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

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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. - PowerPoint PPT Presentation

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<ul><li><p>Didier Swingedouw</p><p>CERFACS, Toulouse, France </p><p>Response of the AMOC to global</p><p> warming: role of ice sheets</p><p> melting and AMOC feedbacks</p></li><li><p>Thermohaline circulation (THC)Ocean circulation related to salinity and temperature gradientsRahsmtorf 2002</p></li><li><p>Effect Effect +Meridional advectionHeat transportSalt transportTHCiTHCoTHC internal feedbacksStocker et al., 2001</p></li><li><p>Stocker et al., 2001Heat transportMeridional advectionMerdional temperature gradient in the atmosphereSea ice budgetSea ice productionEkman divergenceAtmopheric freshwater transportSea ice transportHeat transportSalt transportSurface salinity fluxTHCiTHCoTHC internal feedbacksEffect Effect +</p></li><li><p>Global warming</p></li><li><p>Future of the THC (or AMOC): what role for ice sheet melting?Ice sheets melting neglected in most of the IPCC coupled models</p></li><li><p>Polar ice sheets GreenlandIce volume equivalent to 7 meters of sea-level riseSurface area of 2 millions km (81% ice covered)</p><p>AntarcticaIce volume equivalent to 61 meters of sea-level riseSurface area of 14 millions km (98% ice covered)Massive ice shelves</p></li><li><p>ProblematicCan the Greenland ice sheet (GIS) melting accelerate the weakening of the AMOC?</p><p>How to quantify the key feedbacks of the AMOC?</p><p>Can the Antarctic ice sheet (AIS) melting stabilize this weakening?</p><p>How works the bipolar ocean seesaw?</p></li><li><p>OutlinesImpact of future GIS melting on the AMOC</p><p>Quantifying the AMOC feedbacks</p><p>Impact of future AIS melting on the AMOC</p><p>Revisiting the concept of the bipolar ocean seesaw</p></li><li><p>OutlinesImpact of future GIS melting on the AMOC</p><p>Quantifying the AMOC feedbacks</p><p>Impact of future AIS melting on the AMOC</p><p>Revisiting the concept of the bipolar ocean seesaw</p></li><li><p>Tool No1: IPSL-CM4 coupled modelLMDzGCM, 3.75x2.519 levelsORCA-LIMGCM, 2 horizontal 31 niveaux verticauxORCHIDEEOASIS</p></li><li><p>AMOC in IPSL-CM4Two oceanic cells in the Atlantic: NADW and AABW</p><p>Maximum for NADW cell: 11 Sv</p><p>Smaller than observations-based estimates (13-23 Sv)</p><p>No convection in the Labrador Sea in the modelSv</p></li><li><p>Experimental design 1Two versions of the IPSL-CM4:</p><p>1) With ice sheet melting</p><p>Simple thermodynamical parametrisation of ice sheet melting</p></li><li><p>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). </p></li><li><p>AMOC and convectionCorrelation of 0.98 between density anomalies and THC variationsSwingedouw et al. Quantifying the AMOC feedbacks during a 2xCO2 stabilization experiment with land-ice melting. Climate Dynamics, 2007</p></li><li><p>Density budget in the convection sitesTransportSurface</p><p>In NoGIS: the main decreasing term for the AMOC is the change in heat flux in the convection sitesMain processes that help the AMOC to recover: Transport of salinity anomalies from the tropicsDecrease of sea-ice melting in the convection sites In GIS =&gt; Role of feedbacks?</p></li><li><p>OutlinesImpact of future GIS melting on the AMOC</p><p>Quantifying the AMOC feedbacks</p><p>Impact of future AIS melting on the AMOC</p><p>Revisiting the concept of the bipolar ocean seesaw</p></li><li><p>AMOC related feedbacksWe consider differences between the scenarios to isolate feedbacks effects</p></li><li><p>AMOC feedbacks quantification</p><p>Graph1</p><p>1.13</p><p>-0.53</p><p>Feuil1</p><p>1.130.91-4.28</p><p>-0.530.323.74</p><p>0.280.26</p><p>Feuil1</p><p>Feuil2</p><p>Feuil3</p></li><li><p>A model dependent resultUsing LOVECLIM model, Driesschaert et al. (2007) found a very moderate effect of ice sheet melting (at 4XCO2)</p><p>Causes ?</p><p>Different GIS melting?</p><p>Different THC dynamics?</p><p>Different GIS melting localisation ?</p><p>Different AIS melting? </p><p>AMOC max. (Driesschaert et al. 2007)</p></li><li><p>OutlinesImpact of future GIS melting on the AMOC</p><p>Quantifying the AMOC feedback</p><p>Impact of future AIS melting on the AMOC</p><p>Revisiting the concept of the bipolar ocean seesaw</p></li><li><p>Tool No2: LOVECLIMECBILTQG, T21 with 3 levelsCLIOGCM, 3x3 20 levelsVECODE</p></li><li><p>LOVECLIM climatologyTwo cells with larger magnitudes than in IPSL-CM4</p><p>Convection in the labrador Sea</p><p>Better agreement with observations</p></li><li><p>Experimental design 2Anaysis of 4XCO2 projections:</p><p>Without polar ice sheets melting (fixed)</p><p>With Greenland and Antarctic ice sheet melting (AGIS)</p><p>With Greenland ice sheet melting only (GIS)</p><p>With Antarctic ice sheet melting only (AIS)</p></li><li><p>AABW response in the projectionsAABW cell is weakened the first 300 years</p><p>Then it increases</p><p>The cell stabilizes around its inital value with Antarctic ice sheet melting (AGIS, AIS)</p><p>And 25% above without (GIS, fixed)</p><p>AIS looses mass and put around 0.1 Sv in the SO</p><p>Export AABW at 30SCTRLAGISAISGISfixed</p></li><li><p>NADW cell response in the projectionsWithout AIS melting the weakening of the NADW cell is largerSwingedouw et al., AIS melting provides negative feedbacks for global warming. GRL, 2008</p></li><li><p>OutlinesImpact of future GIS melting on the AMOC</p><p>Quantifying the AMOC feedback</p><p>Impact of future AIS melting on the AMOC</p><p>Revisiting the concept of the bipolar ocean seesaw</p></li><li><p>The ocean bipolar seesawIn ocean model: Stocker et al. (1992)</p><p>In paleoclimatic reconstructions: Broecker (1998)</p><p>Confirmed in OGCMs: Seidov et al. (2001)</p><p>Not true in AOGCM in transient phase: Stouffer et al. (2007)Why?</p></li><li><p>Stouffer et al.s experimental designFreswater input of 1 Sv south of 60S during 100 years (Southern Ocean Hosing: Hos1)</p><p>Equivalent freshwater amount larger than GIS volume</p><p>Stouffer et al. noticed a slight weakening of the NADW cell</p><p>They attributed this weakening to the salinity anomalies transport from the Southern Ocean to the North Atlantic</p></li><li><p>Response of NADW cell to a freshwater input in the Southern OceanWe reproduce the same experiement using LOVECLIM (Hos1) en utilisant le modle LOVECLIM (sans calotte polaire)</p><p>A dipole of streamfunction anomalies:</p><p>Weakening of the northern cell</p><p>Enhancement of the cell south of 30N</p><p>Swingedouw et al., Impact of transient freshwater releases in the Southern Ocean on the AMOC and climate. Climate Dynamics, 2009</p></li><li><p>Climatic response to a freshwater input in the Southern OceanCooling of the Southern Hemisphere</p><p>Increase of the meridional temperature gradient</p><p>Increase in westerly winds</p><p>Potential impact on NADW cell (Toggweiler and Samuels 1995)</p><p>We test this effect with a similar experiment to Hos1 but with fixed wind (CTRL) called HosWind Surface temperature and windHos1 CTRL (100 yr mean)</p></li><li><p>Three main processesThree main processes influence the NADW cell:The wind increase explains part of the NADW cell increaseHos1 - HosWindBipolar ocean seesaw: a weakening of AABW cell enhances the NADW cell</p><p>Salinity anomalies advection, from the south up to the North Atlantic convection sites</p><p>Increase in the Southern winds, which pushes (ekman) surface water towards the Atlantic basinAtlantic meridional streamfunction</p></li><li><p>Quantification of the impact of each processWe use the density binning analysis, which quantifes the formation-consumption of water massesWhich reconcile dynamical and thermodynamical approach</p></li><li><p>Process 1: bipolar ocean seesaw+4.5 Sv </p></li><li><p>-3 Sv Process 2: salinity anomalies advection</p></li><li><p>+1.5 Sv Process 3: Southern wind increase</p></li><li><p>Phase diagrammImpact of the rate for the freswater release for a given amount (100 Sv.yr)</p><p>Rate &lt; 0.2 Sv = process 2 is very small</p><p>In the projections, we are in this side of the phase diagrammHosing Perturbation (Sv)0 0.4 0.8 1.2 1.6 2</p></li><li><p>ConclusionsGIS melting induces a collapse of the AMOC in the IPSL-CM4 after 500 years</p><p>This is due to a large positive salinity feedback and a weak temperature negative one</p><p>In LOVECLIM the AIS melts after a few centuries at 4XCO2, which stabilises the AMOC weakening</p><p>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</p></li><li><p>Quantifying the AMOC feedbacks among different CGCMsFor a given freshwater input, large spread among AOGCMs (Stouffer et al. 2006)</p><p>Methodology of feedbacks quantification useful (Swingedouw et al. 2007)</p><p>To be done using the Thor project framework (FP7)Stouffer et al. 2006</p></li><li><p>OutlooksCompare the AMOC feedbacks in different CGCMs: THOR project.</p><p>Coupling of ice sheets in a higher resolution cimate model (IPSL-CM4) can improve our:Projections of sea-level riseProjections of THC changes</p><p>Compare the trend in salinity in both observations and models</p></li><li><p>Thank you Mailto: swingedo@cerfacs.fr Web: http://dods.ipsl.jussieu.fr/dssce/public_html/index.html</p></li><li><p>AMOC feedbacksSurface heat flux strongly damps the heat transport feedback</p><p>Graph3</p><p>0.910.320.28</p><p>-4.283.740.26</p><p>Feuil1</p><p>0.91-4.28</p><p>0.323.74</p><p>0.950.91-4.280.280.26</p><p>-0.280.323.74</p><p>0.280.26</p><p>Feuil1</p><p>Feuil2</p><p>Feuil3</p><p>SensitivitySensitivityDidier Swingedouw*Didier SwingedouwSensitivitySensitivityDidier Swingedouw*Reprsentation du tapis roulant de la circulation globaleUne partie est li au vent lautre au forage thermohalin (temprature et salinit)En effet cest T et S qui conditionne le phnomne de CV aux hautes latitudes qui transforme les eau de surface en eaux profondes et gnre une circulation mridienne de retournement dans lAtlantique. Cest cette circulation Atlantique de retournement que nous nommerons THC dans la suite de lexpos.Didier SwingedouwSensitivity*Def : une rtroaction cest laction en retour dun effet sur le mcanisme qui lui a donn naissanceDidier SwingedouwSensitivity*Dcrire une rtroaction : celle lie la glace de mer et leffet local.Message: compliqu car pas de comparaison entre diffrentes rtroactions: clarifier</p><p>Prciser : FCMVoir sil est possible dcrire plus gros lgende des botesDidier SwingedouwSensitivity*Dcrire une rtroaction : celle lie la glace de mer et leffet local.Message: compliqu car pas de comparaison entre diffrentes rtroactions: clarifier</p><p>Prciser : FCMVoir sil est possible dcrire plus gros lgende des botesDidier SwingedouwSensitivitySensitivityDidier Swingedouw*Changement climatique rapide diffrent du passCauses dune diminution ?Effet clim ? 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