antarctic circumpolar current
DESCRIPTION
The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and, at approximately 125 Sverdrups, the largest ocean current.The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.TRANSCRIPT
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Lecture 12: The Antarctic Circumpolar CurrentAtmosphere, Ocean, Climate
DynamicsEESS 146B/246B
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The Antarctic Circumpolar Current
Fronts and jets and the zonal circulation in the ACC
Wind-driven meridional circulation Available potential energy Eddy-driven circulation and subduction Zonal force balance of the ACC
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Surface circulation
The Antarctic Circumpolar Current (ACC) is a strong nearly zonal flow in the Southern Ocean.
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Density section crossing the ACC
Isopycnals are slanted in the ACC.
By the thermal wind balance this implies that the current is surface intensified.
Isopycnal outcrops mark the location of fronts.
Subtropical Front
SubantarcticFront
Polar Front
South ACC Front
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Fronts in the ACC
Fronts are regions where the SSH slopes steeply locations of surface jets.
The location of SSH contours can be used to trace the location of fronts around the ACC.
Fronts mark the boundaries between water masses.
SAF:Subantarctic Mode Water (SAMW) and AAIW
PF: AAIW and Circumpolar Deep Water (UCDW).
Figure from Sokolov and Rintoul (2009)
SAMW AAIW CDW
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Location of fronts in the ACC
Figure from Orsi et al 2005
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Zonal transport in the ACC
Figure from Olbers et al 2004
The transport in the ACC is ~140 Sv
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Evidence of a meridional circulation in the ACC
The interleaving of water masses in the Southern Ocean suggests that there is a meridional overturning circulation that both upwells and downwells water.
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Subduction and the sequestration of anthropogenic CO2 at ocean fronts
In the Southern Ocean anthropogenic CO2 is subducted along density surfaces that outcrop at the strong ocean fronts and that bound the Antarctic Intermediate Water.
What drives this subduction?
Figure from Sabine et al, Science 2004
Anthropogenic CO2 (mol kg-1)
60 S Equator 40 N
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Large eddy variability in the ACC
kinetic energy of mean circulation
kinetic energy of eddies
One can split the circulation into a mean and eddy components:
mean, time average
eddy, time-variable
EKE is as large as the mean KE in the ACC
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The ACC is wind-driven
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Wind-stress curl in the Southern Ocean
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Formation of fronts in the Southern Ocean
Convergence/divergence of the Ekman transport drives downwelling/upwelling which tilts density surfaces (isopycnals) upward, forming a front.
The wind-driven overturning is known as the Deacon Cell
This causes an increase in the potential energy of the system.
Ekman transport
Coriolisparameter
density
Deacon cell
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Available potential energy
FRONT STATE WITH LOWEST PE
total mass of water
center of mass of water
The available potential energy is the PE that can be converted to kinetic energy
Eddies that form at fronts draw their energy from the APE and in doing so reduce the APE by generating a net overturning motion.
LIGHT DENSE
LIGHT
DENSE
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Eddy driven overturning
In releasing the energy associated with the baroclinicity of the flow, eddies drive a net overturning motion which flattens out isopycnals.
This overturning is of the same strength but opposite sense of the Deacon cell
The sum of these two circulations determines the strength of the net upwelling.
Eddy driven overturning
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The residual circulation in the ACC
The sum of the eddy and wind driven circulations is known as the residual circulation.
The interleaving of the water masses reflects the structure of upwelling and downwelling associated with the residual circulation.
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Anthropogenic CO2 (mol kg-1)
Equator
Eddy-induced transport and subduction in the Southern Ocean
South of the Polar Front, in the southwest Pacific, Sallee et al (2009) estimate an eddy-induced volume transport of 1.5 Sverdrups along the AAIW isopycnal layer.
Figure from Sabine et al, Science 200460 S
40 N
1.5 Sv the transport of 5 Amazon rivers
In this small sector of the Southern Ocean, this eddy-induced transport would flux anthropogenic carbon into the interior at a rate ~0.01-0.02 Pg C/ year, about 1-2% of the total CO2 fluxed into the ocean surface.
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Heat transport in the ACCIn contrast to the subtropical gyres where the western boundary currents transport heat, in the SO eddies transport heat.
Mooring observations can be used to calculate eddy heat fluxes by taking correlations between temperature and velocity.
Eddies result in a surface intensified heat flux directed to the south.
Across a latitude circle eddies transport a net amount of heat ~1 PW poleward.
Figure from Olbers et al 2004
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What process can balance the frictional torque supplied by the wind-stress curl?
Advection of planetary vorticity (aka the Sverdrup balance) cannot accomplish this since there are no western boundaries in the center of the Southern Ocean.
The velocities required for bottom friction to achieve this balance are too large.
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Bottom form stress
Bottom topography
A pressure difference across a topographic feature will exert a force on the topography.
By Newtons third law an equal and opposite reaction force will be exerted on the fluid.
The momentum flux associated with this process can be quantified in terms of the bottom form stress:
Figure from Olbers et al 2004
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Evidence of form stress in the Southern Ocean
The water is denser on the lee side of ridges.
For a surface intensified flow, what must the shape of the free surface look like to compensate for the baroclinic pressure gradient?
Figure from Olbers et al 2004
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10 year mean dynamic topography
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Zonal force balance in the ACC
The barotropic pressure gradient is partially compensated by the tilt in isopycnals, but not completely.
This results in a bottom form stress equal to the zonally averaged surface wind stress, yielding a force balance in the zonal direction:
Figure from Olbers et al 2004