X. Zhang et al. (AWI) Two ocean states during the LGM 20th Mar. 2012 /11
Two Ocean states
during the Last Glacial Maximum
Zhang, X., G. Lohmann, G. Knorr, X. Xu., Two Ocean states during the Last Glacial Maximum.
SubmiHed
(OHo-‐Bliesner et al. 2007)
MoLvaLon
Large difference among PMIP2 models !!!
Is it possible that the states of AMOC among PMIP2 models are just in the different
posiLons of a bistable regime?
(Stommel. 1961; Rahmstorf 2000)
X. Zhang et al. (AWI) Two ocean states during the LGM 20th Mar. 2012 /11 Isotope
Models Ice Sheet
Model Vegetation
Atmosphere
MPI-‐OM
HAMOCC5
Chemistry
OASIS
Marine Biogeochemistry
ECHAM5
Ocean Land
Model (COSMOS) configuraLon
PI 6k 21k
MIS3 (32k) Eemian
Pliocene Miocene
(T31L19 )
(GR30L40)
Ini6al Ocean state Glacial Ocean
(Stra&fied ocean from CCSM) Ocean only (3000 y)+coupled (3000y)
Present day Ocean (Levitus et al. 1998)
Coupled (3000 y) The other model setup è 21k experimental design in PMIP3
LGMW LGMS
3
Surface ProperLes of the LGM runs
PI LGMS LGMW
LGMW
The white lines represent winter for each hemisphere, while the red for summer.
The dashed lines indicate 15% SIC, and solid lines 90% SIC.
15%
15%
90%
90%
LGMW: from glacial ocean LGMS: from present day ocean
X. Zhang et al. (AWI) Two ocean states during the LGM 20th Mar. 2012 /11
Internal properLes of the LGM runs
LGMW: from glacial ocean LGMS: from present day oceanpsu
5
°C
(Jungclaus et al. 2006)
Internal properLes of the LGM runs
Glacial-‐like ocean PI-‐like ocean
LGMW: from glacial ocean LGMS: from present day ocean
psu Sv
~18 ~26 ~16
X. Zhang et al. (AWI) Two ocean states during the LGM 20th Mar. 2012 /11
Meridional transect of Salinity in PMIP2 models
Glacial-‐like ocean (LGMW)
PI-‐like ocean (LGMS)
Two ocean states with respect to ocean structure during the LGM.
7
Response of Present day ocean
LGM2PI: from glacial ocean PI: from present day ocean1. PI Ocean state independent on iniLal ocean straLficaLon.
LGM2PI : iniLalized from the glacial ocean.
All the other condiLons are same as PI.
An iniLal ocean state for the LGM simulaLon should be specified for a beHer intercomparison of PMIP models !
2. Two ocean states with respect to ocean structure are unique
during the LGM
X. Zhang et al. (AWI) Two ocean states during the LGM 20th Mar. 2012 /11
Freshwater PerturbaLon
1. Bistability of LGM ocean is not related to FWP.
LGMW: from glacial ocean LGMS: from present day ocean
0 100 200 300 400
0 5 10 15 20 25 30 35
Year
Sv
Freshwater Perturbation
LGMW-1Sv LGMS-1Sv
2. AMOC in LGMW increases slowly by itself. Why?
0 100 200 300 400
0 5 10 15 20 25 30 35
Year
Sv
Freshwater Perturbation
LGMW-0.2Sv LGMW
LGMS-0.2Sv LGMS
9
LGMW
Upwelling in the Southern Ocean
Upwelling in the Southern Ocean during the LGM!
500y later
500y later
LGMW
LGMW
LGMW: from glacial ocean LGMS: from present day ocean
X. Zhang et al. (AWI) Two ocean states during the LGM 20th Mar. 2012 /11
Summary
• Last Glacial Maximum possesses two ocean states
dependent on the ocean stra6fica6on. Only one state is consistent with reconstruc6ons, but is not fully
equilibrated with coeval boundary condi6ons,
represen6ng a new paradigm for the last deglacia6on.
• LGM ocean state has an coherent transient nature.
Upwelling in the Southern Ocean is crucial for us to understand the transi6on from weak mode to strong mode of the LGM.
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