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)

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

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.

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

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

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

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

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

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

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

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

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

Simulated time-latitude variability of SAT (ALL forcing run, relative )

1816 – The year without a summer Following the 1815 Tambora eruption (relative ) Mary Shelley

The winter warming phenomenon Composite of 10 largest tropical eruptions

Simulated time-latitude variability of SAT (ALL forcing run, relative )

Simulated Early Warming in the Arctic 2 m temperature, o N Suo et al, in prog

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

Observed AMO Simulated AMO °C per SD-AMO

Observed AMO Simulated AMO °C per SD-AMO

Similarities between observed and simulated ✓ NH Temperature ( ) ✓ AMO ( ) and ✓ Early Warming ( ) for NATURAL and all members of ALL forcing, but not for CTRL

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

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

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

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

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

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

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

AMO vs other climate parameters

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

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

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

Simulated winter temperature Gardar drift vs AMO-index

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)