Antarctic Current

Antarctic Circumpolar CurrentPhilip SontagThermohaline Circulation; Mix-Master

Circulation below major wind-driven currentsDensity distribution flow patternsTuggweiler & Samuels 1995 Strong ACC upwelling short circuit vertical mixing in low, mid latitudes

2Antarctic Fronts

3Convergence Divergence Patterns

The Antarctic currents are wind driven. Strong west winds with maximum speed near 50S drive the currents, and the north-south gradi- ent of wind speed produces convergence and divergence of Ekman transports. Ekman transports diverge, pulling Circumpolar Atlantic Deep Water to the surface south of the Polar Front, which helps drive the deep circulation

4Antarctic Circumpolar CurrentGeostrophic

Driven by prevailing westerly windsBalance of surface and bottom stressesPotential Vorticity and Bottom Topography

Geostrophic balance = N-S direction interior flow, E-W pressure gradientbelow 2500 mExtending to bottom ocean =speeds of 3-9 cm/s shallow ridges, plateaus of southern oceans current curves equatorwardas it moves deeper water, moves curves poleward5Antarctic Polar Ice, Bottom Water Formation (600S)Penetrative barrier Albedo Positive FeedbackPolynasOrigin, Shallow Weddel and Ross SeasStability of stratified oceanTransport to western basinDecrease CO2 uptakeDriven by HaloclineMelting, formation of iceSources: 1) Surface waters -> Continental Shelf 2) Remnants of Circumpolar deep watersRoss Sea (saltiest waters)Total Antarctic Bottom Water production rate is about 8-9.5 Sv 1Sv=106m3s1Ice albedo = 30-40% -> 95% directly after snow fallPolynas -> significant sensible heat loss, as much as 250 W/m2 in winterIce free areas -> 5-15% in winter, doubled in summerIce thickening and reducing -> analagous to formation and dissipation of thermoclineReversal of sign in the thermal expansion coefficent at 4CDensity of seawater greater than 24.7 psu increases in during temp decrease until freezingHalocline 50 to 200 m -> fresher lighter water cooled to freezing pointIce forming, melting causes halocline

6Eddies

20 km diameter > jetsPrimary mechanism for generating interface displacementDivergence in Ekman transportSheered along flanks of jets

Eddies and Jets, Ocean StormsFocusing of eddies into narrow jetACC -> 2 to 3 narrow jets, sharp frontsMeridional gradients of potential vorticity (PV)Strong along EastwardWeaker along WestwardEastward and westward flow arise from local PV mixingPV staircaseMerge and split, yet persistentBarrier or BlenderStrong PV = weak horizontal mixingWeak PV = strong horizontal mixingTraditionally, ACC jets viewed as circumpolar features with strong horizontal gradientsEddies accelerating and decelerating jets locally locally variability allows jet to manuever around local topographyStructure of jets maintains eddy fieldBarrier = weak exchange cross jet exchange

8Drake Passage

Along line of constant longitude Drake Passage:Major Variability in TransportTypical current speeds are around 10 cm/s with speeds of up to 50 cm/s near some fronts. Although the currents are slow, they transport much more water than western boundary currents because the flow is deep and wide. Max transport, late winter, early spring

9Implications for Primary Production

Spring Bloom in New Zealand Waterscold rivers of water that have branched off from the Antarctic Circumpolar Current flow north past the South Island and converge with warmer waters flowing south past the North Island. 10Melting of Antarctic Shelves

Warm circumpolar water can override continental slope frontRedirection of currents, movement of warm waters into ice-shelf cavityIncrease in surface stress in southeasternOzone depletion and increase in greenhouse gasesIncrease in zonal ACC transportEddy-Saturated Regime0.5C increase

11ReferencesHellmer, H. H., F. Kauker, et al. (2012). Twenty-first-century warming of a large Antarctic ice-shelf cavity by a redirected coastal current. 485: 225-228.Knauss, J. A., Ed. (1997). Introduction to Physical Oceanography. Long Grove, IL, Waveland Press, Inc.NASA/MODIS Rapid Response/Jeff Schmaltz. Caption Credit: Rebecca Lindsey, NASA Earth Observatory. http://www.nasa.gov/multimedia/imagegallery/image_feature_1509.htmlOrsi, A. H., G. C. Johnson, et al. (1999). Circulation, mixing, and production of Antarctic Bottom Water. 43: 55-109.Rintoul, S. R. and J. L. Bullister (1999). A late winter hydrographic section from Tasmania to Antarctica. 46: 1417-1454.Stewart, R. H. (2008). Introduction To Physical Oceanography, Department of Oceanography Texas A & M University.Thompson, A. F. (2008). The atmospheric ocean: eddies and jets in the Antarctic Circumpolar Current. 366: 4529-4541.U.S. Geological Survey First published in the Encyclopedia of Earth March 30, 2010; Last revised Date June 11, 2012; Retrieved December 6, 2012 http://www.eoearth.org/article/Antarctic_Convergence?topic=49523