• Bretherton, C. S., M. Widmann, V. P. Dymnikov, J. M. Wallace, and I. Blade, 1999: The effective number of spatial degrees of freedom of a time-varying field. J. Climate, 12, 19902009.

    • Search Google Scholar
    • Export Citation
  • Buckley, M. W., D. Ferreira, J.-M. Campin, J. Marshall, and R. Tulloch, 2012: On the relationship between decadal buoyancy anomalies and variability of the Atlantic meridional overturning circulation. J. Climate, in press.

    • Search Google Scholar
    • Export Citation
  • Colin de Verdière, A., and T. Huck, 1999: Baroclinic instability: An oceanic wavemaker for interdecadal variability. J. Phys. Oceanogr., 29, 893910.

    • Search Google Scholar
    • Export Citation
  • Czaja, A., and J. C. Marshall, 2000: On the interpretation of AGCMs response to prescribed time-varying SST anomalies. Geophys. Res. Lett., 27, 19271930.

    • Search Google Scholar
    • Export Citation
  • Czaja, A., and J. C. Marshall, 2001: Observations of atmosphere–ocean coupling in the North Atlantic. Quart. J. Roy. Meteor. Soc., 127, 18931916.

    • Search Google Scholar
    • Export Citation
  • Dai, A., A. Hu, G. A. Meehl, W. M. Washington, and W. G. Strand, 2005: Atlantic thermohaline circulation in a coupled general circulation model: Unforced variations versus forced changes. J. Climate, 18, 32703293.

    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., 2008: On multidecadal variability of the meridional overturning circulation in the Community Climate System Model, version 3. J. Climate, 21, 55245544.

    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., S. G. Yeager, Y.-O. Kwon, J. J. Tribbia, A. Phillips, and J. Hurrell, 2012: Variability of the Atlantic meridional overturning circulation in CCSM4. J. Climate, in press.

    • Search Google Scholar
    • Export Citation
  • Delworth, T., and R. J. Greatbatch, 2000: Multidecadal thermohaline circulation variability driven by atmospheric surface flux forcing. J. Climate, 13, 14811495.

    • Search Google Scholar
    • Export Citation
  • Delworth, T., and M. E. Mann, 2000: Observed and simulated multidecadal variability in the Northern Hemisphere. Climate Dyn., 16, 661676.

    • Search Google Scholar
    • Export Citation
  • Delworth, T., S. Manabe, and R. J. Stouffer, 1993: Interdecadal variations of the thermohaline circulation in a coupled ocean–atmosphere model. J. Climate, 6, 19932011.

    • Search Google Scholar
    • Export Citation
  • Delworth, T., S. Manabe, and R. J. Stouffer, 1997: Multidecadal climate variability in the Greenland Sea and surrounding regions: A coupled model simulation. Geophys. Res. Lett., 24, 257260.

    • Search Google Scholar
    • Export Citation
  • Delworth, T., and Coauthors, 2006: GFDL’s CM2 global coupled climate models. Part I: Formulation and simulation characteristics. J. Climate, 19, 643674.

    • Search Google Scholar
    • Export Citation
  • Dong, B., and R. T. Sutton, 2005: Mechanism of interdecadal thermohaline circulation variability in a coupled ocean–atmosphere GCM. J. Climate, 18, 11171135.

    • 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.

    • Search Google Scholar
    • Export Citation
  • Frankcombe, L. M., and H. A. Dijkstra, 2009: Coherent multidecadal variability in North Atlantic sea level. Geophys. Res. Lett., 36, L15604, doi:10.1029/2009GL039455.

    • Search Google Scholar
    • Export Citation
  • Frankignoul, C., and K. Hasselmann, 1977: Stochastic climate models. Part II: Application to sea surface temperature anomalies and thermocline variability. Tellus, 29, 289305.

    • Search Google Scholar
    • Export Citation
  • Frankignoul, C., and P. Müller, 1979: Quasi-geostrophic response of an infinite beta-plane ocean to stochastic forcing by the atmosphere. J. Phys. Oceanogr., 9, 104127.

    • Search Google Scholar
    • Export Citation
  • Frankignoul, C., P. Müller, and E. Zurita, 1997: A simple model of the decadal response of the ocean to stochastic wind forcing. J. Phys. Oceanogr., 27, 15331546.

    • Search Google Scholar
    • Export Citation
  • Griffies, S. M., and E. Tziperman, 1995: A linear thermohaline oscillator driven by stochastic atmospheric forcing. J. Climate, 8, 24402453.

    • Search Google Scholar
    • Export Citation
  • Hawkins, E., and R. Sutton, 2009: Decadal predictability of the Atlantic Ocean in a coupled GCM: Forecast skill and optimal perturbations using linear inverse modeling. J. Climate, 22, 39603978.

    • Search Google Scholar
    • Export Citation
  • Hirschi, J., and J. Marotzke, 2007: Reconstructing the meridional overturning circulation from boundary densities and the zonal wind stress. J. Phys. Oceanogr., 37, 743763.

    • Search Google Scholar
    • Export Citation
  • Hurrell, J. W., and Coauthors, 2006: Atlantic climate variability and predictability: A CLIVAR perspective. J. Climate, 19, 51005121.

    • Search Google Scholar
    • Export Citation
  • Jayne, S. R., and J. Marotzke, 2001: The dynamics of ocean heat transport variability. Rev. Geophys., 3, 384411.

  • Johns, W. E., and Coauthors, 2011: Continuous, array-based estimates of Atlantic Ocean heat transport at 26.5°N. J. Climate, 24, 24292449.

    • 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.

    • Search Google Scholar
    • Export Citation
  • Kushnir, Y., 1994: Interdecadal variations in North Atlantic sea surface temperature and associated atmospheric condition. J. Climate, 7, 141157.

    • 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.

    • Search Google Scholar
    • Export Citation
  • Mahajan, S., R. Zhang, and T. L. Delworth, 2011a: Impact of the Atlantic meridional overturning circulation (AMOC) on arctic surface air temperature and sea ice variability. J. Climate, 24, 65736581.

    • Search Google Scholar
    • Export Citation
  • Mahajan, S., R. Zhang, T. L. Delworth, S. Zhang, A. J. Rosati, and Y.-S. Chang, 2011b: Predicting Atlantic meridional overturning circulation (AMOC) variations using subsurface and surface fingerprints. Deep-Sea Res., 58, 18951903.

    • Search Google Scholar
    • Export Citation
  • Mann, C. R., 1967: The termination of the Gulf Stream and the beginning of the North Atlantic Current. Deep-Sea Res., 14, 337359.

  • Marshall, J., and F. Schott, 1999: Open-ocean convection: Observations, theory and models. Rev. Geophys., 37, 164.

  • Marshall, J., and Coauthors, 2001a: North Atlantic climate variability: Phenomena, impacts, and mechanisms. Int. J. Climatol., 21, 18631898.

    • Search Google Scholar
    • Export Citation
  • Marshall, J., H. Johnson, and J. Goodman, 2001b: A study of the interaction of the North Atlantic Oscillation with ocean circulation. J. Climate, 14, 13991421.

    • Search Google Scholar
    • Export Citation
  • Meinen, C. S., D. R. Watts, and R. A. Clarke, 2000: Absolutely referenced geostrophic velocity and transport on a section across the North Atlantic current. Deep-Sea Res., 47, 309322.

    • 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, doi:10.1029/2010GL044517.

    • Search Google Scholar
    • Export Citation
  • Oka, A., and H. Hasumi, 2006: Effects of model resolution on salt transport through northern high-latitude passages and Atlantic meridional overturning circulation. Ocean Modell., 13, 126147.

    • Search Google Scholar
    • Export Citation
  • Olsen, S. M., B. Hansen, D. Quadfasel, and S. Osterhus, 2008: Observed and modelled stability of overflow across the Greenland-Scotland ridge. Nature, 455, 519522.

    • Search Google Scholar
    • Export Citation
  • Sévellec, F., and A. V. Fedorov, 2011: Stability of the Atlantic meridional overturning circulation and stratification in a zonally-averaged ocean model: The effects of freshwater flux, wind stress, and diapycnal diffusion. Deep-Sea Res., 58, 19271943, doi:10.1016/j.dsr2.2010.10.070.

    • Search Google Scholar
    • Export Citation
  • Te Raa, L. A., J. Gerrits, and H. A. Dijkstra, 2004: Identification of the mechanism of interdecadal variability in the North Atlantic Ocean. J. Phys. Oceanogr., 34, 27922807.

    • Search Google Scholar
    • Export Citation
  • Tulloch, R. T., J. C. Marshall, and K. S. Smith, 2009: Interpretation of the propagation of surface altimetric observations in terms of planetary waves and geostrophic turbulence. J. Geophys. Res., 114, C02005, doi:10.1029/2008JC005055.

    • Search Google Scholar
    • Export Citation
  • Tziperman, E., L. Zanna, and C. Penland, 2008: Non-normal thermohaline circulation dynamics in a coupled ocean–atmosphere GCM. J. Phys. Oceanogr., 38, 588604.

    • Search Google Scholar
    • Export Citation
  • Weaver, A. J., and S. Valcke, 1998: On the variability of the thermohaline circulation in the GFDL coupled model. J. Climate, 11, 759767.

    • Search Google Scholar
    • Export Citation
  • Weaver, A. J., J. Marotzke, P. F. Cummins, and E. S. Sarachik, 1993: Stability and variability of the thermohaline circulation. J. Phys. Oceanogr., 23, 3960.

    • 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.

    • Search Google Scholar
    • Export Citation
  • Yoshimori, M., C. C. Raible, T. F. Stocker, and M. Renold, 2010: Simulated decadal oscillations of the Atlantic meridional overturning circulation in a cold climate state. Climate Dyn., 34, 101121, doi:10.1007/s00382-009-0540-9.

    • Search Google Scholar
    • Export Citation
  • Zhang, D., R. Msadek, M. J. McPhaden, and T. L. Delworth, 2011: Multidecadal variability of the North Brazil Current and its connection to the Alantic meridional overturning circulation. J. Geophys. Res., 116, C04012, doi:10.1029/2010JC006812.

    • Search Google Scholar
    • Export Citation
  • Zhang, R., 2008: Coherent surface-subsurface fingerprint of the Atlantic meridional overturning circulation. Geophys. Res. Lett., 35, L20705, doi:10.1029/2008GL035463.

    • Search Google Scholar
    • Export Citation
  • Zhu, X., and J. Jungclaus, 2008: Interdecadal variability of the meridional overturning circulation as an ocean internal mode. Climate Dyn., 31, 731741, doi:10.1007/s00382-008-0383-9.

    • Search Google Scholar
    • Export Citation
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Exploring Mechanisms of Variability and Predictability of Atlantic Meridional Overturning Circulation in Two Coupled Climate Models

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  • 1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts
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Abstract

Multidecadal variability in the Atlantic meridional overturning circulation (AMOC) of the ocean is diagnosed in the NCAR Community Climate System Model, version 3 (CCSM3), and the GFDL Coupled Model (CM2.1). Common diagnostic approaches are applied to draw out similarities and differences between the two models. An index of AMOC variability is defined, and the manner in which key variables covary with it is determined. In both models the following is found. (i) AMOC variability is associated with upper-ocean (top 1 km) density anomalies (dominated by temperature) on the western margin of the basin in the region of the Mann eddy with a period of about 20 years. These anomalies modulate the trajectory and strength of the North Atlantic Current. The importance of the western margin is a direct consequence of the thermal wind relation and is independent of the mechanisms that create those density anomalies. (ii) Density anomalies in this key region are part of a larger-scale pattern that propagates around the subpolar gyre and acts as a “pacemaker” of AMOC variability. (iii) The observed variability is consistent with the primary driving mechanism being stochastic wind curl forcing, with Labrador Sea convection playing a secondary role. Also, “toy models” of delayed oscillator form are fitted to power spectra of key variables and are used to infer “quality factors” (Q-factors), which characterize the bandwidth relative to the center frequency and hence AMOC predictability horizons. The two models studied here have Q-factors of around 2, suggesting that prediction is possible out to about two cycles, which is likely larger than the real AMOC.

Corresponding author address: Ross Tulloch, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139. E-mail: tulloch@mit.edu

Abstract

Multidecadal variability in the Atlantic meridional overturning circulation (AMOC) of the ocean is diagnosed in the NCAR Community Climate System Model, version 3 (CCSM3), and the GFDL Coupled Model (CM2.1). Common diagnostic approaches are applied to draw out similarities and differences between the two models. An index of AMOC variability is defined, and the manner in which key variables covary with it is determined. In both models the following is found. (i) AMOC variability is associated with upper-ocean (top 1 km) density anomalies (dominated by temperature) on the western margin of the basin in the region of the Mann eddy with a period of about 20 years. These anomalies modulate the trajectory and strength of the North Atlantic Current. The importance of the western margin is a direct consequence of the thermal wind relation and is independent of the mechanisms that create those density anomalies. (ii) Density anomalies in this key region are part of a larger-scale pattern that propagates around the subpolar gyre and acts as a “pacemaker” of AMOC variability. (iii) The observed variability is consistent with the primary driving mechanism being stochastic wind curl forcing, with Labrador Sea convection playing a secondary role. Also, “toy models” of delayed oscillator form are fitted to power spectra of key variables and are used to infer “quality factors” (Q-factors), which characterize the bandwidth relative to the center frequency and hence AMOC predictability horizons. The two models studied here have Q-factors of around 2, suggesting that prediction is possible out to about two cycles, which is likely larger than the real AMOC.

Corresponding author address: Ross Tulloch, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139. E-mail: tulloch@mit.edu
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