Helge DrangeGeofysisk 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

CONTROL600: All forcings kept constant at pre-industrial (1850) levelNATURAL600: Same as CONTROL600, but with historic total solar irradiance (TSI) and volcanic aerosol variations for the last 600 yearsAll150: Same as NATURAL600, 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

MLDLS 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 σθ=27.75) is regressed with AMOC

Max MLD in LS ~8yrs before max AMOC

Lag=-20yrs Lag=-10yrs Lag=0yrs

Lag=10yrs Lag=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

MaunderMinimum

DaltonMinimum

Kuwae

1452 Tambora

1815

Krakatoa

1883

Helge DrangeGeofysisk institutt

Universitetet i Bergen

Reconstructed and observed N Hemisphere temperature

Year

Tem

per

atu

re a

no

mal

y (º

C)

Mann et al. 2008

Settlement on Iceland & Greenland

Little Ice Age

Today

Otterå et al 2009

Simulated NH response

Kuwae

1452 Tambora

1815Krakatoa

1883

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

1816 – The year without a summerFollowing the 1815 Tambora eruption

(relative 1500-1899)

Mary Shelley

The winter warming phenomenon

Composite of 10 largest tropical eruptions

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

Simulated Early Warming in the Arctic2 m temperature, 60-90oN

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 (1400-2000)✓AMO (1860-2000) and ✓Early Warming (1930-50)

for NATURAL and all members of ALLforcing, but not for CTRL

Natural forcing as a pacemaker for Atlantic multidecadal variability?

Otterå et al 2009

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

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

Overflow

Winter 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 summary1. 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)