response of the amoc to global warming: role of ice sheets melting and amoc feedbacks
DESCRIPTION
Response of the AMOC to global warming: role of ice sheets melting and AMOC feedbacks. Didier Swingedouw CERFACS, Toulouse, France. NADW. AABW. Thermohaline circulation (THC). Quadfasel 2005. Rahsmtorf 2002. Rahsmtorf 2002. - PowerPoint PPT PresentationTRANSCRIPT
Didier Swingedouw
CERFACS, Toulouse, France
Response of the AMOC to Response of the AMOC to globalglobal
warming: role of ice sheets warming: role of ice sheets
melting and AMOC melting and AMOC feedbacksfeedbacks
Thermohaline circulation (THC)
Ocean circulation related to salinity and temperature gradients
NADW
AABW Rahsmtorf 2002
Rahsmtorf 2002
Quadfasel 2005
Effect –Effect +
Meridional advection
Heat transport
Salt transport
THCi THCo
THC internal feedbacks
Stocker et al., 2001
Stocker et al., 2001
Heat transportMeridional advection
Merdional temperature gradient in the atmosphere
Sea ice budget
Sea ice production
Ekman divergence
Atmopheric freshwater transport
Sea ice transport
Heat transport
Salt transport
Surface salinity flux
THCi THCo
THC internal feedbacksEffect –Effect +
Heat transportMeridional advection
Merdional temperature gradient in the atmosphere
Sea ice budget
Sea ice production
Ekman divergence
Atmopheric freshwater transport
Sea ice transport
Heat transport
Salt transport
Surface salinity flux
THCi THCo
Global warming
Global warming
Future of the THC (or AMOC): what role for ice sheet melting?
Schneider et al., 2007
Ice sheets melting neglected in most of the « IPCC » coupled models
Polar ice sheets
Greenland Ice volume equivalent to 7
meters of sea-level rise Surface area of 2 millions km²
(81% ice covered)
Antarctica Ice volume equivalent to 61
meters of sea-level rise Surface area of 14 millions km²
(98% ice covered) Massive ice shelves
Problematic
Can the Greenland ice sheet (GIS) melting accelerate the weakening of the AMOC?
How to quantify the key feedbacks of the AMOC?
Can the Antarctic ice sheet (AIS) melting stabilize this weakening?
How works the bipolar ocean seesaw?
Outlines
Impact of future GIS melting on the AMOC
Quantifying the AMOC feedbacks
Impact of future AIS melting on the AMOC
Revisiting the concept of the bipolar ocean seesaw
Outlines
Impact of future GIS melting on the AMOC
Quantifying the AMOC feedbacks
Impact of future AIS melting on the AMOC
Revisiting the concept of the bipolar ocean seesaw
Tool No1: IPSL-CM4 coupled model
LMDz
GCM, 3.75°x2.5°
19 levels
ORCA-LIM
GCM, 2° horizontal
31 niveaux verticaux
ORCHIDEEIce sheet
thermodyn.
module
OASIS
AMOC in IPSL-CM4
Atlantic meridional overturning streamfunction
Latitude
Depth
∗ Two oceanic cells in the Atlantic: NADW and AABW
∗ Maximum for NADW cell: 11 Sv
∗ Smaller than observations-based estimates (13-23 Sv)
∗ No convection in the Labrador Sea in the model
Sv
Mixed layer depth in JFM
Experimental design 1
Two versions of the IPSL-CM4:
1) With ice sheet melting Snow
Land Ocean
Ice
Net surface heat flux
2) Without ice sheet
melting
Simple thermodynamical parametrisation of ice sheet melting
Response of the AMOC after 500 years at
2xCO2
CO2
(ppm)
0 70 500
280
560
CTL
NoGIS
GIS
Time (Year)
Swingedouw D. and Braconnot P., Effect of Greenland ice-sheet melting on the response and stability of the AMOC in the next centuries, AGU monograph "Ocean Circulation: Mechanisms and Impacts" by Schmittner A., 383-392, (2007).
GIS
CTL
NoGIS
AMOC max.
GIS melting = 0.13 Sv after 200 ans. More than half of GIS melted in 500 years
AIS melting < 0.02 Sv = negligible
CTL
NoGIS
GIS
Global temperature
AMOC and convection
€
AMOC = γΔρ
Correlation of 0.98 between density
anomalies and THC variations
€
AMOC
t=0
NoGIS-CTL
GIS-CTLSwingedouw et al. Quantifying the AMOC feedbacks during a 2xCO2 stabilization experiment with land-ice melting. Climate Dynamics, 2007
Density budget in the convection sites
€
TransportS + Δρ Transport
T + ΔρSurfaceS + ΔρSurface
T + Δρ ResiduT + Δρ Residu
S ≈ Δρ
Transport
Surface
∗ In NoGIS: the main decreasing term for the AMOC is
the change in heat flux in the convection sites
∗ Main processes that help the AMOC to recover:
∗ Transport of salinity anomalies from the tropics
∗ Decrease of sea-ice melting in the convection sites
In GIS => Role of feedbacks?
Outlines
Impact of future GIS melting on the AMOC
Quantifying the AMOC feedbacks
Impact of future AIS melting on the AMOC
Revisiting the concept of the bipolar ocean seesaw
AMOC related feedbacks
TOT
Tsurface
TTransport
SOT
Ssurface
STransport
We consider differences between the scenarios to isolate feedbacks effects
01
1
ii
iii
with
i
i 0 0with Land-ice melting anomaly
AMOC feedbacks quantification
€
G =1
1− (λ S + λT )
⎛
⎝ ⎜
⎞
⎠ ⎟= 2.5
Dynamical gain
S T
Feedback gain
Temperaturechanges
Salinity changes
THCe THCs
)( S
€
(λT )
+-
€
T
€
S
A model dependent result
Using LOVECLIM model, Driesschaert et al. (2007) found a very moderate effect of ice sheet melting (at 4XCO2)
Causes ?
1. Different GIS melting?
2. Different THC dynamics?
3. Different GIS melting localisation ?
4. Different AIS melting?
After 500 years the GIS melting represents:
• 8% in LOVECLIM
• 50% in IPSL-CM4
1
AMOC max. (Driesschaert et al. 2007)
Outlines
Impact of future GIS melting on the AMOC
Quantifying the AMOC feedback
Impact of future AIS melting on the AMOC
Revisiting the concept of the bipolar ocean seesaw
Tool No2: LOVECLIM
ECBILTQG, T21 with 3 levels
CLIOGCM, 3°x3° 20 levels
VECODE
ISM
10 km
31 levels
LOVECLIM climatology
Two cells with larger magnitudes than in IPSL-CM4
Convection in the labrador Sea
Better agreement with observations
Mixed layer depth JFM
Atlantic meridional streamfunction
Experimental design 2
∗ Anaysis of 4XCO2 projections:
∗ Without polar ice sheets melting (fixed)
∗ With Greenland and Antarctic ice sheet melting (AGIS)
∗ With Greenland ice sheet melting only (GIS)
∗ With Antarctic ice sheet melting only (AIS)
Sans
CO2 (ppm)
0140 3000
280
1120
CTRL
Year
4xCO2
AGISGISAIS
AABW response in the projections
∗ AABW cell is weakened the first 300 years
∗ Then it increases
∗ The cell stabilizes around its inital value with Antarctic ice sheet melting (AGIS, AIS)
∗ And 25% above without (GIS, fixed)
∗ AIS looses mass and put around 0.1 Sv in the SO
Export AABW at 30°S
CTRL
AGIS
AIS
GIS
fixed
NADW cell response in the projections
Without AIS melting the
weakening of the NADW cell is
larger
CTRLNoISAGISGISAIS
Swingedouw et al., AIS melting provides negative feedbacks for global warming. GRL, 2008
AMOC max.
NADW export at 30°S
Outlines
Impact of future GIS melting on the AMOC
Quantifying the AMOC feedback
Impact of future AIS melting on the AMOC
Revisiting the concept of the bipolar ocean seesaw
The ocean bipolar seesaw
In ocean model: Stocker et al. (1992)
In paleoclimatic reconstructions: Broecker (1998)
Confirmed in OGCMs: Seidov et al. (2001)
Not true in AOGCM in transient phase: Stouffer et al. (2007)Why?
NADWAABW
NADWAABW
Stouffer et al.’s experimental design
∗ Freswater input of 1 Sv south of 60°S during 100 years (Southern Ocean Hosing: Hos1)
∗ Equivalent freshwater amount larger than GIS volume
∗ Stouffer et al. noticed a slight weakening of the NADW cell
∗ They attributed this weakening to the salinity anomalies transport from the Southern Ocean to the North Atlantic
AMOC max. (Stouffer et al. 2007)SSS anomalies after 100 yrs
Response of NADW cell to a freshwater input in the Southern
Ocean
∗ We reproduce the same experiement using LOVECLIM (Hos1) en utilisant le modèle LOVECLIM (sans calotte polaire)
∗ A dipole of streamfunction anomalies:
∗ Weakening of the northern cell
∗ Enhancement of the cell south of 30°N
Swingedouw et al., Impact of transient freshwater releases in the Southern Ocean on the AMOC and climate. Climate Dynamics, 2009
Atlantic meridional overturning streamfunction
Hos1 – CTRL (100 yr mean)
Climatic response to a freshwater input in the
Southern Ocean Cooling of the Southern Hemisphere
Increase of the meridional temperature gradient
Increase in westerly winds
Potential impact on NADW cell (Toggweiler and Samuels 1995)
We test this effect with a similar experiment to Hos1 but with fixed wind (CTRL) called HosWind
Surface temperature and wind
Hos1 – CTRL (100 yr mean)
Three main processes
Three main processes influence the NADW cell:
The wind increase explains part of the NADW cell increase
Hos1 - HosWindHosWind - CTRLHos1 - CTRL
1. Bipolar ocean seesaw: a weakening of AABW cell enhances the NADW cell
2. Salinity anomalies advection, from the south up to the North Atlantic convection sites
3. Increase in the Southern winds, which pushes (ekman) surface water towards the Atlantic basin
Atlantic meridional streamfunction
F S F N
F N
AABW
NADW
30°S Latitude
SF NF
Ψ
D1
2D
Pycnocline
Depth
Quantification of the impact of each process
DF=t
Vb
We use the density binning analysis, which quantifes the formation-consumption of water masses
Which reconcile dynamical and thermodynamical approach
F S F N
F N
NADW
30°S Latitude
NF
Ψ
D1
2D
Pycnocline
Depth
F S F N
F N
AABW
NADW
30°S Latitude
SF NF
Ψ
D1
2D
Pycnocline
Depth
NNNtAtl DFV= 1
Budget North of 32°S in the Atlantic for water denser than 27.6 kg/m3
F S F N
F N
AABW
NADW
30°SLatitude
SF NF
Ψ
D1
2D
Pycnocline
Depth
Process 1: bipolar ocean seesaw
F S F N
F N
AABW
NADW
30°SLatitude
Ψ
D1
2D
Pycnocline
Depth
F S F N
F N
AABW
NADW
30°SLatitude
Ψ
D1
2D
Pycnocline
Depth
F S F N
F N
AABW
NADW
30°SLatitude
Ψ
D1
2D
Pycnocline
Depth
F S F N
F N
AABW
NADW
30°SLatitude
Ψ
D1
2D
Pycnocline
Depth
+4.5 Sv
F S F N
F N
AABW
NADW
30°SLatitude
SF NF
Ψ
D1
2D
Pycnocline
Depth
F S F N
F N
AABW
NADW
30°SLatitude
SF NF
Ψ
D1
2D
Pycnocline
Depth
F S F N
F N
AABW
NADW
30°SLatitude
SF NF
Ψ
D1
2D
Pycnocline
Depth
-3 Sv
Process 2: salinity anomalies advection
F S F N
F N
AABW
NADW
30°SLatitude
SF NF
Ψ
D1
2D
Pycnocline
Depth
F S F N
F N
AABW
NADW
30°SLatitude
Ψ
D1
2D
Pycnocline
Depth
F S F N
F N
AABW
NADW
30°SLatitude
Ψ
D1
2D
Pycnocline
Depth
+1.5 Sv
Process 3: Southern wind increase
Phase diagramm
Impact of the rate for the freswater release for a given amount (100 Sv.yr)
Rate < 0.2 Sv = process 2 is very small
In the projections, we are in this side of the phase diagramm
Hosing Perturbation (Sv)0 0.4 0.8 1.2 1.6 2
Conclusions
∗ GIS melting induces a collapse of the AMOC in the IPSL-CM4 after 500 years
∗ This is due to a large positive salinity feedback and a weak temperature negative one
∗ In LOVECLIM the AIS melts after a few centuries at 4XCO2, which stabilises the AMOC weakening
∗ The mechanisms for the response of the AMOC to a Southern Ocean hosing are not trivial and their effects depend on the rate of the hosing
Quantifying the AMOC feedbacks among different CGCMs
For a given freshwater input, large spread among AOGCMs (Stouffer et al. 2006)
Methodology of feedbacks quantification useful (Swingedouw et al. 2007)
To be done using the Thor project framework (FP7)
Feedback gain
Temperaturechanges
Salinity changes
THCi THCo
)( S
€
(λT )
Stouffer et al. 2006
Outlooks
Compare the AMOC feedbacks in different CGCMs: THOR project.
Coupling of ice sheets in a higher resolution cimate model (IPSL-CM4…) can improve our: Projections of sea-level rise Projections of THC changes
Compare the trend in salinity in both observations and models
Thank you
Mailto: [email protected] Web: http://dods.ipsl.jussieu.fr/dssce/public_html/index.html