Intrinsic and Atmospherically Forced Variability of the AMOC: Insights from a Large-Ensemble Ocean Hindcast (bibtex)
by , , , , , , ,
Abstract:
AbstractThis study investigates the origin and features of interannual–decadal Atlantic meridional overturning circulation (AMOC) variability from several ocean simulations, including a large (50 member) ensemble of global, eddy-permitting (1/4°) ocean–sea ice hindcasts. After an initial stochastic perturbation, each member is driven by the same realistic atmospheric forcing over 1960–2015. The magnitude, spatiotemporal scales, and patterns of both the atmospherically forced and intrinsic–chaotic interannual AMOC variability are then characterized from the ensemble mean and ensemble spread, respectively. The analysis of the ensemble-mean variability shows that the AMOC fluctuations north of 40°N are largely driven by the atmospheric variability, which forces meridionally coherent fluctuations reaching decadal time scales. The amplitude of the intrinsic interannual AMOC variability never exceeds the atmospherically forced contribution in the Atlantic basin, but it reaches up to 100% of the latter around 35°S and 60% in the Northern Hemisphere midlatitudes. The intrinsic AMOC variability exhibits a large-scale meridional coherence, especially south of 25°N. An EOF analysis over the basin shows two large-scale leading modes that together explain 60% of the interannual intrinsic variability. The first mode is likely excited by intrinsic oceanic processes at the southern end of the basin and affects latitudes up to 40°N; the second mode is mostly restricted to, and excited within, the Northern Hemisphere midlatitudes. These features of the intrinsic, chaotic variability (intensity, patterns, and random phase) are barely sensitive to the atmospheric evolution, and they strongly resemble the “pure intrinsic” interannual AMOC variability that emerges in climatological simulations under repeated seasonal-cycle forcing. These results raise questions about the attribution of observed and simulated AMOC signals and about the possible impact of intrinsic signals on the atmosphere.
Reference:
Intrinsic and Atmospherically Forced Variability of the AMOC: Insights from a Large-Ensemble Ocean Hindcast (Stephanie Leroux, Thierry Penduff, Laurent Bessières, Jean-Marc Molines, Jean-Michel Brankart, Guillaume Sérazin, Bernard Barnier, Laurent Terray), In Journal of Climate, volume 31, 2018.
Bibtex Entry:
@Article{	  leroux.ea_2018,
  author	= {Stephanie Leroux and Thierry Penduff and Laurent
		  Bessières and Jean-Marc Molines and Jean-Michel Brankart
		  and Guillaume Sérazin and Bernard Barnier and Laurent
		  Terray},
  title		= {Intrinsic and Atmospherically Forced Variability of the
		  AMOC: Insights from a Large-Ensemble Ocean Hindcast},
  journal	= {Journal of Climate},
  volume	= {31},
  number	= {3},
  pages		= {1183-1203},
  year		= {2018},
  doi		= {10.1175/JCLI-D-17-0168.1},
  url		= { https://doi.org/10.1175/JCLI-D-17-0168.1 },
  eprint	= { https://doi.org/10.1175/JCLI-D-17-0168.1 },
  abstract	= { AbstractThis study investigates the origin and features
		  of interannual–decadal Atlantic meridional overturning
		  circulation (AMOC) variability from several ocean
		  simulations, including a large (50 member) ensemble of
		  global, eddy-permitting (1/4°) ocean–sea ice hindcasts.
		  After an initial stochastic perturbation, each member is
		  driven by the same realistic atmospheric forcing over
		  1960–2015. The magnitude, spatiotemporal scales, and
		  patterns of both the atmospherically forced and
		  intrinsic–chaotic interannual AMOC variability are then
		  characterized from the ensemble mean and ensemble spread,
		  respectively. The analysis of the ensemble-mean variability
		  shows that the AMOC fluctuations north of 40°N are largely
		  driven by the atmospheric variability, which forces
		  meridionally coherent fluctuations reaching decadal time
		  scales. The amplitude of the intrinsic interannual AMOC
		  variability never exceeds the atmospherically forced
		  contribution in the Atlantic basin, but it reaches up to
		  100\% of the latter around 35°S and 60\% in the Northern
		  Hemisphere midlatitudes. The intrinsic AMOC variability
		  exhibits a large-scale meridional coherence, especially
		  south of 25°N. An EOF analysis over the basin shows two
		  large-scale leading modes that together explain 60\% of the
		  interannual intrinsic variability. The first mode is likely
		  excited by intrinsic oceanic processes at the southern end
		  of the basin and affects latitudes up to 40°N; the second
		  mode is mostly restricted to, and excited within, the
		  Northern Hemisphere midlatitudes. These features of the
		  intrinsic, chaotic variability (intensity, patterns, and
		  random phase) are barely sensitive to the atmospheric
		  evolution, and they strongly resemble the “pure
		  intrinsic” interannual AMOC variability that emerges in
		  climatological simulations under repeated seasonal-cycle
		  forcing. These results raise questions about the
		  attribution of observed and simulated AMOC signals and
		  about the possible impact of intrinsic signals on the
		  atmosphere. }
}
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