1. Helge Drange Geofysisk institutt Universitetet i Bergen Atlantic Multidecadal Variability and the role of natural forcing in BCM Odd Helge Otterå, Mats Bentsen, Lingling Suo and Helene Langehaug (Nansen/Bjerknes)

2. Bergen Climate Model (version 2) ARPEGE –Resolution: T42, ~2.8x2.8, 31 layers –Volcanic aerosols implemented MICOM –Resolution: ~2.4x2.4, 35 isopycnic layers –New pressure gradient formulation –Reference pressure at 2000 m –Incremental remapping for tracer advection (better conservation) Thermodynamic and dynamic sea-ice modules –GELATO: multi-category ice –NERSC: one ice layer only ARPEGE MICOM

3. Performed simulations with BCM CONTROL 600: All forcings kept constant at pre-industrial (1850) level NATURAL 600: Same as CONTROL 600, but with historic total solar irradiance (TSI) and volcanic aerosol variations for the last 600 years All 150: Same as NATURAL 600, but with variations in well-mixed greenhouse gases and tropospheric sulfate aerosols. Total of 5 ensemble members performed.

4. Atlantic Merdional Overturning Circulation CONTROL600 Otterå et al 2009, GMD, in press 16.6 Sv

5. Comparison to Levitus for control Southern Ocean problem! Otterå et al 2009, GMD, in press

6. Sea ice and NA surface ocean circulation Otterå et al 2009, GMD, in press

7. Ventilation sites in BCM (control run)  Late winter Mixed Layer Depth (MLD) averaged over 700 years.  MLD > 1100 m in 10 winters:  3 convection regions 1. Greenland Sea 2. Labrador Sea 3. Irminger Sea Courtesy of H. Langehaug

8. Regression of MLD & AMOC Max MLD in GS ~17yrs after max AMOC MLD LS is leading AMOC Max MLD in LS ~8yrs before max AMOC Regression between the Mixed Layer Depth averaged over the convection regions and the AMOC. Courtesy of H. Langehaug

9. Lag=30yrs Another way to investigate the propagation of intermediate and deep water masses… Anomalies in the thickness of the intermediate layer (interface σ θ =27.75) is regressed with AMOC Max MLD in LS ~8yrs before max AMOC Lag=-20yrsLag=-10yrsLag=0yrs Lag=10yrsLag=20yrs Max MLD in GS ~17yrs after max AMOC Courtesy of H. Langehaug

10. Natural run: Applied forcing (Crowley et al. 2003) Otterå et al 2009 Effective solar constant Spörer Minimum Maunder Minimum Dalton Minimum Kuwae 1452 Tambora 1815 Krakatoa 1883

11. Helge Drange Geofysisk institutt Universitetet i Bergen Reconstructed and observed N Hemisphere temperature Year Temperature anomaly (ºC) Mann et al. 2008 Settlement on Iceland & Greenland Little Ice Age Today

12. Otterå et al 2009 Simulated NH response Kuwae 1452 Tambora 1815 Krakatoa 1883

13. Simulated time-latitude variability of SAT (ALL forcing run, relative 1961-1990)

14. 1816 – The year without a summer Following the 1815 Tambora eruption (relative 1500-1899) Mary Shelley

15. The winter warming phenomenon Composite of 10 largest tropical eruptions

16. Simulated time-latitude variability of SAT (ALL forcing run, relative 1961-1990)

17. Simulated Early Warming in the Arctic 2 m temperature, 60-90 o N Suo et al, in prog

18. Atlantic Multidecadal Oscillation (AMO) Sutton & Hodson, 2005, Science Average SST 75W-7.5W; 0-60N

19. Observed AMO Simulated AMO °C per SD-AMO

20. Observed AMO Simulated AMO °C per SD-AMO

21. Similarities between observed and simulated ✓ NH Temperature (1400-2000) ✓ AMO (1860-2000) and ✓ Early Warming (1930-50) for NATURAL and all members of ALL forcing, but not for CTRL

22. Natural forcing as a pacemaker for Atlantic multidecadal variability? Otterå et al 2009

23. Power spectrum for AMO and AMOC (shading: 60-100 yr) Otterå et al 2009 Control600 Natural600 More power on 60-100 yr time scales in NATURAL

24. Variability in the simulated strength of AMOC is – mainly – governed by Labrador Sea mixing with a lag of about 8 years. Holds for both CTRL and NATURAL. ~ 8 yr lag

25. Lag-correlations (30 yr filter): AMO vs LabSea/AMOC/RadTOA CONTROL600 NATURAL600 LS density and AMOC lead by 15 and 8 years No lag with Rad TOA; LS density and AMOC lag by 5 and 15 years Otterå et al 2009

26. Lag-correlations (unfiltered time series): Forcing vs LabSea/AMOC/RadTOA Volcanoes plays a key role! NATURAL600

27. Surface T and Atlantic streamfunction regressed onto AMO About 90 yr cycle

28. AMOC linked to the derivative of the AMO (AMO ROC): Atmosphere link? SLP regressed onto the AMO ROC index AMOC

29. AMO vs other climate parameters

30. NAO-index: reconstructed vs model pc1 EOF1 10 yr running mean

31. Reconstructions from Gardar Drift Courtesy of Tor Mjell and U. Ninnemann weak strong cold warm OverflowOverflow Winter tempWinter temp Sortable siltG. inflata

32. Upper ocean (300 m) temperature regressed on AMO-index (lag 0) The Gardar Drift region anti-correlates with the AMO-index in the simulation

33. Simulated winter temperature Gardar drift vs AMO-index

34. Preliminary summary 1.Main features of the observed multidecadal variability in the Atlantic region are simulated by the model 2.The simulated multidecadal variability is strongly linked to changes in the combined effect of solar irradiance and aerosol variations, rather than to internal variability from the ocean component 3.(2) needs to be supported by other models/studies 4.The simulated AMOC in BCM is out of phase with AMO  strong AMOC in cold times and vice versa 5.If these findings are robust, decadal-scale predictability experiments need to take into account future changes in solar irradiance and aerosol variations (volcanoes included)