The ocean’s thermohaline circulation:
an Achilles’ Heel of our Climate system?
Henk Dijkstra
Institute for Marine and Atmospheric research Utrecht,
Department of Physics and Astronomy, Utrecht, The
Netherlands
Annual mean surface wind velocity
Surface Ocean Circulation
Wind-driven circulation: circulation associated with direct forcing of the wind
After: Sverdrup, H.U. et al. (1942)
Temperature + Salinity section
Annual mean surface heat flux
Annual mean freshwater flux
50 mm/month
-100 mm/month
150 mm/month
Global Conveyor Circulation
Ganachaud & Wunsch, Nature, 408, 453, (2000).
1 Sv = 10^6 m^3/s
Thermohaline circulation: circulation associated with the transport of heat and salt
Meridional Overturning Circulation (MOC)
&
ThermoHaline Circulation (THC)
Depth
Model determined
Meridional overturning streamfunction
40S
Contours in Sv
80N
MOC: Total northward/southward transport in latitude/depth (is observable)
THC: Part of MOC driven by heat/freshwater exchange at the surface and subsequent vertical
mixing (is an interpretation)
Conceptual Model of the Atlantic MOC
= (1 (T T
0
T
salinity `driven’
I.M.A.U., Utrecht University
0
) + S ( S S0 ))
thermally `driven’
04/17/08
9
Two box model
surface flow
equal volumes
d Tp
a
V dt = C T (T p T p) +
dT e
a
T
=
(
V dt C T e T e ) +
d Sp
S
a
=
(
V dt C S p S p) +
d Se
a
S
V dt = C (Se Se ) +
deep flow
*
*
e
p
)
(T T )
*
p
e
(S S )
*
e
p
(S T )
*
p
e
= ( p e )
= (1 (T T
0
(T T
T
0
) + S ( S S0 ))
Source: Stommel, H.M., Thermohaline convection with two stable regimes of flow, Tellus, XIII 2, 224-230, 1961.
I.M.A.U., Utrecht University
04/17/08
10
Dimensionless model
T = T(equator) - T(pole)
1.5
T, S
T
1 = 3.0
2 = 0.5
1
3 = 0.3
S = S(equator) - S(pole)
0.5
S
dT
= 1 (1 + M())T
dt
dS
= 2 ( 3 + M())S
dt
0
Which type of flow?
-0.5
0
=TS
warm
3 = C T = S
C
S
2
I.M.A.U., Utrecht University
t
cold
T
Ratio of adjustment time scales
to surface forcing
4
6
8
salt
TH
04/17/08
Equator
10
fresh
SA
Pole
Equator
Pole
11
Multiple equilibria
3.5
T, S
warm
3
cold
salt
fresh
2.5
2
TH
1.5
SA
T
1
1 = 3.0
S
0.5
2 = 1.0
Equator
Pole
Equator
Pole
3 = 0.3
0
-0.5
0
2
4
I.M.A.U., Utrecht University
t
6
8
10
04/17/08
12
Bifurcation diagram
L: saddle - node bifurcations
previous trajectories
stable SA
3.5
T, S
L1
3
TH
S
2.5
2
L2
T
1.5
1
1 = 3.0
0.5
3 = 0.3
0
SA
-0.5
0
0.5
1
2
1.5
2
stable TH
I.M.A.U., Utrecht University
04/17/08
13
The salt-advection feedback
no damping of salinity anomalies
strong damping of heat anomalies
Multiple equilibria in an Atlantic
ocean model ...
TH
SPP
NPP
Source: Bryan, F. High-latitude salinity effects and interhemispheric thermohaline circulations, Nature, 323, 301-304, 1986.
I.M.A.U., Utrecht University
04/17/08
15
... and in global ocean models and EMICs
Rahmstorf et al., GRL, (2005)
I.M.A.U., Utrecht University
04/17/08
16
Bifurcation diagram (global ocean model)
Atlantic MOC (Sv)
12
atl
(Sv)
Reference solution
10
8
L
6
L
4
2
1
2
0
0.1
0.2
0.3
p
Dijkstra and Weijer, JPO, (2005)
(Sv)
0.4
0.5
Freshwater budget over the Atlantic basin
θ
Mov
Freshwater transport of the MOC at a latitude
Σ=
1
2
3
Sv
4
5
1
2
Sv
Data analysis:
Sv
Weijer et al., JPO,
(1999)
3
If the net freshwater transport
is out of the Atlantic basin, then
the flow is in the multiple equilibrium
regime
Rahmstorf, Clim. Dyn, (1996)
De Vries and Weber, GRL, (2005)
Dijkstra, Tellus, (2007)
General notion:
Increasing precipitation in the northern North Atlantic
(as well as increased melt water from Greenland)
may destabilize the MOC!
Central problem:
Can the present Atlantic MOC collapse
and of so, what is the probability that
this occurs before 2100?
IPCC TAR (2001) models
MOC
Change
(Sv)
CO_2
up to 720
ppmv
by 2100
A1B, IPCC - TAR (2001)
IPCC-AR4 (2007) models
CO_2
up to 720
ppmv
by 2100
In the ‘best’ models, the MOC only decreases by a
few Sv from 2000 to 2100
Do coupled models have a bias???
Σ/S
0
Σ=
Net evaporation
MOC transport
Gyre transport
Van de Swaluw et al. GRL, (2007)
ESSENCE project
Ensemble mean
MOC
MPI ECHAM5-OM1
Ensemble mean
Artificial Collapse
induced by 1 Sv
freshwater input
Sterl et al. GRL, 2008
Role of atmospheric feedbacks
Atlantic Multidecadal Oscillation (AMO)
SSTA
Enfield et al., GRL, 28, 2077, 2001
Collapsed
state (yr 100)
Yin & Stouffer, J. Clim, (2007)
Meanwhile: changes in the MOC over the last 50
years
The strength of the MOC has decreased by about 30%!
Bryden et al. Nature, Dec, 1 2005
Monitoring the Atlantic MOC at 26N
RAPID - array (Marotzke, Bryden, Cunningham)
MOC at 26N:
19 ± 5 Sv
Why such a
large variability?
... because the ocean is full of eddies
Sea
surface
height
anomalies
OBS
... and really full of eddies
Tracking: october 1992 - december 2006
Chelton et al. GRL, 2008
Effect of random distribution of
eddies on the MOC
Wunsch, Nature Geosc. 2008
High resolution ocean models
needed to assess MOC stability?
About 0.1 degree
horizontal
resolution required
to model a
‘correct’ Gulf
Stream
MICOM, 1/12 degree
Summary and Perspective
There are no indications that the strength of the
Atlantic MOC has decreased over the last 50 years
Scale interactions between mean flow and eddies
are likely to be important for the large-scale
stability of the MOC
Ocean-atmosphere feedbacks affect the stability of
the MOC
We are still far from a reasonable estimate of the
probability that the MOC will collapse before 2001
Problem requires high-resolution global coupled models!