Combining model simulations and paleoceanographic reconstructions for a process-based understanding of climate variability in the North Atlantic/Arctic region Johann Jungclaus 1 K. Lohmann 1, H. Kleiven 2 1 Max Planck Institute for Meteorology, Hamburg, Germany 2 University of Bergen, Bergen, Norway

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?

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 MPI-ESM-P ECHAM6 T63/L47 MPIOM:GR1.5 (254x220x40) The MPI-ESM Model Jungclaus et al., 2008; 2013 Grid size (km)

Iceland Scotland Overflow intensity and Atlantic surface properties 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 AMO reconstruction (Gray et al., 2004) ISOW

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

Past1000 simulations show similar high positive correlations between AMO and ISOW strength. Visual inspection points to prominent role of external forcing, in particular strong volcanic eruptions (Ottera et al, 2010; Zanchettin et al., 2012) How can we reconcile cold surface condtions with weak overflows? Iceland Scotland Overflow intensity and Atlantic surface properties Lohmann et al., to be submitted AMO ISOW

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) Iceland Scotland Overflow intensity and Atlantic surface properties Lohmann et al., to be submitted

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

A marine sediment core reveals unprecedented warming and enhanced Atlantic Water inflow into the Arctic in modern times. Can such dramatic changes be explained with the help of models? Is the associated increase in heat transport a local or remotely-driven phenomenon? Atlantic water transport into the Arctic Spielhagen et al., 2011

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

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 Atlantic water transport into the Arctic Ocean heat transport trends over 20 th century significantly larger than in unforced control simulations in all MPI-ESM historical experiments in NA subpolar gyre and Nordic Seas

Change in zonal-mean wind stress curl over the historical period Colored lines: MPI-ESM historical simulations Curl τ x10 -7 Nm -3 Wind-driven changes in gyre circulation and heat transports essential (see also Sedlaček and Mysak, 2009; Häkkinen and Rhines) No corresponding changes in AMOC 30N decreases slightly over 20 th century) Atlantic water transport into the Arctic

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) Conclusions

The research leading to these results has received funding from the European Union 7th Framework Programme (FP ), under grant agreement n THOR