Antarctic Circumpolar Current C. Chen General Physical Oceanography MAR 555 School for Marine Sciences and Technology Umass-Dartmouth 1 MAR555 Lecture 9: Antarctic Circumpolar Current 2 Antarctic Circumpolar Region prevails the eastward zonal wind 30oW 0o Africa South America 30oE South Atlantic Ocean 60 60 o E o W n a e c O n 90 W a 90 i o d o n E I h t u o S 60 o S 120 W S ou o t o h P 40 a ci 120 o fi S c O E ce an ia al tr us o E A 150oW 180o 3 150 30oW 0o 30oE The Antarctic Circumpolar A 60 60 n Current (ACC) moves ta o E o W rc tic eastward around the Antarctic Circumpolar Weddle Gyre ACC velocity: 4 to 20 cm/s ACC features two maximum Current 90 jets and 90 o W o E two colder pools; two warmer pools t R n o o e (2-3 C colder or warmer) ss r S r ea u G C y r Propagates along the Antarctic re la o Circumpolar Current (ACC) 120 P and takes 8-9 years to travel o E W o around a circle. 120 ACC is driven by the wind and density gradient. 150oW 180o o E 150 4 Trajectories of drifters (Smith et al., http://oceancurrents.rsmas.miami.edu/southern/antarctic-cp.htm)l (1978-2003) 5 The Sea Surface Height (SSH) (Dynamic Height) From Dr. Sarah Gille at SIO: SSH is calculated by the analysis of drifter,altimeter, GRACE, 6 and hydrographic data (Nikolai Maximenko and collaborator) The Wind-induced Eastward Current Sea Level Pressure Gradient force Coriolis force N Eastward current Eastward Current Ekman transport Wind stress Pressure Gradient Force West East 7 Subpolar front Polar front Subpolar Antarctic frontal Antarctic Antarctic shelf zone zone zone 0 7 34.2 5 2 1.5 34.2 34.2 4 34.3 1 3 34.5 2 34.6 34.73 1 34.72 2 34.7 0.5 Depth (km) 34.72 3 0.25 34.71 4 5 56oS 58oS 60oS 62oS 56oS 58oS 60oS 62oS 8 Locations of density fronts in the southern ocean-from NASA JPL website http://www.jpl.nasa.gov/media/20051206a/images/pic2.jpg 9 Geostrophic Current at A Density Front E Pressure gradient force N S Ho !1 h1 !2 PB L PA U Fc P 10 11 Circumpolar Frontal Subduction Equator South Pole Mixed layer rm he ot Is Horizontal wind-induced current convergence and cooling causes the subduction of the water from the mixed layer to the weekly mixed thermocline layer in the sub- polar frontal zone. These waters will move downward along the isotherm surface (or density surface) to the deep region. 12 January 28,2004 : Chl-a : SST From Kahru et al. (2007)-JGR-Ocean 13 Chl-a concentration (January 26-30, 2005) From Kahru et al. (2007)-JGR-Ocean 14 Geostrophic Instability F P a FP FP P-1 FP FP Fc Fc F c P Fc Fc P+1 Pressure gradient remains unchanged (with uniform space of pressure contours), while the Coriolis force increases with the increase of the velocity. Stable! 15 FP b FP P-3 P-2 P-1 P Fc P+1 FP Fc Fc The increase rate of the horizontal pressure gradient is larger than the increase rate of the Coriolis force: Unstable! 16 The criterion condition of the barotropic instability on the southern hemisphere: $> 0 Unstable %U g ! Vorticity f & = ' a #= 0 Neutral %y ! "< 0 Stable y f < 0 U U ! g ! g Unstable if > f < 0 !y !y x !U0 g > 0 !y stable 17 (Note: this is the supplemental material that is not required for this fundamental PO course...