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Response of the AMOC to global warming: role of ice sheets melting and AMOC feedbacks

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.

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Response of the AMOC to global warming: role of ice sheets melting and AMOC feedbacks

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

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

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

  4. THC internal feedbacks Effect – Effect + Atmopheric freshwater transport Sea ice transport Salt transport Merdional temperature gradient in the atmosphere THCi THCo Meridional advection Heat transport Sea ice budget Heat transport Sea ice production Surface salinity flux Ekman divergence Stocker et al., 2001

  5. Global warming Atmopheric freshwater transport Sea ice transport Salt transport Merdional temperature gradient in the atmosphere THCi THCo Meridional advection Heat transport Sea ice budget Heat transport Sea ice production Surface salinity flux Ekman divergence Global warming

  6. Schneider et al., 2007 Future of the THC (or AMOC): what role for ice sheet melting? 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. Ice sheet thermodyn. module Tool No1: IPSL-CM4 coupled model LMDz GCM, 3.75°x2.5° 19 levels OASIS ORCA-LIM GCM, 2° horizontal 31 niveaux verticaux ORCHIDEE

  12. Atlantic meridional overturning streamfunction Depth Mixed layer depth in JFM Latitude AMOC in IPSL-CM4 • 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

  13. Net surface heat flux 2) Without ice sheet melting Snow Ice Ocean Land Experimental design 1 Two versions of the IPSL-CM4: 1) With ice sheet melting Simple thermodynamical parametrisation of ice sheet melting

  14. CO2 (ppm) GIS 560 NoGIS CTL 280 0 500 70 AMOC max. NoGIS • GIS melting = 0.13 Sv after 200 ans. More than half of GIS melted in 500 years • AIS melting < 0.02 Sv = negligible Global temperature NoGIS GIS CTL CTL GIS Response of the AMOC after 500 years at 2xCO2 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).

  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 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? Transport

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

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

  20. 1 • After 500 years the GIS melting represents: • 8% in LOVECLIM • 50% in IPSL-CM4 A model dependent result AMOC max. (Driesschaert et al. 2007) • Using LOVECLIM model, Driesschaert et al. (2007) found a very moderate effect of ice sheet melting (at 4XCO2) • Causes ? • Different GIS melting? • Different THC dynamics? • Different GIS melting localisation ? • Different AIS melting?

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

  23. Atlantic meridional streamfunction Mixed layer depth JFM LOVECLIM climatology • Two cells with larger magnitudes than in IPSL-CM4 • Convection in the labrador Sea • Better agreement with observations

  24. CO2 (ppm) 1120 4xCO2 CTRL 280 Sans 0 140 3000 Year AGIS GIS AIS 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)

  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 AIS AGIS CTRL GIS fixed

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

  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. AABW NADW NADW AABW 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?

  29. SSS anomalies after 100 yrs AMOC max. (Stouffer et al. 2007) 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

  30. Atlantic meridional overturning streamfunction Hos1–CTRL (100 yr mean) 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

  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. HosWind - CTRL Hos1 - CTRL Atlantic meridional streamfunction Hos1 - HosWind Three main processes The wind increase explains part of the NADW cell increase Three main processes influence the NADW cell: • Bipolar ocean seesaw: a weakening of AABW cell enhances the NADW cell • Salinity anomalies advection, from the south up to the North Atlantic convection sites • Increase in the Southern winds, which pushes (ekman) surface water towards the Atlantic basin

  33. Pycnocline Pycnocline Pycnocline NADW NADW NADW Budget North of 32°S in the Atlantic for water denser than 27.6 kg/m3 AABW AABW Depth Depth Depth 30°S 30°S 30°S Latitude Latitude Latitude 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

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

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

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

  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 0 0.4 0.8 1.2 1.6 2 Hosing Perturbation (Sv)

  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. Salinity changes THCi THCo Feedback gain Temperature changes 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) 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

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