Core Theme 1

WP 1.1

Task 1.1.1: Assessment of millenium-scale simulations and role of external forcing Compare simulated (signatures of) THC variability on interdecadal to centennial time scales with palaeo- observations from WP1.2 [LOCEAN, MET-O, MPI-M, NERSC] Compare simulated key processes of THC dynamics with observations from CT3 [MPI-M] Design a procedure for coordinated model testing [LOCEAN] and apply to the models [IFM-GEOMAR, LOCEAN, MET-O, MPI-M, NERSC] Investigate the role of external forcing on THC variability [MET- O, MPI-M, NERSC, IfM GEOMAR]

Mean Sortable Silt at Gardar drift (this study) Reconstructed AMO based On three rings (Gray et al., 2004) WP 1.2 RESULTS - MEAN SORTABLE SILT AT GARDAR DRIFT Gadar Drift data suggest that basin-wide warm phase is associated with vigorous ISOW flow

Role of processes Monthly mean observed (blue) and modelled (red) Faroe Bank Channel overflow Modeled annual mean Denmark Strait (upper) and FBC (lower) overflow Olsen et al., 2008

Role of processes Modeled annual mean Denmark Strait transport from NCEP forced ocean-only experiment (grey) and assimiltion run with coupled AOGCM (green) Matei et al., in prep.

Internal variability vs. External forcing as a pacemaker for Atlantic multidecadal variability? Otterå et al 2009 …but this finding appears to be model (and forcing) dependent….

Task 1.1.2: THC variability on decadal to centennial time scales Investigate mechanisms responsible for low-frequency THC variability with focus on overflow, deep water formation and its preconditioning [LOCEAN, MET-O, MPI-M, NERSC] Design [MPI-M] sensitivity experiments to investigate the impact of changes in overflow and deep water formation on the THC [LOCEAN, MET-O, MPI-M, NERSC] Assess the role of THC variations on recent changes in North Atlantic heat/fresh water content [MET-O] Design budget and statistical analysis diagnostics [MET-O] and apply to the models [LOCEAN, MET-O, MPI-M, NERSC]

Variability: No consensus among state- of-the-art climate models MPIKCM CSIROGFDL Power spectra: Maximum Atlantic MOC at 30N, CMIP3 pre-industrial control simulations Period (yr) Courtesy: Jin Ba

Role of overflow variations for MOC Denmark Strait Overflow Transp. and MOC 1085m Anomaly (Sv) Jungclaus et al., in prep.

Sensitivity experiment: supress density variations in NS Denmark Strait Overflow Transp. and MOC 1085m Anomaly (Sv) Jungclaus et al., in prep.

Task 1.1.3: Ocean-atmosphere feedbacks and climatic impact of THC changes Statistical analysis of lead/lag relationships to investigate the relative role of (un)coupled modes in explaining the low- frequency THC variability [LOCEAN, MET-O, MPI-M, NERSC], aided by sensitivity experiments [LOCEAN, MPI-M, NERSC] Perform partial coupled experiments with focus to identify to which extent the Atlantic Multidecadal Oscillation is part of a coupled climate mode [LOCEAN, MET-O, MPI-M, NERSC] Investigate the impact of THC changes on European and Arctic climate [LOCEAN, MET-O, MPI-M, NERSC]

Ocean-atmosphere feedbacks Zhu et al., revised Msadek & Frankignoul, 2009

The WP1.1 model zoo NERSC: Bergen Climate Model (BCM): ARPEGE (T42/L31) + MICOM (2.4°, L35) 700yr long control integration solar and volcanic forcing solar, volcanic, GHG and aerosol forcing ensembles for selected periods planned scenario integration MPI-M: MPI-M Earth System Model (COSMOS) ECHAM5 (T31/L19) + MPI-OM, 3°, L40 + carbon cycle) 3000yr long control integration solar, volcanic, land use change, GHG and aerosol forcing (ensemble of 5), single forcing experiments alternative solar forcing (ensemble of 3)

The WP1.1 model zoo LOCEAN: IPSLCM4_v2: Atm: 96x71x19, Ocn: 2°x2° 1000yr long control integration 950yr solar and CO2 forcing solar, volcanic and CO2 forcing (running) higher-resolution runs planned: METO: HadCM3 1.25° ocean,L yr pre-industrial control „natural 500“, solar, orbital, volcanic aerosol, preindustrial GHG (1750), 1750 land surface 1750:2000: „all250“: as natural GHG & aerosol emission history, land-use-change, ozone member anthropogenic + natural ensemble

The WP1.1 model zoo IfM GEOMAR: KCM: ECHAM5 (T31/L19) + NEMO 2°x2°/L31) 5000yr long control integration idealized solar forcing runs higher-resolution runs planned IN SUMMARY: All modelling groups have provided long integrations Cross-model validation is going on using >1000 yr control experiments: Overflow characteristics Sub-polar-gyre characteristics AMO vs. AMOC Sea ice variability

WP1.1 summary All modelling groups have provided long integrations Cross-model validation is going on using >1000 yr control experiments: Overflow characteristics Sub-polar-gyre characteristics AMO vs. AMOC Sea ice variability

Things to do Assess similarities and differences in the THC as represented in the various models and millennium-scale reconstructions representation of processes characteristics of internal variability climate response to THC changes THC response to external forcings What causes the differences between the models? Define common analyses tools and prepare publication strategy

WP 1.2: Participants: BCCR and CNRS (Gif-sur-Yvette) Task Characterize changes in the deep and intermediate return flow of THC; Determine how much it changed, which components, and why. Task Characterize the upper limb of THC—Variations in the inflows to the Nordic Seas. Task Characterize climate and thermocline evolution over the last millennium

Variability in ISOW vigor over the last 1300 years and its relationship to climate U. Ninnemann, T.L. Mjell, H. Kleiven and I. Hall,

Bathymetry of the northern North Atlantic and the Nordic Seas. Location of cores MD /2665 and ODP 983/MC09 are marked with red dots (Modified from Smith and Sandwell, 1997) Linkages to AMOC? How have Nordic Seas overflows varied?

Study Area—ISOW variability on Gardar drift Latitude: 60°19’ N Longitude: 23° 58’ W Depth: 2081 m GS MC-D IR NIIC Iceland-Scotland Overflow Water (ISOW) Curry & Mauritzen, 2005 ~1400 AD

Location in the core of ISOW overflow

Y= – xR= III III IV ~ AD ~ AD~ AD ~ AD ~ AD~ AD WP 1.2 RESULTS - MEAN SORTABLE SILT AT GARDAR DRIFT

Multidecadal to centennial variability in ISOW vigor and chemical properties over the last ~600+ years ISOW flow variability is coherent across a range of depths and space (not a local signal) During the past ~350 years ISOW vigor is in phase with reconstructed AMO on both inter-decadal and centennial timescale—within the error of our age models. This strong coherence suggests that low frequency variability in key components of AMOC is coupled to basin-wide temperature perturbations Summary of Observations

Eirik sediment drift – DSOW & DWBC variability ~ 2006 AD ~600 AD Curry & Mauritzen, 2005 GS MC A Latitude: 57°29’ N Longitude: 48° 37’ W Depth: 3432 m Deep Western Boundary Current (DWBC)

Mann and Jones (2003) NH tmp. reconstructions Benthic oxygen isotopes From MD ; 3 pt.smooth (Kleiven et al., in prep) Natural variability in the deep water masses

WP 1.2.3: Towards the reconstruction of the thermocline variability in the North Atlantic during the last millennium T. Bouinot, E. Cortijo, A. Govin, C. Cléroux LSCE/IPSL (Gif/Yvette, France)

Study sites: sediment cores SST August Already studied Future work

How to reconstruct the thermocline variability? Summer mixed layer Seasonal thermocline Temperature Water depth Permanent thermocline Deep-dwelling foraminifera: 1. Globorotalia inflata 2. Pulleniatina obliquiloculata Planktic foraminifera 1. Globigerinoides ruber 2. Globigerina bulloides 2.1.

Core MD (35°N, 75°W, 620 m) SST August C. Cléroux, PhD thesis

Future work: to better trace the extension of the subtropical & subpolar gyres in the North Atlantic From Hatun et al. Science 2005 Subtropical gyre water Subpolar gyre water MD Q MD Q (56.4°N, 27.8°W, 2830 m) MD Q (58.8°N, 26.0°W, 2603 m) Coretop’s date2000 a is around MD Q671.5 a ± 30 a50 cm MD Q308.5 a ± 30 a35 cm

Deliverables

All WP 1.1 partners have control integrations of 1000 to 6000 years forced integrations over the millennium are accomplished or ongoing, some forced integrations have been run in ensemble mode analyses focus presently on the assessment of THC characteristics and mechanisms Summary WP 1.1

WP1.2: reconstructions of the strength of the ISOW over last millennium ready, upper ocean T and S in progress. Reconstruction of integrated overflows south of Greenland as well as upper ocean T, S, and chemical properties in progress New cores (Gadar Drift and Bay of Biscay) give detailled information on the structure of the thermocline Hydrographic reconstructions from the inflow region (Faroe transect and Norwegian Sea ready for the last years, will be extended back in time Summary WP 1.2

Data from WP3 for process understanding: - Overflow transport timeseries -Watermass characteristics monthly basis / Some key data should be put somewhere together, for instance the data from CT1 on overflow transport / overflow overturning Give information on variability on time scales most relevant for decadal prediction (CT4) CT1 and other CTs