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 o 0 0 _ .,• 0 • .... .. .. .,• ß • .0o• o • .,• • o • _ _ _ NOWLIN AND KLINCK: PHYSICSOF THE ANTARCTIC CIRCUMPOLARCURRENT 471 w E 20 ø 20 ø 40 ø 40 60 ø 8Oø 8Oø 100': 100 ø 120" 120" 140ø 140ø -0.5 160ø 160ø 180" 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