Marisa Montoya 1 , Anders Levermann 2, 3, Andreas Born 4,5

1. Sensitivity of Atlantic large-scale ocean circulation to surface wind-stressfor 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, Germany 3Institute of Physics, Potsdam University 4Bjerknes Centre for Climate Research, Bergen, Norway 5 Geophysical Institute, University of Bergen, Bergen, Norway

2. Motivation Kuhlbrodt et al. RoG (2007)

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

4. 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. CLIMBER-3α CLIMBER-3a 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 Petoukhov et al. (2000) 7.5o x 22.5o Fichefet and Morales Maqueda (1997) Brovkin et al. (2000) 3.75o x 3.75o x L24 Montoya et al. (2005) Trenberth et al. [1989] surface wind-stress climatology × αЄ[0.5, 2] (LGMα).

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

7. Glacial abrupt climate change? Mean annual SAT LGM1.7-strong minus LGM1.7-weak (K) ΔSAT pattern consistent with that expected during DOs Montoya and Levermann GRL (2008)

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

9. Horizontal volume transport equations Bottom pressure term Potential energy term

10. Decomposition of SPG strength

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

12. Mechanism Montoya et al. (in preparation)

13. Conclusions 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.

14. Conclusions

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

24. LGM1.7-weak - LGM1.0 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

26. 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 Difference in salinity (psu) and AMOC (Sv) in Atlantic 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. Glacial abrupt climate change? Mean annual SAT LGM1.7-strong minus LGM1.7-weak (K) LGM1.7-weak LGM1.7-strong Maximum mixed layer depth (m) LGM1.7-strong minus LGM1.7-weak & 80% January-April sea-ice concentration ΔSAT pattern consistent with that expected during DOs Montoya and Levermann GRL (2008)

29. The North Atlantic subpolar gyre

31. 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. 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. Streamfunction of zonally averaged flow

34. Motivation [PO43-] mmol l–1 d13C [Cd] mmol l–1 PMIP2: ±40% range in LGM minus present Atlantic meridional overturning circulation (AMOC) strength Otto-Bliesner et al. GRL (2007); Weber et al. CPD (2007) LGM AMOC: - 30% to slight increase Lynch-Stieglitz et al. Science (2007)

35. Cambio climático abrupto glacial LGM, ~ 21kBP 1kBP DT 0 1 20 17 19 8.2 kBP event 2 18 -20 H1 H5 Eemiense ~ 125 kBP 1-20 :Dansgaard-Oeschger events in Greenland H1-H5: Heinrich events NGRIP members (2004)

36. The North Atlantic subpolar gyre Haekkinen and Rhines Science (2004) McCartney et al. Oceanus (1996) Hátún et al. Science (2005) Thornalley et al. Nature (2009)