1. Mesoscale eddies and deep upwelling in the Southern Ocean
Till Kuhlbrodt, Walker Institute, Dept. of Meteorology, NCAS-Climate

2. Mesoscale eddies in satellite image
½° grid (from Hallberg & Gnanadesikan 2006)

3. Mesoscale eddies in a 1/6° model
Eddy-resolving model (Hallberg & Gnanadesikan 2006)

4. A simplified zonally averaged section of the Atlantic
Drake passage effect
Kuhlbrodt et al. (2007), Rev. Geophys.
Drake Passage Effect

5. Antarctic circumpolar current
Drake passage effect
Antarctic circumpolar current
No net zonal pressure gradient  no net meridional geostrophic flow
Ageostrophic flows:
Wind-driven
Eddies
Drake passage
V. Zlotnicki
JPL

6. An outcropping isopycnal layer
DOWN W E UP S
WIND
Eddy activity means downward and upward motion
More upwelling than downwelling necessary for a net wind-driven upwelling

7. The CMIP3 model database

8. AMOC sensitivity to wind stress
Total tx is the integral of the wind stress between 54°S and 64°S (in 1012 N)
Numbers from Russell et al. (2006), J. Clim.
: observations

9. Antarctic Circumpolar Current
HiGEM
CSIRO
GISS-AOM
GISS-ER
GISS-EH
Mean ACC strength: (135 ± 84) Sv

10. AMOC vs. NADW outflow at 30°S
Correlation is 0.67 without GISS models
“Max. NAOT” is at 50°N

11. Sensitivity to model resolution
Higher resolution  better representation of wind-driven upwelling?
Higher resolution  less numerical diffusion?

12. Meridional overturning on z-levels
Upwelling from depth due to Drake Passage effect (56°S – 62°S)
Sinking between 50°S and 56°S
Deacon cell (40°S – 60°S) X
Kuhlbrodt et al. (2007), Rev. Geophys.

13. The geometry of the Deacon cell

14. The invisible Deacon cell
The Deacon cell does not appear in a streamfunction plot on potential density levels
1/6° hemispheric model, 20 isopycnal layers (MESO)
sinking between 50S and 56S: diapycnal
Sigma_2000 necessary to distinguish deep water masses AABW and NADW
Red line denotes the zonal maximum time-averaged density of the top 50 m
20 levels, (isopyc), diapycnal diffusivity of 10-5 m2 s-1
Model has strong sponges (restoring) at the equator
Hallberg & Gnanadesikan 2006

15. Mesoscale eddies in a 1/6° model
Eddy-resolving model (Hallberg & Gnanadesikan 2006)
southward (transient-)eddy-driven flow off the continental shelves
importance of three-dimensional picture

16. Isopycnal streamfunction in OCCAM
OCCAM: 1/12°, global model, 66 depth levels (z)
Shading: dark – buoyancy gain, light – buoyancy loss
Lee et al. 2007
-63

17. Isopycnal streamfunction in HiGEM
Shaffrey et al. 2008, in prep.

18. Diagnostics Already done: Strength and position of the ACC
Meridional streamfunction in isopycnal coordinates
Eddy activity
Surface wind stress/ Ekman transport
Surface buoyancy fluxes
What could be looked at:
Circulation in individual isopycnal layers
Formation of AABW: location, amount
Relation to residual mean theory (in zonal average)