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The impact of global freshwater forcing on the thermohaline circulation: adjustment of North Atlantic convection sites in a CGCM

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Abstract

On the time scale of a century, the Atlantic thermohaline circulation (THC) is sensitive to the global surface salinity distribution. The advection of salinity toward the deep convection sites of the North Atlantic is one of the driving mechanisms for the THC. There is both a northward and a southward contributions. The northward salinity advection (Nsa) is related to the evaporation in the subtropics, and contributes to increased salinity in the convection sites. The southward salinity advection (Ssa) is related to the Arctic freshwater forcing and tends on the contrary to diminish salinity in the convection sites. The THC changes results from a delicate balance between these opposing mechanisms. In this study we evaluate these two effects using the IPSL-CM4 ocean-atmosphere-sea-ice coupled model (used for IPCC AR4). Perturbation experiments have been integrated for 100 years under modern insolation and trace gases. River runoff and evaporation minus precipitation are successively set to zero for the ocean during the coupling procedure. This allows the effect of processes Nsa and Ssa to be estimated with their specific time scales. It is shown that the convection sites in the North Atlantic exhibit various sensitivities to these processes. The Labrador Sea exhibits a dominant sensitivity to local forcing and Ssa with a typical time scale of 10 years, whereas the Irminger Sea is mostly sensitive to Nsa with a 15 year time scale. The GIN Seas respond to both effects with a time scale of 10 years for Ssa and 20 years for Nsa. It is concluded that, in the IPSL-CM4, the global freshwater forcing damps the THC on centennial time scales.

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Acknowledgments

Stimulating discussions with Gurvan Madec, Paul Williams, Didier Paillard and Gilles Ramstein are acknowledged. We are indebted to Clément de Boyer Montégut for providing data of mixed layer depth climatology and for enriching suggestions. Patrick Brockman kindly provided advice on the Ferret graphics package and its extension FAST. The coupled simulations were carried out on the NEC SX6 of the Centre de Calcul de Recherche et Technologie (CCRT). This work was supported by the Commissariat à l’Energie Atomique (CEA), and the Centre National de la Recherche Scientifique (CNRS)

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Authors and Affiliations

  1. IPSL/Laboratoire des Sciences du Climat et de l’Environnement, Orme des merisiers, 91191, Gif-sur-Yvette, France

    D. Swingedouw, P. Braconnot, P. Delecluse, E. Guilyardi & O. Marti

  2. CGAM, University of Reading, Reading, UK

    E. Guilyardi

Authors
  1. D. Swingedouw
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  2. P. Braconnot
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  3. P. Delecluse
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  4. E. Guilyardi
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  5. O. Marti
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Correspondence to D. Swingedouw.

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Swingedouw, D., Braconnot, P., Delecluse, P. et al. The impact of global freshwater forcing on the thermohaline circulation: adjustment of North Atlantic convection sites in a CGCM. Clim Dyn 28, 291–305 (2007). https://doi.org/10.1007/s00382-006-0171-3

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  • DOI: https://doi.org/10.1007/s00382-006-0171-3

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