1/13 Sensitivity of Atlantic large-scale ocean circulation to surface wind-stress for present and glacial climates Marisa Montoya 1, Anders Levermann 2, 3, Andreas Born 4,5 1 Dpto. Astrofísica y Ciencias de la Atmósfera, Universidad Complutense de Madrid, Spain (PalMA Research Group) 2 Potsdam Institute for Climate Impact Research, Potsdam, Germany 3 Institute of Physics, Potsdam University 4 Bjerknes Centre for Climate Research, Bergen, Norway 5 Geophysical Institute, University of Bergen, Bergen, Norway

2/13 Motivation Kuhlbrodt et al. RoG (2007)

3/13 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)

4/13 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

5/13 Montoya et al. (2005) 7.5 o x 22.5 o 3.75 o x 3.75 o x L24 Petoukhov et al. (2000) Fichefet and Morales Maqueda (1997) Brovkin et al. (2000) PMIP2 boundary conditions for LGM: Insolation Equivalent CO 2 = 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-3α CLIMBER-3 

6/13 LGM1.7-weak LGM1.7-strong Holocene LGMα-weak (initial conditions: LGM1) LGMα-strong (initial conditions: LGM2) α ≡ α c = 1.7 Wind-stress amplification factor (α) Atlantic meridional overturning circulation (AMOC)

7/13 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)

8/13 The North Atlantic subpolar gyre McCartney et al. Oceanus (1996) Hátún et al. Science (2005) Thornalley et al. Nature (2009)

9/13 Horizontal volume transport equations Bottom pressure term Potential energy term

10/13 Decomposition of SPG strength

11/13 LGM1.7weak LGM1.0 LGM1.7weak-LGM1.0 LGM1.7strong LGM1.7weak LGM1.7weak-LGM1.0 Density changes (10 -4 kg m -3 )

12/13 Mechanism Montoya et al. (in preparation)

13/13 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

14/13 Conclusions

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23/13 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).

24/13 Difference in salinity (psu) and surface currents (cms -1 ) averaged from 0-300m 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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26/13 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 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.

27/13 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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29/13 The North Atlantic subpolar gyre

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31/13 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

32/13 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

33/13 Streamfunction of zonally averaged flow

34/13 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) [PO 4 3- ] mmol l –1  13 C [Cd] mmol l –1 Lynch-Stieglitz et al. Science (2007) LGM AMOC: - 30% to slight increase

35/ kBP event LGM, ~ 21kBP Eemiense ~ 125 kBP H5 17 H1  T : Dansgaard-Oeschger events in Greenland H1-H5: Heinrich events NGRIP members (2004) Cambio climático abrupto glacial 1kBP

36/13 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)