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Sensitivity of Atlantic large-scale ocean circulation to surface wind-stress

for present and glacial climates

Marisa Montoya1, Anders Levermann2, 3, Andreas Born 4,5

1Dpto. Astrofísica y Ciencias de la Atmósfera, Universidad Complutense de Madrid, Spain (PalMA Research Group)

2Potsdam Institute for Climate Impact Research, Potsdam, Germany3Institute of Physics, Potsdam University

4Bjerknes Centre for Climate Research, Bergen, Norway5 Geophysical Institute, University of Bergen, Bergen, Norway

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Motivation

Kuhlbrodt et al. RoG (2007)

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Motivation

PMIP2: ±40% range in LGM minus present Atlantic meridional overturning circulation (AMOC) strength

Lynch-Stieglitz et al. Science (2007)Paleodata: - 30% to slight increase

Otto-Bliesner et al. GRL (2007);Weber et al. CPD (2007)

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LGM winds?

Stronger

• Stronger glacial meridional surface temperature gradients.

• Increased glacial aerosol concentrations.

(Crowley and North (1991) and references therein)

Weaker

• Enhanced aerosols might reflect changes in sources, e.g. enhanced aridity.

• Models show enhanced westerlies but not uniformly enhanced surface winds (e.g. Hewitt et al. [2003]; Otto-Bliesner et al. [2007]).

• Reduced glacial CO2 levels result in weaker aloft temperature gradients which might be more relevant to surface winds [Toggweiler 2008].

Glacial wind-stress poorly constrained.

Here we assess impact of surface winds uncertainty on LGM AMOC strength as well as the strength of the SPG

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Montoya et al. (2005)

7.5o x 22.5o

3.75o x 3.75o x L24

Petoukhov et al. (2000)

Fichefet and Morales Maqueda (1997) Brovkin et al.

(2000)

PMIP2 boundary conditions for LGM:

Insolation

Equivalent CO2 = 167 ppmv

Peltier (2004) ICE-5G ice-sheet reconstruction

Land-sea mask

--

Global salinity enhanced by 1psu

Ocean bathymetry, vegetation and river runoff

routing unchanged with respect to Holocene

Trenberth et al. [1989] surface wind-stress climatology × α Є [0.5, 2] (LGMα).

CLIMBER-3αCLIMBER-3

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LGM1.7-weak

LGM1.7-strong

Holocene

LGMα-weak (initial conditions: LGM1)

LGMα-strong (initial conditions: LGM2)

α ≡ αc = 1.7

Wind-stress amplification factor (α)

Atlantic meridional overturning circulation (AMOC)

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Mean annual SAT LGM1.7-strong minus LGM1.7-weak (K)

ΔSAT pattern consistent with that expected during DOs

Glacial abrupt climate change?

Montoya and Levermann GRL (2008)

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The North Atlantic subpolar gyre

McCartney et al. Oceanus (1996)

Hátún et al. Science (2005)

Thornalley et al. Nature (2009)

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Horizontal volume transport equations

Bottom pressure term

Potential energy term

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Decomposition of SPG strength

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LGM1.7weak LGM1.0 LGM1.7weak-LGM1.0

LGM1.7strong LGM1.7weak LGM1.7weak-LGM1.0

Density changes

yz

(10-4 kg m-3 )

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Mechanism

Montoya et al. (in preparation)

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• We have analyzed the sensitivity of the AMOC and the SPG to changes in the wind-stress strength for present and glacial boundary conditions.

• If glacial climate were close to a threshold, small changes in surface wind strength might promote DWF in the Nordic Seas. Our results thus point to a potentially relevant role of changes in surface wind strength in glacial abrupt climate change.

• Glacial abrupt climatic changes are explained through latitudinal shifts in North Atlantic DWF sites, could result both in a drastic reduction of the SPG strength and a sudden change in its sensitivity to wind-stress variations.

Conclusions

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Conclusions

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Global ΔSAT Є [-5.9 K (LGM0.5), -4.1 K (LGM2.0)]

Tropical ΔSST Є [-2.0 K (LGM0.5), -2.6 K (LGM2.0)]

CLIMAP (1976): 1-2 K; Guilderson et al. (1994): 4-5 K; Rosell-Melé et al. (2004): ~2 K;

MARGO Project Members (2009): 1.7 ± 1.0 K ;

Mean annual surface air temperature difference (SAT) LGM1.0 (surface wind-stress: Trenberth et al. [1989]) - Holocene (K).

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Difference in salinity (psu) and

surface currents (cms-1)averaged from 0-300m

LGM1.7-weak - LGM1.0

LGM1.7-weak - LGM1.0

Difference in net freshwater flux

(P – E + runoff, positive into ocean)

LGM1.7-weak - LGM1.0

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Difference in salinity (psu) and AMOC (Sv) in Atlantic

LGM1.7-weak minus LGM1.0

LGM1.7-strong minus LGM1.7-weak

Difference in salinity (psu) and surface currents (cms-1)averaged from 0-300 m

LGM1.7-weak minus LGM1.0

Enhanced subtropical and subpolar horizontal gyre circulation + positive salinity advection feedback increase salt transport to the North Atlantic in upper ocean.

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Mean annual SAT LGM1.7-strong minus LGM1.7-weak (K)

ΔSAT pattern consistent with that expected during DOs

Maximum mixed layer depth (m) LGM1.7-strong minus LGM1.7-weak & 80% January-April sea-

ice concentration

LGM1.7-weak

LGM1.7-strong

Glacial abrupt climate change?

Montoya and Levermann GRL (2008)

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The North Atlantic subpolar gyre

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• Atlantic meridional overturning circulation (AMOC) shows large spread in last glacial maximum (LGM, ca. 21 kyr BP) climate simulations, e.g. PMIP2: ±40% range in LGM minus present AMOC strength [Weber et al., 2007].

• Reconstructions: glacial AMOC strength values ranging from a decrease of up to 30% to a slight increase [Marchal et al., 2000; Lynch-Stieglitz et al., 2007].

• Investigating glacial AMOC requires assessment of its driving mechanisms (surface winds, vertical mixing [Kuhlbrodt et al., 2007]).

• Stronger glacial meridional surface temperature gradients and increased glacial aerosol concentrations have led to assumption of glacial surface winds enhanced by ≥ 50% [Crowley and North,1991]. Yet, enhanced aerosols might also reflect changes in sources, e.g. enhanced aridity and aloft rather than surface temperature gradients might be more relevant to surface winds [Toggweiler 2008]. Models show enhanced westerlies but not uniformly enhanced surface winds (e.g. Hewitt et al. [2003]; Otto-Bliesner et al. [2006]).

• Thus, glacial wind-stress poorly constrained.

• Here we assess impact of surface winds uncertainty on LGM AMOC strength.

Introduction

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• We have investigated the sensitivity of LGM climate simulations to global changes in oceanic surface wind-stress by prescribing these to be proportional to present day observations.

• Caveats: regional wind-stress differences, atmospheric variability not taken into account.

• LGM AMOC strength increases with the surface wind strength, exhibiting a threshold behavior.

• In the North Atlantic pattern and magnitude of the temperature difference between strong and weak AMOC states are consistent with those expected during abrupt climate changes of the last glacial period, in particular DO events.

Summary

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Streamfunction of zonally averaged flow

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Motivation

PMIP2: ±40% range in LGM minus present Atlantic meridional overturning circulation (AMOC)

strength

Otto-Bliesner et al. GRL (2007);Weber et al. CPD (2007)

[PO43-] mmol l–1

13C

[Cd] mmol l–1

Lynch-Stieglitz et al. Science (2007)

LGM AMOC: - 30% to slight increase

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8.2 kBP event

LGM, ~ 21kBPEemiense ~ 125 kBP

20

19

18

1

2

H5

17

H1

T

0

-20

1-20 : Dansgaard-Oeschger events in GreenlandH1-H5: Heinrich events

NGRIP members (2004)

Cambio climático abrupto glacial

1kBP

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The North Atlantic subpolar gyre

McCartney et al. Oceanus (1996)

Hátún et al. Science (2005)

Haekkinen and Rhines Science (2004)

Thornalley et al. Nature (2009)