Climate Dynamics

, Volume 51, Issue 4, pp 1333–1350 | Cite as

Timescales of AMOC decline in response to fresh water forcing

  • Laura C. Jackson
  • Richard A. Wood


The Atlantic meridional overturning circulation (AMOC) is predicted to weaken over the coming century due to warming from greenhouse gases and increased input of fresh water into the North Atlantic, however there is considerable uncertainty as to the amount and rate of AMOC weakening. Understanding what controls the rate and timescale of AMOC weakening may help to reduce this uncertainty and hence reduce the uncertainty surrounding associated impacts. As a first step towards this we consider the timescales associated with weakening in response to idealized freshening scenarios. Here we explore timescales of AMOC weakening in response to a freshening of the North Atlantic in a suite of experiments with an eddy-permitting global climate model (GCM). When the rate of fresh water added to the North Atlantic is small (0.1 Sv; 1 Sv \(=1\times 10^6\) m\(^3\)/s), the timescale of AMOC weakening depends mainly on the rate of fresh water input itself and can be longer than a century. When the rate of fresh water added is large (\(\ge\) 0.3 Sv) however, the timescale is a few decades and is insensitive to the actual rate of fresh water input. This insensitivity is because with a greater rate of fresh water input the advective feedbacks become more important at exporting fresh anomalies, so the rate of freshening is similar. We find advective feedbacks from: an export of fresh anomalies by the mean flow; less volume import through the Bering Strait; a weakening AMOC transporting less subtropical water northwards; and anomalous subtropical circulations which amplify export of the fresh anomalies. This latter circulation change is driven itself by the presence of fresh anomalies exported from the subpolar gyre through geostrophy. This feedback has not been identified in previous model studies and when the rate of freshening is strong it is found to dominate the total export of fresh anomalies, and hence the timescale of AMOC decline. Although results may be model dependent, qualitatively similar mechanisms are also found in a single experiment with a different GCM.


AMOC Timescale Climate 



This work was supported by the Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101).


  1. Bakker P, Schmittner A, Lenaerts JTM, Abe-Ouchi A, Bi D, van den Broeke MR, Chan WL, Hu A, Beadling RL, Marsland SJ, Mernild SH, Saenko OA, Swingedouw D, Sullivan A, Yin J (2016) Fate of the Atlantic Meridional Overturning Circulation: strong decline under continued warming and Greenland melting. Geophys Res Lett 43:12,252–12,260. doi: 10.1002/2016gl070457 CrossRefGoogle Scholar
  2. Boning CW, Behrens E, Biastoch A, Getzlaff K, Bamber JL (2016) Emerging impact of Greenland meltwater on deepwater formation in the North Atlantic Ocean. Nat Geosci 9(7):523–527. doi: 10.1038/ngeo2740 CrossRefGoogle Scholar
  3. Cimatoribus A, Drijfhout S, Dijkstra H (2014) Meridional overturning circulation: stability and ocean feedbacks in a box model. Clim Dyn 42(1–2):311–328. doi: 10.1007/s00382-012-1576-9 CrossRefGoogle Scholar
  4. Clement AC, Peterson LC (2008) Mechanisms of abrupt climate change of the last glacial period. Rev Geophys 46(4):RG4002. doi: 10.1029/2006rg000204 CrossRefGoogle Scholar
  5. den Toom M, Dijkstra HA, Weijer W, Hecht MW, Maltrud ME, van Sebille E (2014) Response of a strongly Eddying global ocean to North Atlantic freshwater perturbations. J Phys Oceanogr 44(2):464–481. doi: 10.1175/jpo-d-12-0155.1 CrossRefGoogle Scholar
  6. de Vries P, Weber SL (2005) The Atlantic freshwater budget as a diagnostic for the existence of a stable shut down of the Meridional Overturning Circulation. Geophys Res Lett. doi: 10.1029/2004GL021450 Google Scholar
  7. Driesschaert E, Fichefet T, Goosse H, Huybrechts P, Janssens I, Mouchet A, Munhoven G, Brovkin V, Weber SL (2007) Modeling the influence of Greenland ice sheet melting on the Atlantic meridional overturning circulation during the next millennia. Geophys Res Lett 34(10):L10707. doi: 10.1029/2007gl029516 CrossRefGoogle Scholar
  8. Gent PR, McWilliams JC (1990) Isopycnal mixing in ocean circulation models. J Phys Oceanogr 20(1):150–155CrossRefGoogle Scholar
  9. Gill AE (1982) Atmosphere-ocean dynamics. International Geophysics Series, Academic, San Fransisco. doi: 10.1002/qj.49711046322
  10. Gordon C, Cooper C, Senior CA, Banks H, Gregory JM, Johns TC, Mitchell JFB, Wood RA (2000) The simulation of SST, sea ice extents and ocean heat transports in a version of the Hadley Centre coupled model without flux adjustments. Clim Dyn 16:147–168CrossRefGoogle Scholar
  11. Gregory JM, Saenko OA, Weaver AJ (2003) The role of the Atlantic freshwater balance in the hysteresis of the meridional overturning circulation. Clim Dyn 21(7–8):707–717. doi: 10.1007/s00382-003-0359-8 CrossRefGoogle Scholar
  12. Hawkins E, Smith RS, Allison LC, Gregory JM, Woollings TJ, Pohlmann H, de Cuevas B (2011) Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport. Geophys Res Lett. doi: 10.1029/2011GL047208 Google Scholar
  13. Hofmann M, Rahmstorf S (2009) On the stability of the Atlantic meridional overturning circulation. Proc Natl Acad Sci 106(49):20584–20589. doi: 10.1073/pnas.0909146106 CrossRefGoogle Scholar
  14. Hu A, Meehl GA, Han W, Yin J (2011) Effect of the potential melting of the Greenland Ice Sheet on the Meridional Overturning Circulation and global climate in the future. Deep Sea Res Part II Top Stud Oceanogr 58(17–18):1914–1926. doi: 10.1016/j.dsr2.2010.10.069 CrossRefGoogle Scholar
  15. Huang RX, Schmitt RW (1993) The GoldsbroughStommel circulation of the World Oceans. J Phys Oceanogr 23(6):1277–1284. doi:10.1175/1520-0485(1993);2Google Scholar
  16. Jackson LC (2013) Shutdown and recovery of the AMOC in a coupled global climate model: the role of the advective feedback. Geophys Res Lett 40(6):1182–1188. doi: 10.1002/grl.50289 CrossRefGoogle Scholar
  17. Jackson LC, Smith RS, Wood RA (2016) Ocean and atmosphere feedbacks affecting AMOC hysteresis in a GCM. Clim Dyn. doi: 10.1007/s00382-016-3336-8 Google Scholar
  18. Liu W, Liu Z, Brady EC (2013) Why is the AMOC monostable in coupled general circulation models? J Clim 27(6):2427–2443. doi: 10.1175/jcli-d-13-00264.1 CrossRefGoogle Scholar
  19. Liu W, Xie SP, Liu Z, Zhu J (2017) Overlooked possibility of a collapsed Atlantic Meridional overturning circulation in warming climate. Sci Adv 3(1):e1601666. doi: 10.1126/sciadv.1601666 CrossRefGoogle Scholar
  20. Madec G (2008) NEMO ocean engine. Institut Pierre-Simon Laplace, FranceGoogle Scholar
  21. Manabe BS, Stouffer RJ (1999) Are two modes of thermohaline circulation stable? Tellus A 51(3):400–411. doi: 10.1034/j.1600-0870.1999.t01-3-00005.x CrossRefGoogle Scholar
  22. Marotzke J, Willebrand J (1991) Multiple equilibria of the global thermohaline circulation. J Phys Oceanogr 21(9):1372–1385. doi:10.1175/1520-0485(1991);2Google Scholar
  23. McCarthy GD, Smeed DA, Johns WE, Frajka-Williams E, Moat BI, Rayner D, Baringer MO, Meinen CS, Collins J, Bryden HL (2015) Measuring the Atlantic meridional overturning circulation at 26N. Progress Oceanogr 130:91–111. doi: 10.1016/j.pocean.2014.10.006 CrossRefGoogle Scholar
  24. McManus JF, Francois R, Gherardi JM, Keigwin LD, Brown-Leger S (2004) Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Nature 428(6985):834–837. doi: 10.1038/nature02494 CrossRefGoogle Scholar
  25. Mecking JV, Drijfhout SS, Jackson LC, Graham T (2016) Stable AMOC off state in an eddy-permitting coupled climate model. Clim Dyn 47(7–8):2455. doi: 10.1007/s00382-016-2975-0 CrossRefGoogle Scholar
  26. Megann A, Storkey D, Aksenov Y, Alderson S, Calvert D, Graham T, Hyder P, Siddorn J, Sinha B (2014) GO5.0: the joint NERCMet Office NEMO global ocean model for use in coupled and forced applications. Geosci Model Dev 7(3):1069–1092. doi: 10.5194/gmd-7-1069-2014 CrossRefGoogle Scholar
  27. Rahmstorf S (1996) On the freshwater forcing and transport of the Atlantic Thermohaline Circulation. Clim Dyn 12:799–811. doi: 10.1007/s003820050144 CrossRefGoogle Scholar
  28. Rahmstorf S (2002) Ocean circulation and climate during the past 120,000 years. Nature 419(6903):207–214. doi: 10.1038/nature01090 CrossRefGoogle Scholar
  29. Rahmstorf S, Crucifix M, Ganopolski A, Goosse H, Kamenkovich I, Knutti R, Lohmann G, Marsh R, Mysak LA, Wang Z et al (2005) Thermohaline circulation hysteresis: a model intercomparison. Geophys Res Lett. doi: 10.1029/2005GL023655 Google Scholar
  30. Reintges A, Martin T, Latif M, Keenlyside N (2016) Uncertainty in twenty-first century projections of the Atlantic Meridional Overturning Circulation in CMIP3 and CMIP5 models. Clim Dyn. doi: 10.1007/s00382-016-3180-x Google Scholar
  31. Roberts CD, Garry FK, Jackson LC (2013) A multimodel study of sea surface temperature and subsurface density fingerprints of the Atlantic meridional overturning circulation. J Clim 26(22):9155–9174. doi: 10.1175/jcli-d-12-00762.1 CrossRefGoogle Scholar
  32. Roberts CD, Jackson L, McNeall D (2014) Is the 2004–2012 reduction of the Atlantic meridional overturning circulation significant? Geophys Res Lett 41(9):3204–3210. doi: 10.1002/2014gl059473 CrossRefGoogle Scholar
  33. Roullet G, Madec G (2000) Salt conservation, free surface, and varying levels: A new formulation for ocean general circulation models. J Geophys Res 105(C10):23927–23942. doi: 10.1029/2000jc900089 CrossRefGoogle Scholar
  34. Sgubin G, Swingedouw D, Drijfhout S, Hagemann S, Robertson E (2015) Multimodel analysis on the response of the AMOC under an increase of radiative forcing and its symmetrical reversal. Clim Dyn 45:1429–1450. doi: 10.1007/s00382-014-2391-2 CrossRefGoogle Scholar
  35. Smith RS, Gregory JM (2009) A study of the sensitivity of ocean overturning circulation and climate to freshwater input in different regions of the North Atlantic. Geophys Res Lett 36(15):L15701. doi: 10.1029/2009gl038607 CrossRefGoogle Scholar
  36. Spence P, Saenko OA, Sijp W, England MH (2012) North Atlantic climate response to Lake Agassiz drainage at coarse and ocean Eddy-permitting resolutions. J Clim 26(8):2651–2667. doi: 10.1175/jcli-d-11-00683.1 CrossRefGoogle Scholar
  37. Stouffer RJ, Yin J, Gregory JM, Dixon KW, Spelman MJ, Hurlin W, Weaver AJ, Eby M, Flato GM, Hasumi H, Hu A, Jungclaus JH, Kamenkovich IV, Levermann A, Montoya M, Murakami S, Nawrath S, Oka A, Peltier WR, Robitaille DY, Sokolov A, Vettoretti G, Weber SL (2006) Investigating the causes of the response of the thermohaline circulation to past and future climate changes. J Clim. doi: 10.1175/JCLI3689.1 Google Scholar
  38. Straneo F (2006) On the connection between dense water formation, overturning, and poleward heat transport in a convective basin. J Phys Oceanogr 36(9):1822–1840. doi: 10.1175/jpo2932.1 CrossRefGoogle Scholar
  39. Swingedouw D, Braconnot P, Delecluse P, Guilyardi E, Marti O (2007) Quantifying the AMOC feedbacks during a 2CO\(_2\) stabilization experiment with land-ice melting. Clim Dyn 29(5):521–534. doi: 10.1007/s00382-007-0250-0 CrossRefGoogle Scholar
  40. Swingedouw D, Rodehacke C, Behrens E, Menary M, Olsen S, Gao Y, Mikolajewicz U, Mignot J, Biastoch A (2013) Decadal fingerprints of freshwater discharge around Greenland in a multi-model ensemble. Clim Dyn 41(3–4):695–720. doi: 10.1007/s00382-012-1479-9 CrossRefGoogle Scholar
  41. Swingedouw D, Rodehacke C, Olsen S, Menary M, Gao Y, Mikolajewicz U, Mignot J (2015) On the reduced sensitivity of the Atlantic overturning to Greenland ice sheet melting in projections: a multi-model assessment. Clim Dyn 44(11–12):3261–3279. doi: 10.1007/s00382-014-2270-x CrossRefGoogle Scholar
  42. Thorpe RB, Gregory JM, Johns TC, Wood RA, Mitchell JFB (2001) Mechanisms determining the Atlantic thermohaline circulation response to Greenhouse gas forcing in a non-flux-adjusted coupled climate model. J Clim 14:3102–3116CrossRefGoogle Scholar
  43. Valdes P (2011) Built for stability. Nat Geosci 4(7):414–416. doi: 10.1038/ngeo1200 CrossRefGoogle Scholar
  44. Vallis GK (2006) Atmospheric and oceanic fluid dynamics. Cambridge University Press, CambridgeCrossRefGoogle Scholar
  45. Vellinga M, Wu P (2004) Low latitude freshwater influence on centennial variability of the Atlantic Thermohaline Circulation. J Clim 17(23):4498–4511CrossRefGoogle Scholar
  46. Weijer W, Maltrud ME, Hecht MW, Dijkstra HA, Kliphuis MA (2012) Response of the Atlantic Ocean circulation to Greenland Ice Sheet melting in a strongly-eddying ocean model. Geophys Res Lett 39(9):L09606. doi: 10.1029/2012gl051611 CrossRefGoogle Scholar
  47. Williams KD, Harris CM, Bodas-Salcedo A, Camp J, Comer RE, Copsey D, Fereday D, Graham T, Hill R, Hinton T, Hyder P, Ineson S, Masato G, Milton SF, Roberts MJ, Rowell DP, Sanchez C, Shelly A, Sinha B, Walters DN, West A, Woollings T, Xavier PK (2015) The met office global coupled model 2.0 (gc2) configuration. Geosci Model Dev Discuss 8(1):521–565. doi: 10.5194/gmdd-8-521-2015 CrossRefGoogle Scholar

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Authors and Affiliations

  1. 1.Met Office Hadley CentreExeterUK

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