On the reduced sensitivity of the Atlantic overturning

to Greenland ice sheet melting in projections: a multi-model assessment

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

Future of Greenland ice sheet

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)

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)

Swingedouw et al. (Clim. Dyn. 2013)

AMOC response at 26°N

Freshwater leakage: A key process to explain the AMOC spread

AMOC changes(Sv)

AMOC changes vs gyre asymmetry

G

yre

asy

mm

etr

iy (

°lat)

Rypina et al. (2011)

Observations

Swingedouw et al. (Clim. Dyn. 2013)

Freshwater leakage

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

Experimental design

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

Model Type Ocean Atmosphere

HadCM3 AOGCM NoName1.25°– L20

HadAM3 2.5° x 3.75° – L19

IPSLCM5A-LR AOGCM NEMO_v3 2° – L31

LMD5 1.8° x 3.75° – L39

MPI-ESM ESM MPI-OM 1.5° – L40

ECHAM6T63 – L47

EcEarth AOGCM NEMO_v3 1° – L42

IFS T159 – L67

BCM2 AOGCM MICOM2.8° – L35

ARPEGE T63 – L31

ORCA05-Kiel OGCM NEMO_v30.5° – L46

X

General response

RCP85 scenario for all models except BCM2 (A1B)

AMOC weakens in response to warming

Further decrease due to hosing

> 2 times lower sensitivity in the

projections / historical

Ensemble mean of SSS response (Hosing-Control, last decade)

Ensemble mean of SST response (Hosing-Control, last decade)

SSS and SST response to hosing

Freshwater leakage

Ventilation decrease in the

projections 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

FutConHisCon

Sea-ice edge

Changes in gyre

asymmetry In response to the global

warming, the barotropic gyres are modified

This is associated with a northward shift of the atmospheric jet in a warmer world

The relation with freshwater leakage still holds

Role of each process 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 %

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

Thank you!

Didier.Swingedouw@u-bordeaux1.fr

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