LGM ice sheets simulated with a complex fully coupled ice sheet – climate modelFlorian Ziemen1,2, Christian Rodehacke2, Uwe Mikolajewicz2

1 International Max Planck Research School on Earth System Modelling, florian.ziemen@zmaw.de; 2 Max Planck Institute for Meteorology

International Max Planck Research School on Earth System Modelling

Max-Planck-Institut für Meteorologie

One major challenge in predicting future climate change is the validation of the numerical models. A particular good time period for testing ice sheet – climate inter-actions is the last glacial maximum (LGM). It combines large ice sheets with good proxy data cover. We study the LGM climate with a coarse resolution complex cli-

mate model coupled with an ice sheet model. We vali-date our setup by comparing glacial as well as pre-in-dustrial equilibrium experiments with reconstructions and the present state. By performing experiments with both setups, we test our model under large perturba-tions that go beyond the linear range.

Introduction

Model setupOur model comprises of the atmosphere-ocean-vege-tation general circulation model ECHAM5/MPIOM/LPJ interactively coupled 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 °), and mPISM on a 20 km grid covering most of the northern hemisphere. We do not use flux correction or anomaly maps in our models.

For the surface mass balance, we use a positive degree day scheme with lapse rate correction and height de-sertification effect.We use an asynchronous coupling with ten years of ice sheet model integrations per year in the climate model.For comparison, we also perform an experiment that fol-lows the PMIP2 protocol and uses the ICE-5G ice sheet reconstruction (Peltier, 2004) instead of mPISM.

In the pre-industrial setup, we model the ice cover of Greenland and the Arctic Islands fairly correctly (ICE 1). The ice sheet forming in Alaska is caused by a temper-ature bias of ECHAM5, that also occurs in stand-alone simulations and is lower in simulations with higher res-olution. In general, the temperature differences to the reanalysis are within the expectations for a T31 (3.75 °) atmosphere model (SAT 1).The coupled LGM experiment (ICE 2) features the major ice sheets (Greenland, Laurentide, Fennoscandia) as well as an extra ice-sheet over Siberia that connects the Fen-noscandian ice sheet with the Laurentide ice sheet. The Laurentide ice sheet in the coupled experiment is split into a Cordilleran and an Eastern part by a massive ice stream that drains into the Arctic Ocean, while the ice in the Hudson Bay is repeatedly flushed into the Labrador Sea by the Hudson Strait ice stream (details below).The LGM setups show a global cooling (SAT 2), that is stronger over the land than over the oceans and stron-ger over the ice covered areas than in the ice-free areas. Over the ice sheets, the increased altitude and albedo cause a strong cooling.

The ICE-5G setup (ICE 3) shows much stronger cooling (SAT 3) than the coupled setup and therefore is closer to the proxy data. This is largely due to the effect of the different topography on the atmospheric circulation.In the pre-industrial setup, the Atlantic meridional over-turning circulation (AMOC) shows a North Atlantic deep-water (NADW) cell with a strength of 16.8 Sv (AMOC 1). This compares very well with present day observations. The LGM NADW cell (AMOC 2) is stronger, but does not reach as far north as the pre-industrial one. The stronger NADW cell is at odds with proxies but in line with sever-al other climate models (Weber et al, 2007). The ICE-5G setup shows an NADW cell with a strength between the other two experiments (AMOC 3).There are recurring collapses of the Hudson bay part of the Laurentide ice sheet (COL 1, 4) with a period of 7 kyrs. They cause additional freshwater inputs of up to 0.05 Sv into the Labrador sea (COL 5). From there, the freshwater spreads into the open Atlantic (COL 2). It weakens the deep convection and thereby causes a re-gional cooling (COL 3).

Results

We have performed long-time coupled ice sheet – cli-mate model studies under pre-industrial as well as LGM boundary conditions without any flux-correction terms.The shape of the ice sheets has a strong influence on

the wind systems and thereby on the global climate. Our model shows ice sheet collapses as regular part of the ice sheet behavior. These pulses create strong sig-nals in the ocean.

Conclusion

Ice sheets

COL 3: Surface air temperature change during the collapse.We show the annual mean surface air temperature difference. The averaging periods match those in COL 1.

COL 2: Sea surface salinity change during the collapse.We show the salinity difference in the topmost ocean layer. The averaging periods match those in COL 1.

ICE 1: Ice sheet elevations in the pre-industrial experiment.Colored areas mark the ice sheets averaged over 5000 years,dark gray the average annual minimum sea ice extent, light gray the average annual maximum sea ice extent in the experiment.

ICE 2: Ice sheet elevations in the LGM experiment.Colored areas mark the ice sheets averaged over 29 000 years, dark gray the average annual minimum sea ice extent, light gray the average annual maximum sea ice extent in the experiment.

ICE 3: Surface elevations in the ICE-5G reconstruction.Colored areas mark the ice sheets in the reconstruction. Dark gray areas show the average annual minimum sea ice extent, light gray the average annual maximum sea ice extent in the experiment.

LiteratureBueler and Brown, 2009, The shallow shelf approximation as a “sliding law” in a thermomechanically coupled ice sheet modelCalov et al, 2002, Large-scale instabilities of the Laurentide Ice Sheet simulated in a fully coupled climate-system modelMacAyeal, 1993, Binge/Purge oscillations of the Laurentide 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.Weber et al, 2007, The modern and glacial overturning circulation in the Atlantic ocean in PMIP coupled model simulations

Acknowledgement:The simulations were performed on the Blizzard supercomputer of the DKRZ. The resources were provided by BMBF project 675.

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Surface air temperatures

SAT 1: Temperature difference pre-industrial – ERA Interim JJA.We show northern hemisphere summer surface air temperature differences, since they are decisive for ice sheet melt. The temper-atures are averaged over 500 years of climate model output.

SAT 2: Temperature difference coupled LGM – pre-industrial.We show the annual mean surface air temperature difference and proxy data for LGM – present day from Kim et al, 2008. The tem-peratures are averaged over 2900 years of climate model output.

SAT 3: Temperature difference ICE-5G – pre-industrial.We show the annual mean surface air temperature difference and proxy data for LGM – present day from Kim et al, 2008. The tem-peratures are averaged over 500 years of climate model output.

AMOC 1: Atlantic meridional overturning circulation pre-ind.The colors change every 2 Sv. The data is averaged over 500 years of climate model output. The maximum strength of the North At-lantic deep water (NADW) cell is 16.8 Sv. The maximum Antarctic bottom water (AABW) cell strength is –2.9 Sv.

AMOC 2: Atlantic meridional overturning circulation LGM.The colors change every 2 Sv. The data is averaged over 2900 years of climate model output. The maximum NADW cell strength is 22.1 Sv. The maximum AABW cell strength is –3.6 Sv.

AMOC 3: Atlantic meridional overturning circulation ICE-5G.The colors change every 2 Sv. The data is averaged over 500 years of climate model output. The maximum NADW cell strength is 18.6 Sv, the maximum AABW cell strength is –3.5 Sv.

Ocean circulation Collapse events

COL 1: Surface elevation change during the collapse.For the collapse, we average over ice model years 88 500 to 89 000; for the reference period we combine the years 85 000 to 86 000 with the years 91 500 to 92 500.

COL 4: Ice volume changes during the experiment.We show the ice sheet volumes relative to their values at the start of the experiment. The ice sheets in Russia keep growing, while the Laurentide ice sheets pulsates.

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COL 5: Freshwater flux into the Labrador sea.We show the net freshwater flux into the Labrador sea. The col-lapse events can be clearly seen.

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