• Abernathey, R., and J. C. Marshall, 2013: Global surface eddy diffusivities derived from satellite altimetry. J. Geophys. Res. Oceans, 118, 901916, https://doi.org/10.1002/jgrc.20066.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Bates, M., R. Tulloch, J. Marshall, and R. Ferrari, 2014: Rationalizing the spatial distribution of mesoscale eddy diffusivity in terms of mixing length theory. J. Phys. Oceanogr., 44, 15231540, https://doi.org/10.1175/JPO-D-13-0130.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Bellucci, A., A. Mariotti, and S. Gualdi, 2017: The role of forcings in the twentieth-century North Atlantic multidecadal variability: The 1940–75 North Atlantic cooling case study. J. Climate, 30, 73177337, https://doi.org/10.1175/JCLI-D-16-0301.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Bentsen, M., H. Drange, T. Furevik, and T. Zhou, 2004: Simulated variability of the Atlantic meridional overturning circulation. Climate Dyn., 22, 701720, https://doi.org/10.1007/s00382-004-0397-x.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Biastoch, A., C. W. Böning, J. Getzlaff, J.-M. Molines, and G. Madec, 2008: Causes of interannual–decadal variability in the meridional overturning circulation of the mid-latutude North Atlantic Ocean. J. Climate, 21, 65996615, https://doi.org/10.1175/2008JCLI2404.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Booth, B. B., N. J. Dunstone, P. R. Halloran, T. Andrews, and N. Bellouin, 2012: Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature, 484, 228232, https://doi.org/10.1038/nature10946.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Briegleb, B. P., G. Danabasoglu, and W. G. Large, 2010: An overflow parameterization for the ocean component of the Community Climate System Model. NCAR Tech. Note NCAR/TN-481+STR, 72 pp.

  • Bryan, F. O., G. Danabasoglu, N. Nakashiki, Y. Yoshida, D. H. Kim, J. Tsutsui, and S. C. Doney, 2006: Response of North Atlantic thermohaline circulation and ventilation to increasing carbon dioxide in CCSM3. J. Climate, 19, 23822397, https://doi.org/10.1175/JCLI3757.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Buckley, M. W., and J. Marshall, 2016: Observations, inferences, and mechanisms of Atlantic meridional overturning circulation variability: A review. Rev. Geophys., 54, 563, https://doi.org/10.1002/2015RG000493.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Cane, M. A., A. C. Clement, L. N. Murphy, and K. Bellomo, 2017: Low-pass filtering, heat flux, and Atlantic multidecadal variability. J. Climate, 30, 75297553, https://doi.org/10.1175/JCLI-D-16-0810.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Clement, A., K. Bellomo, L. N. Murphy, M. A. Cane, T. Mauritsen, G. Rädel, and B. Stevens, 2015: The Atlantic Multidecadal Oscillation without a role for ocean circulation. Science, 350, 320324, https://doi.org/10.1126/science.aab3980.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Cole, S. T., C. Wortham, E. Kunze, and W. B. Owens, 2015: Eddy stirring and horizontal diffusivity from Argo float observations: Geographic and depth variability. Geophys. Res. Lett., 42, 39893997, https://doi.org/10.1002/2015GL063827.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., 2008: On multidecadal variability of the Atlantic meridional overturning circulation in the Community Climate System Model version 3. J. Climate, 21, 55245544, https://doi.org/10.1175/2008JCLI2019.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., and J. C. McWilliams, 1995: Sensitivity of the global ocean circulation to parameterizations of mesoscale tracer transports. J. Climate, 8, 29672987, https://doi.org/10.1175/1520-0442(1995)008<2967:SOTGOC>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., and J. Marshall, 2007: Effects of vertical variations of thickness diffusivity in an ocean general circulation model. Ocean Modell., 18, 122141, https://doi.org/10.1016/j.ocemod.2007.03.006.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., W. G. Large, J. J. Tribbia, P. R. Gent, B. P. Briegleb, and J. C. McWilliams, 2006: Diurnal coupling in the tropical oceans of CCSM3. J. Climate, 19, 23472365, https://doi.org/10.1175/JCLI3739.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., R. Ferrari, and J. C. McWilliams, 2008: Sensitivity of an ocean general circulation model to a parameterization of near-surface eddy fluxes. J. Climate, 21, 11921208, https://doi.org/10.1175/2007JCLI1508.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., W. G. Large, and B. P. Briegleb, 2010: Climate impacts of parameterized Nordic Sea overflows. J. Geophys. Res., 115, C11005, https://doi.org/10.1029/2010JC006243.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., S. C. Bates, B. P. Briegleb, S. R. Jayne, M. Jochum, W. G. Large, S. Peacock, and S. G. Yeager, 2012a: The CCSM4 ocean component. J. Climate, 25, 13611389, https://doi.org/10.1175/JCLI-D-11-00091.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., S. G. Yeager, Y.-O. Kwon, J. J. Tribbia, A. S. Phillips, and J. W. Hurrell, 2012b: Variability of the Atlantic meridional overturning circulation in CCSM4. J. Climate, 25, 51535172, https://doi.org/10.1175/JCLI-D-11-00463.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., and et al. , 2014: North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states. Ocean Modell., 73, 76107, https://doi.org/10.1016/j.ocemod.2013.10.005.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., and et al. , 2016: North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part II: Inter-annual to decadal variability. Ocean Modell., 97, 6590, https://doi.org/10.1016/j.ocemod.2015.11.007.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Delworth, T. L., and M. E. Mann, 2000: Observed and simulated multidecadal variability in the Northern Hemisphere. Climate Dyn., 16, 661676, https://doi.org/10.1007/s003820000075.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Delworth, T. L., and F. Zeng, 2016: The impact of the North Atlantic Oscillation on climate through its influence on the Atlantic meridional overturning circulation. J. Climate, 29, 941962, https://doi.org/10.1175/JCLI-D-15-0396.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Duchon, C. E., 1979: Lanczos filtering in one and two dimensions. J. Appl. Meteor., 18, 10161022, https://doi.org/10.1175/1520-0450(1979)018<1016:LFIOAT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Eden, C., and J. Willebrand, 2001: Mechanism of interannual to decadal variability of the North Atlantic circulation. J. Climate, 14, 22662280, https://doi.org/10.1175/1520-0442(2001)014<2266:MOITDV>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Enfield, D. B., A. M. Mestas-Nunez, and P. J. Trimble, 2001: The Atlantic multidecadal oscillation and its relationship to rainfall and river flows in the continental U.S. Geophys. Res. Lett., 28, 20772080, https://doi.org/10.1029/2000GL012745.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Eyring, V., S. Bony, G. A. Meehl, C. A. Senior, B. Stevens, R. J. Stouffer, and K. E. Taylor, 2016: Overview of the Coupled Model Intercomparison Project phase 6 (CMIP6) experimental design and organization. Geosci. Model Dev., 9, 19371958, https://doi.org/10.5194/gmd-9-1937-2016.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ferrari, R., J. C. McWilliams, V. M. Canuto, and M. Dubovikov, 2008: Parameterization of eddy fluxes near oceanic boundaries. J. Climate, 21, 27702789, https://doi.org/10.1175/2007JCLI1510.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ferreira, D., J. Marshall, and P. Heimbach, 2005: Estimating eddy stresses by fitting dynamics to observations using a residual-mean ocean circulation model and its adjoint. J. Phys. Oceanogr., 35, 18911910, https://doi.org/10.1175/JPO2785.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Fox-Kemper, B., R. Ferrari, and R. Hallberg, 2008: Parameterization of mixed layer eddies. Part I: Theory and diagnosis. J. Phys. Oceanogr., 38, 11451165, https://doi.org/10.1175/2007JPO3792.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Fox-Kemper, B., and et al. , 2011: Parameterization of mixed layer eddies. Part III: Implementation and impact in global ocean climate simulations. Ocean Modell., 39, 6178, https://doi.org/10.1016/j.ocemod.2010.09.002.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Frajka-Williams, E., and et al. , 2019: Atlantic meridional overturning circulation: Observed transport and variability. Front. Mar. Sci., 6, 260, https://doi.org/10.3389/FMARS.2019.00260.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gent, P. R., and J. C. McWilliams, 1990: Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr., 20, 150155, https://doi.org/10.1175/1520-0485(1990)020<0150:IMIOCM>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gent, P. R., and et al. , 2011: The Community Climate System Model version 4. J. Climate, 24, 49734991, https://doi.org/10.1175/2011JCLI4083.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hawkins, E., and R. Sutton, 2008: Potential predictability of rapid changes in the Atlantic meridional overturning circulation. Geophys. Res. Lett., 35, L11603, https://doi.org/10.1029/2008GL034059.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Holland, M. M., D. A. Bailey, B. P. Briegleb, B. Light, and E. Hunke, 2012: Improved sea ice shortwave radiation physics in CCSM4: The impact of melt ponds and aerosols on Arctic sea ice. J. Climate, 25, 14131430, https://doi.org/10.1175/JCLI-D-11-00078.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hurrell, J. W., and et al. , 2013: The Community Earth System Model: A framework for collaborative research. Bull. Amer. Meteor. Soc., 94, 13391360, https://doi.org/10.1175/BAMS-D-12-00121.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Johns, W. E., and et al. , 2011: Continuous, array-based estimates of Atlantic Ocean heat transport at 26.5°N. J. Climate, 24, 24292449, https://doi.org/10.1175/2010JCLI3997.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Jungclaus, J. H., H. Haak, M. Latif, and U. Mikolajewicz, 2005: Arctic–North Atlantic interactions and multidecadal variability of the meridional overturning circulation. J. Climate, 18, 40134031, https://doi.org/10.1175/JCLI3462.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Karspeck, A. R., and et al. , 2017: Comparison of the Atlantic meridional overturning circulation between 1960 and 2007 in six ocean reanalysis products. Climate Dyn., 49, 957982, https://doi.org/10.1007/s00382-015-2787-7.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kay, J. E., and et al. , 2015: The Community Earth System Model (CESM) Large Ensemble project: A community resource for studying climate change in the presence of internal climate variability. Bull. Amer. Meteor. Soc., 96, 13331349, https://doi.org/10.1175/BAMS-D-13-00255.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Klavans, J. M., A. C. Clement, and M. A. Cane, 2019: Variable external forcing obscures the weak relationship between the NAO and North Atlantic multi-decadal SST variability. J. Climate, 32, 38473864, https://doi.org/10.1175/JCLI-D-18-0409.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kwon, Y.-O., and C. Frankignoul, 2012: Stochastically-driven multidecadal variability of the Atlantic meridional overturning circulation in CCSM3. Climate Dyn., 38, 859876, https://doi.org/10.1007/s00382-011-1040-2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kwon, Y.-O., and C. Frankignoul, 2014: Mechanisms of multidecadal Atlantic meridional overturning circulation variability diagnosed in depth versus density space. J. Climate, 27, 93599376, https://doi.org/10.1175/JCLI-D-14-00228.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Large, W. G., J. C. McWilliams, and S. C. Doney, 1994: Oceanic vertical mixing: A review and a model with a nonlocal boundary layer parameterization. Rev. Geophys., 32, 363403, https://doi.org/10.1029/94RG01872.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Large, W. G., G. Danabasoglu, J. C. McWilliams, P. R. Gent, and F. O. Bryan, 2001: Equatorial circulation in a global ocean climate model with anisotropic horizontal viscosity. J. Phys. Oceanogr., 31, 518536, https://doi.org/10.1175/1520-0485(2001)031<0518:ECOAGO>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lawrence, D. M., K. W. Oleson, M. G. Flanner, C. G. Fletcher, P. J. Lawrence, S. Levis, S. C. Swenson, and G. B. Bonan, 2012: The CCSM4 land simulation, 1850–2005: Assessment of surface climate and new capabilities. J. Climate, 25, 22402260, https://doi.org/10.1175/JCLI-D-11-00103.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, F., M. S. Lozier, G. Danabasoglu, N. P. Holliday, Y.-O. Kwon, A. Romanou, S. G. Yeager, and R. Zhang, 2019: Local and downstream relationships between Labrador Sea Water volume and North Atlantic meridional overturning circulation variability. J. Climate, 32, 38833898, https://doi.org/10.1175/JCLI-D-18-0735.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Liu, Z., 2012: Dynamics of interdecadal climate variability: A historical perspective. J. Climate, 25, 19631995, https://doi.org/10.1175/2011JCLI3980.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lozier, M. S., and et al. , 2019: A sea change in our view of overturning in the subpolar North Atlantic. Science, 363, 516521, https://doi.org/10.1126/science.aau6592.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • MacKinnon, J. A., and et al. , 2017: Climate process team on internal wave-driven ocean mixing. Bull. Amer. Meteor. Soc., 98, 24292454, https://doi.org/10.1175/BAMS-D-16-0030.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Msadek, R., and C. Frankignoul, 2009: Atlantic multidecadal oceanic variability and its influence on the atmosphere in a climate model. Climate Dyn., 33, 4562, https://doi.org/10.1007/s00382-008-0452-0.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Msadek, R., K. W. Dixon, T. L. Delworth, and W. Hurlin, 2010: Assessing the predictability of the Atlantic meridional overturning circulation and associated fingerprints. Geophys. Res. Lett., 37, L19608, https://doi.org/10.1029/2010GL044517.

    • Search Google Scholar
    • Export Citation
  • Msadek, R., W. E. Johns, S. G. Yeager, G. Danabasoglu, T. L. Delworth, and A. Rosati, 2013: The Atlantic meridional heat transport at 26.5°N and its relationship with the MOC in the RAPID array and the GFDL and NCAR coupled models. J. Climate, 26, 43354356, https://doi.org/10.1175/JCLI-D-12-00081.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Murphy, L. N., K. Bellomo, M. Cane, and A. Clement, 2017: The role of historical forcings in simulating the observed Atlantic multidecadal oscillation. Geophys. Res. Lett., 44, 24722480, https://doi.org/10.1002/2016GL071337.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Neale, R. B., and et al. , 2012: Description of the NCAR Community Atmosphere Model (CAM5.0). NCAR Tech. Note NCAR/TN-486+STR, 274 pp.

  • Pickart, R. S., and M. A. Spall, 2007: Impact of Labrador Sea convection on the North Atlantic meridional overturning circulation. J. Phys. Oceanogr., 37, 22072227, https://doi.org/10.1175/JPO3178.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schlesinger, M. E., and N. Ramankutty, 1994: An oscillation in the global climate system of period 65–70 years. Nature, 367, 723726, https://doi.org/10.1038/367723a0.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Smith, R. D., and et al. , 2010: The Parallel Ocean Program (POP) reference manual, ocean component of the Community Climate System Model (CCSM). Los Alamos National Laboratory Tech. Rep. LAUR-10-01853, 141 pp., http://www.cesm.ucar.edu/models/cesm1.0/pop2/doc/sci/POPRefManual.pdf.

  • Sutton, R. T., G. D. McCarthy, J. Robson, B. Sinha, A. T. Archibald, and L. J. Gray, 2018: Atlantic multidecadal variability and the U.K. ACSIS program. Bull. Amer. Meteor. Soc., 99, 415425, https://doi.org/10.1175/BAMS-D-16-0266.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Tulloch, R., and J. Marshall, 2012: Exploring mechanisms of variability and predictability of Atlantic meridional overturning circulation in two coupled climate models. J. Climate, 25, 40674080, https://doi.org/10.1175/JCLI-D-11-00460.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Waterhouse, A. F., and et al. , 2014: Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate. J. Phys. Oceanogr., 44, 18541872, https://doi.org/10.1175/JPO-D-13-0104.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wittenberg, A. T., 2009: Are historical records sufficient to constrain ENSO simulations? Geophys. Res. Lett., 36, L12702, https://doi.org/10.1029/2009GL038710.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Xu, X., P. B. Rhines, and E. P. Chassignet, 2016: Temperature–salinity structure of the North Atlantic circulation and associated heat and freshwater transports. J. Climate, 29, 77237742, https://doi.org/10.1175/JCLI-D-15-0798.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Xu, X., E. P. Chassignet, and F. Wang, 2019: On the variability of the Atlantic meridional overturning circulation transports in coupled CMIP5 simulations. Climate Dyn., 52, 65116531, https://doi.org/10.1007/S00382-018-4529-0.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yeager, S. G., and G. Danabasoglu, 2012: Sensitivity of Atlantic meridional overturning circulation variability to parameterized Nordic Sea overflows in CCSM4. J. Climate, 25, 20772103, https://doi.org/10.1175/JCLI-D-11-00149.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yeager, S. G., and G. Danabasoglu, 2014: The origins of late-twentieth-century variations in the large-scale North Atlantic circulation. J. Climate, 27, 32223247, https://doi.org/10.1175/JCLI-D-13-00125.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhang, L., and C. Wang, 2013: Multidecadal North Atlantic sea surface temperature and Atlantic meridional overturning circulation variability in CMIP5 historical simulations. J. Geophys. Res. Oceans, 118, 57725791, https://doi.org/10.1002/jgrc.20390.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhang, R., 2010: Latitudinal dependence of Atlantic meridional overturning circulation AMOC variations. Geophys. Res. Lett., 37, L16703, https://doi.org/10.1029/2010GL044474.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhang, R., R. Sutton, G. Danabasoglu, Y.-O. Kwon, R. Marsh, S. G. Yeager, D. E. Amrhein, and C. M. Little, 2019: A review of the role of Atlantic meridional overturning circulation in Atlantic multidecadal variability and associated climate impacts. Rev. Geophys., 57, 316375, https://doi.org/10.1029/2019RG000644.

    • Crossref
    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 73 73 12
PDF Downloads 53 53 10

Robust and Nonrobust Aspects of Atlantic Meridional Overturning Circulation Variability and Mechanisms in the Community Earth System Model

View More View Less
  • 1 National Center for Atmospheric Research, Boulder, Colorado
© Get Permissions
Restricted access

Abstract

Robust and nonrobust aspects of Atlantic meridional overturning circulation (AMOC) variability and mechanisms are analyzed in several 600-yr simulations with the Community Earth System Model. The simulations consist of a set of cases where a few loosely constrained ocean model parameter values are changed, a pair of cases where round-off level perturbations are applied to the initial atmospheric temperature field, and a millennium-scale integration. The time scales of variability differ among the cases with the dominant periods ranging from decadal to centennial. These dominant periods are not stationary in time, indicating that a robust characterization of AMOC temporal variability requires long, multimillennium-scale simulations. A robust aspect is that positive anomalies of the Labrador Sea (LS) upper-ocean density and boundary layer depth and the positive phase of the North Atlantic Oscillation lead AMOC strengthening by 2–3 years. Respective contributions of temperature and salinity to these density anomalies vary across the simulations, but in a majority of the cases temperature contributions dominate. Following an AMOC intensification, all cases show that advection of warm and salty waters into the LS region results in near-neutral density anomalies. Analysis of the LS heat budget indicates that temperature acts to increase density in all cases prior to an AMOC intensification, primarily due to losses by sensible and latent heat fluxes. The accompanying salt budget analysis reveals that the salt contribution to density anomalies varies across the cases, taking both positive and negative values.

© 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Gokhan Danabasoglu, gokhan@ucar.edu

Abstract

Robust and nonrobust aspects of Atlantic meridional overturning circulation (AMOC) variability and mechanisms are analyzed in several 600-yr simulations with the Community Earth System Model. The simulations consist of a set of cases where a few loosely constrained ocean model parameter values are changed, a pair of cases where round-off level perturbations are applied to the initial atmospheric temperature field, and a millennium-scale integration. The time scales of variability differ among the cases with the dominant periods ranging from decadal to centennial. These dominant periods are not stationary in time, indicating that a robust characterization of AMOC temporal variability requires long, multimillennium-scale simulations. A robust aspect is that positive anomalies of the Labrador Sea (LS) upper-ocean density and boundary layer depth and the positive phase of the North Atlantic Oscillation lead AMOC strengthening by 2–3 years. Respective contributions of temperature and salinity to these density anomalies vary across the simulations, but in a majority of the cases temperature contributions dominate. Following an AMOC intensification, all cases show that advection of warm and salty waters into the LS region results in near-neutral density anomalies. Analysis of the LS heat budget indicates that temperature acts to increase density in all cases prior to an AMOC intensification, primarily due to losses by sensible and latent heat fluxes. The accompanying salt budget analysis reveals that the salt contribution to density anomalies varies across the cases, taking both positive and negative values.

© 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Gokhan Danabasoglu, gokhan@ucar.edu
Save