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Volume 10, issue 2
Ocean Sci., 10, 227–241, 2014
https://doi.org/10.5194/os-10-227-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ocean Sci., 10, 227–241, 2014
https://doi.org/10.5194/os-10-227-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 14 Apr 2014

Research article | 14 Apr 2014

The role of subpolar deep water formation and Nordic Seas overflows in simulated multidecadal variability of the Atlantic meridional overturning circulation

K. Lohmann1, J. H. Jungclaus1, D. Matei1, J. Mignot2,3, M. Menary4, H. R. Langehaug5,6, J. Ba7, Y. Gao5,6, O. H. Otterå6,8, W. Park7, and S. Lorenz1 K. Lohmann et al.
  • 1Max Planck Institute for Meteorology, Hamburg, Germany
  • 2LOCEAN, Institute Pierre Simon Laplace, University Pierre and Marie Curie, Paris, France
  • 3Climate and Environmental Physics, and Oeschger Centre of Climate Change Research, University of Bern, Bern, Switzerland
  • 4Met Office Hadley Center, Exeter, UK
  • 5Nansen Environmental and Remote Sensing Center, Bergen, Norway
  • 6Bjerknes Centre for Climate Research, Bergen, Norway
  • 7Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 8Uni Research, Bergen, Norway

Abstract. We investigate the respective role of variations in subpolar deep water formation and Nordic Seas overflows for the decadal to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This is partly done by analysing long (order of 1000 years) control simulations with five coupled climate models. For all models, the maximum influence of variations in subpolar deep water formation is found at about 45° N, while the maximum influence of variations in Nordic Seas overflows is rather found at 55 to 60° N. Regarding the two overflow branches, the influence of variations in the Denmark Strait overflow is, for all models, substantially larger than that of variations in the overflow across the Iceland–Scotland Ridge. The latter might, however, be underestimated, as the models in general do not realistically simulate the flow path of the Iceland–Scotland overflow water south of the Iceland–Scotland Ridge. The influence of variations in subpolar deep water formation is, on multimodel average, larger than that of variations in the Denmark Strait overflow. This is true both at 45° N, where the maximum standard deviation of decadal to multidecadal AMOC variability is located for all but one model, and at the more classical latitude of 30° N. At 30° N, variations in subpolar deep water formation and Denmark Strait overflow explain, on multimodel average, about half and one-third respectively of the decadal to multidecadal AMOC variance. Apart from analysing multimodel control simulations, we have performed sensitivity experiments with one of the models, in which we suppress the variability of either subpolar deep water formation or Nordic Seas overflows. The sensitivity experiments indicate that variations in subpolar deep water formation and Nordic Seas overflows are not completely independent. We further conclude from these experiments that the decadal to multidecadal AMOC variability north of about 50° N is mainly related to variations in Nordic Seas overflows. At 45° N and south of this latitude, variations in both subpolar deep water formation and Nordic Seas overflows contribute to the AMOC variability, with neither of the processes being very dominant compared to the other.

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