Swingedouw D. , Rodehacke C., Olsen S., Menary M. Gao Y., Mikolajewicz U., Mignot J.

1. On the reduced sensitivity of the Atlantic overturning to Greenland ice sheet melting in projections: a multi-model assessment Swingedouw D., Rodehacke C., OlsenS., MenaryM. Gao Y., MikolajewiczU., Mignot J.

2. Future of Greenlandicesheet • Greenland ice sheet (GrIS) is melting at an increasing rate (Rignot et al. 2011) • Up to 0.07 Sv in 2100 if the accelerating trend continues • Large uncertainty concerning the potential impact of an input of 0.1 Sv on the Atlantic Meridional Overturning Circulation (AMOC) (cf. Stouffer et al. 2006)

3. Impact of freshwater release under historical conditions • 6 models participating to the EU-FP7 THOR project • 0.1 Sv hosing over 40 years around Greenland for the historical period 1965-2004 • Includes an ocean-only model • Large spread found for AMOC response as in Stouffer et al. (2006) AMOC responseat 26°N Swingedouw et al. (Clim. Dyn. 2013)

4. Freshwater leakage: A key process to explain the AMOC spread Observations AMOC changes vs gyre asymmetry Gyre asymmetriy (°lat) Freshwater leakage Rypina et al. (2011) Swingedouw et al. (Clim. Dyn. 2013) AMOC changes(Sv)

5. Central questions • Is the impact of GrIS melting underestimated in most IPCC-type climate models? (due to wrong representation of gyre asymmetry and freshwater leakage) • Does the mean state impact the sensitivity to freshwater input? • Same models, same freshwater input, but within future climate projections

6. Experimental design 0.1 Sv freshwater input homogenously released around Greenland for the historical (1965-2004) and future (2050-2089) periods

7. General response • RCP85 scenario for all modelsexcept BCM2 (A1B) • AMOC weakens in response to warming • Furtherdecrease due to hosing > 2 times lower sensitivity in the projections / historical

8. Ensemble mean of SSS response (Hosing-Control, last decade) SSS and SST response to hosing Freshwater leakage Ensemble mean of SST response (Hosing-Control, last decade)

9. Ventilation decrease in the projections Sea-ice edge • Increased stratification in the North Atlantic mainly due to temperature changes rather than salinity changes (cf. Gregory et al. 2005) • Decoupling between upper and deeper ocean can explain the lower response in projections HisCon FutCon

10. Changes in gyre asymmetry • In response to the global warming, the barotropic gyres are modified • This isassociatedwith a northward shift of the atmospheric jet in a warmer world • The relation withfreshwaterleakagestillholds

11. Role of eachprocess • We hypothesize that two main processes can explain the lower AMOC sensitivity to FW input in a warmer world: • Decrease in ventilation • Changes in gyre asymmetry • We propose a bilinear model to quantify the contribution of each process: • According to this model, changes in asymmetry explain 62 ± 8 % of the lower sensitivity in warmer climate and decrease in ventilation 38 ± 8 %

12. Conclusions • Large GrIS melting (0.1 Sv) doubles AMOC weakening between 2050-2090 in our 5-model ensemble mean • Impact of freshwater input is lower in a warmer world by around two times in our 5-model ensemble mean • This is due both to • the decrease of ventilation that saturates the AMOC decrease • the change in gyre shape that limits freshwater leakage along the Canary current

13. Thankyou! Didier.Swingedouw@u-bordeaux1.fr

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