1. Didier Swingedouw CERFACS, Toulouse, France Response of the AMOC to global warming: role of ice sheets melting and AMOC feedbacks

2. Thermohaline circulation (THC) Ocean circulation related to salinity and temperature gradients NADW AABW Rahsmtorf 2002 Quadfasel 2005

3. Effect – Effect + Meridional advection Heat transport Salt transport THCiTHCo THC internal feedbacks Stocker et al., 2001

4. Heat transport Meridional 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 THCiTHCo THC internal feedbacks Effect – Effect +

5. Heat transport Meridional 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 THCiTHCo Global warming

6. 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

7. 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

8. 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?

9. 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

10. 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

11. Tool No1: IPSL-CM4 coupled model LMDz GCM, 3.75°x2.5° 19 levels ORCA-LIM GCM, 2° horizontal 31 niveaux verticaux ORCHIDEE Ice sheet thermodyn. module OASIS

12. 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

13. 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

14. 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

15. AMOC and convection Correlation of 0.98 between density anomalies and THC variations t=0 NoGIS-CTL GIS-CTL Swingedouw et al. Quantifying the AMOC feedbacks during a 2xCO2 stabilization experiment with land-ice melting. Climate Dynamics, 2007

16. Density budget in the convection sites 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?

17. 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

18. AMOC related feedbacks We consider differences between the scenarios to isolate feedbacks effects with Land-ice melting anomaly

19. AMOC feedbacks quantification Dynamical gain Feedback gain Temperature changes Salinity changes THCeTHCs + -

20. 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)

21. 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

22. Tool No2: LOVECLIM ECBILT QG, T21 with 3 levels CLIO GCM, 3°x3° 20 levels VECODE ISM 10 km 31 levels

23. 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

24. 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 ) 0 1403000 280 1120 CTRL Year 4xCO2 AGISGIS AIS

25. 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

26. NADW cell response in the projections Without AIS melting the weakening of the NADW cell is larger CTRL NoIS AGIS GIS AIS Swingedouw et al., AIS melting provides negative feedbacks for global warming. GRL, 2008 AMOC max. NADW export at 30°S

27. 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

28. 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? NADW AABW NADW AABW

29. 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

30. 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)

31. 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)

32. Three main processes Three main processes influence the NADW cell: The wind increase explains part of the NADW cell increase Hos1 - HosWind HosWind - CTRL Hos1 - 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

33. AABW NADW 30°S Latitude Pycnocline Depth Quantification of the impact of each process We use the density binning analysis, which quantifes the formation- consumption of water masses Which reconcile dynamical and thermodynamical approach NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth Budget North of 32°S in the Atlantic for water denser than 27.6 kg/m 3

34. AABW NADW 30°S Latitude Pycnocline Depth Process 1: bipolar ocean seesaw AABW NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth +4.5 Sv

35. AABW NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth -3 Sv Process 2: salinity anomalies advection

36. AABW NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth AABW NADW 30°S Latitude Pycnocline Depth +1.5 Sv Process 3: Southern wind increase

37. 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

38. 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

39. 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 Temperature changes Salinity changes THCiTHCo Stouffer et al. 2006

40. 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

41. Thank you Mailto: swingedo@cerfacs. fr Web: http://dods.ipsl.jussieu.fr/dssce/public_html/index.html

42. AMOC feedbacks TransportSurface Résidu Salinité Température Transport Surface Résidu Surface heat flux strongly damps the heat transport feedback Oceanic transport Temperature densityflux Salinity Density flux THCi THCo + - Local atmospheric response