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Simulated variability of the Atlantic meridional overturning circulation

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Abstract

To examine the multi-annual to decadal scale variability of the Atlantic Meridional Overturning Circulation (AMOC) we conducted a four-member ensemble with a daily reanalysis forced, medium-resolution global version of the isopycnic coordinate ocean model MICOM, and a 300-years integration with the fully coupled Bergen Climate Model (BCM). The simulations of the AMOC with both model systems yield a long-term mean value of 18 Sv and decadal variability with an amplitude of 1–3 Sv. The power spectrum of the inter-annual to decadal scale variability of the AMOC in BCM generally follows the theoretical red noise spectrum, with indications of increased power near the 20-years period. Comparison with observational proxy indices for the AMOC, e.g. the thickness of the Labrador Sea Water, the strength of the baroclinic gyre circulation in the North Atlantic Ocean, and the surface temperature anomalies along the mean path of the Gulf Stream, shows similar trends and phasing of the variability, indicating that the simulated AMOC variability is robust and real. Mixing indices have been constructed for the Labrador, the Irminger and the Greenland-Iceland-Norwegian (GIN) seas. While convective mixing in the Labrador and the GIN seas are in opposite phase, and linked to the NAO as observations suggest, the convective mixing in the Irminger Sea is in phase with or leads the Labrador Sea. Newly formed deep water is seen as a slow, anomalous cold and fresh, plume flowing southward along the western continental slope of the Atlantic Ocean, with a return flow of warm and saline water on the surface. In addition, fast-travelling topographically trapped waves propagate southward along the continental slope towards equator, where they go east and continue along the eastern rim of the Atlantic. For both types of experiments, the Northern Hemisphere sea level pressure and 2 m temperature anomaly patterns computed based on the difference between climate states with strong and weak AMOC yields a NAO-like pattern with intensified Icelandic low and Azores high, and a warming of 0.25–0.5 °C of the central North Atlantic sea-surface temperature (SST). The reanalysis forced simulations indicate a coupling between the Labrador Sea Water production rate and an equatorial Atlantic SST index in accordance with observations. This coupling is not identified in the coupled simulation.

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Acknowledgements

The model development and analysis have been supported by the Research Council of Norway through several projects, and in particular RegClim, NOClim and KlimaProg’s “Spissforskningsmidler”, and the Programme of Supercomputing. The work has also received support from the EU-project PREDICATE (EVK2-CT-1999–00020). The authors would like to thank C. Frankignoul and R. G. Curry for providing the Gulf Stream and the transport indices, respectively. The authors are grateful to the BCM group for help and guidance throughout the work. Support from the G. C. Rieber Foundations is also acknowledged with thanks. Comments from two anonymous reviewers improved the manuscript. This is contribution A0041 from the Bjerknes Centre for Climate Research.

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Authors and Affiliations

  1. Nansen Environmental and Remote Sensing Center, Bergen, Norway

    M. Bentsen & H. Drange

  2. Bjerknes Centre for Climate Research, Bergen, Norway

    M. Bentsen, H. Drange & T. Furevik

  3. Geophysical Institute, University of Bergen, Bergen, Norway

    H. Drange & T. Furevik

  4. Nansen-Zhu International Research Centre, Beijing, China

    H. Drange

  5. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG)/Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

    T. Zhou

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  1. M. Bentsen
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Correspondence to M. Bentsen.

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Bentsen, M., Drange, H., Furevik, T. et al. Simulated variability of the Atlantic meridional overturning circulation. Climate Dynamics 22, 701–720 (2004). https://doi.org/10.1007/s00382-004-0397-x

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