Manabe and Stouffer (1997) showed with a coupled ocean-atmosphere general circulation model that a 500-year freshwater discharge into the high-latitude North Atlantic weakens the Atlantic thermohaline circulation (THC) and produces a Younger Dryas-like cold event, that the THC recovers within a few centuries after the discharge stops, and that the same discharge into the subtropical North Atlantic weakens the THC 4-5 times less. Their discussion posed six unresolved issues: repeating the experiments under a colder deglacial control climate with extensive sea ice; the insufficient northward extent of simulated Southern Hemisphere cooling; the missing tropical (Huascarán ice core) cooling and the possible role of greenhouse-gas changes; the divergent stability of the THC inactive mode across models (stable off state versus recovery versus intermediate behavior) and the needed comparative assessment of diapycnal diffusion, Antarctic bottom water formation, and wind-stress feedback; the debated origin of meltwater pulse 1A versus the demonstrated inefficacy of subtropical discharge; and the timing of Southern Hemisphere cooling relative to the Byrd record. Later work has substantially resolved the deglacial-boundary-condition and phasing components (transient deglacial simulations; the thermal bipolar seesaw), reframed the Southern Hemisphere and tropical components, and left two precise cores open: attributing inter-model differences in the stability, hysteresis, and recovery of the inactive overturning mode, and partitioning the source and routing of meltwater pulse 1A consistently with the strong dependence of THC weakening on discharge location. Residual model-proxy disagreement in the Younger Dryas cooling pattern also persists.

Background

The Younger Dryas (YD, roughly 12.9-11.7 thousand years before present) was an abrupt return to near-glacial conditions during the last deglaciation, most strongly expressed around the North Atlantic. The leading explanation, dating to the late 1980s, is that meltwater from the decaying continental ice sheets freshened the North Atlantic and weakened the thermohaline circulation - the density-driven overturning now usually called the Atlantic meridional overturning circulation (AMOC) - which transports heat northward; its weakening cools the North Atlantic region and teleconnects climate change worldwide. Manabe and Stouffer (1988) had earlier shown with a coupled ocean-atmosphere general circulation model (GCM, a model simulating atmosphere and ocean dynamics and their fluxes synchronously) that such a coupled system admits two stable equilibria under identical forcing: an active mode and an inactive mode with a collapsed overturning. Manabe and Stouffer (1997) discharged freshwater at 0.1 Sverdrup (1 Sv = 10610^{6} cubic meters per second) for 500 model years into the high-latitude North Atlantic between 50 and 70 degrees north (their FWN experiment, in which the overturning weakened from roughly 18.5 Sv to roughly 3.5 Sv) and, in a paired experiment with the same rate and duration, into the subtropical North Atlantic, comparing the response with the YD as reconstructed from ice cores and marine and lake sediments. Ice-core oxygen-isotope ratios (δ18\delta^{18}O, a standard temperature proxy measured on the ice) at Summit, Greenland were one benchmark; the Huascarán (Peru) tropical ice core of Thompson et al. (1995) another; the Byrd Station (Antarctica) core a third. The comparison exposed six unresolved issues. Two concepts are needed for the later literature: the bipolar seesaw, the idea that when the AMOC weakens, heat that no longer crosses the equator in the Atlantic warms the Southern Hemisphere so that the two hemispheres vary out of phase on millennial scales; and meltwater pulse 1A (mwp-1A), an episode of extremely rapid global sea-level rise (order 10-20 m within a few centuries) near 14.6 thousand years ago whose ice-sheet source - Laurentide versus Antarctic - was already debated by Clark et al. (1996). The stability question involves hysteresis (the overturning collapsing and recovering at different freshwater forcing values) and candidate controlling factors named in 1997: vertical diapycnal diffusion (mixing across ocean density surfaces, which sets the deep density structure), Antarctic Bottom Water formation (the dense water mass formed around Antarctica that competes with North Atlantic deep water), and wind-stress feedback (how the wind field responds to, and feeds back on, circulation changes).

Problem Statement

The overarching objective, as posed in the discussion of Manabe and Stouffer (1997), is to reconcile the coupled ocean-atmosphere response to deglacial freshwater forcing with the observed Younger Dryas record and with the behavior of other coupled models - that is, to determine under what boundary conditions the freshwater-weakening mechanism for the Younger Dryas reproduces the observed pattern, magnitude, timing and hemispheric structure of the event, and why coupled models disagree about the stability of the collapsed circulation state. Concretely, the paper left six linked issues: (1) In the FWN experiment (a 500-year, 0.1 Sv freshwater discharge over the North Atlantic between 50 and 70 degrees north) the simulated cooling pattern is shifted poleward relative to the Younger Dryas proxy pattern and the cooling at Summit, Greenland is smaller than ice-core δ18\delta^{18}O estimates; the experiments should be repeated using a simulated climate of the last deglaciation (colder, with extensive sea ice) as the control, to test whether the cooling pattern shifts equatorward and intensifies. (2) The simulated cooling of the Southern Hemisphere Circumpolar Ocean does not extend far enough north to account for the major Younger Dryas advance of the Franz Josef Glacier in New Zealand; a colder deglacial control with more extensive sea ice and stronger albedo (surface-reflectivity) feedback might allow the cooling to reach lower latitudes. (3) The tropical cooling inferred from the Huascarán ice core is not reproduced in the FWN experiment, suggesting that other mechanisms, such as changes in atmospheric methane or other greenhouse gases, may be required; their role remains to be explored. (4) The stability of the inactive mode of the thermohaline circulation after massive freshwater forcing differs among models - Manabe and Stouffer (1988) found a stable inactive mode that did not recover, the 1997 experiments show recovery after discharge termination, and Schiller et al. (1997) and Fanning and Weaver (1997) show intermediate behavior - and a comparative assessment of candidate factors (vertical diapycnal diffusion, Antarctic bottom water formation, wind-stress feedback) is needed to determine the cause of these differences. (5) The origin of the first meltwater pulse (mwp-1A) remains debated - Clark et al. (1996) argued the Laurentide Ice Sheet could not supply enough meltwater and that Antarctica was the primary source, yet the 1997 experiments show subtropical discharge is much less effective at weakening the THC - and additional modeling is required to reconcile these findings. (6) The model's Southern Hemisphere cooling lags the North Atlantic by several centuries, while the Byrd Station record shows a temperature drop between 12.5 and 14 kyr before present that may correspond to the late Bølling cooling (the Bølling-Allerød being the warm deglacial interval, roughly 14.7-12.9 thousand years before present, that immediately preceded the Younger Dryas) rather than the YD; this hypothesis needs testing with deglacial boundary conditions. The open cores that remain are component (4), the factor or factors responsible for the divergent inactive-mode stability, hysteresis and recovery, and component (5), the source partition and routing of meltwater pulse 1A, together with the residual model-proxy mismatch in the Younger Dryas cooling pattern noted under (1) and (2).

References

  1. Manabe, S., and R. J. Stouffer (1997), 'Coupled ocean-atmosphere model response to freshwater input: Comparison to Younger Dryas Event', Paleoceanography, DOI 10.1029/96PA03932, https://doi.org/10.1029/96PA03932
  2. Manabe, S., and R. J. Stouffer (1988), 'Two Stable Equilibria of a Coupled Ocean-Atmosphere Model', Journal of Climate, DOI 10.1175/1520-0442(1988)001<0841:TSEOAC>2.0.CO;2, https://doi.org/10.1175/1520-0442(1988)001%3C0841:TSEOAC%3E2.0.CO;2
  3. Clark, P. U., R. B. Alley, L. D. Keigwin, J. M. Licciardi, S. J. Johnsen, and H. Wang (1996), 'Origin of the first global meltwater pulse following the Last Glacial Maximum', Paleoceanography, DOI 10.1029/96PA01419, https://doi.org/10.1029/96PA01419
  4. Thompson, L. G., E. Mosley-Thompson, M. E. Davis, P.-N. Lin, K. A. Henderson, J. Cole-Dai, J. F. Bolzan, and K.-b. Liu (1995), 'Late Glacial Stage and Holocene Tropical Ice Core Records from Huascarán, Peru', Science, DOI 10.1126/science.269.5220.46, https://doi.org/10.1126/science.269.5220.46
  5. Schiller, A., U. Mikolajewicz, and R. Voss (1997), 'The stability of the North Atlantic thermohaline circulation in a coupled ocean-atmosphere general circulation model', Climate Dynamics, DOI 10.1007/s003820050169, https://doi.org/10.1007/s003820050169
  6. Fanning, A. F., and A. J. Weaver (1997), 'Temporal-geographical meltwater influences on the North Atlantic conveyor: Implications for the Younger Dryas', Paleoceanography 12, 307-320, DOI 10.1029/96PA03726, https://doi.org/10.1029/96PA03726
  7. Broecker, W. S. (1998), 'Paleocean circulation during the Last Deglaciation: A bipolar seesaw?', Paleoceanography 13, 119-121, DOI 10.1029/97PA03707, https://doi.org/10.1029/97PA03707
  8. Blunier, T., and E. J. Brook (2001), 'Timing of Millennial-Scale Climate Change in Antarctica and Greenland During the Last Glacial Period', Science, DOI 10.1126/science.291.5501.109, https://doi.org/10.1126/science.291.5501.109
  9. Stocker, T. F., and S. J. Johnsen (2003), 'A minimum thermodynamic model for the bipolar seesaw', Paleoceanography, DOI 10.1029/2003PA000920, https://doi.org/10.1029/2003PA000920
  10. Weaver, A. J., O. A. Saenko, P. U. Clark, and J. X. Mitrovica (2003), 'Meltwater Pulse 1A from Antarctica as a Trigger of the Bølling-Allerød Warm Interval', Science, DOI 10.1126/science.1081002, https://doi.org/10.1126/science.1081002
  11. Tarasov, L., and W. R. Peltier (2005), 'Arctic freshwater forcing of the Younger Dryas cold reversal', Nature, DOI 10.1038/nature03617, https://doi.org/10.1038/nature03617
  12. Stouffer, R. J., J. Yin, J. M. Gregory, K. W. Dixon, M. J. Spelman, W. Hurlin, A. J. Weaver, M. Eby, G. M. Flato, H. Hasumi, A. Hu, J. H. Jungclaus, I. V. Kamenkovich, A. Levermann, M. Montoya, S. Murakami, S. Nawrath, A. Oka, W. R. Peltier, D. Y. Robitaille, A. Sokolov, G. Vettoretti, and S. L. Weber (2006), 'Investigating the Causes of the Response of the Thermohaline Circulation to Past and Future Climate Changes', Journal of Climate, DOI 10.1175/JCLI3689.1, https://doi.org/10.1175/JCLI3689.1
  13. Yin, J., and R. J. Stouffer (2007), 'Comparison of the Stability of the Atlantic Thermohaline Circulation in Two Coupled Atmosphere-Ocean General Circulation Models', Journal of Climate, DOI 10.1175/JCLI4256.1, https://doi.org/10.1175/JCLI4256.1
  14. Liu, Z., B. L. Otto-Bliesner, F. He, E. C. Brady, R. Tomas, P. U. Clark, A. E. Carlson, J. Lynch-Stieglitz, W. Curry, E. Brook, D. Erickson, R. Jacob, J. Kutzbach, and J. Cheng (2009), 'Transient Simulation of Last Deglaciation with a New Mechanism for Bølling-Allerød Warming', Science, DOI 10.1126/science.1171041, https://doi.org/10.1126/science.1171041
  15. Barker, S., P. Diz, M. J. Vautravers, J. Pike, G. Knorr, I. R. Hall, and W. S. Broecker (2009), 'Interhemispheric Atlantic seesaw response during the last deglaciation', Nature, DOI 10.1038/nature07770, https://doi.org/10.1038/nature07770
  16. Condron, A., and P. Winsor (2012), 'Meltwater routing and the Younger Dryas', Proceedings of the National Academy of Sciences, DOI 10.1073/pnas.1207381109, https://doi.org/10.1073/pnas.1207381109
  17. Deschamps, P., N. Durand, E. Bard, B. Hamelin, G. Camoin, A. L. Thomas, G. M. Henderson, J. Okuno, and Y. Yokoyama (2012), 'Ice-sheet collapse and sea-level rise at the Bølling warming 14,600 years ago', Nature, DOI 10.1038/nature10902, https://doi.org/10.1038/nature10902
  18. Gregoire, L. J., A. J. Payne, and P. J. Valdes (2012), 'Deglacial rapid sea level rises caused by ice-sheet saddle collapses', Nature, DOI 10.1038/nature11257, https://doi.org/10.1038/nature11257
  19. Hu, A., G. A. Meehl, W. Han, A. Timmermann, B. Otto-Bliesner, Z. Liu, W. M. Washington, W. Large, A. Abe-Ouchi, M. Kimoto, K. Lambeck, and B. Wu (2012), 'Role of the Bering Strait on the hysteresis of the ocean conveyor belt circulation and glacial climate stability', Proceedings of the National Academy of Sciences, DOI 10.1073/pnas.1116014109, https://doi.org/10.1073/pnas.1116014109
  20. He, F., J. D. Shakun, P. U. Clark, A. E. Carlson, Z. Liu, B. L. Otto-Bliesner, and J. E. Kutzbach (2013), 'Northern Hemisphere forcing of Southern Hemisphere climate during the last deglaciation', Nature, DOI 10.1038/nature11822, https://doi.org/10.1038/nature11822
  21. Renssen, H., A. Mairesse, H. Goosse, P. Mathiot, O. Heiri, D. M. Roche, K. H. Nisancioglu, and P. J. Valdes (2015), 'Multiple causes of the Younger Dryas cold period', Nature Geoscience, DOI 10.1038/ngeo2557, https://doi.org/10.1038/ngeo2557
  22. Liu, J., G. A. Milne, R. E. Kopp, P. U. Clark, and I. Shennan (2016), 'Sea-level constraints on the amplitude and source distribution of Meltwater Pulse 1A', Nature Geoscience, DOI 10.1038/ngeo2616, https://doi.org/10.1038/ngeo2616
  23. Renssen, H., H. Goosse, D. M. Roche, and H. Seppä (2018), 'The global hydroclimate response during the Younger Dryas event', Quaternary Science Reviews, DOI 10.1016/j.quascirev.2018.05.033, https://doi.org/10.1016/j.quascirev.2018.05.033
  24. Brendryen, J., H. Haflidason, Y. Yokoyama, K. A. Haaga, and B. Hannisdal (2020), 'Eurasian Ice Sheet collapse was a major source of Meltwater Pulse 1A 14,600 years ago', Nature Geoscience, DOI 10.1038/s41561-020-0567-4, https://doi.org/10.1038/s41561-020-0567-4
  25. Weijer, W., W. Cheng, O. A. Garuba, A. Hu, and B. T. Nadiga (2020), 'CMIP6 Models Predict Significant 21st Century Decline of the Atlantic Meridional Overturning Circulation', Geophysical Research Letters, DOI 10.1029/2019GL086075, https://doi.org/10.1029/2019GL086075
  26. Renssen, H. (2020), 'Comparison of Climate Model Simulations of the Younger Dryas Cold Event', Quaternary 3(4), 29, DOI 10.3390/quat3040029, https://doi.org/10.3390/quat3040029
  27. van Westen, R. M., and H. A. Dijkstra (2023), 'Asymmetry of AMOC Hysteresis in a State-Of-The-Art Global Climate Model', Geophysical Research Letters 50, DOI 10.1029/2023GL106088, https://doi.org/10.1029/2023GL106088 (preprint arXiv:2308.14098, https://arxiv.org/abs/2308.14098)
  28. Vanderborght, E., R. M. van Westen, and H. A. Dijkstra (2025), 'Feedback Processes Causing an AMOC Collapse in the Community Earth System Model', Journal of Climate 38, DOI 10.1175/JCLI-D-24-0570.1, https://doi.org/10.1175/JCLI-D-24-0570.1 (preprint arXiv:2410.03236, https://arxiv.org/abs/2410.03236)