Summary 26 September 2012 Core Theme 1

Deliverables

WP 1.1

Tools: Millennium-scale coupled atmosphere- ocean model simulations (2009) Model nameHorizontal grid (Atm) Horizontal grid (Ocn) Vertical levels (Ocn) Vertical levels (Atm) Length of control run BCM128 x x 150 (conform al) 35 (isopycn al) 31 (hybrid)600 HadCM396 x x (z)19 (hybrid)5000 IPSL-CM496 x x 149 (conformal) 31 (z)19 (hybrid)1000 KCM96x48181x149 (conformal) 31 (z)19 (hybrid)5000 MPI-M ESM96 x x 120 (conformal) 40 (z)19 (hybrid)3000

Tools: Millennium-scale coupled atmosphere- ocean model simulations (CMIP5) Model nameHorizontal grid (Atm) Horizontal grid (Ocn) Vertical levels (Ocn) Vertical levels (Atm) Length of control run IPSL-CM596 x x 149 (conformal) 31 (z)39 (hybrid)1000 MPI-M ESM192X96254 x 220 (conformal) 40 (z)47 (hybrid)2X1000

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] Characteristics of low-frequency variability in simulations, AMOC and climate modes: Marini et al. (2010), Zanchettin et al. (2010), Park and Latif (2010), Marini et al. (in prep), Park et al. (in prep) Comparison between simulations and reconstructions: Sicre et al. (2011), Ottera et al. (2010), Zanchettin et al. (2012), Nuñez-Riboni et al. (2012), Lohmann et al. (in prep), Mjell et al. (in prep)

Task 1.1.1: Assessment of millenium-scale simulations and role of external forcing Investigate the role of external forcing on THC variability [MET-O, MPI-M, NERSC, GEOMAR] Role of strong volcanic eruptions for shaping AMOC variability: Ottera et al. (2010); Zanchettin et al. (2011), Mignot et al. (2011, in prep) Solar forcing of AMOC changes: Menary et al. (in prep.) Response to idealized external forcing: Park and Latif (2011)

Millennium simulations: role of external forcing Ottera et al., 2010 Zanchettin et al., 2011

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] Mechanisms of interdecadal to centennial variability: Menary et al. (2011); Marini et al. (2010), Park and Latif (2010), Born and Mignot (2011), Medhaug et al. (2012), Langehaug et al. (2012), Escudier et al. (2012), Latif et al. (in rev.), Martin et al. (in rev.), Ba et al. (subm), Brandstator et al. (2011), Keenlyside and Ba (2010)

Task 1.1.2: THC variability on decadal to centennial time scales

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] Lohmann et al. in preparation

Task 1.1.2: THC variability on decadal to centennial time scales Assess the role of THC variations on recent changes in North Atlantic heat/fresh water content [MET-O] Fresh water transports in HadCM3: CT report 2010 Ocean heat content changes: Palmer et al. (2011), Robson et al. (2012) Arctic/Atlantic exchanges: Eldevik et al. (2009), Jungclaus and Koenigk (2010), Langehaug et al. (2012b), Langehaug and Falck (2011), Arthun et al. (2012)

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] Atmospheric response to MOC variations: Gastineau and Frankignoul (2011a), Gastineau et al. (2011b), Semenov et al. (2010), Jungclaus and Koenigk (2010), Msadek et al. (2011) Atm leading ocean ocean leading atm

Task 1.1.3: Ocean-atmosphere feedbacks and climatic impact of THC changes Perform partially 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] experiments turned out to be too demanding in coupled mode. Experiments with slab ocean done (Msadek et al, 2011), SST- driven atmosphere model experiments at LOCEAN (in prep).

WP1.1 has assessed 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 WP1.1 demonstrated that model simulation and process study are instrumental to understand mechanisms identified in observations and reconstructions WP1.1 demonstrated that (simulated) natural variability of THC has a significant impact on the winter atmospheric circulation in the NH WP1.1 demonstrated that there is more to the THC than just the conveyor-belt image!

Core Theme 1 J. Jungclaus & H. Kleiven “Quantifying and modeling THC variability using palaeoclimate observations and Simulations” WP 1.2 (Kleiven, Geo and BCCR) THC and related climate variables during the last Millennium from Palaeo observations Deliverables D13 Providing a time series of the variability of integrated exchanges with the Nordic Seas and the intensity of the individual deep branches of the THC over the last millennium from paleo data (Month 24) D20 Providing a data set describing the spatial evolution of SST and thermocline variability associated with changes in the THC at multi- decadal resolution in the North Atlantic during the last millennium (month 36)

Task Characterize changes in the deep limb of THC-Determine how much it changed, which components, and why? ISOW and DSOW both show multidecadal-centennial variability over the past millennium. ISOW covaried with AMO over the past 600yrs, DSOW changes during largest N.Atlantic coolings Task Characterize the upper limb of THC-Variations in the inflows to the Nordic Sea Inflow covaries with NAO/AMO and simulations suggest role for advective anomalies (intergyre/gyre)? Task Characterize climate and thermocline evolution over the last millenium North Atlantic surface climate covary with AMO over past millennium/signal amplification near SPG points toward sensitivity/involvement of SPG.

Reconstructing ocean circulation and climate based on the “Gardar Drift” Tor Lien Mjell (1), Helene R. Langehaug (2), Odd Helge Otterå (3), Tor Eldevik (4), Ulysses. S. Ninnemann (1,5), Helga (Kikki) F. Kleiven (1,5), Ian Hall (6) in prep Using simulations of the last millennium to understand variability seen in paleo- observations: In-phase variation of Iceland-Scotland overflow strength and Atlantic Multidecadal Oscillation Katja Lohmann1, Juliette Mignot2, Helene R. Langehaug3,4, Johann H. Jungclaus1, Odd Helge Otterå4, Yongqi Gao3,4, Tor Lien Mjell4,5, Ulysses Ninnemann4,5 and Helga (Kikki) Flesche Kleiven4,5, to be submitted ISOW vigor over the last millennia and its relationship to climate. Mjell, T.L., Ninnemann, U.S., Kleiven, H.F., Rosenthal, Y. and Hall, I.R. Variability in In prep. North Atlantic climate and deep water variability since 600 A.D. Kleiven, H.F., Rosenthal, Y. and Ninnemann, U.S. In prep

Lund et al. (Nature, 2006) Changed volume transport (1SV=10 6 cubic m./sec) in the Gulfstream outside Florida over the Little Ice Age State of the art pre THOR: Changes in the volume transport of the Gulfstream during the Little Ice Age

On a mission…. R/V Mariond Dufresne

Eirik sediment drift, south of Greenland New marine ocean data spanning the Little Ice Age ~ 1980 AD ~600 AD Curry & Mauritzen, 2005 GS MC A 57°29’ N, 48° 37’ W Depth: 3432 m (Figure modified after Curry and Mauritzen, 2005) 15 yrs sample resolution, 13 AMS 14 C dates

Reconstructed sea surface temperatures A.D. Western settlement declined Eastern settlement declined

Bottom water flow speed (mean sortable silt in RED) bottom water chemistry (benthic  13 C in GREEN) Deep water variability

Observe a close coupling between surface climate and the properties of proto North Atlantic Deep Water, primarily on centennial timescales. The surface property changes clearly affect the density and could strongly influence vertical convection. Today vertical convection in the subpolar gyre and Labrador Sea are important for global deep water ventilation and contribute to the meridional overturning circulation. Climate – deep water coupling

The presence and evolution of these drifts are intimately linked with the deep ocean circulation pattern and intensity, sediment supply, and the local topography of the area Drift pattern mirror the path of the bottom currents FD= Feni Drift HAD= Hatton Drift GD= Gardar Drift BD= Bjørn Drift SD= Snorri Drift GLD= Gloria Drift ED= Eirik Drift Modified after Faugères et al., 1993 STUDY AREA NAD W Modified after Kleiven et al. (2008)

Red = GS MC-D Blue = Gray et al., 2004 ISOW AND CLIMATE RECONSTRUCTIONS OVER THE LAST 2KYR This ocean-climate coupling supports the hypothesis that AMO involves, and is potentially driven by, variations in AMOC

THOR is a project financed by the European Commission through the 7th Framework Programme for Research, Theme 6 Environment, Grant agreement