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..