Download presentation
Presentation is loading. Please wait.
Published byNorman Bridges Modified over 6 years ago
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.
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.