Johann Jungclaus 1 K. Lohmann 1 , H. Kleiven 2

1. Combining model simulations and paleoceanographic reconstructions for a process-based understanding of climate variability in the North Atlantic/Arctic region Johann Jungclaus1 K. Lohmann1, H. Kleiven2 1Max Planck Institute for Meteorology, Hamburg, Germany 2University of Bergen, Bergen, Norway johann.jungclaus@zmaw.de

2. Motivation More sophisticated paleoceanographic reconstructions reflecting dynamical oceanic quantities (e.g., overflow transports, heat transports) become increasingly available How representative are local proxies, how is their recorded variability related to the large-scale circulation? Are CMIP5/PMIP3-style model simulations suitable to interpret the mechanisms behind the observed/reconstructed variations?

3. The MPI-ESM Model The model system: MPI-Earth System Model Atmosphere: ECHAM6. Ocean: MPIOM. Land: JSBACH. Ocean biogeochemistry: HAMOCC5. Coupler: OASIS3 Documentation: Journal of Advances in Modeling Earth Systems (JAMES, 2013) special issue on the MPI-ESM as used in CMIP5/PMIP3 Gridsize (km) MPI-ESM-P ECHAM6 T63/L47 MPIOM:GR1.5 (254x220x40) Jungclaus et al., 2008; 2013

4. Iceland Scotland Overflow intensity and Atlantic surface properties AMO reconstruction (Gray et al., 2004) ISOW Reconstruction of overflow strength based on sortable silt suggests pronounced variations in phase with basin-wide temperature changes (Atlantic Multidecadal Oscillation/Variability) Mjell et al., to be submitted

5. Iceland Scotland Overflow intensity and Atlantic surface properties Possible explanation: strong overturning drives enhanced MOC, MOC carries more heat to North Atlantic → positive AMO anomaly

6. Iceland Scotland Overflow intensity and Atlantic surface properties AMO ISOW • Past1000 simulationsshowsimilar high positive correlationsbetween AMO and ISOW strength. • Visual inspectionpointsto prominent roleofexternalforcing, in particular strong volcaniceruptions (Ottera et al, 2010; Zanchettin et al., 2012) • Howcanwereconcilecoldsurfacecondtionswithweakoverflows? Lohmann et al., to be submitted

7. Iceland Scotland Overflow intensity and Atlantic surface properties Strong Iceland-Scotland overflow associated with warm Nordic Seas, less convection in Greenland Sea, but enhanced barotropic pressure gradient (contrasting with traditional view based on 2-layer (baroclinic) hydraulics) Lohmann et al., to be submitted

8. Iceland Scotland Overflow intensity and Atlantic surface properties Spin-up of gyre circulation Direct radiative forcing Warm Nordic Seas (AMO) Lighter surface density (T dominated) Less convection in Nordic Seas Anticyclonic circulation anomaly Higher SSH in Nordic Seas Deeper isopycnals, less doming Higher (barotropic) pressure north of ISR Strong ISR overflow Lohmann et al., to be submitted

9. Atlantic water transport into the Arctic A marine sedimentcorerevealsunprecedentedwarmingandenhancedAtlanticWaterinflowintotheArctic in modern times. Can such dramaticchangesbeexplainedwiththehelpofmodels? Istheassociatedincrease in heattransport a localorremotely-drivenphenomenon? Spielhagen et al., 2011

10. Atlantic water transport into the Arctic Model simulations exhibit consistenly positive anomalies exceeding internal variability range Ocean heat transport anomalies in 20th century up to 50% higher than in pre-industrial times. Anomalies are associated with gyre (horizontal) circulation rather than overturning Atl. Water in Fram Strait: Model 2σfrom Ctrl. expmt Heat transport to Arctic Heat transport @75N MPI-ESM-P past1000-r1 MPI-ESM-P past1000-r2 gyre MOC

11. Atlantic water transport into the Arctic Trends in total heat transport (TW/100yr) as function of latitude 5-95%-range for 100-yr trends from control run colored lines: MPI-ESM historical simulations 1850-2005 Oceanheattransporttrendsover 20thcenturysignificantly larger than in unforcedcontrolsimulations in all MPI-ESM historicalexperiments in NA subpolar gyreand Nordic Seas

12. Atlantic water transport into the Arctic Change in zonal-mean wind stress curl over the historical period Curl τ x10-7 Nm-3 Colored lines: MPI-ESM historical simulations 1850-2005 Wind-drivenchanges in gyrecirculationandheattransports essential (see also SedlačekandMysak, 2009; Häkkinen and Rhines) Nocorrespondingchanges in AMOC (AMOC @ 30N decreasesslightlyover 20thcentury)

13. Conclusions Last millennium simulations with state-of-the-art models provide detailed representation of important mechanisms to suggest explanations for the diagnosed local property and dynamical changes. CAVEAT: model-dependency in details is diagnosed (Lohmann et al., 2013) and evaluation should be done in a multi-model framework The analyses of THC variations in the THOR project have demonstrated a relative prominent of external forcing shaping the AMO and North Atlantic climate over the millennium (Ottera et al., 2010; Zanchettin et al., 2012) There is more to long-term variations in the North Atlantic/Arctic climate than what we often infer from the simplifying conveyor-belt image (see also Wunsch, QSR 2010)

14. The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.212643 THOR www.eu-thor.eu