combining model simulations and paleoceanographic reconstructions for a process-based understanding...
TRANSCRIPT
Combining model simulations and paleoceanographic reconstructions for
a process-based understanding of climate variability in the North
Atlantic/Arctic region
Johann Jungclaus1
K. Lohmann1, H. Kleiven2
1Max Planck Institute for Meteorology, Hamburg, Germany2University of Bergen, Bergen, Norway
Motivation
More sophisticated paleoceanographic reconstructions reflecting dynamical oceanic quantities (e.g., overflow transports, heat transports) become increasingly available
How representative are local proxies, how is their recorded variability related to the large-scale circulation?
Are CMIP5/PMIP3-style model simulations suitable to interpret the mechanisms behind the observed/reconstructed variations?
The model system: MPI-Earth System ModelAtmosphere: ECHAM6. Ocean: MPIOM. Land: JSBACH.Ocean biogeochemistry: HAMOCC5.Coupler: OASIS3Documentation: Journal of Advances in Modeling Earth Systems (JAMES, 2013)special issue on the MPI-ESM as used in CMIP5/PMIP3
MPI-ESM-PECHAM6 T63/L47MPIOM:GR1.5 (254x220x40)
The MPI-ESM Model
Jungclaus et al., 2008; 2013
Grid size (km)
Iceland Scotland Overflow intensity and Atlantic surface properties
Reconstruction of overflow strength based on sortable silt suggests pronounced variations in phase with basin-wide temperature changes (Atlantic Multidecadal Oscillation/Variability)
Mjell et al., to be submitted
AMO reconstruction (Gray et al., 2004)
ISOW
Iceland Scotland Overflow intensity and Atlantic surface properties
Possible explanation: strong overturning drives enhanced
MOC, MOC carries more heat to North Atlantic → positive AMO anomaly
• Past1000 simulations show similar high positive correlations between AMO and ISOW strength.
• Visual inspection points to prominent role of external forcing, in particular strong volcanic eruptions (Ottera et al, 2010; Zanchettin et al., 2012)
• How can we reconcile cold surface condtions with weak overflows?
Iceland Scotland Overflow intensity and Atlantic surface properties
Lohmann et al., to be submitted
AMO
ISOW
Strong Iceland-Scotland overflow associated with warm Nordic Seas, less convection in Greenland Sea, but enhanced barotropic pressure gradient (contrasting with traditional view based on 2-layer (baroclinic) hydraulics)
Iceland Scotland Overflow intensity and Atlantic surface properties
Lohmann et al., to be submitted
Less convection in Nordic Seas
Deeper isopycnals, less doming
Higher SSHin Nordic Seas
Anticyclonic circulationanomaly
Strong ISR overflow Higher (barotropic)pressure north of ISR
Spin-up of gyre circulation
Warm Nordic Seas (AMO)
Direct radiative forcing
Lighter surface density (T dominated)
Iceland Scotland Overflow intensity and Atlantic surface properties
Lohmann et al., to be submitted
A marine sediment core reveals unprecedented warming and enhanced Atlantic Water inflow into the Arctic in modern times.
Can such dramatic changes be explained with the help of models?
Is the associated increase in heat transport a local or remotely-driven phenomenon?
Atlantic water transport into the Arctic
Spielhagen et al., 2011
Atl. Water in Fram Strait: Model
MPI-ESM-P past1000-r1
Atlantic water transport into the Arctic
Heat transport to Arctic Heat transport @75N
MPI-ESM-P past1000-r2 MOC gyre
2σ from Ctrl. expmt
Model simulations exhibit consistenly positive anomalies exceeding internal variability range
Ocean heat transport anomalies in 20th century up to 50% higher than in pre-industrial times. Anomalies are associated with gyre (horizontal) circulation rather than overturning
Trends in total heat transport (TW/100yr) as function of latitude
5-95%-range for 100-yr trends from control run
colored lines: MPI-ESM historical simulations 1850-2005
Atlantic water transport into the Arctic
Ocean heat transport trends over 20th century significantly larger than in unforced control simulations in all MPI-ESM historical experiments in NA subpolar gyre and Nordic Seas
Change in zonal-mean wind stress curl over the historical period
Colored lines: MPI-ESM historical simulations 1850-2005
Cur
l τ x
10-7 N
m-3
Wind-driven changes in gyre circulation and heat transports essential (see also Sedlaček and Mysak, 2009; Häkkinen and Rhines)No corresponding changes in AMOC (AMOC @ 30N decreases slightly over 20th century)
Atlantic water transport into the Arctic
Last millennium simulations with state-of-the-art models provide detailed representation of important mechanisms to suggest explanations for the diagnosed local property and dynamical changes. CAVEAT: model-dependency in details is diagnosed (Lohmann et al., 2013) and evaluation should be done in a multi-model framework
The analyses of THC variations in the THOR project have demonstrated a relative prominent of external forcing shaping the AMO and North Atlantic climate over the millennium (Ottera et al., 2010; Zanchettin et al., 2012)
There is more to long-term variations in the North Atlantic/Arctic climate than what we often infer from the simplifying conveyor-belt image (see also Wunsch, QSR 2010)
Conclusions
The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.212643 THOR www.eu-thor.eu