Linking Ocean Forcing and Atmospheric Interactions to Atlantic Multidecadal Variability in MPI‐ESM1.2

AbstractThe question of whether ocean dynamics are relevant for basin-scale North Atlantic decadal temperature variability is the subject of ongoing discussions. Here, we analyze a set of simulations with a single climate model consisting of a 2000-yr preindustrial control experiment, a 100-member historical ensemble, and a 100-member ensemble forced with an incremental CO2 increase by 1% yr−1. Compared to previous approaches, our setup offers the following advantages: First, the large ensemble size allows us to robustly separate internally and externally forced variability and to robustly detect statistical links between different quantities. Second, the availability of different scenarios allows us to investigate the role of the background state for drivers of the variability. We find strong evidence that ocean dynamics, particularly ocean heat transport variations, form an important contribution to generate the Atlantic multidecadal variability (AMV) in the Max Planck Institute Earth System Model (MPI-ESM). Particularly the northwest North Atlantic is substantially affected by ocean circulation for the historical and preindustrial simulations. Anomalies of the Labrador Sea deep ocean density precede a change of the Atlantic meridional overturning circulation (AMOC) and heat advection to the region south of Greenland. Under strong CO2 forcing, the AMV–SST regression pattern shows crucial changes: SST variability in the northwestern part of the North Atlantic is strongly reduced, so that the AMV pattern in this scenario is dominated by the low-latitude branch. We found a connection to changes in the deep-water formation that cause a strong reduction of the mean AMOC and its variability. Consequently, ocean heat transport convergence becomes less important for the SST variability south of Greenland.

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Atlantic Multidecadal Variability in a model with an improved North Atlantic Current

Geophysical Research Letters ◽

10.1002/2016gl069815 ◽

2016 ◽

Vol 43 (15) ◽

pp. 8199-8206 ◽

Cited By ~ 31

Author(s):

Annika Drews ◽

Richard J. Greatbatch

Keyword(s):

North Atlantic ◽

Atlantic Multidecadal Variability ◽

North Atlantic Current ◽

Multidecadal Variability ◽

Atlantic Current

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Correcting North Atlantic sea surface salinity biases in the Kiel Climate Model: influences on ocean circulation and Atlantic Multidecadal Variability

Climate Dynamics ◽

10.1007/s00382-016-2982-1 ◽

2016 ◽

Vol 47 (7-8) ◽

pp. 2543-2560 ◽

Cited By ~ 17

Author(s):

T. Park ◽

W. Park ◽

M. Latif

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Climate Model ◽

Sea Surface ◽

Sea Surface Salinity ◽

Atlantic Multidecadal Variability ◽

Surface Salinity ◽

Multidecadal Variability

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A Review of the Role of the Atlantic Meridional Overturning Circulation in Atlantic Multidecadal Variability and Associated Climate Impacts

Reviews of Geophysics ◽

10.1029/2019rg000644 ◽

2019 ◽

Vol 57 (2) ◽

pp. 316-375 ◽

Cited By ~ 54

Author(s):

Rong Zhang ◽

Rowan Sutton ◽

Gokhan Danabasoglu ◽

Young‐Oh Kwon ◽

Robert Marsh ◽

...

Keyword(s):

Atlantic Meridional Overturning Circulation ◽

Climate Impacts ◽

Meridional Overturning Circulation ◽

Atlantic Multidecadal Variability ◽

Overturning Circulation ◽

Multidecadal Variability ◽

Meridional Overturning

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Assessing External and Internal Sources of Atlantic Multidecadal Variability Using Models, Proxy Data, and Early Instrumental Indices

Journal of Climate ◽

10.1175/jcli-d-19-0177.1 ◽

2019 ◽

Vol 32 (22) ◽

pp. 7727-7745 ◽

Cited By ~ 4

Author(s):

Christopher H. O’Reilly ◽

Laure Zanna ◽

Tim Woollings

Keyword(s):

Atmospheric Circulation ◽

Ocean Circulation ◽

Radiative Forcing ◽

Internal Source ◽

External Forcing ◽

Atlantic Multidecadal Variability ◽

Anthropogenic Aerosols ◽

Multidecadal Variability ◽

Internal Climate Variability ◽

Internal Sources

Abstract Atlantic multidecadal variability (AMV) of sea surface temperature exhibits an important influence on the climate of surrounding continents. It remains unclear, however, the extent to which AMV is due to internal climate variability (e.g., ocean circulation variability) or changes in external forcing (e.g., volcanic/anthropogenic aerosols or greenhouse gases). Here, the sources of AMV are examined over a 340-yr period using proxy indices, instrumental data, and output from the Last Millennium Ensemble (LME) simulation. The proxy AMV closely follows the accumulated atmospheric forcing from the instrumental North Atlantic Oscillation (NAO) reconstruction (r = 0.65)—an “internal” source of AMV. This result provides strong observational evidence that much of the AMV is generated through the oceanic response to atmospheric circulation forcing, as previously demonstrated in targeted modeling studies. In the LME there is a substantial externally forced AMV component, which exhibits a modest but significant correlation with the proxy AMV (i.e., r = 0.37), implying that at least 13% of the AMV is externally forced. In the LME simulations, however, the AMV response to accumulated NAO forcing is weaker than in the proxy/observational datasets. This weak response is possibly related to the decadal NAO variability, which is substantially weaker in the LME than in observations. The externally forced component in the proxy AMV is also related to the accumulated NAO forcing, unlike in the LME. This indicates that the external forcing is likely influencing the AMV through different mechanistic pathways: via changes in radiative forcing in the LME and via changes in atmospheric circulation in the observational/proxy record.

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The Role of Forcings in the Twentieth-Century North Atlantic Multidecadal Variability: The 1940–75 North Atlantic Cooling Case Study

Journal of Climate ◽

10.1175/jcli-d-16-0301.1 ◽

2017 ◽

Vol 30 (18) ◽

pp. 7317-7337 ◽

Cited By ~ 25

Author(s):

A. Bellucci ◽

A. Mariotti ◽

S. Gualdi

Keyword(s):

Twentieth Century ◽

Greenhouse Gases ◽

Surface Temperature ◽

Sea Surface Temperature ◽

North Atlantic ◽

Sea Surface ◽

Atlantic Multidecadal Variability ◽

Anthropogenic Aerosols ◽

Multimodel Ensemble ◽

Multidecadal Variability

Abstract Results from a study inspecting the origins of multidecadal variability in the North Atlantic sea surface temperature (NASST) are presented. The authors target in particular the 1940–75 “warm-to-cold” transition, an event that is generally framed in the context of the longer-term Atlantic multidecadal variability (AMV) cycle, in turn associated with the Atlantic meridional overturning circulation (AMOC) internal variability. Here the authors examine the ability of uninitialized, historical integrations from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) archive to retrospectively reproduce this specific episode of twentieth-century climatic history, under a hierarchy of forcing conditions. For this purpose, both standard and so-called historical Misc CMIP5 simulations of the historical climate (combining selected natural and anthropogenic forcings) are exploited. Based on this multimodel analysis, evidence is found for a significant influence of anthropogenic agents on multidecadal sea surface temperature (SST) fluctuations across the Atlantic sector, suggesting that anthropogenic aerosols and greenhouse gases might have played a key role in the 1940–75 North Atlantic cooling. However, the diagnosed forced response in CMIP5 models appears to be affected by a large uncertainty, with only a limited subset of models displaying significant skill in reproducing the mid-twentieth-century NASST cooling. Such uncertainty originates from the existence of well-defined behavioral clusters within the analyzed CMIP5 ensembles, with the bulk of the models splitting into two main clusters. Such a strong polarization calls for some caution when using a multimodel ensemble mean in climate model analyses, as averaging across fairly distinct model populations may result, through mutual cancellation, in a rather artificial description of the actual multimodel ensemble behavior. A potentially important role for both anthropogenic aerosols and greenhouse gases with regard to the observed North Atlantic multidecadal variability has clear implications for decadal predictability and predictions. The uncertainty associated with alternative aerosol and greenhouse gas emission scenarios should be duly accounted for in designing a common protocol for coordinated decadal forecast experiments.

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