1. Advertisement Ocean Circulation in Three Dimensions
By Barry A. Klinger and Thomas W. N. Haine
In Preparation, Cambridge University Press
feedback welcome

2. Example: Subtropical Cells
Observations….

3. … Concepts ….

4. …Theory

5. Surface Conditions and Projections of 21st
Century Meridional Overturning Circulations
Barry A. Klinger
George Mason University (Fairfax, VA) &
Center for Ocean-Land-Atmosphere Studies (COLA;
Calverton, MD)

6. 21st century climate projections show decline in Atlantic meridional
overturning circulation.
AR4 projections under SRES A1b show reductions of 0 – 50%
(relative to mean).
Meehl et al. (2007, Climate Change 2007: The Physical Science Basis) Fig ,
adapted from Schmittner, Latif & Schneider (2005, GRL)

7. What accounts for the variation?
21st century climate projections show decline in Atlantic meridional
overturning circulation.
AR4 projections under SRES A1b show reductions of 0 – 50%
(relative to mean).
What accounts for the variation?
Meehl et al. (2007, Climate Change 2007: The Physical Science Basis) Fig ,
adapted from Schmittner, Latif & Schneider (2005, GRL)

8. GHG emissions increase thru 21st cent models respond with
A1B scenario:
GHG emissions increase thru 21st cent
models respond with
~ 3C global mean surface warming
strong winter warming NH hi lats
precip increase hi lats and eq,
decrease in subtropics
Meehl et al. (2007, Climate Change 2007:
The Physical Science Basis) Fig. 10.4
from Meehl et al. (2007) Fig 10.9

9. Review: What determines overturning strength?
Single closed basin:
surface buoyancy difference Δb,
mixing coefficient κ
Stommel and Robinson (19??), Bryan and Cox
(1967, Tellus), Bryan (1987, JPO)

10. Review: What determines overturning strength?
Single closed basin:
surface buoyancy difference Δb,
mixing coefficient κ
Stommel and Robinson (19??), Bryan and Cox
(1967, Tellus), Bryan (1987, JPO)
Multiple DWF sites:
Small Δb between local dens max
big change in overturning
N Atl vs S Atl, N Atl vs N Pac
Rooth (1982, Prog Ocean.), Welander (1986)
Manabe & Stouffer (1988, J Clim), Marotzke
& Willebrand (1991, JPO), Hughes & Weaver
(1994, JPO), Weaver Saenko Clark Mitrovica
(2003, Sci), etc
T= T= T=6 T= T= T=.6
Klinger & Marotzke (1999, JPO)

11. Southern Ocean wind stress
strengthens NADW formation
(Toggweiler & Samuels, 1995, DSR)
overturning strengthened by
SO wind and global mixing
weakened by SO lateral eddy mixing
(Gnanadesikan, 1999, Science)

12. Southern Ocean wind stress strengthens NADW formation
(Toggweiler & Samuels, 1995, DSR)
overturning strengthened by
SO wind and global mixing
weakened by SO lateral eddy mixing
(Gnanadesikan, 1999, Science)
In single basin with “Southern Ocean” channel, overturning sensitive to
surface dens difference between N edge of SO and N Hem density maximum.
Approx. basin geometry (Klinger) Meridional Overturning Streamfunction
NH denser than SO SO denser than NH
periodic boundary condition
Vallis (2000, JPO)

13. What determines overturning decline in climate projections?
HadCM3 GHG experiments 
overturning increases with
vertically integrated
density difference 30S and 60N.
(Thorpe Gregory Johns Wood &
Mitchell, 2001)
vol trans (Sv)
steric gradient (cm/deg)

14. What determines overturning decline in climate projections?
HadCM3 GHG experiments 
overturning increases with
vertically integrated
density difference 30S and 60N.
(Thorpe Gregory Johns Wood &
Mitchell, 2001)
Surface properties vs vert. integral:
Surface properties isolate interaction
between atmosphere and ocean
response of ocean to SST and SSS is well-defined problem;
∫dz includes ocean “solution” as well
∫dz includes change in pycnocline thickness which can be caused
by different mechanisms (surface density, wind, etc) operating at
different locations
vol trans (Sv)
steric gradient (cm/deg)

15. What determines overturning decline in climate projections?
HadCM3 GHG experiments 
overturning increases with
vertically integrated
density difference 30S and 60N.
(Thorpe Gregory Johns Wood &
Mitchell, 2001)
Surface properties vs vert. integral:
Surface properties isolate interaction
between atmosphere and ocean
response of ocean to SST and SSS is well-defined problem;
∫dz includes ocean “solution” as well
∫dz includes change in pycnocline thickness which can be caused
by different mechanisms (surface density, wind, etc) operating at
different locations
Is variation among models due to variations in
surface properties?
vol trans (Sv)
steric gradient (cm/deg)

16. Forcing from ECHAM4/OPYC and GFDL R30 shows importance of
Denmark Strait Overflow water density for overturning decline.
(Schweckendiek & Willebrand, 2005, J Clim)

17. Examine models from AR4.
20c3m and A1b experiments.
Selected GCM’s w/o flux adjustments.
Compare overturning and surface properties among runs.

21. A few measures of Meridional Overturning Streamfunction Strength

22. Max and Outflow Volume Transport (Sv), 10 yr avgs
Decline generally small
until 21st century.
In 20th century,
and r2 = .90
Change in
and quite
variable among runs:
r2 = .37

23. SSD change
2090s – 20th
century

24. SSD change
2090s – 20th
century (March)
Detailed pattern
varies among
runs.
Which index most
relevant? Can
avoid answer:
Nordic and N Atl
very correlated
(r2 = .82)
Drake and S Atl
very correlated.
(r2=.96)
Choose Nordic
and Drake boxes.

25. SSD

26. Observed March Climatology
c.i.=2C
c.i.=.5psu
c.i.=.5

27. How does overturning decline depend on density decline?
Nordic – Drake; Nordic – Pacific
Drake – Tropical (20th Cent.)
Outflow (~35S) Maximum

28. A few measures of Meridional Overturning Streamfunction Strength

29. How does overturning decline depend on density decline?
Nordic – Drake; Nordic – Pacific
Drake – Tropical (20th Cent.)
Outflow (~35S) Maximum Northern

30. Much of variation in
overturning
(as opposed to
overturning decrease)
also explained by
density difference between
Nordic Sea and
Drake Passage.
filled: 20th Cent
open: 2090s

31. How does surface density decrease depend on SST and SSS decreases?
T contribution to
S contribution to

32. Projections have large changes in S Hem Westerlies.
Do these affect overturning?

34. Ekman effect on overturning change? NO (r2 = .09)
Change in Outflow
= Ekman transport at N bndry
of Drake Passage

35. Ekman effect on overturning change? NO (r2 = .09)
But may affect variation among 20th Cent and 2090s overturning.
Change in Outflow Outflow
solid: 20th cent.
open: 2090s
= Ekman transport at N bndry
of Drake Passage

36. Shouldn’t wind stress influence be bigger?
Delworth & Zheng (2008, GRL) use GFDL AOGCM A1B wind  2 Sv change

37. Shouldn’t wind stress influence be bigger?
Delworth & Zheng (2008, GRL) use GFDL AOGCM A1B wind  2 Sv change
BUT…
Delworth & Zheng used switch-on wind
century-scale response in overturning
[Klinger and Cruz (2009, JPO) also show century-scale response]
response to O(1 century) change in wind ~ 1 Sv
Other models had smaller wind stress change.
change in overturning from wind ~ 1 Sv
< ~3 Sv variations in overturning changes in models

38. Conclusions Most runs examined here (non-flux-adjusted AOGCMs):
similar decline in (17-24% at southern end of Atlantic)

39. Conclusions Most runs examined here (non-flux-adjusted AOGCMs):
similar decline in (17-24% at southern end of Atlantic)
For outliers (9% and 36% declines), most of difference
is due to difference in N Atl sea surface density (SSD).

40. Conclusions Most runs examined here (non-flux-adjusted AOGCMs):
similar decline in (17-24% at southern end of Atlantic)
For outliers (9% and 36% declines), most of difference
is due to difference in N Atl sea surface density (SSD).
Max streamfunction may be misleading indicator of
overturning strength; not correlated with density,
but avg at (800 m, 25-45N) is correlated.

41. Conclusions Most runs examined here (non-flux-adjusted AOGCMs):
similar decline in (17-24% at southern end of Atlantic)
For outliers (9% and 36% declines), most of difference
is due to difference in N Atl sea surface density (SSD).
Max streamfunction may be misleading indicator of
overturning strength; not correlated with density,
but avg at (800 m, 25-45N) is correlated.
For all models, most of range in N Atl SSD is due to SSS range.

42. Conclusions Most runs examined here (non-flux-adjusted AOGCMs):
similar decline in (17-24% at southern end of Atlantic)
For outliers (9% and 36% declines), most of difference
is due to difference in N Atl sea surface density (SSD).
Max streamfunction may be misleading indicator of
overturning strength; not correlated with density,
but avg at (800 m, 25-45N) is correlated.
For all models, most of range in N Atl SSD is due to SSS range.
Overturning depends on northern N Atl density: that is
regional climate.