Advertisement

Climate Dynamics

, Volume 29, Issue 7–8, pp 821–836 | Cite as

Reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria

  • Helen L. JohnsonEmail author
  • David P. Marshall
  • David A. J. Sproson
Article

Abstract

It has recently been suggested that the structure and strength of the meridional overturning circulation in the global ocean is governed by the input of mechanical energy to the system by winds and tides. However, it is not clear how this suggestion relates to the existence of multiple equilibria of the meridional overturning circulation, which depends on thermohaline feedbacks and is more consistent with a buoyancy-driven view of the circulation. Both theories have been illustrated by box models in the past (Stommel in Tellus 13:224–230, 1961; Gnanadesikan in Science 283:2077–2079, 1999). Here we incorporate these two theories into a single box model in an attempt to reconcile the roles of mechanical and buoyancy forcing in driving the meridional overturning circulation. The box model has two equilibrium solutions, one with sinking at high northern latitudes as in the present-day Atlantic, and one without. The circulation is mechanically driven, but the northern sinking can be thought of as a release valve which acts as a sink of potential energy when the surface water at high northern latitudes is dense enough to convect. While the source of energy comes from mechanical forcing, the presence or otherwise of multiple equilibria is therefore determined by thermohaline feedbacks. In some areas of parameter space an oscillation between the model’s two circulation regimes occurs, reminiscent of a bipolar seesaw.

Keywords

Southern Ocean Equilibrium Solution Deep Ocean Volume Flux Thermocline Depth 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

HLJ is funded under a Royal Society University Research Fellowship. DPM acknowledges support from the RAPID programme of the UK Natural Environment Research Council.

References

  1. Broecker WS (1997) Thermohaline circulation, the Achilles heel of our climate system: will man-made CO2 upset the current balance? Science 278:1582–1588CrossRefGoogle Scholar
  2. Danabasoglu G, McWilliams JC, Gent PR (1994) The role of mesoscale tracer transports in the global ocean circulation. Science 264:1123–1126CrossRefGoogle Scholar
  3. Dansgaard W, Johnsen SJ, Clausen HB, Dahl-Jensen D, Gundestrup NS, Hammer CU, Hvidberg CS, Steffensen JP, Sveinbjrnsdottir AE, Jouzel J, Bond G (1993) Evidence for general instability of past climate from a 250-kyr ice-core record. Nature 364:218–220CrossRefGoogle Scholar
  4. Dijkstra HA, Ghil M (2005) Low frequency variability of the large-scale ocean circulation: a dynamical system approach. Rev Geophys 43:RG3002, doi:10.1029/2002RG000122CrossRefGoogle Scholar
  5. Fanning AF, Weaver AJ (1997) Temporal-geographical meltwater influences on the North Atlantic conveyor: implications for the Younger Dryas. Paleoceanography 12:307–320CrossRefGoogle Scholar
  6. Gent PR, McWilliams JC (1990) Isopycnal mixing in ocean circulation models. J Phys Oceanogr 20:150–155CrossRefGoogle Scholar
  7. Gnanadesikan A (1999) A simple predictive model for the structure of the oceanic pycnocline. Science 283:2077–2079CrossRefGoogle Scholar
  8. Gnanadesikan A, Slater RD, Samuels BL (2003) Sensitivity of water mass transformation and heat transport to subgridscale mixing in coarse-resolution ocean models. Geophys Res Lett 30:1967 doi:10.1029/2003GL018036CrossRefGoogle Scholar
  9. Gnanadesikan A, Slater RD, Swathi PS, Vallis GK (2005) The energetics of ocean heat transport. J Clim 18:2604–2616CrossRefGoogle Scholar
  10. Johnson HL, Marshall DP (2002) A theory for the surface Atlantic response to thermohaline variability. J Phys Oceanogr 32:1121–1132CrossRefGoogle Scholar
  11. Klinger BA, Drijfhout S, Marotzke J, Scott JR (2003) Sensitivity of basinwide meridional overturning to diapycnal diffusion and remote wind forcing in an idealized Atlantic-Southern Ocean geometry. J Phys Oceanogr 33:249–266CrossRefGoogle Scholar
  12. Kuhlbrodt T, Griesel A, Montoya M, Levermann A, Hofmann M, Rahmstorf S (2006). On the driving processes of the Atlantic meridional overturning circulation. Rev Geophys (in press)Google Scholar
  13. Manabe S, Stouffer RJ (1988) Two stable equilibria of a coupled ocean-atmosphere model. J Clim 1:841–866CrossRefGoogle Scholar
  14. Manabe S, Stouffer RJ (1999) Are two modes of thermohaline circulation stable? Tellus 51:400–411CrossRefGoogle Scholar
  15. Marotzke J (1994) Ocean models in climate problems. In: Malanotte-Rizzoli P, Robinson AR (eds) Ocean processes in climate dynamics: global and mediterranean examples. Kluwer, Dordrecht, pp 79–109Google Scholar
  16. Marotzke J (1997) Boundary mixing and the dynamics of three-dimensional thermohaline circulations. J Phys Oceanogr 27:1713–1728CrossRefGoogle Scholar
  17. Marotzke J, Scott JR (1999) Convective mixing and the thermohaline circulation. J Phys Oceanogr 29:2962–2970CrossRefGoogle Scholar
  18. Marotzke J, Willebrand J (1991) Multiple equilibria of the global thermohaline circulation. J Phys Oceanogr 21:1372–1385CrossRefGoogle Scholar
  19. Marshall DP (2007) Hydrographic variations along the sloping western and eastern boundaries of the oceans (in preparation)Google Scholar
  20. Marshall DP, Naveira-Garabato AC (2007) A conjecture on the role of bottom-enhanced diapycnal mixing in the parameterization of geostrophic eddies. J Phys Oceanogr (submitted)Google Scholar
  21. Marzeion B, Drange H (2006) Diapycnal mixing in a conceptual model of the Atlantic meridional overturning circulation. Deep Sea Res II 53:226–238CrossRefGoogle Scholar
  22. Marzeion B, Levermann A, Mignot J (2007) The role of stratification-dependent mixing for the stability of the Atlantic overturning in a global climate model J Phys Oceanogr (in press)Google Scholar
  23. Mignone BK, Gnanadesikan A, Sarmiento JL, Slater RD (2006) Central role of Southern Hemisphere winds and eddies in modulating the oceanic uptake of anthropogenic carbon. Geophys Res Lett 33:L01604 doi:10.1029/2005GL024464CrossRefGoogle Scholar
  24. Molemaker MJ, McWilliams J (2005) Baroclinic instability and loss of balance. J Phys Oceanogr 35:1505–1517CrossRefGoogle Scholar
  25. Munk W, Wunsch C (1998) Abyssal recipes II: energetics of tidal and wind mixing. Deep Sea Res I 45:1977–2010CrossRefGoogle Scholar
  26. Nilsson J, Walin G (2001) Freshwater forcing as a booster of the thermohaline circulation. Tellus A 53:629–641CrossRefGoogle Scholar
  27. Nilsson J, Brostrom G, Walin G (2003) The thermohaline circulation and vertical mixing: does weaker density stratification give stronger overturning? J Phys Oceanogr 33:2781–2795CrossRefGoogle Scholar
  28. Park YG (1999) The stability of thermohaline circulation in a two-box model. J Phys Oceanogr 29:3101–3110CrossRefGoogle Scholar
  29. Pasquero C, Tziperman E (2004) Effects of a wind-driven gyre on thermohaline circulation variability. J Phys Oceanogr 34:805–816CrossRefGoogle Scholar
  30. Philander SG, Fedorov AV (2003) Role of tropics in changing the response to Milankovich forcing some three million years ago. Paleoceanography 18:1045, doi:10.1029/2002PA000837CrossRefGoogle Scholar
  31. Rooth C (1982) Hydrology and ocean circulation. Prog Oceanogr 11:131–149CrossRefGoogle Scholar
  32. Ruddick B, Zhang L (1996) Qualitative behaviour and non-oscillation of Stommel’s thermohaline box model. J Clim 9:2768–2777CrossRefGoogle Scholar
  33. Samelson RM (2004) Simple mechanistic models of middepth meridional overturning. J Phys Oceanogr 34:2096–2103CrossRefGoogle Scholar
  34. Sandström JW (1916) Meteorologische Studien im schwedischen Hochebirge. Goteborgs K. Vetensk. Vitterhets-Samh Handl., Series 4 22(2):48Google Scholar
  35. Schmittner A, Weaver AJ (2001) Dependence of multiple climate states on ocean mixing parameters. Geophys Res Lett 28:1027–1030CrossRefGoogle Scholar
  36. Stocker TF (2000) Past and future reorganizations in the climate system. Quat Sci Rev 19:301–319CrossRefGoogle Scholar
  37. Stocker TF, Wright DG, Mysak LA (1992) A zonally averaged, coupled ocean-atmosphere model for paleoclimate studies. J Clim 5:773–797CrossRefGoogle Scholar
  38. Stommel HM (1961) Thermohaline convection with two stable regimes of flow. Tellus 13:224–230CrossRefGoogle Scholar
  39. Stommel H, Rooth C (1968) On the interaction of gravitational and dynamic forcing in simple circulation models. Deep Sea Res 15:165–170Google Scholar
  40. Toggweiler JR, Samuels B (1995) Effect of Drake passage on the global thermohaline circulation. Deep Sea Res 42:477–500CrossRefGoogle Scholar
  41. Whitehead JA (1995) Thermohaline ocean processes and models. Annu Rev Fluid Mech 27:89–113CrossRefGoogle Scholar
  42. Wunsch C (1998) The work done by the wind on the oceanic general circulation. J Phys Oceanogr 28:2332–2340CrossRefGoogle Scholar
  43. Wunsch C (2005) Thermohaline loops, Stommel box models, and the Sandström theorem. Tellus 57A:84–99Google Scholar
  44. Wunsch C, Ferrari R (2004) Vertical mixing, energy, and the general circulation of the oceans. Annu Rev Fluid Mech 36:281–314CrossRefGoogle Scholar
  45. Yuan S, Wunsch C (2005) Stress-driven thermohaline loops. Phys Fluids 17:066601CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Helen L. Johnson
    • 1
    Email author
  • David P. Marshall
    • 2
  • David A. J. Sproson
    • 3
  1. 1.Department of MeteorologyUniversity of ReadingReadingUK
  2. 2.Atmospheric, Oceanic and Planetary Physics, Department of PhysicsUniversity of Oxford OxfordUK
  3. 3.School of Environmental SciencesUniversity of East AngliaNorwichUK

Personalised recommendations