Antarctic Circumpolar Current Philip Sontag

Thermohaline Circulation; “Mix-Master” Circulation below major wind-driven currents Density distribution – flow patterns Tuggweiler & Samuels 1995 Strong ACC upwelling short circuit vertical mixing in low, mid latitudes

Antarctic Fronts

Convergence Divergence Patterns The Antarctic currents are wind driven. Strong west winds with maximum speed near 50◦S 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

Antarctic Circumpolar Current Geostrophic Driven by prevailing westerly winds Balance of surface and bottom stresses Potential Vorticity and Bottom Topography Geostrophic balance = N-S direction interior flow, E-W pressure gradient below 2500 m Extending to bottom ocean =speeds of 3-9 cm/s shallow ridges, plateaus of southern oceans current curves equatorward as it moves deeper water, moves curves poleward

Antarctic Polar Ice, Bottom Water Formation (600S) Penetrative barrier – Albedo – Positive Feedback Polynas Origin, Shallow Weddel and Ross Seas Stability of stratified ocean Transport to western basin Decrease CO2 uptake Driven by Halocline Melting, formation of ice Sources: 1) Surface waters -> Continental Shelf 2) Remnants of Circumpolar deep waters Ross Sea (saltiest waters) Total Antarctic Bottom Water production rate is about 8-9.5 Sv 1 Sv=106 m3 s−1 Ice albedo = 30-40% -> 95% directly after snow fall Polynas -> significant sensible heat loss, as much as 250 W/m2 in winter Ice free areas -> 5-15% in winter, doubled in summer Ice thickening and reducing -> analagous to formation and dissipation of thermocline Reversal of sign in the thermal expansion coefficent at 4C Density of seawater greater than 24.7 psu increases in during temp decrease until freezing Halocline 50 to 200 m -> fresher lighter water cooled to freezing point Ice forming, melting causes halocline

Eddies 20 km diameter –> jets Primary mechanism for generating interface displacement Divergence in Ekman transport Sheered along flanks of jets

Eddies and Jets, Ocean Storms Focusing of eddies into narrow jet ACC -> 2 to 3 narrow jets, sharp fronts Meridional gradients of potential vorticity (PV) Strong along Eastward Weaker along Westward Eastward and westward flow arise from local PV mixing “PV staircase” Merge and split, yet persistent “Barrier” or “Blender” Strong PV = weak horizontal mixing Weak PV = strong horizontal mixing Traditionally, ACC jets viewed as circumpolar features with strong horizontal gradients Eddies accelerating and decelerating jets locally – locally variability allows jet to manuever around local topography Structure of jets maintains eddy field “Barrier” = weak exchange cross jet exchange

Drake Passage Along line of constant longitude Drake Passage: Major Variability in Transport Typical 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

Implications for Primary Production Spring Bloom in New Zealand Waters cold 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.

Melting of Antarctic Shelves Warm circumpolar water can override continental slope front Redirection of currents, movement of warm waters into ice-shelf cavity Increase in surface stress in southeastern Ozone depletion and increase in greenhouse gases Increase in zonal ACC transport “Eddy-Saturated Regime” 0.5°C increase

References Hellmer, 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.html Orsi, 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