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Transcript
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: