A 2000-year integration of a flux-adjusted GFDL coupled ocean-atmosphere model exhibits internally generated 40-80-year oscillations of Greenland Sea temperature and salinity, tied to Arctic freshwater anomalies that propagate through the East Greenland Current around the subpolar gyre and are coherent with multidecadal fluctuations of the North Atlantic thermohaline circulation (THC), which lag the Greenland Sea anomalies by roughly ten years. Whether this oscillation reflects a real mechanism of the climate system remains unresolved: what generates it, what sets its period and its slower amplitude modulation, whether Arctic freshwater export drives the THC or vice versa, whether the atmosphere forces or merely responds to the ocean, and how the modeled signal maps onto observed and proxy records. Later work answered the model-internal questions for this specific GFDL model (surface-flux forcing associated with atmospheric variability drives its THC fluctuations) and showed multidecadal Atlantic variability exists in modern models without flux adjustment; however, the mechanism, time-scale control, and attribution of the observed Atlantic Multidecadal Variability - internal ocean-ice dynamics including the Arctic freshwater pathway, stochastic atmospheric forcing, or external aerosol and volcanic forcing - remain actively contested, as a 2012-2023 literature including published disagreements in Science shows. The problem asks for a quantitatively supported, cross-model, observationally validated mechanistic account of the multidecadal oscillation and its 40-80-year time scale.

Background

General circulation models (GCMs) that couple an atmospheric model to an ocean and sea-ice model can generate climate variability internally, without any change in external forcing such as solar input or volcanic aerosols; a 'control integration' is such a run with constant forcing. Because early coupled models drifted toward unrealistic climates, the generation used in the studies below applied 'flux adjustments' - spatially and seasonally varying, but time-constant, corrections to the heat and freshwater exchanged between atmosphere and ocean, computed from preliminary uncoupled runs and held fixed during the coupled integration. The North Atlantic thermohaline circulation (THC; in modern usage the Atlantic meridional overturning circulation, AMOC) is the basin-scale circulation by which warm, salty surface water flows northward, cools and densifies, sinks in the Labrador and Nordic Seas, and returns southward at depth; fluctuations in its strength redistribute heat and are a leading candidate source of low-frequency climate variability. In a 2000-year control integration of the flux-adjusted Geophysical Fluid Dynamics Laboratory (GFDL) coupled model, Delworth, Manabe and Stouffer (1993) had found irregular but distinct multidecadal fluctuations of the THC with a time scale of roughly 50 years.

Delworth, Manabe and Stouffer (1997) re-examined that integration from the perspective of the Greenland Sea, using a Denmark Strait index (annual-mean sea surface temperature, SST, averaged over the band 15°W-26°W at 70°N). They found pronounced oscillations of SST and sea surface salinity (SSS) with periods of roughly 40-80 years, peak-to-peak SST amplitudes exceeding 3 °C, and an SST-SSS correlation of 0.92. Lagged regression analysis showed that these anomalies are preceded, by about 15 years, by near-surface salinity anomalies in the Arctic, which then propagate through the East Greenland Current (EGC - the boundary current carrying polar water and sea ice southward along eastern Greenland) and around the subpolar gyre (the cyclonic wind-driven circulation between roughly 50°N and 65°N) into the Labrador Sea and central North Atlantic, with an anomalously strong EGC during cold, fresh phases. The Greenland Sea oscillation is coherent with the THC fluctuations (squared coherency - a frequency-domain measure of the linear association between two time series - reaching 0.50 near a 57-year period), with the THC lagging the Greenland Sea SST/SSS by about ten years, and the modeled THC is weakest when salinity in its sinking region (50°N-70°N) is lowest. The proposed mechanism is 'fresh-water capping': a fresh surface layer inhibits open-ocean deep convection, trapping heat below and amplifying surface anomalies while reducing the buoyancy supply that drives the overturning. Spectral analysis against a red-noise null (a null hypothesis of a memory-laden random process), wavelet analysis, and singular spectrum analysis (SSA - a data-adaptive decomposition of a time series into oscillatory components) showed the spectral peak to be significant within this single realization, but its amplitude to be modulated on centennial time scales. Associated surface-pressure anomalies lead the SST by about two years and resemble the pressure pattern that preceded the observed 'Great Salinity Anomaly' of the 1970s - a well-documented low-salinity episode that propagated around the subpolar gyre.

The model was crude by modern standards - a T30, 9-level atmosphere (spectral truncation of roughly 2100 km horizontal resolution) over a 3.75-degree-grid, 12-level ocean with simplified convection and sea ice - and the analysis rested on a single realization, so the causal structure remained open. This question-set seeded what is now called Atlantic Multidecadal Variability (AMV; also the Atlantic Multidecadal Oscillation, AMO) research. Later work on the same GFDL model (Delworth and Greatbatch 2000) showed that its multidecadal THC fluctuations can be driven by surface heat and freshwater flux anomalies associated with the North Atlantic Oscillation (NAO - the leading mode of sea-level-pressure variability over the North Atlantic), i.e. the atmosphere acting on the ocean rather than only responding to it, and multidecadal overturning variability also arises in modern coupled models without flux adjustment (Jungclaus et al. 2005). Yet the central issues remain actively contested: whether the observed roughly 40-80-year Atlantic variability is generated by ocean-ice dynamics (with an Arctic freshwater pathway as in the 1997 model), by the ocean's integration of stochastic (effectively random, red-noise-like) atmospheric weather forcing, or by external aerosol and volcanic forcing - positions argued, respectively and variously, by Zhang et al. (2019), Clement et al. (2015), and Mann, Steinman and Miller (2020).

Problem Statement

The problem, as posed by the open issues of Delworth, Manabe and Stouffer (1997), is to determine the physical mechanism of the roughly 40-80-year multidecadal climate oscillation of the Greenland Sea and North Atlantic found in coupled ocean-atmosphere models, and whether that mechanism operates in the real climate system. The objective is to establish, with quantitatively supported evidence: (i) the direction of causality among Arctic near-surface salinity and freshwater export, East Greenland Current transport, Greenland Sea and Labrador Sea SST/SSS and convection, and the strength of the Atlantic overturning circulation - specifically, whether Arctic freshwater export and an intensified East Greenland Current drive multidecadal reductions in the overturning (for example through fresh-water capping of convection), whether internal overturning variations generate the Greenland Sea anomalies, or whether both are driven by atmospheric surface-flux variability; (ii) which physical processes set the oscillation's 40-80-year time scale and its slower, century-scale amplitude modulation - advective transit times, sea-ice and freshwater feedbacks, convective adjustment, ocean memory, or the statistics of atmospheric forcing; (iii) how sensitive the oscillation's existence, amplitude, and phase relationships are to model configuration, including spatial resolution, physical parameterizations, and the use of flux adjustments; (iv) whether the atmosphere acts as an active forcing of, or primarily a response to, the oceanic anomalies; and (v) whether the modeled mechanism, timing, and amplitude correspond to the observed and proxy-reconstructed Atlantic multidecadal variability. These five facets are jointly posed and share one scientific objective - a validated mechanistic account of the multidecadal oscillation and its time scale.

References

  1. Delworth, Thomas L., Syukuro Manabe, and Ronald J. Stouffer (1997). Multidecadal climate variability in the Greenland Sea and surrounding regions: A coupled model simulation. Geophysical Research Letters 24(3), 257-260. DOI: 10.1029/96gl03927. https://doi.org/10.1029/96gl03927
  2. Delworth, T., S. Manabe, and R. J. Stouffer (1993). Interdecadal variations of the thermohaline circulation in a coupled ocean-atmosphere model. Journal of Climate 6(11), 1993-2011. DOI: 10.1175/1520-0442(1993)006<1993:IVOTTC>2.0.CO;2. https://doi.org/10.1175/1520-0442(1993)006%3C1993:IVOTTC%3E2.0.CO;2
  3. Weaver, Andrew J., and Sophie Valcke (1998). On the variability of the thermohaline circulation in the GFDL coupled model. Journal of Climate 11(4), 759-767. DOI: 10.1175/1520-0442(1998)011<0759:OTVOTT>2.0.CO;2. https://doi.org/10.1175/1520-0442(1998)011%3C0759:OTVOTT%3E2.0.CO;2
  4. Fanning, Augustus F., and Andrew J. Weaver (1998). Thermohaline variability: The effects of horizontal resolution and diffusion. Journal of Climate 11(4), 709-715. DOI: 10.1175/1520-0442(1998)011<0709:TVTEOH>2.0.CO;2. https://doi.org/10.1175/1520-0442(1998)011%3C0709:TVTEOH%3E2.0.CO;2
  5. Belkin, Igor M., Sydney Levitus, John Antonov, and Svend-Aage Malmberg (1998). 'Great Salinity Anomalies' in the North Atlantic. Progress in Oceanography 41(1), 1-68. DOI: 10.1016/S0079-6611(98)00015-9. https://doi.org/10.1016/S0079-6611(98)00015-9
  6. Delworth, Thomas L., and Richard J. Greatbatch (2000). Multidecadal thermohaline circulation variability driven by atmospheric surface flux forcing. Journal of Climate 13(9), 1481-1495. DOI: 10.1175/1520-0442(2000)013<1481:MTCVDB>2.0.CO;2. https://doi.org/10.1175/1520-0442(2000)013%3C1481:MTCVDB%3E2.0.CO;2
  7. Delworth, T. L., and M. E. Mann (2000). Observed and simulated multidecadal variability in the Northern Hemisphere. Climate Dynamics 16(9), 661-676. DOI: 10.1007/s003820000075. https://doi.org/10.1007/s003820000075
  8. Holland, Marika M., Cecilia M. Bitz, Michael Eby, and Andrew J. Weaver (2001). The role of ice-ocean interactions in the variability of the North Atlantic thermohaline circulation. Journal of Climate 14(5), 656-675. DOI: 10.1175/1520-0442(2001)014<0656:TROIOI>2.0.CO;2. https://doi.org/10.1175/1520-0442(2001)014%3C0656:TROIOI%3E2.0.CO;2
  9. Jungclaus, Johann H., Helmuth Haak, Mojib Latif, and Uwe Mikolajewicz (2005). Arctic-North Atlantic interactions and multidecadal variability of the meridional overturning circulation. Journal of Climate 18(19), 4013-4031. DOI: 10.1175/JCLI3462.1. https://doi.org/10.1175/JCLI3462.1
  10. Polyakov, I. V., U. S. Bhatt, H. L. Simmons, D. Walsh, J. E. Walsh, and X. Zhang (2005). Multidecadal variability of North Atlantic temperature and salinity during the twentieth century. Journal of Climate 18(21), 4562-4581. DOI: 10.1175/JCLI3548.1. https://doi.org/10.1175/JCLI3548.1
  11. Frankcombe, Leela M., Anna von der Heydt, and Henk A. Dijkstra (2010). North Atlantic multidecadal climate variability: An investigation of dominant time scales and processes. Journal of Climate 23(13), 3626-3638. DOI: 10.1175/2010JCLI3471.1. https://doi.org/10.1175/2010JCLI3471.1
  12. Medhaug, I., and T. Furevik (2011). North Atlantic 20th century multidecadal variability in coupled climate models: sea surface temperature and ocean overturning circulation. Ocean Science 7(3), 389-404. DOI: 10.5194/os-7-389-2011. https://doi.org/10.5194/os-7-389-2011
  13. Booth, Ben B. B., Nick J. Dunstone, Paul R. Halloran, Timothy Andrews, and Nicolas Bellouin (2012). Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature 484(7393), 228-232. DOI: 10.1038/nature10946. https://doi.org/10.1038/nature10946
  14. Clement, Amy, Katinka Bellomo, Lisa N. Murphy, Mark A. Cane, Thorsten Mauritsen, Gaby Rädel, and Bjorn Stevens (2015). The Atlantic Multidecadal Oscillation without a role for ocean circulation. Science 350(6258), 320-324. DOI: 10.1126/science.aab3980. https://doi.org/10.1126/science.aab3980
  15. Zhang, Rong, Rowan Sutton, Gokhan Danabasoglu, Thomas L. Delworth, Who M. Kim, Jon Robson, and Stephen G. Yeager (2016). Comment on 'The Atlantic Multidecadal Oscillation without a role for ocean circulation'. Science 352(6293), 1527. DOI: 10.1126/science.aaf1660. https://doi.org/10.1126/science.aaf1660
  16. Clement, Amy, Mark A. Cane, Lisa N. Murphy, Katinka Bellomo, Thorsten Mauritsen, and Bjorn Stevens (2016). Response to Comment on 'The Atlantic Multidecadal Oscillation without a role for ocean circulation'. Science 352(6293), 1527. DOI: 10.1126/science.aaf2575. https://doi.org/10.1126/science.aaf2575
  17. Buckley, Martha W., and John Marshall (2016). Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review. Reviews of Geophysics 54(1), 5-63. DOI: 10.1002/2015RG000493. https://doi.org/10.1002/2015RG000493
  18. Zhang, Rong, Rowan Sutton, Gokhan Danabasoglu, Young-Oh Kwon, Robert Marsh, Stephen G. Yeager, Daniel E. Amrhein, and Christopher M. Little (2019). A review of the role of the Atlantic Meridional Overturning Circulation in Atlantic Multidecadal Variability and associated climate impacts. Reviews of Geophysics 57(2), 316-375. DOI: 10.1029/2019RG000644. https://doi.org/10.1029/2019RG000644
  19. Mann, Michael E., Byron A. Steinman, and Sonya K. Miller (2020). Absence of internal multidecadal and interdecadal oscillations in climate model simulations. Nature Communications 11, article 49. DOI: 10.1038/s41467-019-13823-w. https://doi.org/10.1038/s41467-019-13823-w
  20. Wei, X., and R. Zhang (2022). A simple conceptual model for the self-sustained multidecadal AMOC variability. Geophysical Research Letters 49(14), e2022GL099800. DOI: 10.1029/2022GL099800. https://doi.org/10.1029/2022GL099800
  21. Liu, F., J. Lu, Y.-O. Kwon, C. Frankignoul, and Y. Luo (2022). Freshwater flux variability lengthens the period of the low-frequency AMOC variability. Geophysical Research Letters 49(20), e2022GL100136. DOI: 10.1029/2022GL100136. https://doi.org/10.1029/2022GL100136
  22. Liu, Zhengyu, Peng Gu, and Thomas L. Delworth (2023). Strong red noise ocean forcing on Atlantic multidecadal variability assessed from surface heat flux: Theory and application. Journal of Climate 36(1), 55-80. DOI: 10.1175/JCLI-D-22-0063.1. https://doi.org/10.1175/JCLI-D-22-0063.1