Heinrich events modeled in a fully coupled ice sheet – climate model Florian Ziemen 1,2 , Christian Rodehacke 1,3 , Uwe Mikolajewicz 1 1 Max Planck Institute for Meteorology, 2 International Max Planck Research School on Earth System Modelling, 3 Danish Meteorolgical Institute, Copenhagen International Max Planck Research School on Earth System Modelling Max-Planck-Institut für Meteorologie Introduction We study is glacial climate variability with a coupled atmosphere-ocean general circulation model (AOGCM)– ice sheet model system, focusing on one of the most prominent features of glacial climate variability, the Heinrich events. Modeling past climates and periods of past climate change is an important test of the capability of climate models to correctly represent future climate changes. Only if we can correctly represent past climates and climate changes, we can be confident about our pre- dictions of future climate changes. Model setup We coupled the AOGCM ECHAM5/MPIOM/LPJ inter- actively with the ice sheet model mPISM. mPISM is a modified version of the Parallel Ice Sheet Model from the University of Alaska, Fairbanks. We run ECHAM5 in T31 resolution (3.75°), MPIOM with a nominal resolution of 3°, and mPISM on a 20 km grid covering most of the northern hemisphere. The models are coupled bidirectionally after every year of climate model integrations. We do not use flux correction or anomaly maps in our models. In the asynchronously coupled experiment, the models are coupled 1:10 and the experiment covers 3000 years in the climate model and 30 000 years in the ice sheet model. The synchro- nously coupled experiment focuses in on the third ice sheet collapse from the asynchronously coupled experi- ment and covers 3200 years in each model. Results The asynchronously coupled experiment is the first ful- ly coupled long-term Last Glacial Maximum (LGM) ice sheet model – AOGCM experiment. In this experiment, we obtain reasonable LGM ice sheets (Fig. 1) and a rea- sonable LGM climate (Fig. 2). The ice streams show peri- odic surges (Figs. 3, 5) because of the internal instability mechanism proposed by MacAyeal (1993). The time- scale of the repeat cycle is set by the time it takes the surface accumulation to rebuild the ice sheet. The fresh- water pulses from the surges weaken the NADW cell and thus strengthen the AABW cell (Fig. 4 for the NADW cell). In the synchronously coupled experiment, we first observe surges of the ice streams at the coasts of the Arctic Ocean. These ice released in these surges (Fig. 6) causes a regional sub-surface warming (Fig. 7), that could have synchronised the surges. This synchro- nization via the sub-surface ocean temperatures was ruled out by a sensitivity study with prescribed cold ocean temperatures. While the Labrador Sea sub-sur- face temperatures sink during the surges of the Arctic Ice Streams, they rise during the surge of the Hudson Strait Ice Stream (Figs. 5, 7). The surges cause large scale cooling on the northern Hemisphere (Fig. 8) and other climate changes consistent with proxy data. Literature Bueler and Brown, 2009, The shallow shelf approxima- tion as a “sliding law” in a thermomechanically coupled ice sheet model Calov et al., 2002, Large-scale instabilities of the Lauren- tide Ice Sheet simulated in a fully coupled climate-sys- tem model MacAyeal, 1993, Binge/Purge oscillations of the Lauren- tide Ice Sheet as a cause of the North Atlantic’s Heinrich events Peltier, 2004, Global glacial isostasy and the surface of the ice-age earth: The ice-5G (VM2) model and grace. Tarasov et al., 2012, A data-calibrated distribution of deglacial chronologies for the North American ice com- plex from glaciological modeling. K Coupled model Tarasov surface elevation in m Peltier (ICE5G) averaged over 30 000 years Reconstructions Fig 3: Ice sheet volumes in the steady-state experiments The surges of the Hudson Strait Ice Stream dominate the variability. The period of 7000 years agrees with proxies for Heinrich events. Sea level equivalent in m 62.5 50 37.5 25 12.5 0 70 75 80 85 90 95 time in kyrs 25 20 15 10 5 0 Ice volume in Mio km 3 Fig. 4: Freshwater fluxes and NADW cell strength The fresh water released in the surges stabilizes the ocean stratification, this weakens deep convection and thus the North Atlantic deep water cell. -40 -20 0 20 40 60 Global FW flux (mSv) 7000 7500 8000 8500 9000 9500 10000 year 19 20 21 22 23 24 25 NADW cell strength (Sv) North Atlantic deep water cell strength at 30°N Global net freshwater flux into the oceans Zero net freshwater flux Fig 8: 2 m air temperature changes The surge of the Hudson Strait Ice Stream causes cooling over the Labrador Sea and the Arctic, this spreads into Eurasia and the Arctic. °C Fig 5: The surge of the Hudson Strait Ice Stream The ice sheets are drawn for year 2350 (start of blue bar) Colors display the vertically averaged horizontal ice velocity. In the ocean, colors indicate annual mean sea ice cover frac- tion. Visualization by Niklas Röber/DKRZ 2000 2500 3000 3500 4000 4500 5000 year 180 160 140 120 100 80 60 40 Flux in mSv Arctic Ocean freshwater flux Labrador Sea freshwater flux Fig. 1: Modeled and reconstructed LGM ice sheets The modeled ice sheets reasonably agree with reconstruc- tions for the Last Glacial Maximum by Peltier (2004) and by Tarasov (priv. comm., see Tarasov (2012) for a description of the methodology). Fig 2: LGM air temperature anomaly Surface air temperature difference between the asynchro- nously coupled LGM experiment and a pre-industrial con- trol run. Dots indicate proxy reconstructions for LGM–pres- ent day by Kim et al. (2008). Asynchronously coupled experiment Synchronously coupled experiment Fig 6: Net freshwater fluxes into the ocean The ice stream surges cause freshwater pulses in the ocean. Dashed lines mark fluxes in the reference period. The col- ored bars mark the averaging intervals for the surges of the Arctic ice streams and the Hudson Strait Ice Stream. °C Fig 7: Sub-surface ocean temperature changes Ice stream surges stabilize the ocean stratification and thus cause regional sub-surface warming. The sub-surface ocean temperature is decisive for ice shelf basal melt and a candi- date for triggering ice stream surges. Arctic ice streams Hudson Strait Ice Stream The simulations were performed on the Blizzard supercomputer of the DKRZ. The resources were provided by BMBF project 675.
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Heinrich events modeled in a fully coupled ice sheet – climate modelFlorian Ziemen1,2, Christian Rodehacke1,3, Uwe Mikolajewicz1
1 Max Planck Institute for Meteorology, 2International Max Planck Research School on Earth System Modelling, 3Danish Meteorolgical Institute, Copenhagen
International Max Planck Research School on Earth System Modelling
Max-Planck-Institut für Meteorologie
IntroductionWe study is glacial climate variability with a coupled atmosphere-ocean general circulation model (AOGCM)– ice sheet model system, focusing on one of the most prominent features of glacial climate variability, the Heinrich events. Modeling past climates and periods of past climate change is an important test of the capability of climate models to correctly represent future climate changes. Only if we can correctly represent past climates and climate changes, we can be confident about our pre-dictions of future climate changes.
Model setupWe coupled the AOGCM ECHAM5/MPIOM/LPJ inter-actively with the ice sheet model mPISM. mPISM is a modified version of the Parallel Ice Sheet Model from the University of Alaska, Fairbanks. We run ECHAM5 in T31 resolution (3.75°), MPIOM with a nominal resolution of 3°, and mPISM on a 20 km grid covering most of the northern hemisphere. The models are coupled bidirectionally after every year of climate model integrations. We do not use flux correction or anomaly maps in our models. In the asynchronously coupled experiment, the models are coupled 1:10 and the experiment covers 3000 years in the climate model and 30 000 years in the ice sheet model. The synchro-nously coupled experiment focuses in on the third ice sheet collapse from the asynchronously coupled experi-ment and covers 3200 years in each model.
ResultsThe asynchronously coupled experiment is the first ful-ly coupled long-term Last Glacial Maximum (LGM) ice sheet model – AOGCM experiment. In this experiment, we obtain reasonable LGM ice sheets (Fig. 1) and a rea-sonable LGM climate (Fig. 2). The ice streams show peri-odic surges (Figs. 3, 5) because of the internal instability mechanism proposed by MacAyeal (1993). The time-scale of the repeat cycle is set by the time it takes the surface accumulation to rebuild the ice sheet. The fresh-water pulses from the surges weaken the NADW cell and thus strengthen the AABW cell (Fig. 4 for the NADW cell). In the synchronously coupled experiment, we first observe surges of the ice streams at the coasts of the Arctic Ocean. These ice released in these surges (Fig. 6) causes a regional sub-surface warming (Fig. 7), that could have synchronised the surges. This synchro-nization via the sub-surface ocean temperatures was ruled out by a sensitivity study with prescribed cold ocean temperatures. While the Labrador Sea sub-sur-face temperatures sink during the surges of the Arctic Ice Streams, they rise during the surge of the Hudson Strait Ice Stream (Figs. 5, 7). The surges cause large scale cooling on the northern Hemisphere (Fig. 8) and other climate changes consistent with proxy data.
LiteratureBueler and Brown, 2009, The shallow shelf approxima-tion as a “sliding law” in a thermomechanically coupled ice sheet modelCalov et al., 2002, Large-scale instabilities of the Lauren-tide Ice Sheet simulated in a fully coupled climate-sys-tem modelMacAyeal, 1993, Binge/Purge oscillations of the Lauren-tide Ice Sheet as a cause of the North Atlantic’s Heinrich eventsPeltier, 2004, Global glacial isostasy and the surface of the ice-age earth: The ice-5G (VM2) model and grace.Tarasov et al., 2012, A data-calibrated distribution of deglacial chronologies for the North American ice com-plex from glaciological modeling.
K
Coupled modelTarasov
surf
ace
elev
atio
n in
m
Peltier (ICE5G)
averaged over 30 000 years
Reconstructions
Fig 3: Ice sheet volumes in the steady-state experimentsThe surges of the Hudson Strait Ice Stream dominate the variability. The period of 7000 years agrees with proxies for Heinrich events.
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Sea
leve
l equ
ival
ent i
n m
62.5
50
37.5
25
12.5
0 70 75 80 85 90 95 time in kyrs
25
20
15
10
5
0
Ice
volu
me
in M
io k
m3
Fig. 4: Freshwater fluxes and NADW cell strengthThe fresh water released in the surges stabilizes the ocean stratification, this weakens deep convection and thus the North Atlantic deep water cell.
-40
-20
0
20
40
60
Glo
bal F
W �
ux (m
Sv)
7000 7500 8000 8500 9000 9500 10000year
19
20
21
22
23
24
25
NA
DW
cel
l str
engt
h (S
v)
North Atlantic deep water cell strength at 30°N
Global net freshwater flux into the oceans
Zero net freshwater flux
Fig 8: 2 m air temperature changesThe surge of the Hudson Strait Ice Stream causes cooling over the Labrador Sea and the Arctic, this spreads into Eurasia and the Arctic.
°C
Fig 5: The surge of the Hudson Strait Ice StreamThe ice sheets are drawn for year 2350 (start of blue bar)Colors display the vertically averaged horizontal ice velocity. In the ocean, colors indicate annual mean sea ice cover frac-tion. Visualization by Niklas Röber/DKRZ
2000 2500 3000 3500 4000 4500 5000 year
180
160
140
120
100
80
60
40
Flux
in m
Sv
Arctic Ocean freshwater flux
Labrador Sea freshwater flux
Fig. 1: Modeled and reconstructed LGM ice sheets The modeled ice sheets reasonably agree with reconstruc-tions for the Last Glacial Maximum by Peltier (2004) and by Tarasov (priv. comm., see Tarasov (2012) for a description of the methodology).
Fig 2: LGM air temperature anomalySurface air temperature difference between the asynchro-nously coupled LGM experiment and a pre-industrial con-trol run. Dots indicate proxy reconstructions for LGM–pres-ent day by Kim et al. (2008).
Asynchronously coupled experiment
Synchronously coupled experiment
Fig 6: Net freshwater fluxes into the oceanThe ice stream surges cause freshwater pulses in the ocean. Dashed lines mark fluxes in the reference period. The col-ored bars mark the averaging intervals for the surges of the Arctic ice streams and the Hudson Strait Ice Stream.
°C
Fig 7: Sub-surface ocean temperature changesIce stream surges stabilize the ocean stratification and thus cause regional sub-surface warming. The sub-surface ocean temperature is decisive for ice shelf basal melt and a candi-date for triggering ice stream surges.
Arctic ice streams Hudson Strait Ice Stream
The simulations were performed on the Blizzard supercomputer of the DKRZ. The resources were provided by BMBF project 675.
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