REVIEWS OF GEOPHYSICS, VOL. 24, NO. 3, PAGES 469-491, AUGUST 1986

The Physicsof the Antarctic Circumpolar Current

WORTH D. NOWLIN, JR., AND JOHN M. KLINCK

Departmentof Oceanooraphy,Texas A&M University, College Station ,

A region of transitionof surfacewater characteristicsfrom subantarcticto antarcticand an associated eastward flowing Antarctic Circumpolar Current (ACC) have long been recognizedto exist as a band around Antarctica. In this review we summarize the most important observational and theoretical findingsof the past decaderegarding the ACC, identify gaps in our knowledge,and recommendstudies to addressthese. The nature of the meridional zonation of the ACC is only now being revealed.The ACC seemsto exist as multiple narrow jets imbedded in, or associatedwith, density fronts (the Subantarctic and Polar fronts) which appear to be circumpolar in extent. These fronts meander, and current rings form from them; lateral frontal shiftsof as much as 100 km in 10 days have been observed.The volume transport of the ACC has been estimatedmany times with disparate results.Recently, yearlong direct measurementsin Drake Passagehave shownthe meantransport to be approximately134 x 106 m3/s, with an uncertainty of not more than 10%. The instantaneoustransport can vary from the mean by as much as 20%, with most of the variation associatedwith changesin the referenceflow at 2500 m rather than in the vertical shear.Meridional exchangesof heat acrossthe ACC are known to be important to the heat balance of the abyssal ocean and consequentlyto global climate. The most likely candidate processfor the required poleward heat exchangeseems to be mesoscaleeddies, though narrow abyssal boundary currentsmay also be important. Observationsfrom ships,drifters, and satellitesreveal surface mean kinetic energyto be at a maximum along the axis of the ACC and eddy kinetic energyto be large mainly in western boundary regions and off the tip of Africa. Eddy variability in the open ocean is consistentwith baroclinic instability of the narrow jets. Calculations using data from Drake Passage show that the necessaryconditions for baroclinic and barotropic instabilitiesare met in the ACC. The basicdynamical balance of the ACC is still not well known, althoughbottom and lateral topographyand dynamic instabilitiesare shown to be important in balancing wind forcing. The ACC is generally concededto be driven by the wind, but the coupling of wind and thermohaline circulationshave not yet been adequatelyinvestigated. The mechanismresponsible for the multiple coresof the ACC has not been identified in detail. It is suggestedthat future studiesaddress: (1) the circumpolar structureand temporal behavior of the SubantarcticFront and Polar Front; (2) the generaldynamical balance of the ACC and specificmechanisms for creation and maintenanceof the major fronts; (3) the representativenessto the entire ACC of the existing estimatesof meridional exchangesof heat and other properties,as well as kinematic and dynamic quantities,made in Drake Passage;(4) the variability of the ACC transport in severalplaces and coherenceof its variability; (5) the climatology of fields of atmosphere-oceanforcing over the southernocean; and (6) the possibilityof identifying and using simple indicesas good indicators of the behavior of the ACC or parts thereof.

CONTENTS have greatly added to our reconnaissancedata, especiallywith Introduction ...... 469 deeper observations.On the basisof the best available station Zonation ...... 471 data selectedfor the Southern Ocean Atlas [Gordon and Mo- Zonal transport ...... 473 linelli, 1982], Gordon et al. [1978] describedthe large-scale Meridionalexchanges ...... 475 relative dynamic topographyof the southernocean. The Ant- Kinematic and dynamical estimates ...... 478 Circumpolardistribution ...... 478 arctic Circumpolar Current (ACC) is clearly visible in their Site specificestimates ...... 480 representationof the geostrophicsurface current relative to Theory and models ...... 483 1000 dbar (Figure 1). The position of the flow is seento vary Basic dynamical problem of the ACC ...... 483 considerablyin latitude, in large measure owing to geomor- Simplifieddynamical models of the ACC ...... 483 The ACC as part of a global ocean model ...... 486 phological influences.Even for this coarse representation(1 ø Mesoscaledynamics in the ACC ...... 487 latitude by 2ø longitude) of data collectedat different times, Summaryremarks and suggestions...... 488 multiple paths and apparent differencesin width are seen. Gordbnet al. [1978] alsoshowed that the baroclinicityof the ACC extends throughout the water column, with varying 1. INTRODUCTION strengthin differentregions. A region of transition betweensurface waters with antarctic The ACC is generallyconceded to be driven principally by and subantarcticcharacteristics has long been recognizedto surfacewind stress,though the coupling and relative impor- exist as a band around Antarctica [Meinardus, 1923]. Associ- tance of wind and thermohaline driving has not been ad- ated in some manner with this transition, a major eastward equately investigated.Figure 2 showsthe annual mean east- flowing surfacecurrent has likewise been known. Most early ward componentof wind stressprepared by S. Patterson (per- data were derived from ships'logs and from vesselsoperating sonal communication, 1985) using the climatological surface in support of the whaling and sealingindustries. More recent- winds prepared by Han and Lee [1981]. Many models have ly, national scientificexpeditions, such as the Eltanin program, shown the wind stress to be sufficient to drive the ACC. How- ever, the mechanismsresponsible for energy dissipationfrom Copyright1986 by the AmericanGeophysical Union. the ACC are as yet unclear. Four major mechanismshave been advanced: form drag due to bottom topography [Munk Paper number 6R0315. 8755-1209/86/006R-0315515.00 and Palm•n, 1951], thermodynamic effects [Fofonoff, 1955],

469 470 NOWLIN AND KLINCK' PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT

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Fig. 2. Annual mean eastwardwind stress(units of 0.1 N/m e) preparedby S. Pattersonfrom climatologicalsurface winds of Han and Lee [1981]. nonzonal dynamics [Stommel, 1957], and water discharged regarding the ACC and, in some instances,to identify out- from Antarctica [Barcilon, 1966, 1967]. The overall vorticity standingdeficiencies in our knowledgeand recommendstud- balanceis still in doubt [Baker, 1982], although a simple Sver- ies to help fill thesegaps. drup balance with dissipativemechanisms of form drag by We consider first, in section 2, the zonation of the southern bottom topography and lateral dissipation in western bound- ocean.Section 3 reviewsattempts to estimatethe transport of ary layersis consistentwith existingdata. the ACC through Drake Passageand presentsthe best esti- The southern ocean is a region where atmosphere-ocean mate to date. Much of the recent study of the southern ocean exchangescause strong near-surfacemodifications and pro- has focusedon meridional exchanges,principally of heat but duction of water masseswhich spread throughout the global also of salt, momentum, and internal energy, by various pro- ocean.A. Gordon (as cited by deSzoekeand Levine [ 1981]) has cesses. These studies are discussed in section 4. Observational estimated the heat transfer from sea to air south of the Polar studies of kinematics and selecteddynamical quantities are Front to be 3 x 10'4 W. This heat lossmust be balancedby consideredin section5. Section6 outlines the dynamicsof the poleward oceanic heat flux across the ACC. The traditional circumpolarcurrent as revealedby theoreticaland numerical concept that this flux is brought about by mean meridional modeling studies. Finally, in section 7 we make some sum- advection of water massesthrough the predominantly zonal mary remarksand suggestionsfor future research. flow of the ACC has been challengedby recent observations 2. ZONATION which suggesta large role for eddy and small-scaleprocesses. A good summary of the understandingof the ACC as of Deacon [1937] noted that the isolinesof salinity and tem- 1972 is provided by the report "Southern ocean dynamics: A peratureslope downward gradually in a seriesof stepstoward strategyfor scientificexploration, 1973-1983" lad Hoc Work- the north acrossthe ACC. Most of the ACC transport seems ing Group on Antarctic Oceanography,1974]. The past decade to be associatedwith two current coresseparated by a transi- has seenrenewed emphasison studiesof the ACC, due largely tion zone in which the near-surface characteristics are inter- to the International Southern Ocean Studies (ISOS) and the mediate between those of the Antarctic Zone south of the Polar Experiment (POLEX) South [Sarukhanyanand Smirnov, current and the Subantarctic Zone to its north. These current 1985; Sarukhanyan,1985]. These programs encouragedand coresare fronts with pronouncedhorizontal gradientsof den- supported a wide variety of southern ocean studies.The ISOS sity and other characteristicssuch as temperature T, salinity program placed specialemphasis on descriptionsof fronts and S, or nutrients; within the upper water column at least, energeticsin the region of Drake Passageand southeastof characteristicrelations (e.g., T-S) change abruptly across the New Zealand and on monitoring the transport of the ACC. fronts. Vertical sectionsof density acrossDrake Passage(e.g., The purpose of this review is to summarize the most impor- Figure 3) show three fronts separatingfour water masszones. tant observationaland theoreticalfindings of the past decade From north to south these are called the Subantarctic Zone NOWLIN AND KLINCK' PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT 473

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Fig. 4. Surfaceregimes of the southernocean. Position of the SubtropicalFront separatingthe subtropicsfrom the SubantarcticZone is after Deacon[1982]. Locationsof the SubantarcticFront and Polar Front boundingthe polar frontal zone are modifiedfrom a figure by Clifford [1983]. The Antarctic Zone is south of the Polar Front. Summerice extent is shown near Antarctica,as are locationswhere a water mass transition near the continentalslope has been observed[Clifford, 1983].

over horizontal scalesof about 60 km and temporal scalesof 3000 dbar for sevencrossings made during 1975, 1976, 1977, 20 days, to have characteristicamplitudes of 20 cm/s at 1000 1979, and 1980 showed the baroclinic field to be relatively m, to be vertically coherentover the observeddepth range of constant,with an averageof 103 x 106 m3/s[Whitworth et al., 1000-5000 m, and to move southeastwardat •, 12 cm/s. 1982]. This is consistentwith estimatesof relative geostrophic transports from earlier sectionsat Drake Passage[Reid and 3. ZONAL TRANSPORT Nowlin, 1971]. Nowlin and Clifford [1982] showed that the transports associatedwith the three fronts in Drake Passage Estimates of the volume transport of the ACC have fre- account for approximately 75% of the total baroclinic trans- quently been used as inputs to, or observational checks on, port relative to 2500 dbar, though the fronts occupy only circulation models. Realistic estimatesof the transport and its about 20% of the width of the passage. variability can be of considerablevalue as a test of the ability The first estimatesof ACC transport by using direct current of our models to account for pertinent dynamics: if a model measurementswere made at Drake Passagein 1969 [Reid and cannot produce reasonabletransport estimates,it must be de- Nowlin, 1971] and 1970 [Foster, 1972]. These were most con- ficient. fusing, but provocative, because the two independent esti- Early estimatesof ACC transport that required the selection matesgave widely disparate results, 237 x 106 m3/seastward of a reference level varied greatly. Becausethe geostrophic and 15 x 106 m3/swestward. shear in the ACC fronts extends practically to the bottom, A principal objective of the ISOS program was to obtain a middepth choices of zero-velocity referencelevels resulted in yearlong record of ACC transport at Drake Passage. Initial westward deep flow under eastward flow and led to poor effort went into the determination of vertical and horizontal transport estimates. Also, when estimating transport at lo- length scalesfor velocity(for example,see Scirernarnrnano et al. cationsother than Drake Passage,it is difficult to separatethe [1980]). As a result of that work, severalpreliminary transport ACC flow from that due to adjacent currents. In the open estimateswere produced from data collectedin 1975' 110-138 ocean the southern limbs of the subtropical gyres and north- x 106 m3/s [Nowlin et al., 1977], 139+ 36 x 106 m3/s ern limbs of the subpolar gyres both flow eastward and are [Bryden and Pillsbury, 1977], and 127 + 14 x 106 m3/s not easily distinguishedfrom the ACC. Moreover, south of [Fandry and Pillsbury, 1979]. All indicated eastward flow, and Australia and Africa other current systems,for example, the the agreement was good. Improved description of the ACC AgulhasCurrent, add to the confusion. also showed that the disparate results of 1969 and 1970 were Geostrophic transport through Drake Passagerelative to caused by undersampling of the reference velocities in the 474 NOWLIN AND KLINCK' PHYSICSOF THE ANTARCTICCIRCUMPOLAR CURRENT

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1979 1980

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. 70 -'-I , I , i , I , I , I , m , i , i , , , I , I , - 70 Jan. Fig. 6. Transporttime series at Drake Passagein 106m3/s [Whitworth and Peterson, 1985-1' (top) net transport;(bottom) geostrophictransport in upper 2500 m relativeto 2500 dbar. NOWLIN AND KLINCK: PHYSICSOF THE ANTARCTICCIRCUMPOLAR CURRENT 475 worth [1983] and Whitworth and Peterson [1985]. The final Fu and Chelton [1984] constructed time series of north- net transport above 2500 m (Figure 6) has a mean of 125 south sea level differences across the ACC from crossover sea X 106 m3/swith a standarddeviation of 10 x 106 m3/s.As level differencesobserved by Seasat during July to October can be seen by comparing the geostrophictransport above 1978. The results reveal a general eastward acceleration of and relative to 2500 m (also shown) with the net transport, much of the ACC over that period with a decreasein the higher-frequencyfluctuations occur in the barotropic mode, intensity of the cyclonicWeddell Gyre (Figure 7). This first but 70% of the net is in the baroclinic mode. direct observational evidencefor large-scalecoherence in the Three shipboarddensity surveys of the passagewere made ACC demonstratesthe great potential of satellitealtimetry for in January 1979, April 1979, and January 1980 while the synopticobservations of temporal variability. transportmeasuring array was in place. Fifteen to 18 stations There have been severalattempts to relate the changesin were made along the current meter line. Referencingthe geo- net ACC transport, or across-passagepressure difference, at strophic transport calculatedfrom those data to the current Drake Passageto wind stressover the southern ocean. Three observationsat 13 moorings resulted [Whitworth et al., 1982] years (1976-1978) of pressure difference across the passage in threenet transport estimates: 117, 144, and 137 x 106m3/s. were interpreted by Wearn and Baker [1980] in terms of fluc- Whitworth [1983] removed the transport below 2500 m from tuations of the ACC. Then they related this transport time the first two of theseestimates, obtaining 113 and 127 x 106 series to zonal wind stressintegrated between 40ø and 65øS m3/s.These compared well with the net transporttime series over the southern ocean. A 28-day running mean filter was (from the monitoring array) averagedover the periods of the appliedto the series.A strongcorrelation between the circum- cruises:112 and 121 x 106m3/s. polar wind stress and pressuredifference was observed,in Pressurerecords from 500-m depths on both sides of the which the current laggedthe integratedstress by about 9 days. passagewere obtainedalso during the 2 yearspreceeding the This comparedwell with a simplemodel result of a 7-day lag 1979 experiment. Wearn and Baker [1980] reported on the which they also presented,suggesting that the pressurediffer- 1977 and 1978 results,which show a strong semiannualsignal. ence is forced globally rather than locally for periods longer These records have been added by Whitworth and Peterson than a month or that time changesin the ACC should be [1985] to the 1979 recordsto extendthe transport time series coherent around the circumpolar system. to 3 years. Comparison between net transport and transport Chelton [1982] then pointed out that the resultswere su- estimatedfrom pressuredifference during 1979 showsa maxi- specton statisticalgrounds: for the proceduresused, the co- mum differenceof 24 x 106 m3/s and suggestsannual and herencemay have resultedbecause both seriescontain strong semiannual fluctuations which are not phase locked. Ad- seasonalsignals. Moreover, it can be seenfrom a later paper ditional pressurerecords from March 1981 to March 1982 by Fu and Chelton[1984] that large-scalefluctuations in the show a pressure-derivedtransport ranging from 95 to 158 ACC may not be coherentin circumpolarextent (see Figure 7 x 106 m3/s, with a standarddeviation of 13 x 106 m3/s of this paper). [Whitworth and Peterson,1985]. However, Whitworth [1983] compared the net transport at Although no monitoring of the ACC with direct measure- Drake Passagewith the circumpolar-averagedzonal windsbe- ments has been done at locations other than Drake Passage, tween 43ø and 65øS for 1979, smoothingboth with a 10-day estimatesof ACC transport have been made elsewhere.Georgi low-passfilter. For periodsbetween 16 and 24 days,the two and Toole [1982] estimated zonal heat and freshwater trans- series were coherent at the 95% significancelevel, and the ports south of America, Africa, and New Zealand to obtain phaseimplied that the wind leadsthe net transportby about interocean exchangesfor comparisonwith those of workers 17 days.Whitworth noted that "neither(his nor Wearn and using different approaches.They calculatedtransport directly Baker's[1980]) time lag may be representativeof the response from hydrographicstation data by taking the deepestsample time to forcingof the circumpolarcurrent systemas a whole" depths as the referencelevel for each adjacent station pair, because localized disturbances which occur at different dis- whichresulted in transportsof 140and 125 x 106m3/s south tancesfrom Drake Passageduring the year are includedin the of Africa and New Zealand, respectively.These geostrophic zonally averagedwind stress.Comparing the 3-year (1977- transportswere adjustedusing a spatially uniform reference 1979) pressuredifference at 500 m with the circumpolar- speedso that the total volume transports matched the mea- averagedzonal wind stress,he has found (T. Whitworth, per- suredtransport at Drake Passage(taken to be 127 x 106 sonal communication,1985) that wind and pressuredifference m3/s).The differencesin resultingeastward zonal transports are coherentfor periodsbetween about 15 and 30 days, with througheach opening gave a netgain of heat(41 x 1013W) wind leadingby about 3 days.At this time one may conclude andloss of freshwater (0.23 x 106m3/s) for the Indian Ocean that different analysisprocedures, longer time series,or differ- but lossesof heat and gains of fresh water for the Atlantic ent typesof data will be requiredto resolvedthis issue. Ocean(23 x 1013W and 0.14x 106m3/s) and PacificOcean (18 x 10•3 W and 0.08 x 106m3/s). Calculations of the trans- ports through Drake Passage.of heat relative to 0øC (13 x 10TM W) and salt (44 x 10TM g/s), as well as of dissolved 4. MERIDIONAL EXCHANGES oxygen(7 x 10TM mL/s) and silicate(104 x 105 g/s),have re- Early in this century, warm deep water was shown to cently been made using simultaneouslymeasured property extend upward to shallow depths near Antarctica and was and absolutevelocity fields from 1975 and 1979 (M. Giuffrida recognized as being related to the formation of an abysssal and W. Nowlin, personal communication, 1985). Such intero- layer of cold antarctic water. Reid [1981] reviewed the devel- cean exchangesof heat and fresh water, when combined with opment of this understanding.The surfacemodifications nec- estimates of air-sea heat and moisture fluxes south of 40øS, essary to produce these dense waters require large heat ex- gave Georgi and Toole the basis for estimating meridional changefrom the oceanto the atmosphere.An equivalentpole- heat and freshwater fluxes from the southern ocean into each ward heat flux must exist within the ocean. of the three major oceans. We may examine this poleward heat flux by consideringthe 476 NOWLIN AND KLINCK: PHYSICSOF THE ANTARCTIC CIRCUMPOLARCURRENT

240 ø

0

180ø s/"• ß

1

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Fig. 7. The low-frequency(periods of longerthan 20 days)change in sealevel difference from Seasatcrossover points during July to October 1978 [Fu and Chelton,1984].

heat balance for the southern ocean south of the ACC in the along such a path giveszero net barotropic flux acrossthe following form: path, to which deSzoekeand Levine assigna worst caseerror of + 1.0 x 10 •'• W. For the baroclinic heat flux their circum- Q = F• + FE• -{-F e -{-Ft, ½ polar integration also gave zero, with a worst case error of where Q is the sum of the ocean-to-atmosphereheat ex- + 2.3 x 10 •'• W. changessouth of the ACC and terms on the right hand side Using averagetemperatures of the upper 100 m, taken from representpoleward heat fluxes across the ACC asfollows: Fg, their hydrographic stations, and Ekman transport from a heat flux by mean geostrophiccurrent, excludingdeep bound- graphical display of annual mean zonal wind stressnormal- ary currents which are not well resolved;FEk, heat flux by ized by Coriolis parameterf [Taylor et al., 1978], deSzoeke surfaceEkman transport; Fe, eddy heat flux; and F•,c, heat and Levine estimate a total equatorward heat flux due to transport by deep boundary currents. Gordon and Taylor Ekmantransport (FE0 of 1.5 x 10•'• W (___50%).The corre- [1975] estimated the annual average heat loss from ocean to spondingestimate of Ekman volumetransport was 28 x 106 atmospheresouth of the Polar Front to be 4 x 10•'• W. A m3/s. The monthlymean windscompiled by Han and Lee value of 54 x 10•3 W for polewardocean heat flux across [1981] were usedby Clifford [1983] to computeaverage equa- 60øS was obtained by Gordon [1981]. This estimate is sup- torward Ekman transport acrosscircumpolar paths which she ported by Hastenrath [1982]. To obtain poleward heat flux identified for the Polar, Subantarctic,and Subtropicalfronts. across the Polar Front, Gordon subtracted the ocean heat Her valueof 30 x 106 m3/sacross the Polar Front is in good gain between 53ø (the averagelatitude of the Polar Front) and agreementwith the estimateby deSzoekeand Levine. 60øS as given by the works of Zillman [1972] and A. F. To summarize, ocean-atmosphereheat exchangesouth of Bunker (personalcommunication with Gordon, 1985).He ob- the Polar Front and equatorward Ekman transport of warm tainedthe value3 x 10•'• W, as wasreported by deSzoekeand surfacewater acrossthe front togetherresult in a mean heat Levine [1981]. This heat exchangeis determinedfrom climato- loss of 4.5 x 10 •4 W from the ocean south of the ACC. The logical studiesof surfaceconditions. Its value is important to balancing poleward heat flux must be by oceanic processes. the heat budget of the southernocean (and that of the world Becausethe effectof meangeostrophic flow (exceptingper•- ocean as well), but the uncertainty of our presentestimate is haps deep narrow outflows) has been shown to be small by likely large (perhaps30 to 50%). deSzoekeand Levine, eddy processesand deep boundary cur- The advectivegeostrophic heat flux (Fg)across a circum- rents remain as the prime candidatesresponsible for this pole- polar path was determined by deSzoekeand Levine [1981], ward heat flux. using a selectionof historical hydrographicdata taken to rep- DeSzoeke and Levine [1981] point out that their calcula- resent the mean conditions.The path chosen,which lies close tions do not account for ageostrophicmotions, such as deep to the mean path of the Polar Front, was that on which the boundary or interior currents with significantinertial or fric- vertically averagedpotential temperatureis 1.3øC.Integration tional terms. They also acknowledgethat the coarsesampling NOWLIN AND KLINCK' PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT 477

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3000 Fig. 8. Distribution of record length mean, across-streameddy heat flux (in kilowatts per square meter) computed from 40-hour to 90-day band-passedtemperature and velocity relative to the 90-day low-passedvelocity. These yearlong 1979 mooringsspanned Drake Passagefrom NS500 on the shelf south of Cape Horn to SS500 on the shelf off Livingston Island. Asterisks denote values significantly different from zero at a 95% confidence level. Equatorward fluxes are cross-hatched[Nowlin et al., 1985.]

(46 blocksof stationsto representa 25 x 103 km long path) bands and coordinate systemselected for the analysis.When coupled with the rugged bottom topography along the path moorings blow over during periods of high current speeds, could have left some deep outflows unaccounted for. As an errors may be introduced into the eddy heat fluxes.Nowlin et extreme example, they calculate a poleward heat flux of 1.5 al. [1985] correctedall the 1979 recordsfrom Drake Passage x 10TM W for a deep outflow of 20 x 106 m3/s with an by usingconcomitant pressure time seriesand vertical temper- average potential temperature of -0.5øC. Direct monitoring ature gradients (after the proceduresof Rojas [1982]) and of the major deep outflows is required to determine whether showed that failure to correct for instrument blow over could or not the heat transport by deep boundary currents is signifi- result in overestimates of meridional eddy heat flux by as cant. much as 20%. Bryden [1979] estimatedmeridional eddy heat flux from six Although short time scaleprocesses (such as tides, internal nearly yearlong measurementsof temperature and velocity waves,and inertial oscillations)are routinely eliminated from from depths near 2700 m at locations spanning Drake Pas- heat flux estimatesby low-pass filtering, low-frequency con- sage. Multiplying his average poleward heat flux estimate of tamination of eddy heat fluxes is not usually considered. 6.7 kW/m2 by a depthof 4 km and a circumference(length of Long-period events can impose unwanted, dominating cross ACC) of 2 x 104 km givesa lateralflux of 5 x 1014W. Scire- correlation (eddy heat flux) between fluctuation temperature mammano[1980] made additional estimatesof eddy heat flux and velocity. In Drake Passage,low-frequency variability in in Drake Passage.He compared estimatesbased on 1976 re- some current recordsis associatedwith sporadic lateral shifts cords from deep meters at northern, central, and southern of the fronts within the Antarctic Circumpolar Current. Defin- passagelocations with those obtained by Bryden using 1975 ing an effective eddy time scale (40 hours to 90 days) and records from the same locations and found little difference in band-passingthe current recordsbefore calculatingeddy heat the northern and southern records. Using 1977 measurements flux, Nowlin et al. [1985] obtained values which are more from a cluster of five moorings in the central passage,Scire- homogenous (consistent with one another) in direction and mammano extendedBryden's estimates over the depth range have higher statisticalsignificance. from 1000 to 2500 m. The variation in estimatededdy heat Finally, for the circumpolar current, they defined "pole- flux at this location over 3 years and all depths was large ward" eddy heat flux as that component perpendicularto the (9-28 kW/m2) with a meanof 17 kW/m2, whichis consider- axis of the current, which forms a continuous but distorted ably larger than Bryden'sdeepwater value of 6.7 kW/m2. It band encircling Antarctica. Because the current direction should be noted that these reported heat flux estimatesare varies in time, across-streameddy heat flux was calculated from records at least 5 months in duration that have not been relative to the 90-day low-pass direction. The resulting heat correctedfor temperaturevariations due to vertical motions of fluxesfrom 1979 are shown in Figure 8; only poleward fluxes the instruments. were statisticallysignificant. Values from the deep instruments During 1979 a large yearlong array of current and temper- and thosein the southernand central passageare of the order ature recorderswas in place in Drake Passage.This data set of 1 kW/m2; thosein the northernpassage are an order of was used by Nowlin et al. [1985] to examine the character- magnitudelarger. The averageacross-stream flux for all avail- istics of eddy heat flux and its distribution at that location. ableinstruments is 3.7 kW/m2 poleward.This value,if repre- The seemingly straightforward calculation of heat flux can sentative of the average eddy heat flux through the ACC, produce misleading results unless measurementalias is re- yields a net (circumpolar) poleward heat flux due to eddy moved and careful considerationis given to the frequency processesin the 2- to 90-dayband of 3 x 10•4 W. 478 NOWLIN AND KLINCK' PHYSICSOF THE ANTARCTICCIRCUMPOLAR CURRENT

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I--I < 200 cm•'s2 I--I 200-500 cm•'s2 I >500 cm•'sz

Fig. 9a. Kinetic energyof mean surfaceflow as indicatedby FGGE buoy data [Patterson, 1985]. Hourly velocitiesare determinedby splinefits to buoy positions.Velocities are averagedover 5ø x 5ø squares.

Cospectraof fluctuation temperature and velocity show that the lateral heat and salt diffusive exchangesacross a water heat flux is separated, on the average, into two frequency mass boundary with the vertical transport acrossboundaries bands with correspondingperiods between 100 and 40 days of intrusions (interleaving),to obtain estimatesof lateral dif- and between16 and 10 days.This pattern comparesfavorably fusivityfor verticalscales of interleavingbetween 1 and 100 m with the cospectrumfrom a 5-year bridged record from the for an assumedrange of verticaldiffusivity. Assuming that the central passage.Comparison with previous estimatesin this Drake Passageestimates are representativeof the circumpolar region showsconsiderable differences, partly due to the use of front, they estimated that the intrusive heat flux acrossa 500- different techniques.However, analysis of five 1-year records m-deepPolar Front would be 3.6 x 1013W, or only about from a single location shows that interannual variability is 10% of the requiredheat flux. On the basisof budgetstudies, large, which urges caution when interpreting the significance the intrusivesalt flux was estimatedto be more significantin of isolated short-term estimatesof eddy heat flux. relation to the likely total salt flux acrossthe ACC than was Brydenand Heath [1985] estimatedmeridional heat flux at the intrusive heat flux in relation to the total heat flux. With the northern edge of the ACC using current and temperature the exceptionof the work by Joyceet al. on intrusivesalt flux, recordsfrom a 2-year array moored southeastof New Zealand few studiesof other processesresponsible for transportof salt (near 49ø30'S and 170øW). Their values varied from 1 to 35 acrossthe ACC have appeared. kW/m2, fallingwithin the rangeof valuesobserved at Drake Passage.However, becauseof the long time scalesof energetic 5. KINEMATIC AND DYNAMICAL ESTIMATE• variability, Bryden and Heath found the overall eddy heat flux was not statisticallysignificant, although the portion in the 20- Circumpolar Distributions to 40-day band is significantlypoleward. During the past 10 years the first estimatesof circumpolar The current rings which detach from the fronts of the ACC distributions of kinematic variability and surface kinetic are assumedto account for much of the meridional transport energyhave becomeavailable. Basedon geopotentialanoma- acrossthis current. Toward the end of section 5 are presented lies, ship drift records,surface drifter trajectories,and satellite someestimates of heat, salt, and energylevels which have been altimetry, these distributionsreveal regional differencesin ki- estimatedfor individual rings. netic energyand variability over the path of the ACC inferred Measurementshave been made by T. Joyce and coworkers to be associatedwith current-topographyinteractions, interac- of small-scale structure at the Polar Front and within the tions of the ACC with other current regimes,and boundary Polar Frontal Zone both at Drake Passage [Joyce et al., effects. 1978] and southeastof New Zealand.The Drake Passage Wyrtki et al. [1976] used observations of surface drift cur- observationswere usedby Joyce [1977], in a model balancing rents made by merchant vesselsto calculate kinetic energy NOWLIN AND KLINCK: PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT 479

0 o Z0 øW Z0 øR

40 øW 10 øS 40 øE

:30 øS 60 øW OøE 5oø!

80 øW ?'0 ø - • 80 øE -

1øøøI / 100øE

120 120øE

140øW ' %•-'-•• 140øE

160øW 160øE 180 ø

7-] < 200 cm•'se EZ3200-500 cm•'se • >500 cm•'sz

Fig. 9b. Eddy kinetic energyof surfaceflow from FGGE buoy data [Patterson,1985] calculatedby subtractingthe mean kinetic energyfrom the total kinetic energy.

(KE) of the mean surface flow and of the fluctuations, inter- linear tracks from the mean profiles were calculated to repre- preted as eddy kinetic energy (EKE), for the world ocean sent variations of sea surfaceheight. These transient features based on a 5ø grid. Although the sparcity of data from high were studied to characterize length scales, dynamic height southern latitudes precluded good coverage of the ACC, a relief, and translational and surface geostrophic velocities. generaldistribution of KE of the mean flow and some indica- Cheneyet al. [1983] presenteda global distribution of meso- tions of regionswith unusuallyhigh EKE (e.g.,Drake Passage scale variability of the sea surfacebased on Seasatdata. The and south of Africa) were revealed. high variability of the ACC extended nearly continuously Using all available hydrographic stations, Lutjeharrnsand around the southern ocean. Only in the extreme southeast Baker [1980] carried out a statisticalanalysis of the mesoscale Pacific and in the central South Atlantic were values of sea variability of the southern ocean. They presentedpatterns in level rms variability lessthan 5 cm basedon 2ø gridded values. the circulation intensityas expressedby the varianceand hori- Values in excess of 6 cm were obtained over much of the ACC zontal structure function of the 0/1000-dbar dynamic height path, decreasingto the north and south.The largestvariability interval. The intensity of the mesoscalefield was shown to be again seemedto be associatedwith areas of major topograph- inverselyproportional to distancefrom the mean ACC axis, ic relief. suggestingthat the Polar or Subantarcticfronts may be gener- Using data from approximately300 drifting buoysdeployed ating eddiesover the length of the ACC. An upper limit of 150 during the First GARP (Global AtmosphericResearch Pro- to 250 km for the horizontal length scaleof mesoscaleturbu- gramme) Global Experiment (FGGE) during 1979, Garrett lence was found for most of the southern ocean. Distinct pat- [1981] computedthe mean and eddy kinetic energy on a 10ø ternsof high intensityof the variability were closelycorrelated latitude by 10ø longitudegrid for the southernocean. He used with prominenttopographic features on the seafloor. differencesbetween buoy positions interpolated at 1200 UT Collinear altimeter data from the last 25 days of Seasat each day to produce24-hour averagedsurface velocities. The have been used in several studies of mesoscalevariability in- results showed a general increasein KE levels, particularly cluding the region of the ACC. Colton and Chase [1983] se- that of the mean flow, associated with the ACC. However, the lected three regionsin which to study sea surfacevariability grid size was too large to permit much definition. Patterson inducedby interactionof the ACC with bottom topography: [1985] fit spline functions to the FGGE buoy positions to the Indian-Antarctic Ridge south of Australia, representing obtain hourly time seriesof buoy velocities.For each 5ø x 5ø zonal flow along a zonal ridge; the Macquarie Ridge south- grid, these were averaged,first to obtain monthly velocity westof New Zealand, representingzonal flow over an isolated componentsand then to obtain record length meansused to bump; and the Indian Mid-Ocean Ridge south of Africa, rep- construct the KE distribution of the mean flow presentedin resentingzonal flow over a meridionalridge. Residuals of col- Figure 9a. The pattern showsthe highly zonal distributionof 480 NOWLIN AND KLINCK: PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT

surface mean kinetic energy associatedwith the ACC. Rela- near surfaceto depths of somewhatmore than 2000 m. There tivelylarge mean KE values(> 500 cm2/s2) appearwithin the is typically bottom intensification over regions of locally ACC near major bathymetricfeatures; relatively small values rugged topography, as is demonstrated by the structure of are found within the ACC in the extreme southeast Pacific vertical empirical orthogonal modes, leading to a decoupling and south of Australia, upstream of major topographicob- between middepth and near-bottom currents in those locales. structions. Sciremammanoet al. found temperaturefluctuations strongly Most of the KE of the surfacecirculation is in the eddyfield correlated for all depth pairings observed except for instru- (Figure 9b). Eddy kinetic energy was calculatedas the differ- ments positionednear the northern extent of the temperature encebetween total and mean kinetic energiesfor each 5ø x 5ø minimum layer (the Polar Front); there the temperaturestruc- box. Unlike the distribution of KE for the mean flow, which is ture is characterizedby inversionsand isolated cold and warm zonal in nature, the highestvalues of EKE are associatedwith featuresof small spatial extent. western boundaries,though secondarymaxima do occur in Sciremammanoet al. [1980] also examined the horizontal patcheswithin the zonal flow of the ACC shown in the mean scalesof velocity and temperaturefluctuation by studyingthe KE pattern. The background meridional gradient seenin the plots of zero-lag, normalized cross correlation versusstation high-frequencyEKE is probably due to the influenceof wind separation. The spatial decorrelation scale was taken as the variance.The distributionsseem generally consistent with the separation distance for which the correlation crosseszero. Ve- distributionsof variability mentionedearlier, suggestingthat locity fluctuationsin both through- and across-passagedirec- the contribution to the variance by local wind effectsis not tions were narrow (30-40 km) normal to their flow direction large. However, this remains to be examinedin detail. and elongated(50-80 km) along the flow. Horizontal scalesof Using trajectoriesof the FGGE surfacedrifters, Holmann temperature fluctuations were found to be approximately [1985] examined the circumpolar structure of the ACC. She equal in through- and across-passagedirection, with valuesof found good correspondencebetween increased surface speeds 80-100 km. and the historical locations of the Polar, Subantarctic, and lnoue [1982] added to the 1977 data used by Sciremam- Subtropical fronts. The mean near-surfacespeeds were ap- mano et al. the data from nine mooringsin the 1979 array proximately 40 cm/s for the PF and SAF, but only about 25 distributedacross the passage.He examinedvertical structure cm/s for the Subtropical Front. Greater drifter densitieswere of 40-hour low-passedcurrents in terms of empiricaland dy- found to be associated with the fronts than with the water namic normal modes. The barotropic and first baroclinic massregions which they separate,implying that there is hori- modes dominated at all locations, together accountingfor zontal convergenceat the surfacein thesefronts. Regions with 83-99% of record variance. The first empirical mode, with large bathymetric featuresgave the clearestindications of a more than 90% of the variance at most moorings,is surface bandedcurrent structureand the greatestmeridional zonation intensifiedand appearsto be a combinationof barotropicand in speed.This suggestedto Holmann that the lateral motions baroclinic modes. Time scalesare 20-50 days for the first of frontsmay be inhibitedby bathymetry. empiricalmode and 7-20 days for the bottom-trappedsecond empirical mode. These relatively short time scalesfor varia- Site SpecificEstimates bility at depth are consistentwith the scales(average of 14 As a result of the ISOS monitoring efforts we now have days)reported by Brydenand Pillsbury[1977] for depthsfrom estimatesfrom the ACC of many kinematic properties,for 2500 to 3100 m. example,integral time scales,temperature and velocitycorre- Inoue found the horizontal scales of coherence for the first lation lengths,and kinetic energy levels.These are, however, two dynamicmodes and the first empirical mode to be similar specificto two locations. A moored array was maintained to scalesdescribed by Sciremammanoet al. [1980]; scalesare from April 1978 to May 1980 near 49ø30'S,170øW southeast perhaps somewhatlonger in the southern passageremoved of New Zealand. The site was chosen to be north of the ACC from the major flow of the ACC but are still shorter than 80 in the hope that northwardeddy momentumflux, indicativeof km, for whichBryden and Pillsbury [1977] foundvelocity fluc- energydissipation from the ACC, would be observed[Bryden tuations to be independent. Inoue found the scales of the and Heath, 1985]. In Drake Passage,five separateyearlong secondempirical mode to be smaller for longitudinalsepara- arrays of moorings were maintained during the years 1975- tions than estimatedby Sciremammanoet al. 1980. Observationsthere spannedthe ACC and are the basis By examiningcross correlations for maximum lag, propaga- for most of our presentinformation. It shouldbe pointed out tion speedsfor the fluctuationscan be estimated.Based on the that although by definition the entire ACC passesthrough midpassagearrays during 1976 and 1977, the propagation of Drake Passage,one cannot observe transitionsin energy fluctuationsin temperatureand velocity componentsis to the levels,scales, etc., equatorward of the current becausethe Sub- east in a through-passagedirection (about 60øT).Propagation antarctic Front is typically located close to the northern speedsaverage 12 cm/s (rangingfrom 9 to 20 cm/s),consistent boundaryof the passage.However, the transitionregion pole- with estimatesof the propagation speedsof rings and mean- ward of the current can be observed in the southern half of the dersthrough the region. passage. Bryden and Heath [1985] have summarized the results of Within the ACC at Drake Passage,the typical time scaleof their study of mesoscalefluctuations using data from the dominant velocity fluctuations estimated as the first zero measurementsite southeastof New Zealand just north of the crossing of the autocorrelation function, varies from 1 to 4 SubantarcticFront. They say that "energeticeddies are found weeks. For records below 2 km the average time scale is 2 to have typical amplitudesof 20 cm/s at 1000-m depth, to be weeks I-Brydenand Pillsbury, 1977]. On the basisof data from vertically coherent throughout the water column, to vary over a cluster of five moorings located near the southern edge of temporal scalesof 30 d and horizontal scalesof 60 km, and to the Polar Frontal Zone in 1977, Sciremammanoet al. [1980] propagate southeastwardat about 12 cm/s." They find their found velocity fluctuationsto be significantlycorrelated from maximumvalue of eddykinetic energy (169 cm2/s:) at their NOWLIN AND KLINCK: PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT 481

rve__] 36% I 64% IMean I I Fluctuatiøn! 46% I 54% T!•29% 39% • 31% I 50d:•T>2dI 33% 38% 27% ' ' 49% 1 I

diurnal tidal bands [InertialOscillations [InternalWaves ] I Indiurnal plussemi- I 10% 27% I 56%•1 44% coh.=rent [ I Notcoherent wit • tidal ! I with tidal pot.=ntial [ Lpotential Fig. 10. Partitioningof kinetic energyof horizontalmotions at Drake Passage.Fractions shown above boxes are averagesfor the entire passage;those below the boxesare averagesfrom centralpassage locations [after Nowlin et al., 1986]. shallowestrecord depth (1000 m). Though the horizontal ring from the SubantarcticFront (as well as severallarge scalesfrom the Pacific location are somewhatlarger than meanders).All of theserings had radii of the order of 50 km, those in Drake Passage,the other resultsare in remarkable surfacespeeds consistent with their formation from the ACC agreement,suggesting similar dynamicalprocesses. Eddy ki- fronts, and deep-reachingdensity structure, and they moved neticenergy values for fluctuationslonger than 2 daysin the northward or northeastwardat speedsof 5-10 cm/s. All but northernDrake Passage,again just north of the Subantarctic oneappeared to be steeredby bottom topography. Front, are over 140 cm2/s2 at depthsbelow 2000 m and in- Many individual ring sightingshave been reported,not all creaseto over 200 cm2/s• near600-800 m. in Drake Passage.For example,Gordon et al. [1977b] inter- The kineticenergy levels in the ACC at Drake Passagehave preted 1975 hydrographicobservations in the eastern Scotia beenstudied in considerabledetail. Nowlin et al. [1981] exam- Sea as indicative of a ring of subantarcticwater within the ined 31 long recordsand found no significantinterannual Polar Frontal Zone, and Savchenkoet al. [1978] observed a variabilitybased on comparisonsover 4 years,but theyfound larger (140-kin diameter) cyclonic ring in the Polar Frontal large spatial variability.Examinations of the fifth year'sre- Zone south of Australia in 1977. cordsconfirms this. The generaltrend is for mean KE values Pillsburyand Bottero [1984] interpretedthe recordsfrom a of 5-15 cm:/s• at 2500-3000m increasingupward to values midpassagelocation south of the Polar Front in the Antarctic near 300 cm:/s2 at 500 m. Eddy kineticenergy values for Zone as showingthat five cyclonicrings and one anticyclonic fluctuationperiods of longerthan 3 daysare ratheruniform in ring passedthe locationbetween June 1975 and January 1976. the southernand centralpassage (near 20 cm:/s:),increasing The cyclonicrings had the propertiesof the continentalwater northward throughthe ACC to maximumvalues at the north- near the Antarctic Peninsulaand were likely derived from the ernmost moorings.(Total EKE in the deep water increases Continental Water Boundary; the anticyclonic (warm core) acrossthe passageby a factorof 5.) The averagepartitioning ring is believedto have separatedfrom the Polar Front. The betweenmean and eddy kinetic energyand betweenthree rings had diametersof from 30 to 130 kin, extendedvertically frequencybands for EKE is shownin Figure 10. Studiesof the to at least 2500 m, and had maximum spin velocitiesof about principalshort-period tides [Nowlin et al., 1982] and of inter- 20 cm/s at 1000 m and 10 cm/s at 2500 m. nal and near-inertialoscillations [Nowlin et al., 1986]have led A ring censushas not proved feasibleusing satelliteimages to estimatesof the fractionsof eddykinetic energy for periods of temperature or color obtained from visible measurements of 2 hoursto 2 dayswhich are associatedwith thesephenome- becauseof the very high incidenceof cloud cover. Based on na at Drake Passage(Figure 10). the studies of time series during 1975 and 1979 at Drake Numerouscurrent rings and meandershave beenreported Passage,however, the number of ACC rings in existenceat at Drake Passage.On severaloccasions, rings have beenob- any time must be large if theseyears are typical and the Drake servedduring formation (to date,always in the Polar Frontal Passageis characteristicof the circumpolarsituation. Zone).Shipboard observation of coldcore rings in the process Available heat and salt anomalies, available potential of formationfrom the Polar Front has occurredtwice, as was energy, and kinetic energy of various rings have been esti- reportedby Joyceand Patterson[1977] and Petersonet al. mated and reported [Joyce et al., 1981; Petersonet al., 1982; [1982]. When last observed,the ring trackedby Petersonet Pillsbury and Bottero, 1984]. Heat and salt anomalies were al. appearedto be pushingnorthward through the Subantarc- calculatedby comparing the temperature and salinity within tic Front. On the basisof weeklysynoptic maps depicting density intervals in the rings with characteristicsat the same changesin frontalpositions for 1979(constructed using tem- densitieswithin the waters surrounding the rings. Available peratureand velocitydata from 15 mooringsin the northern potential energieswere calculatedfollowing the suggestionsof and centralDrake Passage),Holmann and Whitworth[1985] Reid et al. [198 l l. Vertical sectionsof available heat and salt describedthe formation and movement through the passage of anomaliesare shown by Joyce et al. [1981] and Petersonet al. threecold corerings from the Polar Front and one warm core [1982], and the former picture kinetic and available potential 482 NOWLIN AND KLINCK.' PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT

TABLE 1. Estimatesof PropertyAnomalies for SomeSouthern Ocean Current Rings

Petersonet al. [1982]'[' Pillsbury and Bottero [198535 Joyce et al. Relative to PFZ Relative to SAZ [1981]* Characteristics CharacteristicsAnticyclonic Cyclonic

Availablepotential energy, J 5.1 X 1014 ...... 0.9 x 1014 3.9 x 1014 Kineticenergy, J 3.4 X 1014 ...... 0.5 x 10 TM 1.5 x 10 TM Heat anomaly,J 1.2 x 1019 0.8x 10 •9 3 x 1019 1.Ox 1018 -1.9x 1018 Salt anomaly,kg 2.5 x 1011 2.0 x 10 TM 8 x 10 TM ......

*Anomalies,relative to Polar Front Zone characteristics,were integratedbetween a t = 27.0 and 27.7 (approximatelysea surfaceto 1500 m in PFZ) basedon a sectionthrough a 1975 cyclonicring in the Polar Frontal Zone. Kinetic energyis basedon geostrophicspeeds relative to 2800 dbar. •'Anomaliesintegrated from at - 27.0 to 27.8 (approximatelysea surface to 2500 m in the PFZ) based on a vertical sectionthrough a 1979 cyclonicring in the Polar Frontal Zone. $Valuesof velocitiesand temperaturesobtained from a symmetricring model with parametersdeter- mined by time seriesobservations of theseparameters. Values shown on the left are for an anticyclonic ring of Polar Frontal Zone water and those on the right are the average for five cyclonic rings of continental water, all observed in the Antarctic Zone. Anomalies relative to Antarctic Zone character- isticswere integrateddown to 3500 m as a nominal bottom depth.

energiesand potential vorticity are shown as well. In Table 1 hydrography. The stability problem was solved in terms of we present for comparison integrals over the ring volumes east-westpropagations with sinuosidalstructure in the north- given in thesethree papers.Note that rings from two different south direction. The ninth north-south mode, with a width fronts (Polar Front and Continental Water Boundary) are scaleof 78 km, was analyzedfor stability. The most unstable given and that anomalies are estimatedrelative to the charac- wave had a growth rate of 16 days and a length scaleof 143 teristics of different water mass zones. Each set of authors km. Energyconversion estimates from the observationsagreed speculateson the consequencesof ring formation for meridio- well with characteristics of the most unstable wave from the nal transfers across the ACC. model. Several researchershave analyzed the stability of flow in lnoue [1985] used the extensiveDynamic Responseand Drake Passage;most found the flow, even though defined by KinematicsExperiment of 1979 (DRAKE 79) observationsto differentmeasures, to be baroclinicallyunstable. Furthermore, considerthe questionof flow stability.He carefullyconsidered the characteristicsof the instability are similar for the various the characteristics of the different water mass zones in Drake analyses.Fandry [1979] used the averageof all current obser- Passageand found that all of the zones are baroclinically vationsfrom Drake Passagefor the years 1975-1977 to obtain unstable. The fastest-growingwave has a length decreasing a mean speed and mean shear as a function of depth. The from 177 km in the northernpassage to 91 km in the southern Brunt-V/iis/il/i frequencywas estimated from hydrographic passage.Growth rates are slow (15-40 days) between the data. Fandry found the unstable waves to have lengths of fronts and fast (3-5 days) in the fronts. The instability is a between 123 and 390 km and the most unstable wave to have mixture of barotropic and first baroclinic modes, with two a growth rate of 25 days.Bryden [1979] analyzedthe stability thirds of the energygoing to the barotropicmode. of flow in central Drake Passageusing repeated hydrography In a study of current rings in Drake Passage,Pillsbury and and a single current meter mooring to estimate the Bottero [1984] examinedthree long current records(at 1020, Brunt-V/iis/il/iprofile and the mean currentshear, respectively. 1520, and 2520 m) from a single mooring in the Antarctic The flow was found to be unstable,and a detailedcomparison Zone. This is the same mooring from which Bryden's[1979] of theoretical expectationswith observationsyielded mixed analysis of the complete 12-month data set indicated con- results. However, energy conversion was estimated from the ditions favorable for the production of mesoscalewaves by data and found to be within a factor of 2 of the estimated meansof baroclinicinstability. Deleting thoseportions of the wind energy flux from the atmosphere.Peterson et al. [1982] recordscontaining rings, Pillsbury and Bottero found that for examined the necessarycondition for baroclinic instability the period from February to October 1975 the conditions based on the vertical shear of zonal geostrophiccurrent and were indicative of energytransfer from eddy kinetic energyto the hydrostatic stability calculatedfor each of the four water available potential energy.Transfer rates (calculatedfollowing mass zones in Drake Passageby using 1976 hydrographic Wright[1981]) wereof the orderof 2 x 10-5 ergs/cm3/s,with data. They found the necessaryconditions for instability met the valuesdecreasing with increasingdepth. They concludedit in each of thesezones even though they are betweenthe cur- was unlikely that the six observedrings originated under the rentcoi'es (or fronts)and haverelatively weak vertical shear. conditions observed at that site in the Antarctic Zone because Using the samedata set, they also found necessaryconditions the conditionsdid not support the formation and growth of for barotropic instability adjacent to each of the three fronts eddiesthrough baroclinicinstability. in Drake Passage. Severalobservational studies have considered,peripherally, Wright [1981] analyzed observationsfrom a compactclus- to what extent the ACC is in geostrophicbalance. Using 1975 ter of current metersin central Drake Passage(part of the measurementsat Drake Passage,Nowlin et al. [1977] demon- First Dynamic Response and Kinematics Experiment stratedthat geostrophicshear from pairs of hydrographicsta- (FDRAKE 77) observations).Current shearand Brunt-V/iis/il/i tions agrees with measured shear at positions between the profiles were obtained from the 1977 current observationsand stationsif the direct current measurementsare appropriately NOWLIN AND KLINCK: PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT 483 averaged.The sameprocedure was usedsuccessfully by Whit- baroclinicity [Fofonoff, 1955], effect of continental land masses worth et al. [1982] in combining 1979 station data and direct [Stommel, 1957], and discharge of water from Antarctica current measurementsto estimateACC transport. [Barcilon, 1966, 1967]. Using data from the 1979 ISOS transportexperiment, Whit- Even before the Hidaka and Tsuchiya [1953] paper, a dy- worth and Peterson [1985] formed a time seriesof the differ- namical model of the ACC was advanced by Munk and ence between through-passagevelocity componentsat 500 m Palm•n [1951], who realized that viscousdissipation was in- averagedacross the passageand those at 2500 m. They com- sufficientto balance the wind stress.They suggestedthat form pared this serieswith the time seriesof 500- to 2500-m geo- drag, due to flow over bottom topography, provides the force strophic shear obtained from heavily instrumentedvertical necessaryto balance the wind. Moreover, they estimated that mooringsat the northern and southernedges of the passage. the drag force due to the four largest ocean ridgescan balance (The time serieswere smoothedwith a 10-day low-passfilter; the observed surface stress. the series lengths were approximately 12 months.) Even The study by Fofonoff [1955] is a direct extension of the though the estimateof directly measuredshear may not be a work of Hidaka and Tsuchiya [1953] to include stratification good representationbecause some current meterswere lost, and forcing by surfacebuoyancy flux which drives flow west- leavinggaps, the two shearestimates are coherent(at the 95% ward against the prevailing flow in the ACC. The geometry is level) and in phasefor periodsbetween about 25 and 50 days, a zonal channel, and both vertical and horizontal viscous ef- suggestingthat fluctuationsin the ACC are approximatelyin fectsare included.The transport calculatedfor various combi- geostrophicbalance. nations of surface forcing shows that stratification actually increasesthe transport becauseit causesthe wind-driven flow

6. THEORY AND MODELS to be reduced near the bottom, thus reducing the effectiveness of bottom viscousdissipation. The major conclusion is that Basic Dynamical Problem of the ACC the addition of baroclinicityis not sufficientto form a proper balance for a zonal model of the ACC. Hidaka and Tsuchiya[1953] applied the basic ideas of the Stommel [1957] was first to observe that the ACC does not developing theory of wind-driven ocean circulation to the flow in a zonal channelat all but that only a narrow band of southern ocean. A constant wind stresswas applied as the latitudes is not blocked by land barriers and that even this driving force; lateral and vertical viscouseffects provided the band is blocked by bottom topography that comes within dissipation. With reasonable choices for the friction coef- 1000 m of the surface. He maintained that most of the flow is ficients,the transport of the resultingcirculation was about 1 Sverdruplike, since pressure differencesare allowed across order of magnitude too large. The reduction of the transport continental boundaries.He further argued that viscousdissi- to reasonable levels required viscous coefficients approxi- pation takes place in the westernboundary currentsthat exist mately 100 times the largest "acceptable"values. The basic along land boundaries, with the principal dissipation oc- dynamical balance in the ACC appeared to be fundamentally different from the balance in a closed basin. curring downstreamof Drake Passagealong South America. The dischargeof water from the Antarctic continent can McWilliams and Chow [1981] developedan eddy-resolving, drive a westward flow, thereby reducing the transport of the three-layer model of a wind-driven zonal channel, which is ACC [Barcilon, 1966, 1967]. As water is dischargedfrom the equivalent to the Hidaka and Tsuchiya calculation but with continent, a free surface slope develops(down to the north) no explicit turbulenceclosure hypothesis. Eddies due to baro- which drives a westward current. There is a strong amplifi- clinic instabilitiesappeared in the flow, and thoseeddies redis- cation effect, so the circulation created is 100 to 1000 times the tributed momentum. It is possibleto calculate effectiveeddy discharge. Even this amplification is not sufficient to reduce viscositiesfrom correlationsof the velocity fluctuations.The the unrealistically large eastward transport calculated by eddy viscositiescalculated from the model were nearly equal Hidaka and Tsuchiya [1953] to result from the wind driving. to the valuesused by Hidaka and Tsuchiya to get reasonable Furthermore, the dischargemust comefrom melting snow and transports.Therefore it appearsthat subgrid-scaleviscous dis- ice, so the ACC should show strong seasonaltransport vari- sipation, in the absenceof lateral and bottom topography, ations. The observedvariations are not strong enough, and cannot be the only processbalancing the forcing of the surface wind stress. though this mechanismmay be important near the continent, The basic problem is to find a dynamical balance for the it seemsto be of little consequencein ACC dynamics. ACC that allows for observedsurface wind stressas a driving force while maintaining reasonable transport values. Trans- SimplifiedDynamical Models of the ACC port is a key variable used to test the applicability of models Dynamical models of the ACC discussedin this paper are to the prototype. Indeed, this observation has such impor- partitioned into three categories: simplified models, world tance that considerableeffort was expendedin the late 1970s ocean models, and mesoscalemodels. The simplified models to determine the transport of the ACC at Drake Passage.We address either reduced dynamics or a limited region of the now have a 1-year observedtime seriesof this transport and 3 ACC. World ocean models necessarilycontain the ACC, but years of additional good estimates modeled from across- only large-scale(hundreds of kilometers) processescan be in- passagepressure differences [Whitworth and Peterson,1985]. cluded because of computer limitations. Mesoscale models Given this improved estimate of the transport, it is appropri- addresseffects of dynamic instabilities and bottom topogra- ate now to reconsiderthe theoriesadvanced for the dynamical phy on flow in zonal channels.Each type of model considers balance within the ACC. some part of the dynamicsof the ACC, and theseresults will Four dissipativemechanisms have been consideredas possi- lead to more realisticdynamical understandingof the ACC. ble balancesfor the wind stress.Simply stated, theseare form Wyrtki [1960] made a detailedSverdrup transport calcula- drag due to bottom topography [Munk and Palm•n, 1951], tion for the southern ocean using his best estimate for the 484 NOWLIN AND KLINCK' PHYSICSOF THE ANTARCTIC CIRCUMPOLARCURRENT

0" 90'E 180- 90-w 0' A

•)e. WINDSTRESS CURL •:..- ...... 2•

• •*E t80' • •'w •

1.1 1.o 0/4000PREDICTED FROM _.•.... _•__ ..._.._.-..-"- B .9 WINDSTRESS CURL (55øS) //'• I ...... • .8 .7

.5 .4

.2

0.'•

O' 90øE E 180' W 90øW O* 45øE 135øE 135øW 45øW LONGITUDE

Fig. 11. (a) Wind stresscurl calculatedby Taylor [1978] from the data of denneet al. [1971]. Contoursare 10-9 dyn/cm3. (b) Dynamic height at 55øSas a functionof latitudefrom the data of Gordonet al. [1978]and predicted dynamic heightfrom windstress curl data shownin Figure1 la [from Baker,1982].

meridional structure of the zonal wind stress based on the yields a southward transport between 40ø and 90øW of 113 meagerdata availableat the time. The transport was integrat- -{-20 x 106 m3/s, which Baker contrastedwith 103+ 13 ed from the western coast of South America (the eastern x 106 m3/s geostrophictransport through Drake Passage boundary). A linear transport distribution was chosen for above and relative to 3000 m estimatedby Whitworth et al. Drake Passagematching the presumedstructure of the flow. [1982] from sevenISOS densitysections. Using the Sverdrup The resulting flow pattern coincided with the large-scale balance,Baker integratedthe wind stresscurl again along propertiesof the flow as it was then understood.In general, 55øSto obtaina southwardtransport of 190+ 60 x 106m3/s. the model ACC shifted to the south as it flowed eastward Thoughsomewhat large, this valuemay be comparedwith the through the Indian and Pacific oceans.Despite its correspon- four direct measurementsof total transport through Drake dencewith observations,this model is unsatisfyingbecause the Passage by Whitworth et al. [1982] of 117, 144, and 137 ACC transport is imposedon the solution and the important -{-6-15 x 106 m3/sand by Nowlinet al. [1977] of 124+ 15 questionof the "non-Sverdrup"dynamics that take placenear x 106 m3/s. This calculationindicates that the southern Drake Passageis not addressed. oceanis forcedby wind stresscurl which pusheswater across Using the wind climatology of denneet al. [1971] and Han latitude lines. and Lee [1981] and the hydrographic station data selectedby The wind-driventransport of the ACC is presentedby Ka- Gordon et al. [1978], Baker [-1982] made a similar Sverdrup rnenkot,ich [1962] as a combinationof barotropicand Ekman- calculation of the wind-driven flow across 55øS (chosenbe- driven flows. Geostrophicstream lines are used to evaluate cause it is at approximately the tip of South America). If the flow through Drake Passage.No stratificationis included,but Sverdrup balance is appropriate in the southern ocean, then smoothed bottom topography is allowed. Vertical friction is the water driven south across this latitude from 40øW east- usedto balancethe wind stress.The flow obtainedagrees with ward to 70øW should approximately balance the ACC trans- dynamic topography, and the transport is reasonable(130 port through Drake Passage, which then turns northward x 106 m3/s).The calculationshows the importance of bottom before flowing to the east again near 40øW. In Figure 11 are topographyin determiningcirculation in the southernocean. shown circumpolar distributions of wind stress curl and of Rattray [1964] analyzedthe time dependentbehavior of a dynamic height estimatesat 55øS as functions of longitude. two-layerfluid on a beta plane.The linear, primitive equations Integrating the 0/3000-dbar dynamic topography along 55øS are convertedto modal (internal and external)form and ana- NOWLIN AND KLINCK: PHYSICSOF THE ANTARCTIC CIRCUMPOLARCURRENT 485

The first complete model of the ACC [Gill, 1968] filled in the details of the dynamical hypothesisadvanced by Storereel [1957]. The model had linear dynamics, a fiat bottom, hom- ogenousfluid, and linear bottom friction; it is Stommel'sbasin model with a recirculating gap. The model equations are solved numerically and through a perturbation (boundary layer) technique. The wind is assumed to be zonal with a structurecomposed of the sum of a constant stress(Xo) and a part with sinusoidal north-south structure. Examples of the solution for two valuesof Xo (Figure 12) show that the model generally agreeswith observations.A western boundary layer develops,and most of the ACC enters this layer after flowing through the gap. Furthermore, the currents in the northern side of the gap are stronger than those in the south, as is (a) Xo=O •totJ('=l'91(5-7x 108 m3/s) observed in Drake Passage.While the resulting transport values are larger than measured transports, they are much smaller than transportsin strictly zonal models with the same values for the viscosities.This result showsthe importance of meridional boundaries on the flow in the southern ocean. Schulman[1970] presenteda numerical version of the Gill [1968] model including the effectsof bottom topography and nonlinearity. A stretched grid, primitive equation model is used,with the finestresolution (50 km) occurringin Drake Passageand along the straight continentalboundary and with increasingly coarse resolution in the rest of the southern ocean. For a flat bottom (5 km deep), the transport ranges from 70 to 200 x 106 m3/s for variousvalues of the bottom friction coefficient. The addition of nonlinear terms to the model dynamicschanges the appearanceof the flow; in partic- ular, the ACC overshoots the east coast of South America (b) X0=-0.%i3tod/'--0.65(2x108 m3/s) beforeturning north. However, the total transport of the ACC is not greatly changedby nonlinear effects.The most impor- Fig. 12. Numerical solution for a model of the ACC with meridi- tant result of this calculation is the effect of topography in onal walls representingSouth America and the Antarctic Peninsula [from Gill, 1968]. The windstressis zonal and is the sum of a constant Drake Passage on the total transport. The topography is (Xo) and a part that variessinusoidally with y. For the casein Figure added in four pieces:continental slopes(1 km deep), Scotia 12a, X o = 0 and the transportis 570 x 106 m3/s. For the casein Ridge and the South SandwichIslands (3 km deep),a shallow Figure 12b, a westwardstress (X o = -0.9) is applied,resulting in a plateau in Drake Passage(2 km deep), and the continental reducedtransport (200 x 106 m3/s) and an enhancedpolar gyre. (Taken from Figure 5 of Gill [1968].) shelf east of South America (1 km deep). When each of these featuresis separatelyadded to the model, it is found that the lyzed for oscillatorysolutions. A dispersionrelation, repre- plateau topography in Drake Passagehas the strongesteffect sentingtwo gravity wavesand one Rossbywave, is obtained on the transport,reducing it from 130to 25 x 106m3/s, while for waveshaving sinusoidalstructure in both horizontal direc- the other topographies cause only a 10 to 20% reduction. tions. For parameter values representativeof the southern With all four piecesincluded, the total transportis 20 x 106 ocean, the Rossbywave periods range from a few weeks to m3/s.The implicationof thisfirst complete model with topog- severalmonths for the externalmode and from yearsto thou- raphy is that bottom topographyis certainly effective,at least sandsof years for the internal mode. in a one-layercase, in reducingthe total transport. Devine [1972] included stratification in an inviscid, wind- The analytical study of Johnsonand Hill [1975] also shows drivenmodel of the ACC. A "porousplug" supporting a pres- the bottom topography to play a major role in reducing the sure differenceis placedin Drake Passageto avoid explicit wind-driven transport. A three-dimensional,homogeneous considerationof the westernboundary currents.The calcula- model with vertical friction is integrated along geostrophic tion is a combinationof a Sverdrupmodel and a thermocline contours passingthrough a recirculation gap in a rectangular model becausethe advectionof temperatureis included.The basin model. For topographic relief that is 20% of the total resultinggoverning equation is solvedby assumingthat the depth, transport may be reducedby as much as 85% (relative linear and nonlinear parts vanish separately,which implies to the fiat bottom case). Johnson and Hill argue that the that only the baroclinicflow advectstemperature variations, reduction in transport comes from increased bottom dissi- that the horizontalflow is nondivergent,and that verticalad- pation due to faster currents produced as water moves vection of temperatureis balancedby vertical diffusion.The throughshallower regions, such as Drake Passage. resultingACC transportof 260 x 106 m3/s is split about The interaction of stratificationand topography was con- evenly betweenbarotropic and baroclinicparts. The coldest sideredby Gill and Bryan [1971] by usingan eight-levelmodel surfacetemperatures occur east of Drake Passage,across of flow in a flat-bottomed,rectangular basin driven by zonal which there is a strongeast-west temperature gradient (about winds and an imposedsurface temperature distribution. Two 2øC). This model does not addressthe mechanismthat bal- forms were consideredfor the gap through which recirculation ances the wind stress. passes:extending to basin depth (deepgap) and extendingto 486 NOWLIN AND KLINCK: PHYSICSOF THE ANTARCTICCIRCUMPOLAR CURRENT

35-

40-

õ0 •, 0

==• '---: 4 60-

50W 0ø 40E 112E 140E 160E 180E *•)W 6bW Fig. 13. Transportstreamfunction for the southernocean as determined by Veronis[1973] from a Sverdrupcalcula- tion for a two-layerworld ocean model forced by themean wind stress distribution of Hellerman[1967]. This calculation allowedthe interface to reachthe sea surface, exposing the lower layer directly to windstress. The dashed curve along the northernedge of the regionpictured is the trace of that interfaceon the sea surface;thus the entire southernocean is modeledas one (lower) layer driven directly by the wind.

only half the basin depth (shallowgap). For the secondcase tion depthof the wind forcingis determinedby the strengthof the partial wall mimicsthe shallow area in Drake Passage. the wind. The interaction of the zonal current and the wind The Coriolis parameter is chosento be one tenth of the cor- driven gyre give rise to (1) a realisticdensity front betweenthe rect value so that the numerical solution will be stable with a subtropicaland subpolarcirculations and (2) a polewardshift coarse horizontal resolution but without the need for excessive of the centerof the gyre with depth. friction. The verticallyintegrated flow compareswell with the analytical model of Gill [1968]. The transport in the model The A CC as Part of a Global OceanModel ACC for the deepgap caseis 300 x 106 m3/s,while for the The ACC is really a part of the global ocean and shouldbe shallowgap caseit is 836 x 106 m3/s.The increasein trans- modeledin that manner.Currently, it is not possible,because port with topographyis shownto be due to a pressurediffer- of computer limitations, to solve numerical models of the ence acrossthe wall in Drake Passagewhich comes from a world ocean that have all of the dynamical processes,es- temperature difference(warm water to the west and cold water peciallymesoscale, that are thoughtto be importantin large- to the east).It is not clearwhether this effectis importantfor scalecirculation. However, some models exist having approxi- topographic barriers with finite widths. mationsto theseprocesses, and it is appropriateto assesshow Clarke [1982] analyzedthe dynamicsof wind-drivenflow in well they representthe ACC. Four modelsof varying com- a zonal channelwith a flat bottom.The emphasisof the study plexity are considered. is on the interactionof large-scalewind forcingon the large- The simplestmodel [Veronis, 1973] was a modified Sver- scale currents.Scale argumentsare based on east-westscales drup calculationfor a two-layerworld oceanforced by the of 7000 km and north-southscales of 700 km. The full dynam- climatologicalwinds of Hellerman[1967]. The lowerlayer was ics are reducedto the equationsgoverning thermocline dy- motionlessunless exposed to the wind (mainly in the polar namics,and a similaritysolution is proposedwhich partitions regions). Becausethe Sverdrup solution allows no latitude the flow into baroclinicand barotropicparts. The resultsof lines within the flow to continue without meridional bound- the studyindicate that barotropicflow shouldrespond mainly aries, the South Scotia Arc and the Antarctic Peninsula were to the zonally averaged(zero zonal wave number)wind with a assumedto block the ACC. The calculatedtransport through time lag of 10 or so days.This spindowntime is too rapid to Drake Passagewas more than 200 x 106 m3/s,and the struc- be due to bottom Ekman layers,so it is assumedto be due to ture of the flow in the southern ocean bore some resemblance Rossbywave drag on smallscale variations in bottomtopog- to observations(Figure 13). In spite of the reasonablygood raphy. The barotropic part of the transport, accordingto estimate for the total transport, this model addressesthe Clarke, shouldvary by about 65% over a year (measurements forced flow and not the higher order dynamicsof the ACC, show a maximum variation to be about 40% of the mean whichmay be especiallyimportant at Drake Passage.It does [Whitworth et al., 1982]). showthe role of simpleSverdrup dynamics in the largescale Much recent analysisof large-scalecirculation is from the circulation in the southern ocean. point of view of inviscidflow that conservespotential vorti- Bryan and Cox [1972] describeda nine-level,primitive city. In closedbasins the wind forcingmust deflectthe flow equationworld oceanmodel forced by the Hellerman[1967] sufficientlyto causelines of constantpotential vorticity to winds.The wateris assumedto be homogeneouseven though closeupon themselves; otherwise, no circulationis possible.In the verticalstructure of the flow is calculated.Separate polar, the southernocean there is the possibilityof somepotential mid-latitude,and equatorialgrids are used,and the solutions vorticitylines circling the globe(the ACC) allowingzonal flow are patched together betweenthese regions.The horizontal that need not be strongly forced. Haynes [1985] considers resolutionis 2øx 2ø, which gives a distancebetween grid how a gyre and a free zonal current interact. His analysis points which is everywhereless than 220 km. Lateral friction considerstwo-, three-, and four-layer modelsin addition to a is somewhathigh, resulting in a Munk boundarylayer 300 km calculation for continuous stratification. The occurrence of the wide, or 2 to 3 times too large. Solutionswith and without free zonal flow allows penetration to the bottom of the effect bottom topography are presented.The model ACC has a of wind forcing.In the absenceof the zonal flow, the penetra- transportof 32 x 106 m3/swith realisticbottom topography. NOWLIN AND KLINCK: PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT 487

These values do not compare well with the known transport the Atlantic Ocean. A nonlinear calculation shows that some in Drake Passage; stratification is assumed to provide the of this water turns sharply eastward in the region south of missingphysics and thus improve the comparison. Africa and flows back into the Indian Ocean. The structure of In a following study, Cox [1975] consideredthe effect of the wind curl over the ocean south of Africa determines the stratification by calculating the circulation with the average amount of water which moves westward and that which turns temperature and salinity fields.In one calculation the stratifi- back eastward into the Indian Ocean. Therefore the model cation was fixed, and the transport at Drake Passage in- results are quite sensitiveto the shape of the wind curl over creasedto 184 x 106 m3/s.A secondcalculation allowed the this small region of the ocean. stratification to change in response to the circulation. The Smith and Fandry [1978] use a two-layer model of a zonal model was run for only 2.3 model years,which was insufficient channel to compare the effects of stratification and bottom for a steady state. This resulted in a slight increase of ACC topography (essentially a two-layer version of the Kamenko- transportto 186 x 106 m3/s. vich [1962] model). Vertical viscosity is incorporated by in- Using a different primitive equation world ocean model, cluding surface, interface, and bottom Ekman layers; wind Bye and Sag [1972] consideredvertically integrated circu- stressdrives the circulation.The flow followsgeostrophic con- lation, with and without bottom topography, as forced by the tours (lines of constant f/depth) under the assumption that Hellerman [1967] winds. Three different numerical grids were planetary beta is small compared to topographic beta. For used, with horizontal resolutions between 2ø and 5ø. Viscous flow along a ridge, they find that the speed is higher and a dissipationwas providedby bottom friction alone. For the flat density front can develop on the equatorward side of the bottom simulation with realistic bottom friction, the ACC ridge. This effect may play some role in the formation of den- transportwas about 340 x 106 m3/s; with realisticbottom sity fronts in the ACC. topographythe transportreduced to 15-20 x 106 m3/smore McCartney [1976] examined eastward zonal flow of a stra- or less independentof the choice of friction parameter. The tified, rotating fluid over meridional and zonal ridges and over inclusionof stratificationwas suggestedas a means to increase isolatedseamounts. There is a naturallength scale of (u/•) •/2, the transport to a more realisticvalue. where u is the mean speedand • is the gradient of the Coriolis A coupled ocean-atmosphereclimate model [Bryan et al., parameter, associatedwith these flow interactions,implying 1975] includes a stratified (12 level) ocean with both realistic that mesoscaleflow patterns can be created from large-scale coastline and bottom topography. The maximum horizontal oceanic flow. In particular, a meridional ridge produces lee grid spacingfor the oceanis 700 km. A nonlinear equation of waves. A zonal ridge inducesfaster flow on the equatorward state is used to convert model temperature and salinity to side of the ridge and, for finite length, a vortex street down- density. The climatological steady state for the model ocean stream of the ridge. An isolated seamountproduces an anti- has reasonableflow patterns,but the current speedsare slower cyclonic meander on its upstream, equatorward side and a than observed.The ACC, in particular, has a transport of only cyclonic meander on its downstream, poleward side. The in- 22 x 106 m3/s.The zonallyaveraged model surface winds in tensity of thesemeanders increases with increasingheight of the southern hemisphere are weaker than observed surface the topography. winds, and the "Roaring 40's" are missingentirely, which may Boyer and Guala [1972] analyzed flow over the complex accountfor the low transport of the ACC. ridge and trench system south of New Zealand by using a one-layer numerical model and a laboratory model. The nu- MesoscaleDynamics in the ACC merical model usedf plane dynamics and was forced by un- The studies mentioned thus far (except that of McWilliams sheared,eastward flow. Vertical friction (A v = 10-3 m2/s)was and Chow [1981]) have taken a large-scaleview of the dynam- usedto damp the flow. The numericalmodel agreedquite well ics of the ACC; that is, the length scaleshave been longer than with the laboratory simulation. Neglecting beta effects, the severalhundred kilometers.Mesoscale dynamics, occurring on flow disturbance is trapped to the topography (geostrophic scalesof 10-200 km, need to be consideredfor possibleimpor- contoursare identical with topographiccontours), and Rossby tance in the southern ocean. Two general classesof mesoscale lee waves are not permitted. Given the large friction and the f effects are reviewed here: flow-topography interaction and plane assumption,the model cannot be compared easily with eddy-mean flow interaction. Both partial meridional barriers the observedoceanic currents in this region. and bottom topography can produce flow perturbations with Mesoscaleflow featuresare also created by barotropic and length scalesequal to a few internal radii of deformation (say, baroclinic instabilities. The resulting eddies interact with the 40-150 km). Such perturbationscan enhancedissipation in the mean flow to redistribute energy and vorticity both horizon- flow through the creation of mesoscaleeddies. tally and vertically. The analysis of strongly interacting eddy Wind-driven flow in a basin with a partial meridional bar- flows requires the use of numerical models with horizontal rier (simulating Africa) was consideredby de Ruijter [1982]. resolution at least as small as the internal radius of defor- Special emphasiswas placed on the dynamic processesthat mation. Such models put considerablestrain on present com- occur at the tip of the barrier, where the Agulhas Current puter resources,so they are usually employed to demonstrate leaves the continent and turns eastward (so-called "re- the dynamics in reduced domains rather than as complete troflection"). Both linear and nonlinear models with lateral replicasof a natural system. friction were used,in which the interior is in Sverdrupbalance McWilliams et al. [1978] describedan eddy-resolving,two- with the wind. Munk-type boundary layers develop on the layer, wind-forced numerical model of a zonal channel with western side of each gyre and around the tip of the partial partial meridional barriers. In this model the wind is zonal barrier. A spreading,free viscousboundary layer allows water with a sinusoidal distribution that is zero at the northern and that flows around the partial barrier to move to the far west- southern edges of the basin and maximum (eastward) in the ern boundary of the ocean. In a linear calculation, all of the center. Linear bottom friction and biharmonic lateral friction flow on the eastern coast of Africa rounds the tip and enters are specifiedin the model, but eddies,created by flow instabil- 488 NOWLIN AND KLINCK: PHYSICSOF THE ANTARCTICCIRCUMPOLAR CURRENT

_B

GAP

1 L

TB

Fig. 14. Time-averagedstream functions from the two-layereddy-resolving experiments of McWilliarnset al. [19783. The casesare denotedLB (flat bottom, 5 km deep)and TB (500 m, Gaussianridge in the recirculatinggap); •px(left) and Ip3(right) are the time averagedstream functions for the upperand lower layer,with contourintervals of 104m2/s and 0.6 x 104m2/s, respectively.

ities, play a major role in distributingmomentum. Two simu- surfacetopography will be necessaryto get the requiredsyn- lations seemespecially pertinent to the ACC. Both casesare optic coverageof the southernocean. The proposedsatellites for recirculationthrough a gap representingDrake Passage; by the European SpaceAgency (ERS 1) and NASA/Centre there are no other meridional boundaries within the channel. National d'EtudesSpatiales (TOPEX/POSEIDON) can pro- One case(LB) has a flat bottom at 5 km; the other (TB) has a vide the altimetric coverage required for study of time- 500-m-high Gaussian ridge in the recirculating gap. The dependentgeostrophic surface currents. Together with surface model was integrated to a statisticalsteady state, and results wind data from the Navy ResearchOcean Satellite System were analyzedin termsof time-averagedenergy budgets where (NROSS), thesealtimetric data will provideglobal coverageof the averagingtimes are 1200 and 3300 days, for casesLB and surfacecurrent variability. The NASA gravity and magnetic TB, respectively.The time mean solutionsare shownin Figure mission (GRM) being planned would provide data on the 14. The total transports for the two casesare 500 and 100 geoidwhich is neededfor determinationof geostrophicsurface x 106 m3/sfor LB and TB. The principalenergy pathway is currents.Surface temperature will be provided by operational from the wind to the flow in layer 1, which becomesunstable. polar orbiters as well as by these researchsatellites. In situ Eddiesthen transferenergy to layer 2, whereit is dissipatedby observationswill play a crucial role in providing subsurface bottom friction.The major effectof bottom topographyis the informationas well as informationfor calibratingthe satellite productionof form drag, which reducesthe total transportby observations.Such observationsshould include detailed hy- a factor of 5. An additional effect of the barrier is to cause the drographic sections, surface temperature and expendable flow downstreamof the barrier to be considerablymore ener- bathythermographsections from ships of opportunity, and getic than flow elsewhere.A similar pattern is observedin the surfacedrifter observations,as were usedin FGGE. surfaceKE from drifters (Figure 9). These model resultsem- The questionsof heat flux and energy conversionby dy- phasize the importance of eddies and topography on the namic instabilities have been addressed in limited areas of the energybudget of the ACC. ACC. However, it is not known how representativethese measurements are of the entire ACC. Moored instruments 7. SUMMARY REMARKS AND SUGGESTIONS shouldbe placedin regionsof the southernocean that repre- There has been great improvementin our understandingof sent differentdynamical regimes. Such areasmight be selected the structure and dynamics of the ACC in the last decade or from considerationof surface kinetic energy estimatesand so. This enhanced understanding,however, indicates that local bathymetry.Sea surfaceheight variability, as determined sharper and more focusedquestions and experimentsshould from Seasat,can be usedto identify dynamicallydifferent re- be considered.Here severalstudies are suggestedfor continu- gimesof the ACC. The mooring arrays shouldhave adequate ing to improve our descriptionsand understandingof the spacingto estimateeddy heat and momentum fluxes,selected ACC. Discussed first are three observational studies consider- parts of the energy and vorticity budgets,and estimatesof ing the major fronts, meridional exchanges,and total trans- energyconversion rates by instabilities.Conductivity observa- port; these are followed by four commentspertaining to the tions would also be desirable to calculate salt fluxes. generaldynamics and modelingof the ACC. The transport of the ACC is a crucial observationagainst The structure and time behavior of the Subantarctic and which numericalmodels can be tested.Transport at Drake Polar fronts need to be considered in more detail. Satellite Passagehas been determined with some precision,and tech- observations of sea surface temperature, surface winds, and niques are now available to monitor the transport with a NOWLIN AND KLINCK' PHYSICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT 489 minimum of instrumentation. A second transport measure- report, 52 pp., National Academy of Sciences,Washington, D.C., ment is needed south of Australia or New Zealand. Time cor- 1974. Allen, J. S., M. S. McCartney, and T. Yao, Topographically-induced relations between these two transport series will begin to jets in a zonal channel with application to the Drake Passage(ab- answerquestions about the angular momentumof the ACC as stract), Eos Trans. AGU, 56(12), 1011, 1975. a whole. There have seen several estimates of the time lag of Baker, D. J., Jr., A note on Sverdrup balance in the southern ocean, J. the ACC in responseto changesin wind forcing. Such trans- Mar. Res.,40, suppl.,21-26, 1982. port time serieswill help sort out the temporal behaviorof the Baker, D. J., Jr., W. D. Nowlin, Jr., R. D. Pillsbury, and H. L. Bryden, Antarctic Circumpolar Current: Space and time fluctuations in the ACC and its relation to forcing. As a long-range goal, satellite Drake Passage,Nature, 268(5622), 696-699, 1977. altimetry coupledwith sealevel stations(including atmospher- Barcilon, V., On the influence of the peripheral antarctic water dis- ic surfacepressure) and deep pressureobservations, should be charge on the dynamics of the circumpolar current, J. Mar. Res., used to calculate the transport of the ACC at several places 24, 269-275, 1966. Barcilon, V., Further investigationof the influenceof the peripheral about the southern ocean, not just in the narrow parts of the antarctic water dischargeof the circumpolar current, J. Mar. Res., current system. 25, 1-9, 1967. Improved fields of atmosphere-oceanexchanges of momen- Boyer, D. L., and J. R. Guala, Model of the Antarctic Circumpolar tum, heat and moisture are needed to improve our under- Current in the vicinity of the Macquarie Ridge, in Antarctic Ocea- standing of the circulation and heat budget of the southern nology II, The Australian-New Zealand Sector, Antarct. Res. Ser., vol. 19, edited by D. E. Hayes, pp. 79-93, AGU, Washington, D.C., ocean. Climatology for the southern ocean is hampered by its 1972. remotenessas well as its dependably bad weather. It seems Bryan, K., and M. Cox, The circulation of the world ocean: A nu- that satellite observations (for example, the scatterometer on merical study, I, A homogeneousmodel, J. Phys. Oceanogr.,2, NROSS) offer the only hope for obtaining wind stressobser- 319-335, 1972. Bryan, K., S. Manabe, and R. C. Pacanowski, A global ocean- vations with adequate geographical coverage. Heat exchange atmosphere climate model, II, The oceanic circulation, J. Phys. with the atmosphereis an important element of water mass Oceanogr.,5, 30-46, 1975. conversions which occur in many places in the southern Bryden, H. L., Poleward heat flux and conversionof available poten- ocean. Such heat flux estimateswill be necessaryfor the devel- tial energyin Drake Passage,J. Mar. Res.,37, 1-22, 1979. opment of thermodynamicallyactive modelsof the ACC. Bryden, H. L., The southern ocean, in Eddies in Marine Science, edited by A. R. Robinson,pp. 265-277, Springer,New York, 1983. The correct dynamical balance for the ACC is still not Bryden, H. L., and R. A. Heath, Energeticeddies at the northern edge know. The roles of continental boundaries, stratification, of the Antarctic Circumpolar Current in the southwest Pacific, bottom topography, and dynamic instabilities have been Prog. Oceanogr.,14, 65-87, 1985. shown to be important separately,but such effectshave not Bryden, H. L., and R. D. Pillsbury, Variability of deep flow in the Drake Passage from year-long current measurements,J. Phys. been considered all in the same numerical model. As an exam- Oceanogr.,7, 803-810, 1977. ple of the interrelation of thesefactors, the meridional distri- Bye, J. A. T., and T. W. Sag, A numerical model for circulation in a bution of wind forcing in relation to the meridional extent of homogenousworld ocean,J. Phys. Oceanogr.,2, 305-318, 1972. land barriers has been shown to be of considerableimpor- Chelton, D. B., Statistical reliability and the seasonal cycle: Com- tance. Finally, active thermodynamicsshould be included in ments on "Bottom pressuremeasurements across the Antarctic Cir- cumpolar Current and their relation to the wind," Deep Sea Res., these models to consider how newly formed, dense water is Part A, 29(11), 1381-1388, 1982. able to crossthe ACC without losingits distinctivecharacter. Cheney, R. E., J. G. Marsh, and B. D. Beckley, Global mesoscale The dynamicsof the creation and maintenanceof the ACC variability from collinear tracks of SEASAT altimetry data, J. Geo- fronts and their associatedstrong currents should be investi- phys.Res., 88(C7), 4343-4354, 1983. Clarke, A. J., The dynamics of large-scale,wind-driven variations in gated. Such currentsseem to be sensitiveto bottom topogra- the Antarctic Circumpolar Current, J. Phys. Oceanogr.,12, 1092- phy and tend to flow through gapsin bathymetry.These deep- 1105, 1982. reachingcurrents are shown to be unstablein Drake Passage Clifford, M. A., A descriptivestudy of the zonation of the Antarctic where they have been extensivelyinvestigated, and they may Circumpolar Current and its relation to wind stressand ice cover, be unstablethroughout the remainderof the A CC. The stabili- M.S. thesis,93 pp., Tex. A&M Univ., College Station, 1983. Colton, M. T., and R. P. Chase, Interaction of the Antarctic Circum- ty of thesecurrents has an impact on the meridional exchange polar Current with bottom topography:An investigationusing sat- of heat, momentum, and other properties across the ACC. ellite altimetry, J. Geophys.Res., 88(C3), 1825-1843, 1983. Investigationsshould include both theoretical and observa- Cox, M.D., A baroclinic numerical model of the world ocean: Pre- tional points of view. liminary results,in NumericalModels of OceanCirculation, pp. 107- 120, National Academy of Sciences,Washington, D.C., 1975. Simple indiceswhich give good indicationsof the behavior Deacon, G. E. R., The hydrology of the southern ocean, Discovery of the ACC, or someparts thereof,may exist.Specific observa- Rep., 15, 3-122, 1937. tions in the surfacemeteorological fields, for example, surface Deacon, G. E. R., Physical and biological zonation in the Southern winds,may signal variationsin the current fieldsof the south- Ocean, Deep Sea _Res.,Part A, 29, 1-15, 1982. ern ocean. The ACC transport observationsshould be com- de Ruijter, W., Asymptoticanalysis of the Agulhasand Brazil current pared through the use of models with surface wind fields. systems,J. Phys.Oceanogr., 12, 361-373, 1982. Some surface wind fields are available now that are contem- deSzoeke, R. A., and M.D. Levine, The advective flux of heat by mean geostrophicmotions in the southern ocean, Deep Sea _Res., poraneouswith transport observationsin Drake Passage;the 28(10), 1057-1085, 1981. next decadepromises improvements in observationsof both Devine, M., Some aspectsof the dynamics of the Antarctic Circum- wind stress and current variations. polar Current, J. Geophys.Res., 77, 5987-5992, 1972. Emery, W. 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