Oscillatory sensitivity of Atlantic overturning to high‐latitude forcing

Czeschel, Lars; Marshall, David P.; Johnson, H. L.

doi:10.1029/2010gl043177

Cited by 30 publications

(39 citation statements)

References 22 publications

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“…The sensitivity of the MOC to high-latitude perturbations is consistent with other studies (e.g. Tziperman et al, 2008;Hawkins and Sutton, 2009;Czeschel et al, 2010;Heimbach et al, 2011;Zanna et al, 2011). The perturbations are located near the downwelling branch of the steady state MOC and in the region where the slumping of the isopycnals is the steepest (ZHMT11).…”

Section: Spatial Structure Of the Leading Singular Vectorsupporting

confidence: 79%

“…The related sensitivity of the MOC and meridional heat transport to initial conditions and surface forcing has been investigated in 3D ocean models using adjoint methods (Marotzke et al, 1999;Bugnion et al, 2006;Sevellec et al, 2008;Czeschel et al, 2010;Heimbach et al, 2011). However the singular vector approach provides a stricter bound on error growth than an adjoint sensitivity experiment or excitation by adjoint modes.…”

Section: Introductionmentioning

confidence: 99%

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Upper‐ocean singular vectors of the North Atlantic climate with implications for linear predictability and variability

Zanna

Heimbach²,

Moore

et al. 2011

Quart J Royal Meteoro Soc

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The limits of predictability of the meridional overturning circulation (MOC) and upper-ocean temperatures due to errors in ocean initial conditions and model parametrizations are investigated in an idealized configuration of an ocean general circulation model (GCM). Singular vectors (optimal perturbations) are calculated using the GCM, its tangent linear and adjoint models to determine an upper bound on the predictability of North Atlantic climate.The maximum growth time-scales of MOC and upper-ocean temperature anomalies, excited by the singular vectors, are 18.5 and 13 years respectively and in part explained by the westward propagation of upper-ocean anomalies against the mean flow. As a result of the linear interference of non-orthogonal eigenmodes of the non-normal dynamics, the ocean dynamics are found to actively participate in the significant growth of the anomalies. An initial density perturbation of merely 0.02 kg m −3 is found to lead to a 1.7 Sv MOC anomaly after 18.5 years. In addition, Northern Hemisphere upper-ocean temperature perturbations can be amplified by a factor of 2 after 13 years.The growth of upper-ocean temperature and MOC anomalies is slower and weaker when excited by the upper-ocean singular vectors than when the deep ocean is perturbed. This leads to the conclusion that predictability experiments perturbing only the atmospheric initial state may overestimate the predictability time. Interestingly, optimal MOC and upper-ocean temperature excitations are only weakly correlated, thus limiting the utility of SST observations to infer MOC variability. The excitation of anomalies in this model might have a crucial impact on the variability and predictability of Atlantic climate. The limit of predictability of the MOC is found to be different from that of the upper-ocean heat content, emphasizing that errors in ocean initial conditions will affect various measures differently and such uncertainties should be carefully considered in decadal prediction experiments.

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Section: Spatial Structure Of the Leading Singular Vectorsupporting

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Upper‐ocean singular vectors of the North Atlantic climate with implications for linear predictability and variability

Zanna

Heimbach²,

Moore

et al. 2011

Quart J Royal Meteoro Soc

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“…There is also a prominent interdecadal mode of variability found in CMIP5 models associated with the westward propagation of density anomalies in the band 308-608N (Muir and Fedorov 2017). This mode of variability had been anticipated in an earlier study (Sévellec and Fedorov 2013; see also Czeschel et al 2010) and may also contribute to the AMV. A similar mode of variability has been described by Ortega et al (2015).…”

Section: Introductionmentioning

confidence: 81%

Evolution of the Atlantic Multidecadal Variability in a Model with an Improved North Atlantic Current

Drews

Greatbatch

2017

J. Climate

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This article investigates the dynamics and temporal evolution of the Atlantic multidecadal variability (AMV) in a coupled climate model. The model contains a correction to the North Atlantic flow field to improve the path of the North Atlantic Current, thereby alleviating the surface cold bias, a common problem with climate models, and offering a unique opportunity to study the AMV in a model. Changes in greenhouse gas forcing or aerosol loading are not considered. A striking feature of the results is the contrast between the western and eastern sides of the subpolar gyre in the model. On the western side, anomalous heat supply by the ocean plays a major role, with most of this heat being given up to the atmosphere in the warm phase, largely symmetrically about the time of the AMV maximum. By contrast, on the eastern side, the ocean anomalously gains heat from the atmosphere, with relatively little role for ocean heat supply in the years before the AMV maximum. Thereafter, the balance changes with heat now being anomalously removed from the eastern side by the ocean, leading to a reduced ocean heat content, behavior associated with the establishment of an intergyre gyre at the time of the AMV maximum. In the warm phase, melting sea ice leads to a freshening of surface waters northeast of Greenland that travel southward into the Irminger and Labrador Seas, shutting down convection and terminating the AMV warm phase.

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“…Buckley 2010, private communication). Nonetheless, Czeschel et al (2010) found in an adjoint calculation with a realistic OGCM that cold anomalies near the eastern boundary led to a stronger AMOC after about 9 years and caused an oscillatory behavior, although the role of topography in the generation of the anomaly is unclear. However, the delay was due to Rossby wave propagation toward the western boundary followed by northward advection, instead of mean advection along the eastern boundary and the subpolar gyre, as in CCSM3.…”

Section: Summary and Discussionmentioning

confidence: 99%

Stochastically-driven multidecadal variability of the Atlantic meridional overturning circulation in CCSM3

Kwon

Frankignoul

2011

Clim Dyn

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The Atlantic meridional overturning circulation (AMOC) in the last 250 years of the 700-yearlong present-day control integration of the Community Climate System Model version 3 with T85 atmospheric resolution exhibits a red noise-like irregular multi-decadal variability with a persistence longer than 10 years, which markedly contrasts with the preceding ~300 years of very regular and stronger AMOC variability with ~20 year periodicity. The red noise-like multidecadal AMOC variability is primarily forced by the surface fluxes associated with stochastic changes in the North Atlantic Oscillation (NAO) that intensify and shift northward the deep convection in the Labrador Sea. However, the persistence of the AMOC and the associated oceanic anomalies that are directly forced by the NAO forcing does not exceed about 5 years.The additional persistence originates from anomalous horizontal advection and vertical mixing, which generate density anomalies on the continental shelf along the eastern boundary of the subpolar gyre. These anomalies are subsequently advected by the mean boundary current into the northern part of the Labrador Sea convection region, reinforcing the density changes directly forced by the NAO. As no evidence was found of a clear two-way coupling with the atmosphere, the multi-decadal AMOC variability in the last 250 years of the integration is an ocean-only response to stochastic NAO forcing with a delayed positive feedback caused by the changes in the horizontal ocean circulation.

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Oscillatory sensitivity of Atlantic overturning to high‐latitude forcing

Cited by 30 publications

References 22 publications

Upper‐ocean singular vectors of the North Atlantic climate with implications for linear predictability and variability

Upper‐ocean singular vectors of the North Atlantic climate with implications for linear predictability and variability

Evolution of the Atlantic Multidecadal Variability in a Model with an Improved North Atlantic Current

Stochastically-driven multidecadal variability of the Atlantic meridional overturning circulation in CCSM3

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