Lecture 12: The Antarctic Circumpolar CurrentAtmosphere, Ocean, Climate

DynamicsEESS 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

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

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

Location of fronts in the ACC

Figure from Orsi et al 2005

Zonal transport in the ACC

Figure from Olbers et al 2004

The transport in the ACC is ~140 Sv

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

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

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

The ACC is wind-driven

Wind-stress curl in the Southern Ocean

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

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

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

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.

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.

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

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

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

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

10 year mean dynamic topography

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