Download Lecture 12: The Antarctic Circumpolar Current

Lecture 12: The Antarctic
Circumpolar Current
Atmosphere, Ocean, Climate
Dynamics
EESS 146B/246B
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
Surface circulation
•The Antarctic Circumpolar Current (ACC) is a strong nearly zonal flow in the
Southern Ocean.
Density section crossing the ACC
South ACC
Front
Polar
Front
Subantarctic
Front
Subtropical
Front
•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.
Fronts in the ACC
•Fronts are regions where the SSH
slopes steeplyÆ 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
SAMW
AAIW
CDW
•PF: AAIW and Circumpolar
Deep Water (UCDW).
Figure from Sokolov and
Rintoul (2009)
Location of fronts in the ACC
Figure from Orsi et al 2005
Zonal transport in the ACC
The transport
in the ACC is
~140 Sv
Figure from Olbers et al 2004
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.
Subduction and the sequestration of
anthropogenic CO2 at ocean fronts
Anthropogenic CO2 (µmol kg-1)
60 S
Equator
40 N
Figure from
Sabine et al,
Science 2004
•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?
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, timevariable
•EKE is as large as the
mean KE in the ACC
The ACC is wind-driven
Wind-stress curl in the Southern Ocean
Formation of fronts in the Southern Ocean
Ekman transport
Deacon cell
Coriolis
parameter
density
•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.
Available potential energy
total mass of water
center of mass of water
FRONT
STATE WITH LOWEST PE
LIGHT
LIGHT
DENSE
DENSE
•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.
Eddy driven overturning
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.
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.
Eddy-induced transport and subduction in
the Southern Ocean
Anthropogenic CO2 (µmol kg-1)
1.5 Sv≈ the
transport of 5
Amazon rivers
Equator
60 S
40 N
Figure from
Sabine et al,
Science 2004
•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.
•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.
Heat transport in the ACC
•In 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
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.
Bottom form stress
Bottom topography
Figure from Olbers et al 2004
•A pressure difference across a topographic feature will exert a force on the
topography.
•By Newton’s 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:
Evidence of form stress in the Southern
Ocean
Figure from Olbers et al 2004
•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?
10 year mean dynamic topography
Zonal force balance in the ACC
Figure from Olbers et al 2004
•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: