AOMIP Goals and deliverables

1
Sensitivity of the ACC transport in IPCC and CORE
simulations
Rüdiger Gerdes Alfred-Wegener-Institut,
Bremerhaven
2
Sensitivity

• Role of wind
• ACC NADW
• Role of surface buoyancy fluxes
• Parameterizations

3
Sensitivity

• Role of wind
• ACC NADW
• Role of surface buoyancy fluxes
• Parameterizations

4
ACC transport vs. Zonal mean wind stress
Gnanadesikan Hallberg, 2000
5
Brix and Gerdes, 2003
6
Russel et al., submitted
0.10
0.12
0.20
0.18
0.16
0.14
7
The wind is nothing without thermohaline forcing
Borowski, Gerdes Olbers, JPO, 2002
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Positive zonal wind stress
Borowski, Gerdes Olbers, JPO, 2002
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Zero wind stress
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Negative zonal wind stress
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Russel et al., submitted
0.2
0.3
0.4
0.5
0.6
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Sensitivity

• Role of wind
• ACC NADW
• Role of surface buoyancy fluxes
• Parameterizations

13
Drake Passage effect
Gnanadesikan Hallberg, 2000
14
Drake Passage effect
Brix and Gerdes, 2003
15
Drake Passage effect
Brix and Gerdes, 2003
16
Change in NADW
MOC North Atlantic
ACC Drake Passage
Griffies et al. in prep.
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Change in NADW

• More NADW decreases ACC strength
• Negative density anomalies have little effect
• .... even over 1500 years adjustment time
• Increased southern dense water production has the
  largest effect

Brix and Gerdes, 2003
18
Enhanced NADW overturning cools (and freshens)
the depth range 1000-3000m (?)
LAB
19
Enhanced NADW overturning warms (and salinifies)
the depth range 1000-3000m (?)
TAU
20
Sensitivity

• Role of wind
• ACC NADW
• Role of surface buoyancy fluxes (later)
• Parameterizations

21
Sensitivity

• Role of wind
• ACC NADW
• Role of surface buoyancy fluxes
• Parameterizations

22
Effect of parameterizations
OMIP, Fritzsch et al., 2000
Isoneutral diffusion
Isoneutral diffusion GM
23
Sensitivities after 10 years
M.Losch, pers.comm.
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Sensitivities after 100 years
M.Losch, pers.comm.
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SynthesisY - c
26
ACC transport is geostrophic and the deep
velocities are small
Borowski, Gerdes Olbers, JPO, 2002
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Channel model
Borowski, Gerdes Olbers, JPO, 2002
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Y - c
Borowski, Gerdes Olbers, JPO, 2002
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Global model, Y vs. c/fo
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Shallow vs. deep density changes
Density anomaly over mixed layer
Density anomaly over whole water column
Deep changes are more effective by a factor H/HML
31
Sea ice formation

• Anomaly of 20 northward sea ice transport (0.02
  Sv) and corresponding ice growth
• Change in density of 0.1kgm-3 over the whole
  water column (south of 65oS) within 10 years
• Change in ACC transport of around 20 Sv

32
Change in Ekman transport
Ekman transport across the latitude of Drake
Passage 40 Sv Deepening of subtropical
pycnocline O(400m/100years) Change in ACC
transport 20 Sv
33
Change in NADW
dru
s2
S
?
34
CORE (GFDL)
CORE Coordinated Ocean Reference
Experiments WGOMD activity/protocol (CORE
I) Large Yeager normal year forcing, based on
NCEP Surface salinity restoring GFDL MOM4 ocean
sea ice, basically as used in coupled climate
model (IPCC runs) Griffies et al., Ocean
Science, 2005
35
f/H (MOM4, OM3)
36
NA overturning and Drake Passage transport
37
Y after 100 and 500 years of integration
38
Mixed layer depth
39
Changes in PE and Y
Dc/fo
DY
40
Changes in density
Changes in density 500-100 years
41
Atlantic sector, 500a vs. 100a
S
dr u
dS
d?
42
OM2 conguration, water hosing experiments Gerdes
et al., Ocean Modelling, 2006
43
Changes in the Atlantic sector over the first 100y
S
dr u
dS
d?
44
...compared to climatology
SC
?C
S
?
45
CORE (Kiel)
CORE Coordinated Ocean Reference
Experiments WGOMD activity/protocol (CORE
I) Large Yeager normal year forcing, based on
NCEP Surface salinity restoring Kiel OPA-9.0
ocean model LIM sea ice model
46
Changes in Y between 100y and 500y
47
Atlantic sector after 100y ...compared to
climatology
SC
?C
S
?
48
Changes in the Atlantic sector density over the
first 100 years (Kiel)
S
dr
dS
d?
49
Changes in the Atlantic sector over the first
100y (GFDL)
S
dr u
dS
d?
50
Atlantic sector, 500a vs. 100a
S
dr
dS
d?
51
Global MOC after 100years
52
IPCC (GFDL, A1B)
53
Drake Passage transport 20th century
54
Drake Passage transport 21st century
55
Sea ice thickness change 2080-2100 vs. 1980-2000
Water flux to ocean change 2080-2100 vs. 1980-2000
56
Northward Ekman transport across the latitude of
Drake Passage increases
57
Drake Passage transport 23hd century
58
Temperature changes 2300-2000
2300
2000
2300-2000
2300-2100
59
Salinity changes 2300-2000
2300
2000
2300-2000
60
Density changes 2300-2000
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Summary

• c is a useful diagnostic
• ACC transport is very stable on time scales of
  100 years or less
• ACC sensitivity to
• Wind (via downwelling, NADW)
• Surface buoyancy fluxes (sea ice formation)
• Parameterizations
• NADW influence is difficult model/configuration/
  forcing dependent
• Tendency to reduce AAIW transport and accumulate
  fresh water south of the subantarctic front

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Stop here
63

• The ACC transport is very stable on time scales
  of 100 years or less
• Wind stress changes over Southern Ocean affect
  ACC most (by changing the depth of the
  subtropical pycnocline).
• Density changes at depth are in principle more
  effective than comparable changes at shallower
  depths.
• NADW influence on the ACC is rather small and is
  not expected to affect ACC on timescales smaller
  than 100 years.
• Look for wind changes and changes in sea ice
  production! The latter are a function of the
  first.

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IPCC
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Delworth et al., 2005
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DJF
JJA
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Der Weddellwirbel und die Lage der Messungen
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Modell BRIOS-2.2

• zirkumpolares gekoppeltes Eis-Ozean-Modell (inkl.
  Schelfeis)
• Meereis mit viskos-plastischer Rheologie
• bodenfolgende Vertikalkoordinaten (24 Schichten)
• 1.5 1.5 cos f horizontale Auflösung
• optimierte Parametrisierung der Konvektion
• Integration über 255 Jahre, angetrieben durch
  Daten der NCEP Reanalyse 1948-2003

Zusätzliche Vergleichsstudien mit dem globalen
gekoppelten Meereis-Ozean-Modell ORCA2-LIM
Timmermann et al., 2004
82
Water mass export from the Weddell Sea
Extreme anomalies and global change
Schodlok et al., 2002
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Baroclinic southward transport in the EGC
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Borowski, Gerdes Olbers, JPO, 2002
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Temperature gradient
Salinity gradient
Russel et al., submitted
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Influence of surface forcing

• Ekman pumping
• thermohaline forcing
• Sea ice formation/melt
• Heat flux
• P-E and river run-off

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Changes in LSW have little effect on
ACC Increased southern dense water production has
the largest effect Negative density anomalies
have little effect .... even over 1500 years
adjustment time
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Borowski, Gerdes Olbers, JPO, 2002
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Global model
Borowski, Gerdes Olbers, JPO, 2002
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Arctic liquid fresh water balance
Borowski, Gerdes Olbers, JPO, 2002