions the Institut f/ir Barotropicflow: There are many definitionsfor Meereskunde, Kiel, has built a picture of the mean this. Here we mean the flow where the densityfield is near-surface circulation and obtained valuable informa- nearly uniform, such as below the main thermocline or tion on the nature of the eddy field. With respectto the in Subpolarwaters. NAC, the drifter data indicate that the Subpolar Front Crest and trough: Those parts of a meandering cannotbe describeda• a baroclinicjet (h la Gulf Stream) current where the curvature is at an extremum: anticy- with various branch flows peeling off, but rather as a clonic (clockwise)and cyclonic(anticlockwise), respec- tively. broad eastward and northward drift with a widening Cyclonicmotion: Flow patternsin whichthe direc- envelopeto the east. We suspectthat the shallowstrat- tion of flow turns to the left (i.e., anticlockwise)in the ification at high latitudes is unable to provide enough northernhemisphere and to the right (clockwise)in the density contrast to maintain permanent baroclinic southernhemisphere. fronts.Any baroclinicfront would be too weak to with- Eddykinetic energy: Definedas ((u '2) + (v'2))/2, standthe disruptiveeffects of the backgroundbarotropic where u' and v' representthe eddyvelocity component, eddy field. The Krauss[1986] paper is a very important i.e., the observedvelocity minus the mean for the region one, both for what it has taught us about the upper in question. ocean circulation of the North Atlantic Ocean and for its [kman transport: The 10- to 100-m-thickboundary systematicuse of direct measurementsof currents. layer at the surfaceof the ocean where fluid motion is The work in progressby the RAFOS float group at directly influencedby the action of the winds. In the the University of Rhode Island is similar to that of the northern hemispherethe transportin the Ekman layer is IFM groupwith our use of Lagrangiantechniques. We directed to the right (up to 90ø ) of the applied wind are focusingon the NAC as a westernboundary current stress. and on how it functions as a boundary between the Eulerianflow: Descriptionof fluid motionin a fixed subtropical and subpolar waters. The isopycnalfloat coordinate system,i.e., U(x, y, z, time). All hydro- trajectorydata indicate that the NAC actseffectively as graphic descriptionsof the ocean use the Eulerian co- a boundary between the two gyres on shallow density ordinate system. surfacesin the sensethat only a very few floats have Geostrophicflow: All large-scalehorizontal flow, crossedthe current entirely from one side to the other, whether in the oceanor the atmosphereis very nearly in and then only along specific pathways.The NAC is geostrophicbalance. This meansthat the pressuregra- highly convoluted(has steep meanders),but with the dient and the are equal and oppositeto exceptionof the trough at 44øN, which can shed cold each other and normal to the direction of flow. The core eddies,warm- and cold-corering formation,which latter force results from the rotation of the Earth. is commonplacein the Gulf Stream, has not been ob- Inertial motion: Circular motion with a period of served.The mean path of the meanderingNAC follows 12 hours/sin(latitude). Thesemotions are ubiquitousin the bathymetryclosely, suggesting conservation of po- the oceanand are usuallyinduced by suddenchanges in tential vorticity by vortex stretchingand positive curva- wind stress at the surface. ture vorticity in deeper waters causingthe current to Isopycnalsurface: A surfacealong which fluid par- turn back inshore, and vice versa in shallower waters. cels move without changein density. The meanders do not propagate downstreamor up- I_agrangianflow: Fluid motiondefined in termsof stream.Almost certainlythis is due to their generation the time historyof labeledfluid parcels,i.e., X(x0, Y0,z0, by prominent topographicfeatures: the SoutheastNew- time) where x0, Y0, z0 representthe parcel's starting foundlandRise and Flemish Cap. Waters are lost from point. The isopycnalRAFOS floats naturally use the the NAC in stronglydifluent and thusdecelerating flows. Lagrangiancoordinate system. There appearsto be no mechanismfor focusingexpelled Potentialvorticity' Conservationof potentialvor- watersinto zonaljets, which may explaintheir absencein ticity is an important constraintto fluid motion in the 480 ß Rossby:THE NORTH ATLANTICCURRENT 34, 4 / REVIEWSOF GEOPHYSICS

ocean. It states that in the absence of friction, the abso- floats as roving hydrographersin the North Atlantic Cur- lute angularmomentum of'a fluidparcel must be con- rent region,ACCP Notes,//(2), NOAA Atl. Oceanogr.and served. Meteorol. Lab., Miami, Fla., 1995. Clarke, R. A., H. W. Hills, R. F. Reiniger, and B. A. Warren, Sigma-t({rt): A commonlyused definition for den- Current systemsouth and east of the Grand Banks of sityof water(1 at unitis equalto 1 kgm-3). Newfoundland,J. Phys.Oceanogr., 10, 25-65, 1980. Streamlineand trajectory' A streamlinerepresents Colin de Verdi•re, A., H. Mercier, and M. Arhan, Mesoscale lines alongwhich the fluid movesat a particularinstant, variabilityfrom the westernto easternAtlantic along48øN, whereasa trajectorytraces out the path of a parcel.The J. Phys.Oceanogr., 19, 1149-1170, 1989. Dietrich, G., K. Kalle, W. Krauss, and G. Siedler, General two are the same only if the flow is steady. Oceanography,626 pp., John Wiley, New York, 1975. $verdrup($v): A widelyused unit for volumetrans- Dutkiewicz,S., and N. Paldor,On the mixingenhancement in portin theocean (1 Sv = 106m 3 s-•). a meanderingjet due to the interactionwith an eddy, J. Phys.Oceanogr., 24, 2418-2423, 1994. Evans, J. C., An intense warm eddy south of the western ACKNOWLEDGMENTS. I would particularlylike to ac- boundarycurrent jet in an idealizedquasi-geostrophic basin knowledgemy colleaguesM. E. Carr and M. Prater for many (abstract),Eos Trans.AGU, 70, 373, 1989. helpful discussions.It is a great pleasureto acknowledgemany Flierl, G., Particle motions in large-amplitudewave fields, stimulatingconversations and exchangeof ideas with R. A. Geophys.Astrophys. Fluid Dyn., 18, 39-74, 1981. Clarke, B. Cushman-Roisin, S. Dutkiewicz, E. Kearns, W. Fofonoff,N. P., and R. M. Hendry, Current variabilitynear the Krauss,L. Rothstein,C. Rowley, and R. Watts. What hasbeen southeastNewfoundland Ridge, J. Phys.Oceanogr., 15, 963- 984, 1985. discussedhere is in large measurethe result of these.I would Fuglister,F. G., Averagetemperature and salinityat a depthof alsolike to expressmy sincereappreciation to R. A. Clarke at 200 m in the North Atlantic, Tellus, 6, 46-58, 1954. the Bedford Institute of Oceanographyfor wonderful collab- Gill, A.,Atmosphere-OceanDynamics, 662 pp., Academic,San oration and assistancewith the field program on the CSS Diego, Calif., 1982. Hudson. The successof a major field program like this is Goodman, L., and E. R. Levine, Vertical motion of neutrally possibleonly through tremendousteamwork: J. Fontaine is buoyantfloats, J. Atmos. OceanicTechnol., 7, 38-49, 1990. responsiblefor the development and preparation of the Gould, W. J., Physicaloceanography of the AzoresFront, Prog. RAFOS floats and has done a truly superbjob. S. Andersson- Oceanogr.,14, 167-190, 1985. Fontana hasbeen responsiblefor the excellentdata processing Harvey, J., and M. Arhan, The water massesof the central and preparationof the float trajectoryplots. The installationof North Atlantic in 1983-1984,J. Phys.Oceanogr., 18, 1856- 1875, 1988. the sound source moorings has been expertly handled by Hogg, N. G., On the transportof the Gulf Stream between J. Kemp at WHOI and T. Orvoshat the Graduate Schoolof Cape Hatteras and the Grand Banks,Deep Sea Res., 39, Oceanography,URI. We alsothank Captain M. Palmieri and 1231-1246, 1992. the crew of the R/V Oceanusfor their tremendoussupport. I Iselin, C. O., A studyof the circulationof the westernNorth particularlywish to acknowledgeJoyce Reusch for her cheerful Atlantic, Pap. Phys.Oceanogr. Meteorol., 4, 101 pp., 1936. assistancewith all thingsadministrative here at URI. Several Isemer, H. J., and L. Hasse, The Bunker ClimateAtlas of the reviewersand the editor, D. Luther, made much appreciated NorthAtlantic Ocean, vol. 2, 252 pp., Springer-Verlag,New suggestionsfor improvingthe text. 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