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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. - PowerPoint PPT Presentation
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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
Gy
re a
sym
met
riy (°
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 design0.1 Sv freshwater input homogenously released around Greenland for the historical (1965-2004) and future (2050-2089) periods
Model Type Ocean AtmosphereHadCM3 AOGCM NoName
1.25°– L20HadAM3 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!
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